Merge pull request #23: promote to main

Seven new games, single-player support and a menu tree. Verified on dev by
ci.yml through run 109 and published as v1.2.112-dev from this tree.

app/src/main/java/org/prolibertate/games/game/Catalog.kt

@@ -1,22 +1,39 @@
 package org.prolibertate.games.game
 
 /**
- * The catalogue the main menu is built from.
+ * The menus the main screen is built from.
  *
- * Every game the app intends to ship appears here, whether or not its rules
- * engine exists yet. [GameDescriptor.available] is what the menu keys off to
- * decide between "play" and "coming soon", so adding a game is a matter of
- * flipping that flag once its engine and screen land.
+ * A tree rather than a list, because "trick-taking" is a kind of card game
+ * rather than a rival to it. [parent] is read from a `when` rather than passed
+ * to the constructor: an enum entry cannot refer to another entry while the
+ * class is still initialising, and a lookup afterwards is free.
  */
-enum class GameCategory(val label: String) {
-    CARD("Playing Card Games"),
-    BOARD("Board Games"),
+enum class GameMenu(val label: String, val blurb: String) {
+    SOLITAIRE("Solitaire", "Played alone, against the deal or the board."),
+    CARDS("Card Games", "Anything played with a pack."),
+    TRICK_TAKING("Trick-taking", "Lead, follow, and count what you took."),
+    BOARD("Board Games", "Boards, pieces and dice."),
+    ;
+
+    val parent: GameMenu? get() = when (this) {
+        TRICK_TAKING -> CARDS
+        else -> null
+    }
+
+    val children: List<GameMenu> get() = entries.filter { it.parent == this }
+
+    val isTopLevel: Boolean get() = parent == null
 }
 
 data class GameDescriptor(
     val id: String,
     val title: String,
-    val category: GameCategory,
+    /**
+     * Every menu this game appears under. A patience is both solitaire and a
+     * card game, and Euchre is both a card game and a trick-taking one — so
+     * this is a set, and the same game is listed in each place it belongs.
+     */
+    val menus: Set<GameMenu>,
     val minPlayers: Int,
     val maxPlayers: Int,
     /** True when players are grouped into partnerships rather than playing solo. */
@@ -25,6 +42,14 @@ data class GameDescriptor(
     val available: Boolean,
 )
 
+/**
+ * The catalogue the menus are built from.
+ *
+ * Every game the app intends to ship appears here, whether or not its rules
+ * engine exists yet. [GameDescriptor.available] is what the menu keys off to
+ * decide between "play" and "coming soon", so adding a game is a matter of
+ * flipping that flag once its engine and screen land.
+ */
 object GameCatalog {
 
     const val EUCHRE = "euchre"
@@ -34,6 +59,15 @@ object GameCatalog {
     const val WIZARD = "wizard"
     const val CRAZY_EIGHTS = "crazy8s"
     const val CRIBBAGE = "cribbage"
+
+    const val HEARTS = "hearts"
+    const val PEG_SOLITAIRE = "pegsolitaire"
+
+    const val YAHTZEE = "yahtzee"
+    const val KLONDIKE = "klondike"
+    const val FREECELL = "freecell"
+    const val SPIDER = "spider"
+    const val PYRAMID = "pyramid"
     const val SEQUENCE = "sequence"
     const val MASTERMIND = "mastermind"
     const val BACKGAMMON = "backgammon"
@@ -47,7 +81,7 @@ object GameCatalog {
         GameDescriptor(
             id = EUCHRE,
             title = "Euchre",
-            category = GameCategory.CARD,
+            menus = setOf(GameMenu.CARDS, GameMenu.TRICK_TAKING),
             minPlayers = 4,
             maxPlayers = 4,
             teamBased = true,
@@ -57,7 +91,7 @@ object GameCatalog {
         GameDescriptor(
             id = KAISER,
             title = "Kaiser",
-            category = GameCategory.CARD,
+            menus = setOf(GameMenu.CARDS, GameMenu.TRICK_TAKING),
             minPlayers = 4,
             maxPlayers = 4,
             teamBased = true,
@@ -67,7 +101,7 @@ object GameCatalog {
         GameDescriptor(
             id = PRESIDENT,
             title = "President",
-            category = GameCategory.CARD,
+            menus = setOf(GameMenu.CARDS),
             minPlayers = 3,
             maxPlayers = 7,
             teamBased = false,
@@ -77,7 +111,7 @@ object GameCatalog {
         GameDescriptor(
             id = GOLF,
             title = "Golf",
-            category = GameCategory.CARD,
+            menus = setOf(GameMenu.CARDS),
             minPlayers = 2,
             maxPlayers = 6,
             teamBased = false,
@@ -87,7 +121,7 @@ object GameCatalog {
         GameDescriptor(
             id = WIZARD,
             title = "Wizard",
-            category = GameCategory.CARD,
+            menus = setOf(GameMenu.CARDS, GameMenu.TRICK_TAKING),
             minPlayers = 3,
             maxPlayers = 6,
             teamBased = false,
@@ -97,7 +131,7 @@ object GameCatalog {
         GameDescriptor(
             id = CRAZY_EIGHTS,
             title = "Crazy 8s",
-            category = GameCategory.CARD,
+            menus = setOf(GameMenu.CARDS),
             minPlayers = 2,
             maxPlayers = 6,
             teamBased = false,
@@ -107,7 +141,7 @@ object GameCatalog {
         GameDescriptor(
             id = CRIBBAGE,
             title = "Cribbage",
-            category = GameCategory.CARD,
+            menus = setOf(GameMenu.CARDS),
             minPlayers = 2,
             maxPlayers = 4,
             // Only at four, where the two pairs share a board between them.
@@ -116,10 +150,65 @@ object GameCatalog {
                 "twice round the board to 121.",
             available = true,
         ),
+        GameDescriptor(
+            id = HEARTS,
+            title = "Hearts",
+            menus = setOf(GameMenu.CARDS, GameMenu.TRICK_TAKING),
+            minPlayers = 4,
+            maxPlayers = 4,
+            teamBased = false,
+            blurb = "Take no trick worth taking. Every heart costs a point, the queen " +
+                "of spades costs thirteen, and taking all of them costs everybody else.",
+            available = true,
+        ),
+        GameDescriptor(
+            id = KLONDIKE,
+            title = "Klondike",
+            menus = setOf(GameMenu.SOLITAIRE, GameMenu.CARDS),
+            minPlayers = 1,
+            maxPlayers = 1,
+            teamBased = false,
+            blurb = "Patience, as everyone means it. Seven columns down to the aces, " +
+                "and a pack that decides most of it before you start.",
+            available = true,
+        ),
+        GameDescriptor(
+            id = FREECELL,
+            title = "FreeCell",
+            menus = setOf(GameMenu.SOLITAIRE, GameMenu.CARDS),
+            minPlayers = 1,
+            maxPlayers = 1,
+            teamBased = false,
+            blurb = "Every card face up from the start, and four cells to park what is " +
+                "in the way. Nearly every deal can be won by someone good enough.",
+            available = true,
+        ),
+        GameDescriptor(
+            id = SPIDER,
+            title = "Spider",
+            menus = setOf(GameMenu.SOLITAIRE, GameMenu.CARDS),
+            minPlayers = 1,
+            maxPlayers = 1,
+            teamBased = false,
+            blurb = "Two packs, ten columns, eight runs to build from king to ace. " +
+                "One suit is a pastime; four is a fight.",
+            available = true,
+        ),
+        GameDescriptor(
+            id = PYRAMID,
+            title = "Pyramid",
+            menus = setOf(GameMenu.SOLITAIRE, GameMenu.CARDS),
+            minPlayers = 1,
+            maxPlayers = 1,
+            teamBased = false,
+            blurb = "Take cards away in pairs that make thirteen, kings alone. " +
+                "Nothing is built and nothing goes home — the pyramid just has to go.",
+            available = true,
+        ),
         GameDescriptor(
             id = SEQUENCE,
             title = "Sequence",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD, GameMenu.CARDS),
             minPlayers = 2,
             maxPlayers = 12,
             teamBased = true,
@@ -129,7 +218,7 @@ object GameCatalog {
         GameDescriptor(
             id = MASTERMIND,
             title = "Mastermind",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD),
             minPlayers = 2,
             maxPlayers = 2,
             teamBased = false,
@@ -140,7 +229,7 @@ object GameCatalog {
         GameDescriptor(
             id = BACKGAMMON,
             title = "Backgammon",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD),
             minPlayers = 2,
             maxPlayers = 2,
             teamBased = false,
@@ -150,7 +239,7 @@ object GameCatalog {
         GameDescriptor(
             id = CHESS,
             title = "Chess",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD),
             minPlayers = 2,
             maxPlayers = 2,
             teamBased = false,
@@ -160,7 +249,7 @@ object GameCatalog {
         GameDescriptor(
             id = CHECKERS,
             title = "Checkers",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD),
             minPlayers = 2,
             maxPlayers = 2,
             teamBased = false,
@@ -170,17 +259,41 @@ object GameCatalog {
         GameDescriptor(
             id = MORRIS,
             title = "Nine Men's Morris",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD),
             minPlayers = 2,
             maxPlayers = 2,
             teamBased = false,
             blurb = "Three in a row takes a piece. Reduce them to two and the game is yours.",
             available = true,
         ),
+        GameDescriptor(
+            id = PEG_SOLITAIRE,
+            title = "Peg Solitaire",
+            menus = setOf(GameMenu.SOLITAIRE, GameMenu.BOARD),
+            // The first game here played alone. Everything downstream reads the
+            // seat count from these two numbers, so one seat is all it takes.
+            minPlayers = 1,
+            maxPlayers = 1,
+            teamBased = false,
+            blurb = "The tee game from the table, and four larger boards. Jump a peg, " +
+                "take the one you passed, and try to finish with one standing.",
+            available = true,
+        ),
+        GameDescriptor(
+            id = YAHTZEE,
+            title = "Yahtzee",
+            menus = setOf(GameMenu.BOARD),
+            minPlayers = 1,
+            maxPlayers = 6,
+            teamBased = false,
+            blurb = "Five dice, three throws, thirteen boxes to fill. Every box can be " +
+                "written in once, so the hard part is what to give up on.",
+            available = true,
+        ),
         GameDescriptor(
             id = PIRATES,
             title = "Pirates and Bulgars",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD),
             minPlayers = 2,
             maxPlayers = 2,
             teamBased = false,
@@ -191,7 +304,7 @@ object GameCatalog {
         GameDescriptor(
             id = TAYU,
             title = "Ta Yü",
-            category = GameCategory.BOARD,
+            menus = setOf(GameMenu.BOARD),
             minPlayers = 2,
             maxPlayers = 4,
             // Two play solo, four play in partnerships across the two axes.
@@ -213,13 +326,24 @@ object GameCatalog {
      * production build lists only what can actually be played, so nobody
      * downloads a release and taps into a dead end.
      */
-    fun byCategory(
-        category: GameCategory,
+    fun inMenu(
+        menu: GameMenu,
         includeComingSoon: Boolean = true,
     ): List<GameDescriptor> = all.filter {
-        it.category == category && (includeComingSoon || it.available)
+        it.menus.contains(menu) && (includeComingSoon || it.available)
     }
 
-    /** Everything currently playable, regardless of category. */
+    /**
+     * Whether a menu is worth showing at all.
+     *
+     * A section with nothing in it and no populated child under it is a heading
+     * over an empty room, which is what a production build would get if every
+     * game in a menu were still unfinished.
+     */
+    fun hasAnything(menu: GameMenu, includeComingSoon: Boolean = true): Boolean =
+        inMenu(menu, includeComingSoon).isNotEmpty() ||
+            menu.children.any { inMenu(it, includeComingSoon).isNotEmpty() }
+
+    /** Everything currently playable, whatever menu it is under. */
     val playable: List<GameDescriptor> get() = all.filter { it.available }
 }

app/src/main/java/org/prolibertate/games/game/freecell/FreeCellModel.kt

@@ -0,0 +1,81 @@
+package org.prolibertate.games.game.freecell
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import org.prolibertate.games.game.cards.Card
+
+const val COLUMNS: Int = 8
+const val FOUNDATIONS: Int = 4
+
+@Serializable
+enum class CellKind { CELL, TABLEAU, FOUNDATION }
+
+@Serializable
+data class Place(val kind: CellKind, val index: Int) {
+    companion object {
+        fun cell(index: Int) = Place(CellKind.CELL, index)
+        fun column(index: Int) = Place(CellKind.TABLEAU, index)
+        fun foundation(index: Int) = Place(CellKind.FOUNDATION, index)
+    }
+}
+
+@Serializable
+data class FreeCellOptions(
+    /**
+     * Four cells is the game as everyone knows it. Fewer is the same game made
+     * hard, since the cells are the only place a card can wait.
+     */
+    val freeCells: Int = 4,
+    /**
+     * Move a run in one gesture rather than a card at a time. The rules do not
+     * actually allow it — a run is moved by shuffling cards through the cells —
+     * but every computer version does it for you, because doing it by hand is
+     * bookkeeping rather than a decision.
+     */
+    val allowSupermoves: Boolean = true,
+) {
+    init {
+        require(freeCells in 1..6) { "One to six cells" }
+    }
+}
+
+@Serializable
+data class FreeCellState(
+    val options: FreeCellOptions,
+    val seed: Long,
+    /** Null where a cell is empty. */
+    val cells: List<Card?>,
+    val foundations: List<List<Card>>,
+    /** Every card is face up in FreeCell; there is nothing buried to turn. */
+    val tableau: List<List<Card>>,
+    val moves: Int,
+    val log: List<String>,
+) {
+    val freeCellCount: Int get() = cells.count { it == null }
+    val emptyColumns: Int get() = tableau.count { it.isEmpty() }
+    val isWon: Boolean get() = foundations.all { it.size == 13 }
+}
+
+@Serializable
+sealed interface FreeCellMove
+
+@Serializable
+@SerialName("move")
+data class MoveTo(val from: Place, val to: Place, val count: Int = 1) : FreeCellMove
+
+/**
+ * How many cards can travel together.
+ *
+ * A run is really moved one card at a time through the free cells and the empty
+ * columns, so what can be shifted is what those could have held: one per free
+ * cell plus the card itself, doubled for every empty column that could be used
+ * as a staging pile. Moving *into* an empty column cannot also use it, which is
+ * the fiddly part and the reason [intoEmptyColumn] exists.
+ */
+fun maxRunLength(freeCells: Int, emptyColumns: Int, intoEmptyColumn: Boolean): Int {
+    val usable = if (intoEmptyColumn) emptyColumns - 1 else emptyColumns
+    if (usable < 0) return 1
+    var limit = freeCells + 1
+    repeat(usable) { limit *= 2 }
+    return limit
+}

app/src/main/java/org/prolibertate/games/game/freecell/FreeCellRules.kt

@@ -0,0 +1,193 @@
+package org.prolibertate.games.game.freecell
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Decks
+import org.prolibertate.games.game.cards.shuffledWith
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+import org.prolibertate.games.game.solitaire.acceptsOnFoundation
+import org.prolibertate.games.game.solitaire.buildsDownAlternating
+import org.prolibertate.games.game.solitaire.isRun
+import kotlin.random.Random
+
+/**
+ * FreeCell as a pure state machine. See RULES-freecell.md.
+ *
+ * The whole pack is face up from the first move, so unlike Klondike there is
+ * nothing hidden and nothing to be lucky about — very nearly every deal can be
+ * won by someone who plays it well enough.
+ */
+object FreeCellRules : GameRules<FreeCellState, FreeCellMove> {
+
+    override val gameId: String = GameCatalog.FREECELL
+    const val SOLO_SEAT: Int = 0
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): FreeCellState {
+        val options = json.decodeFromString<FreeCellOptions>(config.optionsJson)
+        require(config.seats.size == 1) { "FreeCell seats one, not ${config.seats.size}" }
+        return deal(options, config.seed)
+    }
+
+    private fun deal(options: FreeCellOptions, seed: Long): FreeCellState {
+        val deck = Decks.standard52().shuffledWith(Random(seed))
+        // Fifty-two into eight leaves four columns of seven and four of six.
+        val columns = mutableListOf<List<Card>>()
+        var at = 0
+        repeat(COLUMNS) { index ->
+            val size = if (index < 4) 7 else 6
+            columns += deck.subList(at, at + size).toList()
+            at += size
+        }
+        return FreeCellState(
+            options = options,
+            seed = seed,
+            cells = List(options.freeCells) { null },
+            foundations = List(FOUNDATIONS) { emptyList() },
+            tableau = columns,
+            moves = 0,
+            log = listOf("Dealt, with ${options.freeCells} cells."),
+        )
+    }
+
+    override fun currentSeat(state: FreeCellState): Int? =
+        if (isFinished(state)) null else SOLO_SEAT
+
+    /** The longest run sitting on the end of a column, by the build rule. */
+    fun movableTail(column: List<Card>): List<Card> {
+        if (column.isEmpty()) return emptyList()
+        var take = 1
+        while (take < column.size) {
+            val candidate = column.takeLast(take + 1)
+            if (!isRun(candidate, ::buildsDownAlternating)) break
+            take++
+        }
+        return column.takeLast(take)
+    }
+
+    override fun legalMoves(state: FreeCellState, seat: Int): List<FreeCellMove> {
+        if (seat != SOLO_SEAT || state.isWon) return emptyList()
+        val moves = mutableListOf<FreeCellMove>()
+
+        fun offer(from: Place, cards: List<Card>) {
+            val head = cards.first()
+            if (cards.size == 1) {
+                val foundation = head.suit.ordinal
+                if (acceptsOnFoundation(state.foundations[foundation], head)) {
+                    moves += MoveTo(from, Place.foundation(foundation), 1)
+                }
+                state.cells.indices.forEach { cell ->
+                    if (state.cells[cell] == null && from.kind != CellKind.CELL) {
+                        moves += MoveTo(from, Place.cell(cell), 1)
+                    }
+                }
+            }
+            state.tableau.indices.forEach { column ->
+                if (from.kind == CellKind.TABLEAU && from.index == column) return@forEach
+                val target = state.tableau[column]
+                val fits = target.lastOrNull()?.let { buildsDownAlternating(head, it) } ?: true
+                if (!fits) return@forEach
+                // Moving a whole column into an empty one is a no-op that would
+                // let a game shuffle for ever.
+                if (target.isEmpty() && from.kind == CellKind.TABLEAU &&
+                    cards.size == state.tableau[from.index].size
+                ) {
+                    return@forEach
+                }
+                val allowed = maxRunLength(
+                    freeCells = state.freeCellCount,
+                    emptyColumns = state.emptyColumns,
+                    intoEmptyColumn = target.isEmpty(),
+                ).let { if (state.options.allowSupermoves) it else 1 }
+                if (cards.size <= allowed) moves += MoveTo(from, Place.column(column), cards.size)
+            }
+        }
+
+        state.cells.forEachIndexed { index, card ->
+            card?.let { offer(Place.cell(index), listOf(it)) }
+        }
+        state.tableau.forEachIndexed { index, column ->
+            val tail = movableTail(column)
+            // Every length of the tail run can travel, not just the whole of it.
+            for (count in 1..tail.size) {
+                offer(Place.column(index), tail.takeLast(count))
+            }
+        }
+        return moves
+    }
+
+    override fun applyMove(state: FreeCellState, seat: Int, move: FreeCellMove): FreeCellState {
+        require(seat == SOLO_SEAT) { "FreeCell has one seat" }
+        require(legalMoves(state, seat).contains(move)) { "$move is not legal here" }
+        move as MoveTo
+        val cards = when (move.from.kind) {
+            CellKind.CELL -> listOfNotNull(state.cells[move.from.index])
+            CellKind.TABLEAU -> state.tableau[move.from.index].takeLast(move.count)
+            CellKind.FOUNDATION -> state.foundations[move.from.index].takeLast(move.count)
+        }
+        var next = when (move.from.kind) {
+            CellKind.CELL -> state.copy(cells = state.cells.replacing(move.from.index, null))
+            CellKind.TABLEAU -> state.copy(
+                tableau = state.tableau.replacing(
+                    move.from.index,
+                    state.tableau[move.from.index].dropLast(move.count),
+                ),
+            )
+
+            CellKind.FOUNDATION -> state.copy(
+                foundations = state.foundations.replacing(
+                    move.from.index,
+                    state.foundations[move.from.index].dropLast(move.count),
+                ),
+            )
+        }
+        next = when (move.to.kind) {
+            CellKind.CELL -> next.copy(cells = next.cells.replacing(move.to.index, cards.single()))
+            CellKind.TABLEAU -> next.copy(
+                tableau = next.tableau.replacing(
+                    move.to.index,
+                    next.tableau[move.to.index] + cards,
+                ),
+            )
+
+            CellKind.FOUNDATION -> next.copy(
+                foundations = next.foundations.replacing(
+                    move.to.index,
+                    next.foundations[move.to.index] + cards,
+                ),
+            )
+        }
+        return next.copy(
+            moves = state.moves + 1,
+            log = if (next.isWon) next.log + "Out, in ${state.moves + 1} moves." else next.log,
+        )
+    }
+
+    private fun <T> List<T>.replacing(index: Int, value: T): List<T> =
+        toMutableList().also { it[index] = value }
+
+    override fun isFinished(state: FreeCellState): Boolean =
+        state.isWon || legalMoves(state, SOLO_SEAT).isEmpty()
+
+    override fun summary(state: FreeCellState): String = when {
+        state.isWon -> "Out in ${state.moves} moves"
+        isFinished(state) -> "Blocked, ${state.foundations.sumOf { it.size }} cards home"
+        else -> "${state.foundations.sumOf { it.size }} of 52 home"
+    }
+
+    override fun viewFor(state: FreeCellState, seat: Int): FreeCellState = state
+
+    override fun encodeState(state: FreeCellState): String = json.encodeToString(state)
+    override fun decodeState(json: String): FreeCellState = this.json.decodeFromString(json)
+    override fun encodeMove(move: FreeCellMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): FreeCellMove = this.json.decodeFromString(json)
+
+    fun restart(state: FreeCellState): FreeCellState = deal(state.options, state.seed)
+}

app/src/main/java/org/prolibertate/games/game/hearts/HeartsAi.kt

@@ -0,0 +1,108 @@
+package org.prolibertate.games.game.hearts
+
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.engine.GameAi
+
+/**
+ * A cautious Hearts player.
+ *
+ * It plays to avoid points rather than to shoot the moon: it sheds the queen of
+ * spades and the guards above her at the first opportunity, ducks under whoever
+ * is winning a trick when it can, and takes a trick as cheaply as possible when
+ * it cannot get out of the way. That is enough to make it an opponent worth
+ * beating without pretending to read the table.
+ */
+object HeartsAi : GameAi<HeartsState, HeartsMove> {
+
+    override fun chooseMove(state: HeartsState, seat: Int, legal: List<HeartsMove>): HeartsMove {
+        require(legal.isNotEmpty()) { "No legal move for seat $seat" }
+        return when (state.phase) {
+            HeartsPhase.PASSING -> choosePass(state, seat, legal)
+            else -> choosePlay(state, seat, legal)
+        }
+    }
+
+    private fun choosePass(state: HeartsState, seat: Int, legal: List<HeartsMove>): HeartsMove {
+        val hand = state.hands[seat]
+        val wanted = hand.sortedByDescending { passDanger(it, hand) }.take(PASS_SIZE).toSet()
+        // Prefer the exact bundle the heuristic asked for, but fall back rather
+        // than fail: the caller's list is what the host will validate against.
+        return legal.firstOrNull { it is PassCards && it.cards.toSet() == wanted } ?: legal.first()
+    }
+
+    /**
+     * How much a card wants to be somebody else's problem.
+     *
+     * The queen is the whole game, and the ace and king of spades are only
+     * dangerous because they can be forced to capture her — so a long spade
+     * holding, which gives room to duck, makes all three far less urgent.
+     */
+    private fun passDanger(card: Card, hand: List<Card>): Int {
+        val spades = hand.count { it.suit == Suit.SPADES }
+        val guarded = spades >= 5
+        return when {
+            card == QUEEN_OF_SPADES -> if (guarded) 45 else 100
+            card.suit == Suit.SPADES && card.rank.order > Rank.QUEEN.order ->
+                if (guarded) 30 else 90
+
+            card.suit == Suit.HEARTS -> 40 + card.rank.order
+            else -> card.rank.order
+        }
+    }
+
+    private fun choosePlay(state: HeartsState, seat: Int, legal: List<HeartsMove>): HeartsMove {
+        val cards = legal.filterIsInstance<PlayCard>().map { it.card }
+        if (cards.isEmpty()) return legal.first()
+        val chosen = when {
+            state.trick.isEmpty() -> lead(cards)
+            else -> follow(state, cards)
+        }
+        return PlayCard(chosen)
+    }
+
+    /**
+     * Leads low, and keeps off spades while the queen is still unaccounted for —
+     * leading a spade under her is how a seat ends up holding her.
+     */
+    private fun lead(cards: List<Card>): Card {
+        val offSpades = cards.filter { it.suit != Suit.SPADES }
+        val pool = if (offSpades.isNotEmpty()) offSpades else cards
+        return pool.minBy { it.rank.order }
+    }
+
+    private fun follow(state: HeartsState, cards: List<Card>): Card {
+        val led = state.trick.first().card.suit
+        val best = state.trick.maxOf { trickStrength(it.card, led) }
+        val pot = pointsIn(state.trick.map { it.card })
+        val last = state.trick.size == state.options.playerCount - 1
+
+        val ducking = cards.filter { trickStrength(it, led) < best }
+        if (ducking.isNotEmpty()) {
+            // Safe under the current winner, so shed the most expensive card
+            // that still loses.
+            return ducking.maxBy { it.rank.order + pointsOf(it) * 2 }
+        }
+
+        val void = cards.none { it.suit == led }
+        if (void) return discard(cards)
+
+        // Every card left would take the trick. Playing last with nothing in the
+        // pot means it costs nothing, so there is no reason to waste a high card
+        // on it; otherwise take it as cheaply as possible.
+        return if (last && pot == 0) cards.maxBy { it.rank.order } else cards.minBy { it.rank.order }
+    }
+
+    /** Void in the led suit and free to throw anything, so throw the worst of it. */
+    private fun discard(cards: List<Card>): Card {
+        cards.firstOrNull { it == QUEEN_OF_SPADES }?.let { return it }
+        cards.filter { it.suit == Suit.SPADES && it.rank.order > Rank.QUEEN.order }
+            .maxByOrNull { it.rank.order }
+            ?.let { return it }
+        cards.filter { it.suit == Suit.HEARTS }
+            .maxByOrNull { it.rank.order }
+            ?.let { return it }
+        return cards.maxBy { it.rank.order }
+    }
+}

app/src/main/java/org/prolibertate/games/game/hearts/HeartsModel.kt

@@ -0,0 +1,164 @@
+package org.prolibertate.games.game.hearts
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+import org.prolibertate.games.game.cards.Suit
+
+@Serializable
+data class HeartsOptions(
+    /**
+     * Four hands of thirteen, which is the game as it is normally played. Three
+     * and five handed Hearts exist, but they need cards struck from the deck to
+     * divide evenly, and a seat that holds a different number of cards from its
+     * neighbours changes which discards are safe — a different game to bid and
+     * play, not a seat count.
+     */
+    val playerCount: Int = 4,
+    /** The round in which somebody crosses this ends the game. */
+    val targetScore: Int = 100,
+    /**
+     * Taking all twenty-six points scores nothing and gives everyone else the
+     * lot. Switching this off scores a moon like any other hand, which is to say
+     * it becomes the worst result on the table rather than the best.
+     */
+    val allowShootTheMoon: Boolean = true,
+) {
+    init {
+        require(playerCount == 4) { "Hearts is dealt four handed" }
+        require(targetScore > 0) { "targetScore must be positive" }
+    }
+}
+
+/**
+ * Where the three passed cards go. The cycle repeats every fourth round, and
+ * the hold round is what stops a seat from planning around a known direction
+ * forever.
+ */
+@Serializable
+enum class PassDirection(val label: String) {
+    LEFT("left"),
+    RIGHT("right"),
+    ACROSS("across"),
+    HOLD("nobody"),
+    ;
+
+    companion object {
+        /** Rounds run left, right, across, hold, and back to left. */
+        fun forRound(round: Int): PassDirection = entries[round % entries.size]
+    }
+}
+
+@Serializable
+enum class HeartsPhase { PASSING, PLAYING, ROUND_OVER, GAME_OVER }
+
+@Serializable
+data class PlayedCard(val seat: Int, val card: Card)
+
+@Serializable
+data class HeartsState(
+    val options: HeartsOptions,
+    val seed: Long,
+    /** Zero-based. Only used to pick the pass direction and to number the log. */
+    val round: Int,
+    val phase: HeartsPhase,
+    val hands: List<List<Card>>,
+    /** Survives redaction so a client can draw the backs of other hands. */
+    val handCounts: List<Int>,
+    /** What each seat has chosen to pass, before the swap happens. */
+    val passSelections: List<List<Card>>,
+    val turn: Int,
+    val leader: Int,
+    /**
+     * Which trick of the round is being played, counting from zero. Carried
+     * rather than derived because the first trick is the one that bans point
+     * cards, and a rule that important should not depend on reading it back out
+     * of how many cards happen to have been taken.
+     */
+    val trickNumber: Int,
+    val trick: List<PlayedCard>,
+    /** Held on the table so a finished trick can be read before it is swept. */
+    val completedTrick: List<PlayedCard> = emptyList(),
+    /**
+     * Every card each seat has taken in tricks this round. Kept whole rather
+     * than as a running count because shooting the moon is decided on the
+     * twenty-six penalty cards together, not on a total that cannot tell an
+     * unbroken moon from thirteen hearts and a lucky queen elsewhere.
+     */
+    val taken: List<List<Card>>,
+    val heartsBroken: Boolean,
+    val scores: List<Int>,
+    val roundScores: List<Int>,
+    val log: List<String>,
+) {
+    val passDirection: PassDirection get() = PassDirection.forRound(round)
+
+    /** Thirteen, four handed: every card dealt is a card played. */
+    val tricksPerRound: Int get() = DECK_SIZE / options.playerCount
+}
+
+/** An ordinary pack, which Hearts uses whole. */
+const val DECK_SIZE: Int = 52
+
+@Serializable
+sealed interface HeartsMove
+
+/** The three cards this seat is giving away. Order does not matter. */
+@Serializable
+@SerialName("pass")
+data class PassCards(val cards: List<Card>) : HeartsMove
+
+@Serializable
+@SerialName("play")
+data class PlayCard(val card: Card) : HeartsMove
+
+/** How many cards change hands in a pass. */
+const val PASS_SIZE: Int = 3
+
+val QUEEN_OF_SPADES: Card = Card(Rank.QUEEN, Suit.SPADES)
+
+/** The two of clubs always leads the first trick of a round. */
+val TWO_OF_CLUBS: Card = Card(Rank.TWO, Suit.CLUBS)
+
+/** Every heart costs a point; the queen of spades costs thirteen. */
+fun pointsOf(card: Card): Int = when {
+    card == QUEEN_OF_SPADES -> 13
+    card.suit == Suit.HEARTS -> 1
+    else -> 0
+}
+
+/** The whole twenty-six, which is what a moon has to consist of. */
+const val ALL_POINTS: Int = 26
+
+fun pointsIn(cards: List<Card>): Int = cards.sumOf { pointsOf(it) }
+
+/** Which seat receives what [from] passes, given the round's direction. */
+fun passTarget(from: Int, playerCount: Int, direction: PassDirection): Int = when (direction) {
+    PassDirection.LEFT -> (from + 1) % playerCount
+    PassDirection.RIGHT -> (from + playerCount - 1) % playerCount
+    PassDirection.ACROSS -> (from + playerCount / 2) % playerCount
+    PassDirection.HOLD -> from
+}
+
+/**
+ * Ranks a card within a trick. Nothing trumps in Hearts, so a card that did not
+ * follow the led suit cannot win however high it is.
+ */
+fun trickStrength(card: Card, ledSuit: Suit): Int =
+    if (card.suit == ledSuit) card.rank.order else -1
+
+/**
+ * Turns a round's takings into the points each seat adds to its score.
+ *
+ * A seat that took all twenty-six has shot the moon: it scores nothing and
+ * every other seat takes the lot. That is the one case where a round's scores
+ * do not sum to twenty-six.
+ */
+fun scoreRound(taken: List<List<Card>>, allowShootTheMoon: Boolean): List<Int> {
+    val raw = taken.map { pointsIn(it) }
+    if (!allowShootTheMoon) return raw
+    val shooter = raw.indexOfFirst { it == ALL_POINTS }
+    if (shooter < 0) return raw
+    return raw.indices.map { if (it == shooter) 0 else ALL_POINTS }
+}

app/src/main/java/org/prolibertate/games/game/hearts/HeartsRules.kt

@@ -0,0 +1,294 @@
+package org.prolibertate.games.game.hearts
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Decks
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.cards.shuffledWith
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+import kotlin.random.Random
+
+/**
+ * Hearts as a pure state machine. See RULES-hearts.md.
+ */
+object HeartsRules : GameRules<HeartsState, HeartsMove> {
+
+    override val gameId: String = GameCatalog.HEARTS
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): HeartsState {
+        val options = json.decodeFromString<HeartsOptions>(config.optionsJson)
+        require(config.seats.size == options.playerCount) {
+            "Seat count ${config.seats.size} does not match ${options.playerCount} players"
+        }
+        return deal(options, config.seed, 0, List(options.playerCount) { 0 }, listOf("Round 1."))
+    }
+
+    private fun deal(
+        options: HeartsOptions,
+        seed: Long,
+        round: Int,
+        scores: List<Int>,
+        log: List<String>,
+    ): HeartsState {
+        // A fresh generator per round, keyed on the round, so the whole game is
+        // reproducible from the one seed the host shipped with the table.
+        val random = Random(seed + round)
+        val deck = Decks.standard52().shuffledWith(random)
+        val perHand = deck.size / options.playerCount
+        val hands = (0 until options.playerCount).map { seat ->
+            deck.subList(seat * perHand, (seat + 1) * perHand).sortedWith(handOrder)
+        }
+        val direction = PassDirection.forRound(round)
+        val passing = direction != PassDirection.HOLD
+        // A hold round has nothing to swap, so it opens straight onto the lead.
+        val opener = if (passing) 0 else holderOfTwoOfClubs(hands)
+        return HeartsState(
+            options = options,
+            seed = seed,
+            round = round,
+            phase = if (passing) HeartsPhase.PASSING else HeartsPhase.PLAYING,
+            hands = hands,
+            handCounts = hands.map { it.size },
+            passSelections = List(options.playerCount) { emptyList() },
+            turn = opener,
+            leader = opener,
+            trickNumber = 0,
+            trick = emptyList(),
+            completedTrick = emptyList(),
+            taken = List(options.playerCount) { emptyList() },
+            heartsBroken = false,
+            scores = scores,
+            roundScores = List(options.playerCount) { 0 },
+            log = log + if (passing) "Passing ${direction.label}." else "No passing this round.",
+        )
+    }
+
+    /** Sorted by suit then rank, which is how a hand is held rather than dealt. */
+    private val handOrder = compareBy<Card>({ it.suit.ordinal }, { it.rank.order })
+
+    private fun holderOfTwoOfClubs(hands: List<List<Card>>): Int =
+        hands.indexOfFirst { it.contains(TWO_OF_CLUBS) }
+
+    override fun currentSeat(state: HeartsState): Int? = when (state.phase) {
+        // Passing is simultaneous at the table, but the engine hands out one
+        // seat at a time. The lowest seat that has not chosen goes next, and
+        // the swap happens once the last one has.
+        HeartsPhase.PASSING -> state.passSelections.indexOfFirst { it.isEmpty() }.takeIf { it >= 0 }
+        HeartsPhase.PLAYING -> state.turn
+        HeartsPhase.ROUND_OVER, HeartsPhase.GAME_OVER -> null
+    }
+
+    override fun legalMoves(state: HeartsState, seat: Int): List<HeartsMove> =
+        when (state.phase) {
+            HeartsPhase.PASSING ->
+                if (state.passSelections[seat].isNotEmpty()) {
+                    emptyList()
+                } else {
+                    combinations(state.hands[seat], PASS_SIZE).map { PassCards(it) }
+                }
+
+            HeartsPhase.PLAYING ->
+                if (state.turn != seat) emptyList() else playableCards(state, seat).map { PlayCard(it) }
+
+            HeartsPhase.ROUND_OVER, HeartsPhase.GAME_OVER -> emptyList()
+        }
+
+    /**
+     * The cards [seat] may legally play into the trick as it stands.
+     *
+     * Four restrictions, in the order they bite: the two of clubs opens the
+     * round, a seat that can follow the led suit must, no point card may be
+     * discarded on the first trick, and hearts cannot be led until one has been
+     * played. Each of the last three lifts when a seat holds nothing else.
+     */
+    fun playableCards(state: HeartsState, seat: Int): List<Card> {
+        val hand = state.hands[seat]
+        if (hand.isEmpty()) return emptyList()
+        val firstTrick = state.trickNumber == 0
+
+        if (state.trick.isEmpty()) {
+            if (firstTrick) return listOf(TWO_OF_CLUBS).filter { hand.contains(it) }
+            val offHearts = hand.filter { it.suit != Suit.HEARTS }
+            // Hearts may only be led once broken — unless that is all there is
+            // left to lead, in which case the restriction cannot be honoured.
+            return if (state.heartsBroken || offHearts.isEmpty()) hand else offHearts
+        }
+
+        val led = state.trick.first().card.suit
+        val following = hand.filter { it.suit == led }
+        if (following.isNotEmpty()) return following
+
+        // Void in the led suit, so anything goes — except that the first trick
+        // takes no points while the seat has a card that costs none.
+        if (firstTrick) {
+            val harmless = hand.filter { pointsOf(it) == 0 }
+            if (harmless.isNotEmpty()) return harmless
+        }
+        return hand
+    }
+
+    override fun applyMove(state: HeartsState, seat: Int, move: HeartsMove): HeartsState =
+        when (move) {
+            is PassCards -> applyPass(state, seat, move)
+            is PlayCard -> applyPlay(state, seat, move)
+        }
+
+    private fun applyPass(state: HeartsState, seat: Int, move: PassCards): HeartsState {
+        require(state.phase == HeartsPhase.PASSING) { "Not passing" }
+        require(state.passSelections[seat].isEmpty()) { "Seat $seat has already passed" }
+        require(move.cards.size == PASS_SIZE) { "A pass is $PASS_SIZE cards" }
+        require(move.cards.distinct().size == PASS_SIZE) { "Cannot pass the same card twice" }
+        val hand = state.hands[seat]
+        require(move.cards.all { hand.contains(it) }) { "Cannot pass a card not held" }
+
+        val selections = state.passSelections.toMutableList()
+        selections[seat] = move.cards
+        if (selections.any { it.isEmpty() }) {
+            return state.copy(passSelections = selections)
+        }
+        return completePass(state.copy(passSelections = selections))
+    }
+
+    /** Everyone has chosen, so the three-card bundles change hands at once. */
+    private fun completePass(state: HeartsState): HeartsState {
+        val direction = state.passDirection
+        val count = state.options.playerCount
+        val incoming = MutableList(count) { emptyList<Card>() }
+        for (from in 0 until count) {
+            incoming[passTarget(from, count, direction)] = state.passSelections[from]
+        }
+        val hands = state.hands.mapIndexed { seat, hand ->
+            (hand - state.passSelections[seat].toSet() + incoming[seat]).sortedWith(handOrder)
+        }
+        val opener = holderOfTwoOfClubs(hands)
+        return state.copy(
+            phase = HeartsPhase.PLAYING,
+            hands = hands,
+            handCounts = hands.map { it.size },
+            turn = opener,
+            leader = opener,
+            log = state.log + "Cards passed ${direction.label}.",
+        )
+    }
+
+    private fun applyPlay(state: HeartsState, seat: Int, move: PlayCard): HeartsState {
+        require(state.phase == HeartsPhase.PLAYING) { "Not playing" }
+        require(state.turn == seat) { "Seat $seat is not on the clock" }
+        require(playableCards(state, seat).contains(move.card)) { "${move.card} is not playable" }
+
+        val hands = state.hands.toMutableList()
+        hands[seat] = hands[seat] - move.card
+        val trick = state.trick + PlayedCard(seat, move.card)
+        val broken = state.heartsBroken || move.card.suit == Suit.HEARTS
+
+        if (trick.size < state.options.playerCount) {
+            return state.copy(
+                hands = hands,
+                handCounts = hands.map { it.size },
+                trick = trick,
+                completedTrick = emptyList(),
+                turn = (seat + 1) % state.options.playerCount,
+                heartsBroken = broken,
+            )
+        }
+
+        val led = trick.first().card.suit
+        val winner = trick.maxBy { trickStrength(it.card, led) }.seat
+        val taken = state.taken.toMutableList()
+        taken[winner] = taken[winner] + trick.map { it.card }
+        val swept = state.copy(
+            hands = hands,
+            handCounts = hands.map { it.size },
+            trick = emptyList(),
+            completedTrick = trick,
+            taken = taken,
+            heartsBroken = broken,
+            turn = winner,
+            leader = winner,
+            trickNumber = state.trickNumber + 1,
+        )
+        return if (hands.all { it.isEmpty() }) finishRound(swept) else swept
+    }
+
+    private fun finishRound(state: HeartsState): HeartsState {
+        val gained = scoreRound(state.taken, state.options.allowShootTheMoon)
+        val scores = state.scores.mapIndexed { seat, score -> score + gained[seat] }
+        val moon = state.taken.indexOfFirst { pointsIn(it) == ALL_POINTS }
+        val over = scores.any { it >= state.options.targetScore }
+        val notes = buildList {
+            if (moon >= 0 && state.options.allowShootTheMoon) add("Seat $moon shot the moon.")
+            add("Round ${state.round + 1} scored.")
+            if (over) add("Game over — lowest score wins.")
+        }
+        return state.copy(
+            phase = if (over) HeartsPhase.GAME_OVER else HeartsPhase.ROUND_OVER,
+            scores = scores,
+            roundScores = gained,
+            log = state.log + notes,
+        )
+    }
+
+    /** Advances from [HeartsPhase.ROUND_OVER] into the next deal. */
+    fun startNextRound(state: HeartsState): HeartsState {
+        check(state.phase == HeartsPhase.ROUND_OVER) { "Round is not over" }
+        val next = state.round + 1
+        return deal(state.options, state.seed, next, state.scores, state.log + "Round ${next + 1}.")
+    }
+
+    override fun isFinished(state: HeartsState): Boolean = state.phase == HeartsPhase.GAME_OVER
+
+    override fun summary(state: HeartsState): String = when (state.phase) {
+        HeartsPhase.PASSING -> "Passing ${state.passDirection.label}"
+        HeartsPhase.PLAYING -> "Trick ${state.trickNumber + 1} of ${state.tricksPerRound}"
+        HeartsPhase.ROUND_OVER -> "Round ${state.round + 1}: " + state.roundScores.joinToString(", ")
+        HeartsPhase.GAME_OVER -> {
+            val best = state.scores.min()
+            "Winner: seat ${state.scores.indexOf(best)} on $best"
+        }
+    }
+
+    /**
+     * Other hands go, and so do the cards they have chosen to pass — a pass is
+     * secret until it lands, and a client that could read the selections would
+     * know exactly what is coming.
+     */
+    override fun viewFor(state: HeartsState, seat: Int): HeartsState = state.copy(
+        hands = state.hands.mapIndexed { index, hand -> if (index == seat) hand else emptyList() },
+        handCounts = state.handCounts,
+        passSelections = state.passSelections.mapIndexed { index, cards ->
+            if (index == seat) cards else emptyList()
+        },
+    )
+
+    override fun encodeState(state: HeartsState): String = json.encodeToString(state)
+    override fun decodeState(json: String): HeartsState = this.json.decodeFromString(json)
+    override fun encodeMove(move: HeartsMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): HeartsMove = this.json.decodeFromString(json)
+
+    /** Every distinct [size]-card selection from [cards], order ignored. */
+    private fun combinations(cards: List<Card>, size: Int): List<List<Card>> {
+        if (size == 0) return listOf(emptyList())
+        if (cards.size < size) return emptyList()
+        val result = mutableListOf<List<Card>>()
+        fun walk(start: Int, chosen: List<Card>) {
+            if (chosen.size == size) {
+                result.add(chosen)
+                return
+            }
+            // Stop early once too few cards remain to finish the selection.
+            for (i in start..cards.size - (size - chosen.size)) {
+                walk(i + 1, chosen + cards[i])
+            }
+        }
+        walk(0, emptyList())
+        return result
+    }
+}

app/src/main/java/org/prolibertate/games/game/klondike/KlondikeAi.kt

@@ -0,0 +1,50 @@
+package org.prolibertate.games.game.klondike
+
+import org.prolibertate.games.game.engine.GameAi
+
+/**
+ * Plays the deal the way a person does when they are not thinking hard.
+ *
+ * Nothing asks it to during an ordinary game — the only seat belongs to the
+ * player — but a seat has to be playable by something, and this is what a
+ * "suggest a move" button would offer.
+ *
+ * The order matters more than the numbers. Sending a card home always wins,
+ * turning one up comes next, and aimless shuffling between columns scores below
+ * drawing so it is only reached when there is nothing else at all. That last
+ * ordering is what stops it walking a run back and forth between two columns for
+ * ever.
+ */
+object KlondikeAi : GameAi<KlondikeState, KlondikeMove> {
+
+    override fun chooseMove(state: KlondikeState, seat: Int, legal: List<KlondikeMove>): KlondikeMove {
+        require(legal.isNotEmpty()) { "No legal move for seat $seat" }
+        return legal.maxBy { worth(state, it) }
+    }
+
+    private fun worth(state: KlondikeState, move: KlondikeMove): Int = when (move) {
+        Draw -> 30
+        Redeal -> 20
+        is MoveCards -> when {
+            move.to.kind == SpotKind.FOUNDATION -> 100
+            // Taking a card back off a foundation is occasionally right and
+            // usually not, so it sits below doing nothing much at all.
+            move.from.kind == SpotKind.FOUNDATION -> -50
+            move.from.kind == SpotKind.TABLEAU && turnsACardUp(state, move) -> 80
+            move.from.kind == SpotKind.TABLEAU && emptiesAColumn(state, move) -> 60
+            move.from.kind == SpotKind.WASTE -> 40
+            else -> 15
+        }
+    }
+
+    /** True when the move strips a column back to its buried cards. */
+    private fun turnsACardUp(state: KlondikeState, move: MoveCards): Boolean {
+        val pile = state.tableau[move.from.index]
+        return move.count == pile.faceUp.size && pile.faceDown.isNotEmpty()
+    }
+
+    private fun emptiesAColumn(state: KlondikeState, move: MoveCards): Boolean {
+        val pile = state.tableau[move.from.index]
+        return move.count == pile.faceUp.size && pile.faceDown.isEmpty()
+    }
+}

app/src/main/java/org/prolibertate/games/game/klondike/KlondikeModel.kt

@@ -0,0 +1,145 @@
+package org.prolibertate.games.game.klondike
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.solitaire.buildsDownAlternating
+
+const val TABLEAU_PILES: Int = 7
+const val FOUNDATIONS: Int = 4
+
+/**
+ * Where a card can be, and which one of them.
+ *
+ * A kind and an index rather than a sealed hierarchy: every spot in this game is
+ * one of four things and at most one number identifies it, so a pair says
+ * everything a subclass per kind would, and serialises as two plain fields.
+ */
+@Serializable
+enum class SpotKind { STOCK, WASTE, TABLEAU, FOUNDATION }
+
+@Serializable
+data class Spot(val kind: SpotKind, val index: Int = 0) {
+    companion object {
+        val stock = Spot(SpotKind.STOCK)
+        val waste = Spot(SpotKind.WASTE)
+        fun tableau(index: Int) = Spot(SpotKind.TABLEAU, index)
+        fun foundation(index: Int) = Spot(SpotKind.FOUNDATION, index)
+    }
+}
+
+/**
+ * A tableau pile: what is still turned down, and the run turned up on top of it.
+ *
+ * Kept as two lists because the split is the whole tension of the game. A pile
+ * is only worth what has been turned up, and the buried part is the thing being
+ * played for.
+ */
+@Serializable
+data class TableauPile(
+    val faceDown: List<Card> = emptyList(),
+    val faceUp: List<Card> = emptyList(),
+) {
+    val isEmpty: Boolean get() = faceDown.isEmpty() && faceUp.isEmpty()
+    val top: Card? get() = faceUp.lastOrNull()
+}
+
+@Serializable
+data class KlondikeOptions(
+    /**
+     * One card off the stock at a time, or three. Three is the harder game and
+     * the older one; one is what most people mean by patience now.
+     */
+    val drawCount: Int = 1,
+    /**
+     * How many times the waste may be turned back into the stock. Null is as
+     * often as you like; zero is a single pass through the pack, which is the
+     * gambling version.
+     */
+    val redealLimit: Int? = null,
+    /**
+     * Only a king may be moved into an empty column. Turning this off is a
+     * common kindness that makes far more deals winnable.
+     */
+    val kingsOnlyInSpaces: Boolean = true,
+) {
+    init {
+        require(drawCount in 1..3) { "Draw one to three" }
+        require(redealLimit == null || redealLimit >= 0) { "redealLimit cannot be negative" }
+    }
+}
+
+@Serializable
+data class KlondikeState(
+    val options: KlondikeOptions,
+    val seed: Long,
+    /** Face down, dealt from the end. */
+    val stock: List<Card>,
+    /** Face up; the last card is the one in play. */
+    val waste: List<Card>,
+    /** One per suit, in [Suit] order, each running ace upwards. */
+    val foundations: List<List<Card>>,
+    val tableau: List<TableauPile>,
+    val redealsUsed: Int,
+    val moves: Int,
+    val log: List<String>,
+) {
+    /** Every foundation full is the game won. */
+    val isWon: Boolean get() = foundations.all { it.size == Rank.standard.size }
+
+    val canRedeal: Boolean
+        get() = stock.isEmpty() &&
+            waste.isNotEmpty() &&
+            (options.redealLimit == null || redealsUsed < options.redealLimit)
+}
+
+@Serializable
+sealed interface KlondikeMove
+
+/** Turns the next card or three off the stock and onto the waste. */
+@Serializable
+@SerialName("draw")
+data object Draw : KlondikeMove
+
+/** Turns the waste back over to make a new stock, once the stock is out. */
+@Serializable
+@SerialName("redeal")
+data object Redeal : KlondikeMove
+
+/**
+ * Moves [count] cards from one spot to another.
+ *
+ * [count] is only ever more than one when moving between tableau columns, where
+ * a whole descending run travels together.
+ */
+@Serializable
+@SerialName("move")
+data class MoveCards(val from: Spot, val to: Spot, val count: Int = 1) : KlondikeMove
+
+/** Foundations are indexed by suit, so a card always knows which one it wants. */
+fun foundationFor(card: Card): Int = card.suit.ordinal
+
+/**
+ * Down by one and in the other colour.
+ *
+ * Named here because it is what this game builds with, but the rule itself is
+ * shared: FreeCell builds the same way, Spider builds in suit, and all of them
+ * need the same ace-low ordering underneath.
+ */
+fun buildsDown(lower: Card, onto: Card): Boolean = buildsDownAlternating(lower, onto)
+
+/**
+ * Whether [cards] is a run that can travel as one: each card a step below the
+ * one before it and in the opposite colour.
+ */
+fun isMovableRun(cards: List<Card>): Boolean =
+    cards.isNotEmpty() && cards.zipWithNext().all { (upper, lower) -> buildsDown(lower, upper) }
+
+/** What an empty column will take. */
+fun acceptsInSpace(card: Card, kingsOnly: Boolean): Boolean =
+    !kingsOnly || card.rank == Rank.KING
+
+/** Suit order is the foundation order, so this names them for the screen. */
+fun foundationSuit(index: Int): Suit = Suit.entries[index]

app/src/main/java/org/prolibertate/games/game/klondike/KlondikeRules.kt

@@ -0,0 +1,250 @@
+package org.prolibertate.games.game.klondike
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Decks
+import org.prolibertate.games.game.cards.shuffledWith
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+import org.prolibertate.games.game.solitaire.acceptsOnFoundation
+import kotlin.random.Random
+
+/**
+ * Klondike as a pure state machine. See RULES-klondike.md.
+ *
+ * One seat, like peg solitaire: the player is not playing against anybody, only
+ * against the deal.
+ */
+object KlondikeRules : GameRules<KlondikeState, KlondikeMove> {
+
+    override val gameId: String = GameCatalog.KLONDIKE
+
+    const val SOLO_SEAT: Int = 0
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): KlondikeState {
+        val options = json.decodeFromString<KlondikeOptions>(config.optionsJson)
+        require(config.seats.size == 1) { "Klondike seats one, not ${config.seats.size}" }
+        return deal(options, config.seed)
+    }
+
+    private fun deal(options: KlondikeOptions, seed: Long): KlondikeState {
+        val deck = Decks.standard52().shuffledWith(Random(seed)).toMutableList()
+        val piles = (0 until TABLEAU_PILES).map { index ->
+            // Column n gets n+1 cards, and only the last of them is turned up.
+            val taken = List(index + 1) { deck.removeAt(deck.size - 1) }
+            TableauPile(faceDown = taken.dropLast(1), faceUp = listOf(taken.last()))
+        }
+        return KlondikeState(
+            options = options,
+            seed = seed,
+            stock = deck.toList(),
+            waste = emptyList(),
+            foundations = List(FOUNDATIONS) { emptyList() },
+            tableau = piles,
+            redealsUsed = 0,
+            moves = 0,
+            log = listOf("Dealt. Drawing ${options.drawCount} at a time."),
+        )
+    }
+
+    override fun currentSeat(state: KlondikeState): Int? =
+        if (isFinished(state)) null else SOLO_SEAT
+
+    override fun legalMoves(state: KlondikeState, seat: Int): List<KlondikeMove> {
+        if (seat != SOLO_SEAT || state.isWon) return emptyList()
+        val moves = mutableListOf<KlondikeMove>()
+        if (state.stock.isNotEmpty()) moves += Draw
+        if (state.canRedeal) moves += Redeal
+
+        state.waste.lastOrNull()?.let { card ->
+            val foundation = foundationFor(card)
+            if (acceptsOnFoundation(state.foundations[foundation], card)) {
+                moves += MoveCards(Spot.waste, Spot.foundation(foundation))
+            }
+            state.tableau.indices.forEach { pile ->
+                if (tableauAccepts(state, pile, card)) {
+                    moves += MoveCards(Spot.waste, Spot.tableau(pile))
+                }
+            }
+        }
+
+        state.tableau.forEachIndexed { index, pile ->
+            pile.top?.let { card ->
+                val foundation = foundationFor(card)
+                if (acceptsOnFoundation(state.foundations[foundation], card)) {
+                    moves += MoveCards(Spot.tableau(index), Spot.foundation(foundation))
+                }
+            }
+            for (count in 1..pile.faceUp.size) {
+                val run = pile.faceUp.takeLast(count)
+                if (!isMovableRun(run)) continue
+                val head = run.first()
+                state.tableau.indices.forEach { target ->
+                    if (target == index || !tableauAccepts(state, target, head)) return@forEach
+                    // Shifting a whole column into an empty one gains nothing and
+                    // would let a game shuffle back and forth for ever, so it is
+                    // not offered as a move at all.
+                    val wholeColumn = pile.faceDown.isEmpty() && count == pile.faceUp.size
+                    if (wholeColumn && state.tableau[target].isEmpty) return@forEach
+                    moves += MoveCards(Spot.tableau(index), Spot.tableau(target), count)
+                }
+            }
+        }
+
+        state.foundations.forEachIndexed { index, foundation ->
+            foundation.lastOrNull()?.let { card ->
+                state.tableau.indices.forEach { pile ->
+                    if (tableauAccepts(state, pile, card)) {
+                        moves += MoveCards(Spot.foundation(index), Spot.tableau(pile))
+                    }
+                }
+            }
+        }
+        return moves
+    }
+
+    /** Whether column [index] will take [card] on top of it. */
+    fun tableauAccepts(state: KlondikeState, index: Int, card: Card): Boolean {
+        val pile = state.tableau[index]
+        val top = pile.top ?: return acceptsInSpace(card, state.options.kingsOnlyInSpaces)
+        return buildsDown(card, top)
+    }
+
+    override fun applyMove(state: KlondikeState, seat: Int, move: KlondikeMove): KlondikeState {
+        require(seat == SOLO_SEAT) { "Klondike has one seat" }
+        require(legalMoves(state, seat).contains(move)) { "$move is not legal here" }
+        val next = when (move) {
+            Draw -> draw(state)
+            Redeal -> redeal(state)
+            is MoveCards -> moveCards(state, move)
+        }
+        val won = next.isWon
+        return next.copy(
+            moves = state.moves + 1,
+            log = if (won) next.log + "Out, in ${next.moves + 1} moves." else next.log,
+        )
+    }
+
+    private fun draw(state: KlondikeState): KlondikeState {
+        val count = minOf(state.options.drawCount, state.stock.size)
+        val taken = state.stock.takeLast(count)
+        return state.copy(
+            stock = state.stock.dropLast(count),
+            // Turning a packet over reverses it, so the card that was deepest of
+            // the three is the one left showing.
+            waste = state.waste + taken.reversed(),
+        )
+    }
+
+    private fun redeal(state: KlondikeState): KlondikeState = state.copy(
+        // The waste is turned face down as one, so its top card ends up at the
+        // bottom of the new stock.
+        stock = state.waste.reversed(),
+        waste = emptyList(),
+        redealsUsed = state.redealsUsed + 1,
+        log = state.log + "Waste turned back over.",
+    )
+
+    private fun moveCards(state: KlondikeState, move: MoveCards): KlondikeState {
+        val cards = cardsAt(state, move.from, move.count)
+        val without = removeFrom(state, move.from, move.count)
+        return addTo(without, move.to, cards)
+    }
+
+    private fun cardsAt(state: KlondikeState, from: Spot, count: Int): List<Card> = when (from.kind) {
+        SpotKind.WASTE -> state.waste.takeLast(count)
+        SpotKind.TABLEAU -> state.tableau[from.index].faceUp.takeLast(count)
+        SpotKind.FOUNDATION -> state.foundations[from.index].takeLast(count)
+        SpotKind.STOCK -> throw IllegalArgumentException("Cards are not taken from the stock by hand")
+    }
+
+    private fun removeFrom(state: KlondikeState, from: Spot, count: Int): KlondikeState =
+        when (from.kind) {
+            SpotKind.WASTE -> state.copy(waste = state.waste.dropLast(count))
+
+            SpotKind.TABLEAU -> {
+                val pile = state.tableau[from.index]
+                val left = pile.faceUp.dropLast(count)
+                // Emptying the turned-up run exposes whatever was under it, and
+                // turning that card is the point of most of the game.
+                val flipped = if (left.isEmpty() && pile.faceDown.isNotEmpty()) {
+                    TableauPile(
+                        faceDown = pile.faceDown.dropLast(1),
+                        faceUp = listOf(pile.faceDown.last()),
+                    )
+                } else {
+                    pile.copy(faceUp = left)
+                }
+                state.copy(tableau = state.tableau.replacing(from.index, flipped))
+            }
+
+            SpotKind.FOUNDATION -> state.copy(
+                foundations = state.foundations.replacing(
+                    from.index,
+                    state.foundations[from.index].dropLast(count),
+                ),
+            )
+
+            SpotKind.STOCK -> throw IllegalArgumentException("Nothing is taken off the stock by hand")
+        }
+
+    private fun addTo(state: KlondikeState, to: Spot, cards: List<Card>): KlondikeState =
+        when (to.kind) {
+            SpotKind.TABLEAU -> {
+                val pile = state.tableau[to.index]
+                state.copy(
+                    tableau = state.tableau.replacing(
+                        to.index,
+                        pile.copy(faceUp = pile.faceUp + cards),
+                    ),
+                )
+            }
+
+            SpotKind.FOUNDATION -> state.copy(
+                foundations = state.foundations.replacing(
+                    to.index,
+                    state.foundations[to.index] + cards,
+                ),
+            )
+
+            SpotKind.WASTE, SpotKind.STOCK ->
+                throw IllegalArgumentException("Nothing is put back on the ${to.kind}")
+        }
+
+    private fun <T> List<T>.replacing(index: Int, value: T): List<T> =
+        toMutableList().also { it[index] = value }
+
+    /**
+     * Won, or with nothing left to try.
+     *
+     * A game can dry up entirely: no stock, no redeal left, and not a card that
+     * will go anywhere. That is a finish as much as winning is, and the screen
+     * needs to be able to say so rather than leaving the player hunting.
+     */
+    override fun isFinished(state: KlondikeState): Boolean =
+        state.isWon || legalMoves(state, SOLO_SEAT).isEmpty()
+
+    override fun summary(state: KlondikeState): String = when {
+        state.isWon -> "Out in ${state.moves} moves"
+        isFinished(state) -> "Blocked, ${state.foundations.sumOf { it.size }} cards home"
+        else -> "${state.foundations.sumOf { it.size }} of 52 home"
+    }
+
+    /** One seat, and it is the player's, so there is nothing to strip. */
+    override fun viewFor(state: KlondikeState, seat: Int): KlondikeState = state
+
+    override fun encodeState(state: KlondikeState): String = json.encodeToString(state)
+    override fun decodeState(json: String): KlondikeState = this.json.decodeFromString(json)
+    override fun encodeMove(move: KlondikeMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): KlondikeMove = this.json.decodeFromString(json)
+
+    /** Deals the same pack again, so a lost game can be replayed from the start. */
+    fun restart(state: KlondikeState): KlondikeState = deal(state.options, state.seed)
+}

app/src/main/java/org/prolibertate/games/game/pegsolitaire/PegSolitaireAi.kt

@@ -0,0 +1,54 @@
+package org.prolibertate.games.game.pegsolitaire
+
+import org.prolibertate.games.game.engine.GameAi
+
+/**
+ * Plays the board for the one seat.
+ *
+ * Nothing normally asks it to: the single seat belongs to the person holding the
+ * phone, so this never runs during an ordinary game. It exists because a seat
+ * that could be left empty has to be playable by something, and because a
+ * one-move-deep chooser is exactly what a "suggest a jump" button would want.
+ *
+ * The heuristic is the one every good peg player uses by hand: keep the pegs
+ * together. A jump that leaves more jumps behind it has not painted the board
+ * into a corner, and pulling toward the middle stops a stranded peg being left
+ * out on an arm where nothing can ever reach it.
+ */
+object PegSolitaireAi : GameAi<PegSolitaireState, PegSolitaireMove> {
+
+    override fun chooseMove(
+        state: PegSolitaireState,
+        seat: Int,
+        legal: List<PegSolitaireMove>,
+    ): PegSolitaireMove {
+        require(legal.isNotEmpty()) { "No jump available" }
+        val jumps = legal.filterIsInstance<PegJump>()
+        if (jumps.isEmpty()) return legal.first()
+        return jumps.maxBy { score(state, it) }
+    }
+
+    private fun score(state: PegSolitaireState, jump: PegJump): Int {
+        val after = state.copy(pegs = state.pegs - jump.from - jump.over + jump.to)
+        // Solving outright beats every other consideration.
+        if (after.solved) return Int.MAX_VALUE
+        val followUps = PegSolitaireRules.legalJumps(after).size
+        return followUps * 10 - spread(after)
+    }
+
+    /**
+     * How far the pegs sit from their own centre, in whole holes.
+     *
+     * A smaller number is a tighter cluster. Manhattan distance is the right
+     * measure on both board shapes here, because a jump moves two steps along one
+     * of the grid's own directions either way.
+     */
+    private fun spread(state: PegSolitaireState): Int {
+        if (state.pegs.isEmpty()) return 0
+        val meanRow = state.pegs.sumOf { it.row } / state.pegs.size
+        val meanCol = state.pegs.sumOf { it.col } / state.pegs.size
+        return state.pegs.sumOf {
+            kotlin.math.abs(it.row - meanRow) + kotlin.math.abs(it.col - meanCol)
+        }
+    }
+}

app/src/main/java/org/prolibertate/games/game/pegsolitaire/PegSolitaireModel.kt

@@ -0,0 +1,153 @@
+package org.prolibertate.games.game.pegsolitaire
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * A hole in the board, by row and column.
+ *
+ * Both geometries the boards come in are indexed this way, which is what lets
+ * one jump rule serve them all: a triangular row is simply shorter than the one
+ * below it, and a cross is a square with pieces bitten out of the corners.
+ */
+@Serializable
+data class Hole(val row: Int, val col: Int)
+
+/**
+ * The boards this plays on.
+ *
+ * Two shapes of grid, and the difference between them is only how many
+ * directions a peg can jump in. On a triangle a hole has six neighbours, because
+ * the rows are staggered; on a square board it has four, because they are not.
+ * Everything else — the jump, the goal, the dead end — is the same game.
+ */
+@Serializable
+enum class PegBoard(val label: String, val holeCount: Int) {
+    /** The tee game found on a restaurant table: five rows, fifteen holes. */
+    TRIANGLE_15("Triangle of 15", 15),
+    TRIANGLE_21("Triangle of 21", 21),
+
+    /** The classic cross: a seven by seven square with two-by-two corners cut. */
+    ENGLISH_CROSS("English cross", 33),
+
+    /** The continental board, with a shallower cut that leaves four more holes. */
+    FRENCH_CROSS("French cross", 37),
+    DIAMOND_41("Diamond of 41", 41),
+    ;
+
+    val isTriangular: Boolean get() = this == TRIANGLE_15 || this == TRIANGLE_21
+
+    /** Every hole on this board. */
+    fun holes(): Set<Hole> = when (this) {
+        TRIANGLE_15 -> triangle(5)
+        TRIANGLE_21 -> triangle(6)
+        // 49 less the four two-by-two corners.
+        ENGLISH_CROSS -> square(7) { r, c -> r in 2..4 || c in 2..4 }
+        // 49 less a three-hole triangle at each corner.
+        FRENCH_CROSS -> square(7) { r, c ->
+            val fromEdgeRow = minOf(r, 6 - r)
+            val fromEdgeCol = minOf(c, 6 - c)
+            fromEdgeRow + fromEdgeCol >= 2
+        }
+
+        DIAMOND_41 -> square(9) { r, c -> kotlin.math.abs(r - 4) + kotlin.math.abs(c - 4) <= 4 }
+    }
+
+    /**
+     * The directions a jump may run in, as row and column steps.
+     *
+     * Diagonals are not among the four on a square board: a peg moves along a
+     * rank or a file and nowhere else, which is what makes the cross boards hard.
+     */
+    fun directions(): List<Pair<Int, Int>> = if (isTriangular) {
+        listOf(0 to -1, 0 to 1, -1 to -1, -1 to 0, 1 to 0, 1 to 1)
+    } else {
+        listOf(0 to -1, 0 to 1, -1 to 0, 1 to 0)
+    }
+
+    /** Where the one empty hole sits when nothing else is chosen. */
+    fun defaultStart(): Hole = when {
+        // The apex, which is how the tee game is set out on the table.
+        isTriangular -> Hole(0, 0)
+        this == DIAMOND_41 -> Hole(4, 4)
+        else -> Hole(3, 3)
+    }
+
+    private fun triangle(rows: Int): Set<Hole> =
+        (0 until rows).flatMap { r -> (0..r).map { c -> Hole(r, c) } }.toSet()
+
+    private fun square(size: Int, keep: (Int, Int) -> Boolean): Set<Hole> =
+        (0 until size).flatMap { r ->
+            (0 until size).mapNotNull { c -> Hole(r, c).takeIf { keep(r, c) } }
+        }.toSet()
+}
+
+/**
+ * What counts as having won.
+ *
+ * Leaving one peg anywhere is the game as it is usually played. Leaving it in
+ * the hole the board started empty is the harder problem, and on the English
+ * cross it is the one the puzzle is famous for.
+ */
+@Serializable
+enum class PegGoal(val label: String) {
+    ONE_PEG("Leave one peg"),
+    ONE_PEG_IN_START("Leave one peg, in the starting hole"),
+}
+
+@Serializable
+data class PegSolitaireOptions(
+    val board: PegBoard = PegBoard.TRIANGLE_15,
+    val goal: PegGoal = PegGoal.ONE_PEG,
+    /**
+     * The hole left empty at the start. Null takes the board's own default,
+     * which keeps a saved table valid if the default ever moves.
+     */
+    val startEmpty: Hole? = null,
+) {
+    /** The empty hole this table actually opens with. */
+    fun start(): Hole = startEmpty ?: board.defaultStart()
+
+    init {
+        require(board.holes().contains(start())) {
+            "${start()} is not a hole on the ${board.label}"
+        }
+    }
+}
+
+@Serializable
+data class PegSolitaireState(
+    val options: PegSolitaireOptions,
+    /** Which holes still hold a peg. Everything else on the board is empty. */
+    val pegs: Set<Hole>,
+    val jumps: Int,
+    /** Held so the screen can show what just happened. */
+    val lastJump: PegJump? = null,
+    val log: List<String>,
+) {
+    val board: PegBoard get() = options.board
+
+    val remaining: Int get() = pegs.size
+
+    /** True once the board matches what the chosen goal asked for. */
+    val solved: Boolean
+        get() = remaining == 1 && when (options.goal) {
+            PegGoal.ONE_PEG -> true
+            PegGoal.ONE_PEG_IN_START -> pegs.first() == options.start()
+        }
+}
+
+@Serializable
+sealed interface PegSolitaireMove
+
+/**
+ * A peg jumps a neighbour and lands two holes away, and the peg it jumped is
+ * taken off. [over] is not carried because a jump is always exactly two steps in
+ * one direction, so the hole between the ends is the only one it can be —
+ * storing it would be a second copy of the same fact, free to disagree.
+ */
+@Serializable
+@SerialName("jump")
+data class PegJump(val from: Hole, val to: Hole) : PegSolitaireMove {
+    val over: Hole get() = Hole((from.row + to.row) / 2, (from.col + to.col) / 2)
+}

app/src/main/java/org/prolibertate/games/game/pegsolitaire/PegSolitaireRules.kt

@@ -0,0 +1,127 @@
+package org.prolibertate.games.game.pegsolitaire
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+
+/**
+ * Peg solitaire as a pure state machine. See RULES-pegsolitaire.md.
+ *
+ * The first game here played by one person, which the engine takes in its
+ * stride: there is one seat, it is always on the clock until the board runs out
+ * of jumps, and [viewFor] has nothing to hide because there is nobody to hide it
+ * from.
+ */
+object PegSolitaireRules : GameRules<PegSolitaireState, PegSolitaireMove> {
+
+    override val gameId: String = GameCatalog.PEG_SOLITAIRE
+
+    /** The one seat. Named rather than written as a bare 0 at each use. */
+    const val SOLO_SEAT: Int = 0
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): PegSolitaireState {
+        val options = json.decodeFromString<PegSolitaireOptions>(config.optionsJson)
+        require(config.seats.size == 1) {
+            "Peg solitaire seats one, not ${config.seats.size}"
+        }
+        val start = options.start()
+        return PegSolitaireState(
+            options = options,
+            pegs = options.board.holes() - start,
+            jumps = 0,
+            lastJump = null,
+            log = listOf("${options.board.label}, opening at row ${start.row + 1}."),
+        )
+    }
+
+    /**
+     * Every jump available on the board as it stands.
+     *
+     * A jump needs three holes in a line: a peg to move, a peg beside it to take,
+     * and an empty hole beyond. Checking that the landing hole is on the board is
+     * what keeps a jump from running off a triangle's short row or out through a
+     * cross's missing corner.
+     */
+    fun legalJumps(state: PegSolitaireState): List<PegJump> {
+        val holes = state.board.holes()
+        val directions = state.board.directions()
+        return state.pegs.flatMap { from ->
+            directions.mapNotNull { (dr, dc) ->
+                val over = Hole(from.row + dr, from.col + dc)
+                val to = Hole(from.row + 2 * dr, from.col + 2 * dc)
+                // `over` holding a peg already implies it is a hole, since a peg
+                // can only ever sit in one.
+                val ok = state.pegs.contains(over) &&
+                    holes.contains(to) &&
+                    !state.pegs.contains(to)
+                if (ok) PegJump(from, to) else null
+            }
+        }
+    }
+
+    override fun currentSeat(state: PegSolitaireState): Int? =
+        if (isFinished(state)) null else SOLO_SEAT
+
+    override fun legalMoves(state: PegSolitaireState, seat: Int): List<PegSolitaireMove> =
+        if (seat != SOLO_SEAT) emptyList() else legalJumps(state)
+
+    override fun applyMove(
+        state: PegSolitaireState,
+        seat: Int,
+        move: PegSolitaireMove,
+    ): PegSolitaireState {
+        require(seat == SOLO_SEAT) { "Peg solitaire has one seat" }
+        val jump = move as PegJump
+        require(legalJumps(state).contains(jump)) { "$jump is not a legal jump" }
+        val pegs = state.pegs - jump.from - jump.over + jump.to
+        val next = state.copy(
+            pegs = pegs,
+            jumps = state.jumps + 1,
+            lastJump = jump,
+        )
+        val note = when {
+            next.solved -> "Solved in ${next.jumps} jumps."
+            legalJumps(next).isEmpty() -> "No jumps left, ${next.remaining} pegs standing."
+            else -> null
+        }
+        return if (note == null) next else next.copy(log = next.log + note)
+    }
+
+    /** The board is finished when nothing can jump, whether or not it was solved. */
+    override fun isFinished(state: PegSolitaireState): Boolean = legalJumps(state).isEmpty()
+
+    override fun summary(state: PegSolitaireState): String = when {
+        state.solved -> "Solved in ${state.jumps} jumps"
+        isFinished(state) -> "Stuck with ${state.remaining} pegs"
+        else -> "${state.remaining} pegs, ${legalJumps(state).size} jumps available"
+    }
+
+    /**
+     * Nothing to strip. One person plays, the whole board is face up in front of
+     * them, and there is no second seat that could be told less than the truth.
+     */
+    override fun viewFor(state: PegSolitaireState, seat: Int): PegSolitaireState = state
+
+    override fun encodeState(state: PegSolitaireState): String = json.encodeToString(state)
+    override fun decodeState(json: String): PegSolitaireState = this.json.decodeFromString(json)
+    override fun encodeMove(move: PegSolitaireMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): PegSolitaireMove = this.json.decodeFromString(json)
+
+    /** Deals the same board again, so a lost game can be retried from the start. */
+    fun restart(state: PegSolitaireState): PegSolitaireState {
+        val start = state.options.start()
+        return state.copy(
+            pegs = state.options.board.holes() - start,
+            jumps = 0,
+            lastJump = null,
+            log = state.log + "Board reset.",
+        )
+    }
+}

app/src/main/java/org/prolibertate/games/game/pyramid/PyramidModel.kt

@@ -0,0 +1,98 @@
+package org.prolibertate.games.game.pyramid
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.solitaire.patienceOrder
+
+/** Seven rows of one to seven cards. */
+const val ROWS: Int = 7
+const val PYRAMID_SIZE: Int = ROWS * (ROWS + 1) / 2
+
+/** A pair has to make this, and a king makes it alone. */
+const val TARGET: Int = 13
+
+@Serializable
+enum class PyramidZone { PYRAMID, WASTE }
+
+@Serializable
+data class PyramidSpot(val zone: PyramidZone, val index: Int = 0)
+
+@Serializable
+data class PyramidOptions(
+    /** How many times the waste may be turned back over. */
+    val redeals: Int = 2,
+) {
+    init {
+        require(redeals >= 0) { "redeals cannot be negative" }
+    }
+}
+
+@Serializable
+data class PyramidState(
+    val options: PyramidOptions,
+    val seed: Long,
+    /** Twenty-eight places; null where the card has been taken. */
+    val pyramid: List<Card?>,
+    val stock: List<Card>,
+    val waste: List<Card>,
+    val redealsUsed: Int,
+    val moves: Int,
+    val log: List<String>,
+) {
+    val isWon: Boolean get() = pyramid.all { it == null }
+
+    val canRedeal: Boolean
+        get() = stock.isEmpty() && waste.isNotEmpty() && redealsUsed < options.redeals
+}
+
+@Serializable
+sealed interface PyramidMove
+
+@Serializable
+@SerialName("draw")
+data object DrawCard : PyramidMove
+
+@Serializable
+@SerialName("redeal")
+data object RecycleWaste : PyramidMove
+
+/** A king is worth thirteen on its own, so it needs no partner. */
+@Serializable
+@SerialName("king")
+data class TakeKing(val spot: PyramidSpot) : PyramidMove
+
+@Serializable
+@SerialName("pair")
+data class TakePair(val first: PyramidSpot, val second: PyramidSpot) : PyramidMove
+
+/** Ace one through king thirteen, which is what the pairing is counted in. */
+fun valueOf(card: Card): Int = card.rank.patienceOrder
+
+/** Where a row starts in the flattened pyramid. */
+fun rowStart(row: Int): Int = row * (row + 1) / 2
+
+/** Which row a place is in. */
+fun rowOf(index: Int): Int {
+    var row = 0
+    while (rowStart(row + 1) <= index) row++
+    return row
+}
+
+/**
+ * The two places resting on [index], if any.
+ *
+ * A card is only in play once both of them are gone, which is the whole shape of
+ * the game: the pyramid has to be taken apart from the bottom.
+ */
+fun coveredBy(index: Int): List<Int> {
+    val row = rowOf(index)
+    if (row == ROWS - 1) return emptyList()
+    val offset = index - rowStart(row)
+    val below = rowStart(row + 1) + offset
+    return listOf(below, below + 1)
+}
+
+/** Whether the card at [index] has nothing left sitting on it. */
+fun isExposed(pyramid: List<Card?>, index: Int): Boolean =
+    pyramid[index] != null && coveredBy(index).all { pyramid[it] == null }

app/src/main/java/org/prolibertate/games/game/pyramid/PyramidRules.kt

@@ -0,0 +1,150 @@
+package org.prolibertate.games.game.pyramid
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Decks
+import org.prolibertate.games.game.cards.shuffledWith
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+import kotlin.random.Random
+
+/**
+ * Pyramid as a pure state machine. See RULES-pyramid.md.
+ *
+ * The odd one out among the patiences here: nothing is built and nothing goes
+ * home in order. Cards are taken away in pairs that make thirteen, and the only
+ * structure is which of them are still buried.
+ */
+object PyramidRules : GameRules<PyramidState, PyramidMove> {
+
+    override val gameId: String = GameCatalog.PYRAMID
+    const val SOLO_SEAT: Int = 0
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): PyramidState {
+        val options = json.decodeFromString<PyramidOptions>(config.optionsJson)
+        require(config.seats.size == 1) { "Pyramid seats one, not ${config.seats.size}" }
+        return deal(options, config.seed)
+    }
+
+    private fun deal(options: PyramidOptions, seed: Long): PyramidState {
+        val deck = Decks.standard52().shuffledWith(Random(seed))
+        return PyramidState(
+            options = options,
+            seed = seed,
+            pyramid = deck.take(PYRAMID_SIZE),
+            stock = deck.drop(PYRAMID_SIZE),
+            waste = emptyList(),
+            redealsUsed = 0,
+            moves = 0,
+            log = listOf("Dealt."),
+        )
+    }
+
+    override fun currentSeat(state: PyramidState): Int? =
+        if (isFinished(state)) null else SOLO_SEAT
+
+    /** Every card that can be reached: the exposed pyramid, and the waste's top. */
+    fun available(state: PyramidState): List<Pair<PyramidSpot, Card>> {
+        val fromPyramid = state.pyramid.indices
+            .filter { isExposed(state.pyramid, it) }
+            .map { PyramidSpot(PyramidZone.PYRAMID, it) to state.pyramid[it]!! }
+        val fromWaste = state.waste.lastOrNull()
+            ?.let { listOf(PyramidSpot(PyramidZone.WASTE) to it) }
+            .orEmpty()
+        return fromPyramid + fromWaste
+    }
+
+    override fun legalMoves(state: PyramidState, seat: Int): List<PyramidMove> {
+        if (seat != SOLO_SEAT || state.isWon) return emptyList()
+        val moves = mutableListOf<PyramidMove>()
+        if (state.stock.isNotEmpty()) moves += DrawCard
+        if (state.canRedeal) moves += RecycleWaste
+
+        val reachable = available(state)
+        reachable.forEach { (spot, card) ->
+            if (valueOf(card) == TARGET) moves += TakeKing(spot)
+        }
+        // Each pair once: the second card is only ever looked for after the
+        // first, so a pair cannot be offered both ways round.
+        for (i in reachable.indices) {
+            for (j in i + 1 until reachable.size) {
+                val (spotA, cardA) = reachable[i]
+                val (spotB, cardB) = reachable[j]
+                if (valueOf(cardA) + valueOf(cardB) == TARGET) {
+                    moves += TakePair(spotA, spotB)
+                }
+            }
+        }
+        return moves
+    }
+
+    override fun applyMove(state: PyramidState, seat: Int, move: PyramidMove): PyramidState {
+        require(seat == SOLO_SEAT) { "Pyramid has one seat" }
+        require(legalMoves(state, seat).contains(move)) { "$move is not legal here" }
+        val next = when (move) {
+            DrawCard -> state.copy(
+                stock = state.stock.dropLast(1),
+                waste = state.waste + state.stock.last(),
+            )
+
+            RecycleWaste -> state.copy(
+                stock = state.waste.reversed(),
+                waste = emptyList(),
+                redealsUsed = state.redealsUsed + 1,
+                log = state.log + "Waste turned back over.",
+            )
+
+            is TakeKing -> remove(state, listOf(move.spot))
+            is TakePair -> remove(state, listOf(move.first, move.second))
+        }
+        return next.copy(
+            moves = state.moves + 1,
+            log = if (next.isWon) next.log + "Pyramid cleared, in ${state.moves + 1} moves." else next.log,
+        )
+    }
+
+    /**
+     * Takes cards off the table.
+     *
+     * The pyramid places are blanked rather than compacted, because a place is
+     * what tells the two cards above it that they are free — an index that
+     * shifted would uncover the wrong cards.
+     */
+    private fun remove(state: PyramidState, spots: List<PyramidSpot>): PyramidState {
+        val pyramid = state.pyramid.toMutableList()
+        var waste = state.waste
+        spots.forEach { spot ->
+            when (spot.zone) {
+                PyramidZone.PYRAMID -> pyramid[spot.index] = null
+                // Only ever the top card, so dropping the last is enough.
+                PyramidZone.WASTE -> waste = waste.dropLast(1)
+            }
+        }
+        return state.copy(pyramid = pyramid.toList(), waste = waste)
+    }
+
+    override fun isFinished(state: PyramidState): Boolean =
+        state.isWon || legalMoves(state, SOLO_SEAT).isEmpty()
+
+    override fun summary(state: PyramidState): String = when {
+        state.isWon -> "Cleared in ${state.moves} moves"
+        isFinished(state) -> "Blocked, ${state.pyramid.count { it != null }} cards left"
+        else -> "${state.pyramid.count { it != null }} of $PYRAMID_SIZE left"
+    }
+
+    override fun viewFor(state: PyramidState, seat: Int): PyramidState = state
+
+    override fun encodeState(state: PyramidState): String = json.encodeToString(state)
+    override fun decodeState(json: String): PyramidState = this.json.decodeFromString(json)
+    override fun encodeMove(move: PyramidMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): PyramidMove = this.json.decodeFromString(json)
+
+    fun restart(state: PyramidState): PyramidState = deal(state.options, state.seed)
+}

app/src/main/java/org/prolibertate/games/game/solitaire/HintAi.kt

@@ -0,0 +1,24 @@
+package org.prolibertate.games.game.solitaire
+
+import org.prolibertate.games.game.engine.GameAi
+
+/**
+ * Takes the first move on offer.
+ *
+ * Every patience here has one seat and it belongs to the person playing, so
+ * nothing asks this during an ordinary game. [MatchController] needs an AI to
+ * construct, though, and a seat that could be left empty has to be playable by
+ * something.
+ *
+ * It is deliberately not clever. A greedy patience player that looked good would
+ * be worse than this: it would be trusted as a hint, and a hint that walks a run
+ * back and forth is more annoying than no hint at all. Klondike has a real one
+ * because its move ordering is obvious; these three do not, and pretending
+ * otherwise would be the wrong kind of helpful.
+ */
+class FirstLegalAi<S : Any, M : Any> : GameAi<S, M> {
+    override fun chooseMove(state: S, seat: Int, legal: List<M>): M {
+        require(legal.isNotEmpty()) { "No legal move for seat $seat" }
+        return legal.first()
+    }
+}

app/src/main/java/org/prolibertate/games/game/solitaire/Patience.kt

@@ -0,0 +1,41 @@
+package org.prolibertate.games.game.solitaire
+
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+
+/**
+ * Where a rank sits when the ace is low.
+ *
+ * [Rank.order] puts the ace at fourteen, which is right for the games this deck
+ * was built for: in Euchre, Kaiser, Wizard and the rest it is the highest card
+ * there is. Every patience in this app counts it the other way — foundations run
+ * ace upwards, columns run down to an ace, and Pyramid needs it to be worth one
+ * when a pair has to make thirteen.
+ *
+ * It lives here rather than on [Rank] because ace high is correct everywhere
+ * outside these games, and it is shared rather than copied because four
+ * separate definitions of "the ace is low" would be four chances to disagree.
+ */
+val Rank.patienceOrder: Int get() = if (this == Rank.ACE) 1 else order
+
+/** Down by one, and in the other colour: the ordinary tableau build. */
+fun buildsDownAlternating(lower: Card, onto: Card): Boolean =
+    lower.rank.patienceOrder == onto.rank.patienceOrder - 1 &&
+        lower.suit.isRed != onto.suit.isRed
+
+/** Down by one in the same suit, which is what Spider pays for. */
+fun buildsDownInSuit(lower: Card, onto: Card): Boolean =
+    lower.rank.patienceOrder == onto.rank.patienceOrder - 1 && lower.suit == onto.suit
+
+/** Ace first, then strictly upwards in the same suit. */
+fun acceptsOnFoundation(foundation: List<Card>, card: Card): Boolean {
+    val top = foundation.lastOrNull() ?: return card.rank == Rank.ACE
+    return card.suit == top.suit && card.rank.patienceOrder == top.rank.patienceOrder + 1
+}
+
+/**
+ * Whether [cards] is a run that travels as one, by whatever rule the game
+ * builds with.
+ */
+fun isRun(cards: List<Card>, builds: (Card, Card) -> Boolean): Boolean =
+    cards.isNotEmpty() && cards.zipWithNext().all { (upper, lower) -> builds(lower, upper) }

app/src/main/java/org/prolibertate/games/game/spider/SpiderModel.kt

@@ -0,0 +1,107 @@
+package org.prolibertate.games.game.spider
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.solitaire.patienceOrder
+
+const val COLUMNS: Int = 10
+
+/** Eight complete runs, king down to ace, is the whole pack cleared. */
+const val RUNS_TO_WIN: Int = 8
+const val RUN_LENGTH: Int = 13
+
+@Serializable
+data class SpiderOptions(
+    /**
+     * How many suits the two packs are made of. One suit is the game people
+     * actually finish; four is the real thing and is brutal. Two sits between
+     * them. The pack is always a hundred and four cards either way — only how
+     * many different suits those cards are drawn from changes.
+     */
+    val suits: Int = 1,
+) {
+    init {
+        require(suits == 1 || suits == 2 || suits == 4) { "Spider is played with one, two or four suits" }
+    }
+}
+
+@Serializable
+data class SpiderPile(
+    val faceDown: List<Card> = emptyList(),
+    val faceUp: List<Card> = emptyList(),
+) {
+    val isEmpty: Boolean get() = faceDown.isEmpty() && faceUp.isEmpty()
+    val top: Card? get() = faceUp.lastOrNull()
+}
+
+@Serializable
+data class SpiderState(
+    val options: SpiderOptions,
+    val seed: Long,
+    val stock: List<Card>,
+    val tableau: List<SpiderPile>,
+    /** Runs sent away complete. Eight of them wins. */
+    val completed: Int,
+    val moves: Int,
+    val log: List<String>,
+) {
+    val isWon: Boolean get() = completed == RUNS_TO_WIN
+
+    /**
+     * A row cannot be dealt onto an empty column — the rule that stops a deal
+     * from being used to dodge a position with nowhere to put anything.
+     */
+    val canDeal: Boolean get() = stock.isNotEmpty() && tableau.none { it.isEmpty }
+}
+
+@Serializable
+sealed interface SpiderMove
+
+/** One card face up onto every column at once. */
+@Serializable
+@SerialName("deal")
+data object DealRow : SpiderMove
+
+@Serializable
+@SerialName("move")
+data class MoveRun(val from: Int, val to: Int, val count: Int) : SpiderMove
+
+/**
+ * Down by one in the same suit.
+ *
+ * Spider lets a card *land* on any suit a rank higher, but only a same-suit run
+ * may travel as a unit — which is the whole difficulty of the four-suit game.
+ */
+fun sameSuitRun(cards: List<Card>): Boolean =
+    cards.isNotEmpty() && cards.zipWithNext().all { (upper, lower) ->
+        lower.suit == upper.suit && lower.rank.patienceOrder == upper.rank.patienceOrder - 1
+    }
+
+/** What a column will take on top: anything one rank higher, of any suit. */
+fun landsOn(card: Card, onto: Card): Boolean =
+    card.rank.patienceOrder == onto.rank.patienceOrder - 1
+
+/** King down to ace in one suit, which is a run ready to go away. */
+fun isCompleteRun(cards: List<Card>): Boolean =
+    cards.size == RUN_LENGTH &&
+        cards.first().rank == Rank.KING &&
+        cards.last().rank == Rank.ACE &&
+        sameSuitRun(cards)
+
+/**
+ * A hundred and four cards drawn from [suits] suits.
+ *
+ * One suit means eight identical spade packs; four means two of each ordinary
+ * suit. The count never changes, so the deal is the same shape whatever the
+ * table chose.
+ */
+fun spiderDeck(suits: Int): List<Card> {
+    val used = Suit.entries.take(suits)
+    val copies = 8 / suits
+    return used.flatMap { suit ->
+        List(copies) { Rank.standard.map { Card(it, suit) } }.flatten()
+    }
+}

app/src/main/java/org/prolibertate/games/game/spider/SpiderRules.kt

@@ -0,0 +1,177 @@
+package org.prolibertate.games.game.spider
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.shuffledWith
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+import kotlin.random.Random
+
+/**
+ * Spider as a pure state machine. See RULES-spider.md.
+ */
+object SpiderRules : GameRules<SpiderState, SpiderMove> {
+
+    override val gameId: String = GameCatalog.SPIDER
+    const val SOLO_SEAT: Int = 0
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): SpiderState {
+        val options = json.decodeFromString<SpiderOptions>(config.optionsJson)
+        require(config.seats.size == 1) { "Spider seats one, not ${config.seats.size}" }
+        return deal(options, config.seed)
+    }
+
+    private fun deal(options: SpiderOptions, seed: Long): SpiderState {
+        val deck = spiderDeck(options.suits).shuffledWith(Random(seed)).toMutableList()
+        // Fifty-four cards go out: six to the first four columns, five to the
+        // rest. The remaining fifty are the five rows still to come.
+        val piles = (0 until COLUMNS).map { index ->
+            val size = if (index < 4) 6 else 5
+            val taken = List(size) { deck.removeAt(deck.size - 1) }
+            SpiderPile(faceDown = taken.dropLast(1), faceUp = listOf(taken.last()))
+        }
+        return SpiderState(
+            options = options,
+            seed = seed,
+            stock = deck.toList(),
+            tableau = piles,
+            completed = 0,
+            moves = 0,
+            log = listOf("Dealt, ${options.suits} suit${if (options.suits == 1) "" else "s"}."),
+        )
+    }
+
+    override fun currentSeat(state: SpiderState): Int? =
+        if (isFinished(state)) null else SOLO_SEAT
+
+    /** The longest same-suit descending run on the end of a column. */
+    fun movableTail(pile: SpiderPile): List<Card> {
+        if (pile.faceUp.isEmpty()) return emptyList()
+        var take = 1
+        while (take < pile.faceUp.size) {
+            if (!sameSuitRun(pile.faceUp.takeLast(take + 1))) break
+            take++
+        }
+        return pile.faceUp.takeLast(take)
+    }
+
+    override fun legalMoves(state: SpiderState, seat: Int): List<SpiderMove> {
+        if (seat != SOLO_SEAT || state.isWon) return emptyList()
+        val moves = mutableListOf<SpiderMove>()
+        if (state.canDeal) moves += DealRow
+
+        state.tableau.forEachIndexed { from, pile ->
+            val tail = movableTail(pile)
+            for (count in 1..tail.size) {
+                val run = tail.takeLast(count)
+                val head = run.first()
+                state.tableau.indices.forEach { to ->
+                    if (to == from) return@forEach
+                    val target = state.tableau[to]
+                    val fits = target.top?.let { landsOn(head, it) } ?: true
+                    if (!fits) return@forEach
+                    // Emptying one column only to fill another gains nothing.
+                    if (target.isEmpty && pile.faceDown.isEmpty() && count == pile.faceUp.size) {
+                        return@forEach
+                    }
+                    moves += MoveRun(from, to, count)
+                }
+            }
+        }
+        return moves
+    }
+
+    override fun applyMove(state: SpiderState, seat: Int, move: SpiderMove): SpiderState {
+        require(seat == SOLO_SEAT) { "Spider has one seat" }
+        require(legalMoves(state, seat).contains(move)) { "$move is not legal here" }
+        val next = when (move) {
+            DealRow -> dealRow(state)
+            is MoveRun -> moveRun(state, move)
+        }
+        return next.copy(
+            moves = state.moves + 1,
+            log = if (next.isWon) next.log + "Out, in ${state.moves + 1} moves." else next.log,
+        )
+    }
+
+    private fun dealRow(state: SpiderState): SpiderState {
+        var stock = state.stock
+        val piles = state.tableau.map { pile ->
+            val card = stock.lastOrNull() ?: return@map pile
+            stock = stock.dropLast(1)
+            pile.copy(faceUp = pile.faceUp + card)
+        }
+        // A dealt row can complete a run in any column it landed on.
+        return sweep(state.copy(stock = stock, tableau = piles), piles.indices.toList())
+    }
+
+    private fun moveRun(state: SpiderState, move: MoveRun): SpiderState {
+        val source = state.tableau[move.from]
+        val run = source.faceUp.takeLast(move.count)
+        val left = source.faceUp.dropLast(move.count)
+        val stripped = if (left.isEmpty() && source.faceDown.isNotEmpty()) {
+            SpiderPile(source.faceDown.dropLast(1), listOf(source.faceDown.last()))
+        } else {
+            source.copy(faceUp = left)
+        }
+        val target = state.tableau[move.to]
+        val piles = state.tableau.toMutableList().also {
+            it[move.from] = stripped
+            it[move.to] = target.copy(faceUp = target.faceUp + run)
+        }
+        return sweep(state.copy(tableau = piles), listOf(move.to))
+    }
+
+    /**
+     * Sends away any king-to-ace run finished by the last move.
+     *
+     * Only the columns that changed are looked at, and only their ends: a
+     * complete run can only ever be sitting on the bottom of a pile, because
+     * anything laid on an ace would have to be lower than one.
+     */
+    private fun sweep(state: SpiderState, touched: List<Int>): SpiderState {
+        var piles = state.tableau
+        var completed = state.completed
+        var notes = state.log
+        touched.forEach { index ->
+            val pile = piles[index]
+            if (pile.faceUp.size < RUN_LENGTH) return@forEach
+            if (!isCompleteRun(pile.faceUp.takeLast(RUN_LENGTH))) return@forEach
+            val left = pile.faceUp.dropLast(RUN_LENGTH)
+            val stripped = if (left.isEmpty() && pile.faceDown.isNotEmpty()) {
+                SpiderPile(pile.faceDown.dropLast(1), listOf(pile.faceDown.last()))
+            } else {
+                pile.copy(faceUp = left)
+            }
+            piles = piles.toMutableList().also { it[index] = stripped }
+            completed++
+            notes = notes + "A run went away — $completed of $RUNS_TO_WIN."
+        }
+        return state.copy(tableau = piles, completed = completed, log = notes)
+    }
+
+    override fun isFinished(state: SpiderState): Boolean =
+        state.isWon || legalMoves(state, SOLO_SEAT).isEmpty()
+
+    override fun summary(state: SpiderState): String = when {
+        state.isWon -> "All eight runs, in ${state.moves} moves"
+        isFinished(state) -> "Blocked with ${state.completed} runs away"
+        else -> "${state.completed} of $RUNS_TO_WIN runs away"
+    }
+
+    override fun viewFor(state: SpiderState, seat: Int): SpiderState = state
+
+    override fun encodeState(state: SpiderState): String = json.encodeToString(state)
+    override fun decodeState(json: String): SpiderState = this.json.decodeFromString(json)
+    override fun encodeMove(move: SpiderMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): SpiderMove = this.json.decodeFromString(json)
+
+    fun restart(state: SpiderState): SpiderState = deal(state.options, state.seed)
+}

app/src/main/java/org/prolibertate/games/game/yahtzee/YahtzeeAi.kt

@@ -0,0 +1,86 @@
+package org.prolibertate.games.game.yahtzee
+
+import org.prolibertate.games.game.engine.GameAi
+
+/**
+ * Plays the dice by eye rather than by arithmetic.
+ *
+ * It deliberately does not try each way of keeping and look at what came back.
+ * The throw is a pure function of the state and what is held, so an AI that
+ * enumerated the thirty-two subsets and applied them would be reading the roll
+ * before making it — beating the player by seeing the future rather than by
+ * playing well. Everything below decides from the dice already on the table.
+ */
+object YahtzeeAi : GameAi<YahtzeeState, YahtzeeMove> {
+
+    override fun chooseMove(state: YahtzeeState, seat: Int, legal: List<YahtzeeMove>): YahtzeeMove {
+        require(legal.isNotEmpty()) { "No legal move for seat $seat" }
+        val rolls = legal.filterIsInstance<RollDice>()
+        val writes = legal.filterIsInstance<ScoreIn>()
+
+        // Nothing on the table yet, so there is only one thing to do.
+        if (!state.hasRolled) return rolls.firstOrNull() ?: writes.first()
+
+        val best = writes.maxByOrNull { scoreOf(it.category, state.dice) }
+        val bestScore = best?.let { scoreOf(it.category, state.dice) } ?: 0
+
+        // A hand already worth taking, or no rolls left, ends the turn.
+        if (rolls.isEmpty()) return best ?: writes.first()
+        if (bestScore >= WORTH_KEEPING) return best!!
+
+        val keep = whatToKeep(state.dice)
+        return rolls.firstOrNull { it.keep.sorted() == keep.sorted() }
+            ?: rolls.first()
+    }
+
+    /**
+     * Good enough to stop throwing. Set at a full house, which is the point
+     * where a hand is worth more than an average reroll of it.
+     */
+    private const val WORTH_KEEPING = 25
+
+    /**
+     * What to hold on to: a run if one is nearly there, otherwise the face there
+     * is most of. Holding the biggest group is what turns three of a kind into
+     * four, and it is the whole of ordinary dice play.
+     */
+    private fun whatToKeep(dice: List<Int>): List<Int> {
+        val run = runIndices(dice)
+        if (run.size >= 4) return run
+
+        val counts = countsOf(dice)
+        val most = counts.entries
+            .sortedWith(compareByDescending<Map.Entry<Int, Int>> { it.value }.thenByDescending { it.key })
+            .first()
+        if (most.value >= 2) {
+            return dice.indices.filter { dice[it] == most.key }
+        }
+        // Five different faces and no run worth keeping: hold the big ones and
+        // throw the rest.
+        return dice.indices.filter { dice[it] >= 5 }
+    }
+
+    /** Positions making up the longest run of consecutive faces, one die per face. */
+    private fun runIndices(dice: List<Int>): List<Int> {
+        val faces = dice.toSortedSet().toList()
+        var bestStart = 0
+        var bestLength = 0
+        var start = 0
+        for (i in faces.indices) {
+            if (i > 0 && faces[i] != faces[i - 1] + 1) start = i
+            val length = i - start + 1
+            if (length > bestLength) {
+                bestLength = length
+                bestStart = start
+            }
+        }
+        if (bestLength < 4) return emptyList()
+        val wanted = faces.subList(bestStart, bestStart + bestLength).toSet()
+        // One die per face: a second five adds nothing to a straight.
+        val taken = mutableSetOf<Int>()
+        return dice.indices.filter { index ->
+            val face = dice[index]
+            face in wanted && taken.add(face)
+        }
+    }
+}

app/src/main/java/org/prolibertate/games/game/yahtzee/YahtzeeModel.kt

@@ -0,0 +1,192 @@
+package org.prolibertate.games.game.yahtzee
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/** Five dice, thirteen boxes, and thirteen turns to fill them. */
+const val DICE_COUNT: Int = 5
+const val ROLLS_PER_TURN: Int = 3
+
+@Serializable
+enum class YahtzeeSection { UPPER, LOWER }
+
+/**
+ * The thirteen boxes on the card, in the order they are printed.
+ *
+ * The upper six score only the die they name, which is what the bonus is for:
+ * three of each is exactly sixty-three, so the bonus rewards a card that did not
+ * give up on any of them.
+ */
+@Serializable
+enum class YahtzeeCategory(val label: String, val section: YahtzeeSection) {
+    ONES("Ones", YahtzeeSection.UPPER),
+    TWOS("Twos", YahtzeeSection.UPPER),
+    THREES("Threes", YahtzeeSection.UPPER),
+    FOURS("Fours", YahtzeeSection.UPPER),
+    FIVES("Fives", YahtzeeSection.UPPER),
+    SIXES("Sixes", YahtzeeSection.UPPER),
+    THREE_OF_A_KIND("Three of a kind", YahtzeeSection.LOWER),
+    FOUR_OF_A_KIND("Four of a kind", YahtzeeSection.LOWER),
+    FULL_HOUSE("Full house", YahtzeeSection.LOWER),
+    SMALL_STRAIGHT("Small straight", YahtzeeSection.LOWER),
+    LARGE_STRAIGHT("Large straight", YahtzeeSection.LOWER),
+    YAHTZEE("Yahtzee", YahtzeeSection.LOWER),
+    CHANCE("Chance", YahtzeeSection.LOWER),
+    ;
+
+    /** The face this box counts, for the upper six. */
+    val face: Int? get() = if (section == YahtzeeSection.UPPER) ordinal + 1 else null
+}
+
+/** Three of each of the upper six, which is what the bonus asks for. */
+const val UPPER_BONUS_AT: Int = 63
+const val UPPER_BONUS: Int = 35
+const val YAHTZEE_SCORE: Int = 50
+const val YAHTZEE_BONUS: Int = 100
+
+@Serializable
+data class YahtzeeOptions(
+    /** One plays it as a puzzle against the card; up to six round a table. */
+    val playerCount: Int = 2,
+    /**
+     * A second Yahtzee is worth a hundred on top, once the Yahtzee box itself
+     * holds fifty. Switched off, extra Yahtzees are only worth whatever box they
+     * are written into.
+     */
+    val yahtzeeBonus: Boolean = true,
+) {
+    init {
+        require(playerCount in 1..6) { "Yahtzee seats one to six" }
+    }
+}
+
+/**
+ * One player's card.
+ *
+ * Boxes are held as a list indexed by [YahtzeeCategory.ordinal] rather than a
+ * map, so what crosses the wire is thirteen slots in a fixed order and a box
+ * that has never been filled is plainly null rather than absent.
+ */
+@Serializable
+data class YahtzeeCard(
+    val boxes: List<Int?> = List(YahtzeeCategory.entries.size) { null },
+) {
+    operator fun get(category: YahtzeeCategory): Int? = boxes[category.ordinal]
+
+    fun isFilled(category: YahtzeeCategory): Boolean = this[category] != null
+
+    val isComplete: Boolean get() = boxes.none { it == null }
+
+    fun with(category: YahtzeeCategory, score: Int): YahtzeeCard {
+        require(!isFilled(category)) { "${category.label} is already written in" }
+        return copy(boxes = boxes.toMutableList().also { it[category.ordinal] = score })
+    }
+
+    val upperSubtotal: Int
+        get() = YahtzeeCategory.entries
+            .filter { it.section == YahtzeeSection.UPPER }
+            .sumOf { this[it] ?: 0 }
+
+    val upperBonus: Int get() = if (upperSubtotal >= UPPER_BONUS_AT) UPPER_BONUS else 0
+
+    val lowerSubtotal: Int
+        get() = YahtzeeCategory.entries
+            .filter { it.section == YahtzeeSection.LOWER }
+            .sumOf { this[it] ?: 0 }
+
+    /** Does not include Yahtzee bonuses, which are held on the state. */
+    val total: Int get() = upperSubtotal + upperBonus + lowerSubtotal
+}
+
+@Serializable
+data class YahtzeeState(
+    val options: YahtzeeOptions,
+    val seed: Long,
+    /** Zero-based. Thirteen rounds fill thirteen boxes. */
+    val round: Int,
+    val turn: Int,
+    /** Empty until the seat has rolled; five dice after that. */
+    val dice: List<Int>,
+    val rollsUsed: Int,
+    val cards: List<YahtzeeCard>,
+    /** Hundreds earned for extra Yahtzees, counted per seat. */
+    val yahtzeeBonuses: List<Int>,
+    val log: List<String>,
+) {
+    val rollsLeft: Int get() = ROLLS_PER_TURN - rollsUsed
+
+    /** Nothing can be written in until the dice have been thrown at least once. */
+    val hasRolled: Boolean get() = rollsUsed > 0
+
+    fun totalFor(seat: Int): Int = cards[seat].total + yahtzeeBonuses[seat]
+
+    val isOver: Boolean get() = cards.all { it.isComplete }
+}
+
+@Serializable
+sealed interface YahtzeeMove
+
+/**
+ * Throws the dice, keeping the ones at [keep].
+ *
+ * Positions rather than faces: two fives are two different dice, and a player
+ * who keeps one of them and rerolls the other is doing something a set of faces
+ * could not express.
+ */
+@Serializable
+@SerialName("roll")
+data class RollDice(val keep: List<Int> = emptyList()) : YahtzeeMove
+
+@Serializable
+@SerialName("score")
+data class ScoreIn(val category: YahtzeeCategory) : YahtzeeMove
+
+/** How many of each face, indexed one to six. */
+fun countsOf(dice: List<Int>): Map<Int, Int> = dice.groupingBy { it }.eachCount()
+
+/** The longest run of consecutive faces present, however many of each. */
+fun longestRun(dice: List<Int>): Int {
+    val present = dice.toSortedSet()
+    var best = 0
+    var run = 0
+    var previous: Int? = null
+    for (face in present) {
+        run = if (previous != null && face == previous + 1) run + 1 else 1
+        best = maxOf(best, run)
+        previous = face
+    }
+    return best
+}
+
+/**
+ * What [category] is worth for [dice].
+ *
+ * A full house is strictly three of one and two of another, so five of a kind
+ * does not pay for it — that is the printed rule, and the joker variations that
+ * change it are not played here.
+ */
+fun scoreOf(category: YahtzeeCategory, dice: List<Int>): Int {
+    val counts = countsOf(dice)
+    val sum = dice.sum()
+    return when (category) {
+        YahtzeeCategory.ONES, YahtzeeCategory.TWOS, YahtzeeCategory.THREES,
+        YahtzeeCategory.FOURS, YahtzeeCategory.FIVES, YahtzeeCategory.SIXES,
+        -> {
+            val face = category.face!!
+            face * (counts[face] ?: 0)
+        }
+
+        YahtzeeCategory.THREE_OF_A_KIND -> if (counts.values.any { it >= 3 }) sum else 0
+        YahtzeeCategory.FOUR_OF_A_KIND -> if (counts.values.any { it >= 4 }) sum else 0
+        YahtzeeCategory.FULL_HOUSE ->
+            if (counts.values.sorted() == listOf(2, 3)) 25 else 0
+
+        YahtzeeCategory.SMALL_STRAIGHT -> if (longestRun(dice) >= 4) 30 else 0
+        YahtzeeCategory.LARGE_STRAIGHT -> if (longestRun(dice) >= 5) 40 else 0
+        YahtzeeCategory.YAHTZEE -> if (counts.values.any { it == DICE_COUNT }) YAHTZEE_SCORE else 0
+        YahtzeeCategory.CHANCE -> sum
+    }
+}
+
+fun isYahtzee(dice: List<Int>): Boolean =
+    dice.size == DICE_COUNT && dice.toSet().size == 1

app/src/main/java/org/prolibertate/games/game/yahtzee/YahtzeeRules.kt

@@ -0,0 +1,175 @@
+package org.prolibertate.games.game.yahtzee
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+import kotlin.random.Random
+
+/**
+ * Yahtzee as a pure state machine. See RULES-yahtzee.md.
+ *
+ * The dice are thrown from the state rather than from a generator held on the
+ * side: [applyMove] has to return the same thing every time it is handed the
+ * same state and move, or the host and its clients would part company on the
+ * first reroll. Seeding on the round, the seat, the rolls used and what was kept
+ * gives a throw that is unpredictable to play against and identical to replay.
+ */
+object YahtzeeRules : GameRules<YahtzeeState, YahtzeeMove> {
+
+    override val gameId: String = GameCatalog.YAHTZEE
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): YahtzeeState {
+        val options = json.decodeFromString<YahtzeeOptions>(config.optionsJson)
+        require(config.seats.size == options.playerCount) {
+            "Seat count ${config.seats.size} does not match ${options.playerCount} players"
+        }
+        return YahtzeeState(
+            options = options,
+            seed = config.seed,
+            round = 0,
+            turn = 0,
+            dice = emptyList(),
+            rollsUsed = 0,
+            cards = List(options.playerCount) { YahtzeeCard() },
+            yahtzeeBonuses = List(options.playerCount) { 0 },
+            log = listOf("Round 1."),
+        )
+    }
+
+    override fun currentSeat(state: YahtzeeState): Int? =
+        if (state.isOver) null else state.turn
+
+    override fun legalMoves(state: YahtzeeState, seat: Int): List<YahtzeeMove> {
+        if (state.isOver || state.turn != seat) return emptyList()
+        val moves = mutableListOf<YahtzeeMove>()
+        if (state.rollsUsed < ROLLS_PER_TURN) {
+            // The first throw of a turn is all five dice: there is nothing on
+            // the table yet to keep.
+            moves += if (!state.hasRolled) {
+                listOf(RollDice(emptyList()))
+            } else {
+                keepSubsets().map { RollDice(it) }
+            }
+        }
+        if (state.hasRolled) {
+            moves += YahtzeeCategory.entries
+                .filterNot { state.cards[seat].isFilled(it) }
+                .map { ScoreIn(it) }
+        }
+        return moves
+    }
+
+    /** Every set of dice positions a player might hold back — thirty-two of them. */
+    private fun keepSubsets(): List<List<Int>> =
+        (0 until (1 shl DICE_COUNT)).map { mask ->
+            (0 until DICE_COUNT).filter { mask and (1 shl it) != 0 }
+        }
+
+    override fun applyMove(state: YahtzeeState, seat: Int, move: YahtzeeMove): YahtzeeState {
+        require(!state.isOver) { "The game is over" }
+        require(state.turn == seat) { "Seat $seat is not on the clock" }
+        return when (move) {
+            is RollDice -> applyRoll(state, move)
+            is ScoreIn -> applyScore(state, seat, move)
+        }
+    }
+
+    private fun applyRoll(state: YahtzeeState, move: RollDice): YahtzeeState {
+        require(state.rollsUsed < ROLLS_PER_TURN) { "No rolls left this turn" }
+        require(move.keep.distinct().size == move.keep.size) { "Cannot keep a die twice" }
+        require(move.keep.all { it in 0 until DICE_COUNT }) { "No such die" }
+        require(state.hasRolled || move.keep.isEmpty()) { "Nothing to keep before the first roll" }
+        return state.copy(
+            dice = throwDice(state, move.keep),
+            rollsUsed = state.rollsUsed + 1,
+        )
+    }
+
+    private fun throwDice(state: YahtzeeState, keep: List<Int>): List<Int> {
+        val random = Random(
+            state.seed +
+                state.round * 10_000L +
+                state.turn * 1_000L +
+                state.rollsUsed * 100L +
+                keep.sorted().hashCode(),
+        )
+        return (0 until DICE_COUNT).map { index ->
+            if (keep.contains(index) && index < state.dice.size) {
+                state.dice[index]
+            } else {
+                random.nextInt(1, 7)
+            }
+        }
+    }
+
+    private fun applyScore(state: YahtzeeState, seat: Int, move: ScoreIn): YahtzeeState {
+        require(state.hasRolled) { "Roll before writing anything in" }
+        val card = state.cards[seat]
+        require(!card.isFilled(move.category)) { "${move.category.label} is already written in" }
+
+        val written = scoreOf(move.category, state.dice)
+        // A second Yahtzee pays a hundred on top, but only once the box itself
+        // holds the fifty — a zero written there earlier forfeits the bonus, as
+        // the printed rule has it.
+        val earnsBonus = state.options.yahtzeeBonus &&
+            isYahtzee(state.dice) &&
+            card[YahtzeeCategory.YAHTZEE] == YAHTZEE_SCORE
+        val bonuses = if (!earnsBonus) {
+            state.yahtzeeBonuses
+        } else {
+            state.yahtzeeBonuses.toMutableList().also { it[seat] = it[seat] + YAHTZEE_BONUS }
+        }
+
+        val cards = state.cards.toMutableList().also { it[seat] = card.with(move.category, written) }
+        val nextTurn = (seat + 1) % state.options.playerCount
+        val wrapped = nextTurn == 0
+        val round = if (wrapped) state.round + 1 else state.round
+        val notes = buildList {
+            add("Seat $seat wrote $written in ${move.category.label}.")
+            if (earnsBonus) add("Seat $seat rolled another Yahtzee: $YAHTZEE_BONUS more.")
+            if (cards.all { it.isComplete }) add("Cards full.") else if (wrapped) add("Round ${round + 1}.")
+        }
+        return state.copy(
+            round = round,
+            turn = nextTurn,
+            // The dice go back in the cup for whoever is next.
+            dice = emptyList(),
+            rollsUsed = 0,
+            cards = cards,
+            yahtzeeBonuses = bonuses,
+            log = state.log + notes,
+        )
+    }
+
+    override fun isFinished(state: YahtzeeState): Boolean = state.isOver
+
+    override fun summary(state: YahtzeeState): String = when {
+        state.isOver -> {
+            val totals = state.cards.indices.map { state.totalFor(it) }
+            val best = totals.max()
+            "Winner: seat ${totals.indexOf(best)} on $best"
+        }
+
+        !state.hasRolled -> "Seat ${state.turn} to throw"
+        else -> "Seat ${state.turn}, ${state.rollsLeft} rolls left"
+    }
+
+    /**
+     * Nothing is hidden. The dice are on the table and every card is face up —
+     * Yahtzee is a game of what you do with a throw everyone watched, so there
+     * is no private state to strip.
+     */
+    override fun viewFor(state: YahtzeeState, seat: Int): YahtzeeState = state
+
+    override fun encodeState(state: YahtzeeState): String = json.encodeToString(state)
+    override fun decodeState(json: String): YahtzeeState = this.json.decodeFromString(json)
+    override fun encodeMove(move: YahtzeeMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): YahtzeeMove = this.json.decodeFromString(json)
+}

app/src/main/java/org/prolibertate/games/ui/AppRoot.kt

@@ -2,15 +2,17 @@ package org.prolibertate.games.ui
 
 import androidx.activity.compose.BackHandler
 import androidx.compose.foundation.background
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
-import androidx.compose.foundation.layout.ExperimentalLayoutApi
-import androidx.compose.foundation.layout.FlowRow
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.widthIn
 import androidx.compose.foundation.lazy.grid.GridCells
+import androidx.compose.foundation.lazy.grid.GridItemSpan
 import androidx.compose.foundation.lazy.grid.LazyVerticalGrid
 import androidx.compose.foundation.lazy.grid.items
 import androidx.compose.material3.Button
@@ -36,7 +38,7 @@ import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
 import org.prolibertate.games.R
 import org.prolibertate.games.game.GameCatalog
-import org.prolibertate.games.game.GameCategory
+import org.prolibertate.games.game.GameMenu
 import org.prolibertate.games.game.GameDescriptor
 import org.prolibertate.games.game.engine.TableConfig
 import org.prolibertate.games.net.LobbyController
@@ -64,6 +66,9 @@ class AppEnv(
  */
 sealed interface Route {
     data object Menu : Route
+
+    /** One section of the menu: its games, and any sub-section under it. */
+    data class Section(val menu: GameMenu) : Route
     data object Settings : Route
     data object Scorekeeper : Route
     data class Setup(val gameId: String) : Route
@@ -152,12 +157,20 @@ private fun AppContent(
             // Dev builds show the games that are not finished yet; production
             // releases list only what can actually be played.
             showComingSoon = showComingSoon,
-            onPickGame = { push(Route.Setup(it.id)) },
+            onPickSection = { push(Route.Section(it)) },
             onJoinGame = { push(Route.Lobby(gameId = "", optionsJson = "{}", hosting = false)) },
             onScorekeeper = { push(Route.Scorekeeper) },
             onSettings = { push(Route.Settings) },
         )
 
+        is Route.Section -> SectionScreen(
+            menu = route.menu,
+            showComingSoon = showComingSoon,
+            onPickGame = { push(Route.Setup(it.id)) },
+            onPickSection = { push(Route.Section(it)) },
+            onBack = pop,
+        )
+
         is Route.Settings -> SettingsScreen(
             env = env,
             settings = settings,
@@ -202,11 +215,11 @@ private fun AppContent(
 // Main menu
 // ---------------------------------------------------------------------------
 
-@OptIn(ExperimentalMaterial3Api::class, ExperimentalLayoutApi::class)
+@OptIn(ExperimentalMaterial3Api::class)
 @Composable
 fun MainMenuScreen(
     showComingSoon: Boolean,
-    onPickGame: (GameDescriptor) -> Unit,
+    onPickSection: (GameMenu) -> Unit,
     onJoinGame: () -> Unit,
     onScorekeeper: () -> Unit,
     onSettings: () -> Unit,
@@ -221,51 +234,157 @@ fun MainMenuScreen(
             )
         }
     ) { padding ->
-        Column(
+        // Only the top of the tree. Trick-taking is reached through Card Games,
+        // which is where somebody looking for it would go.
+        val sections = GameMenu.entries.filter {
+            it.isTopLevel && GameCatalog.hasAnything(it, includeComingSoon = showComingSoon)
+        }
+
+        // One column, held in the middle. There are only ever a handful of
+        // entries here now that the games themselves live a level down, and a
+        // short list spread across a grid reads as a page half empty rather
+        // than as a menu. Capped in width so it does not become a row of very
+        // wide buttons on a tablet.
+        Box(
             modifier = Modifier
                 .fillMaxSize()
                 .padding(padding)
                 .padding(horizontal = 12.dp),
+            contentAlignment = Alignment.Center,
         ) {
-            // Wraps rather than running off the edge: two buttons and a narrow
-            // phone would otherwise push the second one past the screen.
-            FlowRow(
-                modifier = Modifier.fillMaxWidth().padding(vertical = 8.dp),
-                horizontalArrangement = Arrangement.spacedBy(8.dp),
-                verticalArrangement = Arrangement.spacedBy(4.dp),
+            Column(
+                modifier = Modifier
+                    .widthIn(max = 340.dp)
+                    .fillMaxWidth()
+                    .verticalScroll(rememberScrollState()),
+                verticalArrangement = Arrangement.spacedBy(8.dp),
+                horizontalAlignment = Alignment.CenterHorizontally,
             ) {
-                OutlinedButton(onClick = onJoinGame) { Text("Join a game nearby") }
-                // Not a game, and deliberately not in the catalogue: it keeps
-                // score for whatever is being played on the actual table.
-                OutlinedButton(onClick = onScorekeeper) { Text("Scorekeeper") }
+                sections.forEach { section ->
+                    SectionTile(
+                        menu = section,
+                        count = countIn(section, showComingSoon),
+                        onClick = { onPickSection(section) },
+                    )
                 }
 
-            // The column count follows the available width, so a phone in
-            // portrait gets two and a tablet in landscape gets four or more.
+                // Below the games, and full width like them, so the column
+                // stays one column all the way down.
+                OutlinedButton(
+                    onClick = onJoinGame,
+                    modifier = Modifier.fillMaxWidth().padding(top = 8.dp),
+                ) { Text("Join a game nearby") }
+                // Not a game, and deliberately not in the catalogue: it keeps
+                // score for whatever is being played on the actual table.
+                OutlinedButton(
+                    onClick = onScorekeeper,
+                    modifier = Modifier.fillMaxWidth(),
+                ) { Text("Scorekeeper") }
+            }
+        }
+    }
+}
+
+
+/**
+ * How many games a section offers, counting whatever is under it as well.
+ *
+ * A game in both a section and its sub-section is one game, not two, so this
+ * counts a set rather than adding the lists together.
+ */
+private fun countIn(menu: GameMenu, showComingSoon: Boolean): Int =
+    (GameCatalog.inMenu(menu, showComingSoon) +
+        menu.children.flatMap { GameCatalog.inMenu(it, showComingSoon) })
+        .toSet().size
+
+/**
+ * One section of the menu.
+ *
+ * Sub-sections come first and its own games after, so Card Games opens on
+ * "Trick-taking" and then everything played with a pack — including the
+ * trick-taking games themselves, which belong in both places and are listed in
+ * both. A game reachable one way and not the other would just be a game
+ * somebody could not find.
+ */
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+fun SectionScreen(
+    menu: GameMenu,
+    showComingSoon: Boolean,
+    onPickGame: (GameDescriptor) -> Unit,
+    onPickSection: (GameMenu) -> Unit,
+    onBack: () -> Unit,
+) {
+    val children = menu.children.filter {
+        GameCatalog.hasAnything(it, includeComingSoon = showComingSoon)
+    }
+    val games = GameCatalog.inMenu(menu, includeComingSoon = showComingSoon)
+
+    ScreenScaffold(title = menu.label, onBack = onBack) { modifier ->
         LazyVerticalGrid(
             columns = GridCells.Adaptive(minSize = 172.dp),
             verticalArrangement = Arrangement.spacedBy(8.dp),
             horizontalArrangement = Arrangement.spacedBy(8.dp),
-                modifier = Modifier.fillMaxSize(),
+            modifier = modifier.fillMaxSize(),
         ) {
-                GameCategory.entries.forEach { category ->
-                    val games = GameCatalog.byCategory(category, includeComingSoon = showComingSoon)
-                    // Don't leave a heading stranded over nothing.
-                    if (games.isEmpty()) return@forEach
-
-                    item(span = { androidx.compose.foundation.lazy.grid.GridItemSpan(maxLineSpan) }) {
+            item(span = { GridItemSpan(maxLineSpan) }) {
                 Text(
-                            text = category.label,
+                    text = menu.blurb,
+                    style = MaterialTheme.typography.bodyMedium,
+                    modifier = Modifier.padding(vertical = 8.dp),
+                )
+            }
+            items(children) { child ->
+                SectionTile(
+                    menu = child,
+                    count = countIn(child, showComingSoon),
+                    onClick = { onPickSection(child) },
+                )
+            }
+            if (games.isNotEmpty() && children.isNotEmpty()) {
+                item(span = { GridItemSpan(maxLineSpan) }) {
+                    Text(
+                        text = "All ${menu.label.lowercase()}",
                         style = MaterialTheme.typography.titleMedium,
                         fontWeight = FontWeight.Bold,
                         modifier = Modifier.padding(top = 12.dp, bottom = 4.dp),
                     )
                 }
+            }
             items(games) { game ->
                 GameTile(game = game, onClick = { onPickGame(game) })
             }
         }
     }
+}
+
+/** A way into a section, rather than into a game. */
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+private fun SectionTile(menu: GameMenu, count: Int, onClick: () -> Unit) {
+    Card(
+        onClick = onClick,
+        colors = CardDefaults.cardColors(
+            containerColor = MaterialTheme.colorScheme.secondaryContainer,
+        ),
+        modifier = Modifier.fillMaxWidth(),
+    ) {
+        Column(modifier = Modifier.padding(12.dp)) {
+            Text(
+                text = menu.label,
+                style = MaterialTheme.typography.titleMedium,
+                fontWeight = FontWeight.Bold,
+            )
+            Text(
+                text = menu.blurb,
+                style = MaterialTheme.typography.bodySmall,
+                modifier = Modifier.padding(top = 4.dp),
+            )
+            Text(
+                text = if (count == 1) "1 game" else "$count games",
+                style = MaterialTheme.typography.labelSmall,
+                modifier = Modifier.padding(top = 8.dp),
+            )
         }
     }
 }
@@ -297,14 +416,12 @@ private fun GameTile(game: GameDescriptor, onClick: () -> Unit) {
                 modifier = Modifier.padding(top = 4.dp),
             )
             Text(
-                text = if (game.available) {
-                    if (game.minPlayers == game.maxPlayers) {
-                        "${game.minPlayers} players"
-                    } else {
-                        "${game.minPlayers}–${game.maxPlayers} players"
-                    }
-                } else {
-                    "Coming soon"
+                text = when {
+                    !game.available -> "Coming soon"
+                    // A puzzle for one, rather than "1 players".
+                    game.maxPlayers == 1 -> "On your own"
+                    game.minPlayers == game.maxPlayers -> "${game.minPlayers} players"
+                    else -> "${game.minPlayers}–${game.maxPlayers} players"
                 },
                 style = MaterialTheme.typography.labelSmall,
                 modifier = Modifier.padding(top = 8.dp),

app/src/main/java/org/prolibertate/games/ui/GameSetupScreen.kt

@@ -40,6 +40,15 @@ import org.prolibertate.games.game.crazyeights.CrazyEightsOptions
 import org.prolibertate.games.game.cribbage.CribbageOptions
 import org.prolibertate.games.game.euchre.EuchreOptions
 import org.prolibertate.games.game.golf.GolfOptions
+import org.prolibertate.games.game.hearts.HeartsOptions
+import org.prolibertate.games.game.klondike.KlondikeOptions
+import org.prolibertate.games.game.freecell.FreeCellOptions
+import org.prolibertate.games.game.pyramid.PyramidOptions
+import org.prolibertate.games.game.spider.SpiderOptions
+import org.prolibertate.games.game.yahtzee.YahtzeeOptions
+import org.prolibertate.games.game.pegsolitaire.PegBoard
+import org.prolibertate.games.game.pegsolitaire.PegGoal
+import org.prolibertate.games.game.pegsolitaire.PegSolitaireOptions
 import org.prolibertate.games.game.kaiser.KaiserOptions
 import org.prolibertate.games.game.mastermind.MastermindLevel
 import org.prolibertate.games.game.mastermind.MastermindOptions
@@ -88,6 +97,13 @@ fun GameSetupScreen(
     var backgammon by remember { mutableStateOf(BackgammonOptions()) }
     var mastermind by remember { mutableStateOf(MastermindOptions()) }
     var pirates by remember { mutableStateOf(PiratesOptions()) }
+    var hearts by remember { mutableStateOf(HeartsOptions()) }
+    var pegs by remember { mutableStateOf(PegSolitaireOptions()) }
+    var yahtzee by remember { mutableStateOf(YahtzeeOptions()) }
+    var klondike by remember { mutableStateOf(KlondikeOptions()) }
+    var freecell by remember { mutableStateOf(FreeCellOptions()) }
+    var spider by remember { mutableStateOf(SpiderOptions()) }
+    var pyramid by remember { mutableStateOf(PyramidOptions()) }
     // Not an engine option: which seat the local player takes. Seat 0 is
     // always White, so choosing Black means sitting in seat 1.
     var playWhite by remember { mutableStateOf(true) }
@@ -111,6 +127,13 @@ fun GameSetupScreen(
         GameCatalog.BACKGAMMON -> setupJson.encodeToString(backgammon)
         GameCatalog.MASTERMIND -> setupJson.encodeToString(mastermind)
         GameCatalog.PIRATES -> setupJson.encodeToString(pirates)
+        GameCatalog.HEARTS -> setupJson.encodeToString(hearts)
+        GameCatalog.PEG_SOLITAIRE -> setupJson.encodeToString(pegs)
+        GameCatalog.YAHTZEE -> setupJson.encodeToString(yahtzee)
+        GameCatalog.KLONDIKE -> setupJson.encodeToString(klondike)
+        GameCatalog.FREECELL -> setupJson.encodeToString(freecell)
+        GameCatalog.SPIDER -> setupJson.encodeToString(spider)
+        GameCatalog.PYRAMID -> setupJson.encodeToString(pyramid)
         else -> "{}"
     }
     val seatCount = seatCountFor(descriptor.id, optionsJson)
@@ -162,15 +185,37 @@ fun GameSetupScreen(
                     onSide = { playPirates = it },
                 )
 
+                GameCatalog.HEARTS -> HeartsOptionsEditor(hearts) { hearts = it }
+
+                GameCatalog.PEG_SOLITAIRE -> PegSolitaireOptionsEditor(pegs) { pegs = it }
+
+                GameCatalog.YAHTZEE -> YahtzeeOptionsEditor(yahtzee) { yahtzee = it }
+
+                GameCatalog.KLONDIKE -> KlondikeOptionsEditor(klondike) { klondike = it }
+
+                GameCatalog.FREECELL -> FreeCellOptionsEditor(freecell) { freecell = it }
+
+                GameCatalog.SPIDER -> SpiderOptionsEditor(spider) { spider = it }
+
+                GameCatalog.PYRAMID -> PyramidOptionsEditor(pyramid) { pyramid = it }
+
                 else -> Text("No options yet for this game.")
             }
 
             Divider()
-            Text("Opponents", fontWeight = FontWeight.Bold)
+            // A one-seat game has nobody to play against and no seat anyone
+            // could join, so it is offered as itself rather than as a table
+            // with every other chair filled by the computer.
+            val solo = seatCount == 1
+            Text(if (solo) "On your own" else "Opponents", fontWeight = FontWeight.Bold)
             Text(
-                text = "Play now and every other seat is taken by the computer. Host a game " +
+                text = if (solo) {
+                    "A puzzle for one. There is nobody to wait for and nothing to host."
+                } else {
+                    "Play now and every other seat is taken by the computer. Host a game " +
                         "instead and people can claim those seats as they join — whatever is " +
-                    "still empty when you start stays computer-controlled.",
+                        "still empty when you start stays computer-controlled."
+                },
                 style = MaterialTheme.typography.bodySmall,
             )
 
@@ -182,17 +227,19 @@ fun GameSetupScreen(
                 descriptor.id == GameCatalog.PIRATES && playPirates -> 1
                 else -> 0
             }
-            PrimaryAction(text = "Play against the computer") {
+            PrimaryAction(text = if (solo) "Play" else "Play against the computer") {
                 onPlayOffline(
                     offlineConfig(descriptor.id, optionsJson, seatCount, playerName, localSeat),
                     localSeat,
                 )
             }
+            if (!solo) {
                 PrimaryAction(text = "Host a game for others to join") {
                     onHostOnline(optionsJson)
                 }
             }
         }
+    }
 }
 
 /** [localSeat] is the person at this device; every other seat starts as AI. */
@@ -250,6 +297,9 @@ fun seatCountFor(gameId: String, optionsJson: String): Int {
             GameCatalog.CHESS, GameCatalog.MORRIS, GameCatalog.CHECKERS,
             GameCatalog.BACKGAMMON, GameCatalog.MASTERMIND, GameCatalog.PIRATES -> 2
 
+            GameCatalog.YAHTZEE ->
+                setupJson.decodeFromString<YahtzeeOptions>(optionsJson).playerCount
+
             GameCatalog.TAYU ->
                 setupJson.decodeFromString<TayuOptions>(optionsJson).playerCount
 
@@ -944,3 +994,150 @@ private fun ToggleRow(
         Switch(checked = checked, onCheckedChange = onChange)
     }
 }
+
+@Composable
+private fun HeartsOptionsEditor(options: HeartsOptions, onChange: (HeartsOptions) -> Unit) {
+    Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        ChipRow(
+            label = "Game ends at",
+            values = listOf(50, 100),
+            selected = options.targetScore,
+            display = { "$it points" },
+            onSelect = { onChange(options.copy(targetScore = it)) },
+        )
+        ToggleRow(
+            title = "Shooting the moon",
+            subtitle = "Take all twenty-six and score nothing, while everyone else takes " +
+                "the lot. Switched off, a moon is simply the worst hand at the table.",
+            checked = options.allowShootTheMoon,
+            onChange = { onChange(options.copy(allowShootTheMoon = it)) },
+        )
+    }
+}
+
+@Composable
+private fun PegSolitaireOptionsEditor(
+    options: PegSolitaireOptions,
+    onChange: (PegSolitaireOptions) -> Unit,
+) {
+    Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        ChipRow(
+            label = "Board",
+            values = PegBoard.entries.toList(),
+            selected = options.board,
+            display = { it.label },
+            // The opening hole is cleared as well: a hole that exists on the old
+            // board may be a missing corner on the new one, and the options
+            // refuse to be built that way.
+            onSelect = { onChange(options.copy(board = it, startEmpty = null)) },
+        )
+        ChipRow(
+            label = "Finish",
+            values = PegGoal.entries.toList(),
+            selected = options.goal,
+            display = { it.label },
+            onSelect = { onChange(options.copy(goal = it)) },
+        )
+    }
+}
+
+@Composable
+private fun YahtzeeOptionsEditor(options: YahtzeeOptions, onChange: (YahtzeeOptions) -> Unit) {
+    Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        ChipRow(
+            label = "Players",
+            values = (1..6).toList(),
+            selected = options.playerCount,
+            display = { if (it == 1) "On your own" else "$it" },
+            onSelect = { onChange(options.copy(playerCount = it)) },
+        )
+        ToggleRow(
+            title = "Second Yahtzee scores a hundred",
+            subtitle = "Only once the Yahtzee box holds the fifty. A zero written there " +
+                "earlier forfeits it, as the printed rule has it.",
+            checked = options.yahtzeeBonus,
+            onChange = { onChange(options.copy(yahtzeeBonus = it)) },
+        )
+    }
+}
+
+@Composable
+private fun KlondikeOptionsEditor(options: KlondikeOptions, onChange: (KlondikeOptions) -> Unit) {
+    Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        ChipRow(
+            label = "Turn from the stock",
+            values = listOf(1, 3),
+            selected = options.drawCount,
+            display = { "$it at a time" },
+            onSelect = { onChange(options.copy(drawCount = it)) },
+        )
+        ChipRow(
+            label = "Turning the waste back over",
+            values = listOf(null, 0, 1, 2),
+            selected = options.redealLimit,
+            display = {
+                when (it) {
+                    null -> "As often as you like"
+                    0 -> "Never — one pass"
+                    else -> "$it times"
+                }
+            },
+            onSelect = { onChange(options.copy(redealLimit = it)) },
+        )
+        ToggleRow(
+            title = "Only a king fills a space",
+            subtitle = "Turning this off lets any card into an empty column, which makes " +
+                "far more deals winnable.",
+            checked = options.kingsOnlyInSpaces,
+            onChange = { onChange(options.copy(kingsOnlyInSpaces = it)) },
+        )
+    }
+}
+
+@Composable
+private fun FreeCellOptionsEditor(options: FreeCellOptions, onChange: (FreeCellOptions) -> Unit) {
+    Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        ChipRow(
+            label = "Free cells",
+            values = (1..6).toList(),
+            selected = options.freeCells,
+            display = { "$it" },
+            onSelect = { onChange(options.copy(freeCells = it)) },
+        )
+        ToggleRow(
+            title = "Move a run in one go",
+            subtitle = "The rules move a run one card at a time through the cells. This " +
+                "does the shuffling for you, which is bookkeeping rather than a decision.",
+            checked = options.allowSupermoves,
+            onChange = { onChange(options.copy(allowSupermoves = it)) },
+        )
+    }
+}
+
+@Composable
+private fun SpiderOptionsEditor(options: SpiderOptions, onChange: (SpiderOptions) -> Unit) {
+    ChipRow(
+        label = "Suits",
+        values = listOf(1, 2, 4),
+        selected = options.suits,
+        display = {
+            when (it) {
+                1 -> "One — a pastime"
+                2 -> "Two"
+                else -> "Four — a fight"
+            }
+        },
+        onSelect = { onChange(options.copy(suits = it)) },
+    )
+}
+
+@Composable
+private fun PyramidOptionsEditor(options: PyramidOptions, onChange: (PyramidOptions) -> Unit) {
+    ChipRow(
+        label = "Turns through the pack",
+        values = listOf(0, 1, 2, 3),
+        selected = options.redeals,
+        display = { if (it == 0) "One pass only" else "$it turns back" },
+        onSelect = { onChange(options.copy(redeals = it)) },
+    )
+}

app/src/main/java/org/prolibertate/games/ui/PlayScreen.kt

@@ -4,7 +4,10 @@ import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
 import androidx.compose.runtime.rememberCoroutineScope
 import org.prolibertate.games.game.GameCatalog
 import org.prolibertate.games.game.chess.ChessAi
@@ -26,6 +29,33 @@ import org.prolibertate.games.game.euchre.EuchreMove
 import org.prolibertate.games.game.euchre.EuchrePhase
 import org.prolibertate.games.game.euchre.EuchreRules
 import org.prolibertate.games.game.euchre.EuchreState
+import org.prolibertate.games.game.freecell.FreeCellMove
+import org.prolibertate.games.game.freecell.FreeCellRules
+import org.prolibertate.games.game.freecell.FreeCellState
+import org.prolibertate.games.game.pyramid.PyramidMove
+import org.prolibertate.games.game.pyramid.PyramidRules
+import org.prolibertate.games.game.pyramid.PyramidState
+import org.prolibertate.games.game.solitaire.FirstLegalAi
+import org.prolibertate.games.game.spider.SpiderMove
+import org.prolibertate.games.game.spider.SpiderRules
+import org.prolibertate.games.game.spider.SpiderState
+import org.prolibertate.games.game.klondike.KlondikeAi
+import org.prolibertate.games.game.klondike.KlondikeMove
+import org.prolibertate.games.game.klondike.KlondikeRules
+import org.prolibertate.games.game.klondike.KlondikeState
+import org.prolibertate.games.game.yahtzee.YahtzeeAi
+import org.prolibertate.games.game.yahtzee.YahtzeeMove
+import org.prolibertate.games.game.yahtzee.YahtzeeRules
+import org.prolibertate.games.game.yahtzee.YahtzeeState
+import org.prolibertate.games.game.hearts.HeartsAi
+import org.prolibertate.games.game.hearts.HeartsMove
+import org.prolibertate.games.game.hearts.HeartsPhase
+import org.prolibertate.games.game.hearts.HeartsRules
+import org.prolibertate.games.game.hearts.HeartsState
+import org.prolibertate.games.game.pegsolitaire.PegSolitaireAi
+import org.prolibertate.games.game.pegsolitaire.PegSolitaireMove
+import org.prolibertate.games.game.pegsolitaire.PegSolitaireRules
+import org.prolibertate.games.game.pegsolitaire.PegSolitaireState
 import org.prolibertate.games.game.golf.GolfAi
 import org.prolibertate.games.game.golf.GolfMove
 import org.prolibertate.games.game.golf.GolfPhase
@@ -81,11 +111,18 @@ import org.prolibertate.games.ui.game.CheckersScreen
 import org.prolibertate.games.ui.game.ChessScreen
 import org.prolibertate.games.ui.game.CrazyEightsScreen
 import org.prolibertate.games.ui.game.CribbageScreen
+import org.prolibertate.games.ui.game.HeartsScreen
+import org.prolibertate.games.ui.game.FreeCellScreen
+import org.prolibertate.games.ui.game.KlondikeScreen
+import org.prolibertate.games.ui.game.PyramidScreen
 import org.prolibertate.games.ui.game.EuchreScreen
 import org.prolibertate.games.ui.game.GolfScreen
 import org.prolibertate.games.ui.game.KaiserScreen
 import org.prolibertate.games.ui.game.MastermindScreen
 import org.prolibertate.games.ui.game.MorrisScreen
+import org.prolibertate.games.ui.game.PegSolitaireScreen
+import org.prolibertate.games.ui.game.SpiderScreen
+import org.prolibertate.games.ui.game.YahtzeeScreen
 import org.prolibertate.games.ui.game.PiratesScreen
 import org.prolibertate.games.ui.game.PresidentScreen
 import org.prolibertate.games.ui.game.TRICK_HOLD_MILLIS
@@ -430,6 +467,162 @@ fun PlayScreen(
             PiratesScreen(controller = controller, localSeat = route.localSeat, onExit = onExit)
         }
 
+        GameCatalog.HEARTS -> {
+            val controller = remember(route) {
+                MatchController<HeartsState, HeartsMove>(
+                    rules = HeartsRules,
+                    ai = HeartsAi,
+                    config = route.config,
+                    scope = scope,
+                    role = role,
+                    localSeats = setOf(route.localSeat),
+                    primarySeat = route.localSeat,
+                    advanceIdle = { state ->
+                        if (state.phase == HeartsPhase.ROUND_OVER) {
+                            HeartsRules.startNextRound(state)
+                        } else {
+                            null
+                        }
+                    },
+                    // What a round cost is read before the next deal.
+                    awaitsConfirmation = { state -> state.phase == HeartsPhase.ROUND_OVER },
+                    aiThinkingMillis = { settings.scaled(700L) },
+                    holdBeforeNextMove = { state ->
+                        if (state.completedTrick.isNotEmpty()) {
+                            settings.scaled(TRICK_HOLD_MILLIS)
+                        } else {
+                            0L
+                        }
+                    },
+                )
+            }
+            StartMatch(env, controller, route)
+            HeartsScreen(controller = controller, localSeat = route.localSeat, onExit = onExit)
+        }
+
+        GameCatalog.PEG_SOLITAIRE -> {
+            // Playing again means a fresh board rather than a fresh round, and
+            // it cannot go through advanceIdle: driveIdleSeats returns as soon
+            // as rules.isFinished is true, and a peg board with no jumps left
+            // genuinely is finished. Rebuilding the controller is the honest
+            // way to deal again, and the board is deterministic, so the retry
+            // is the same puzzle.
+            var generation by remember(route) { mutableIntStateOf(0) }
+            val controller = remember(route, generation) {
+                MatchController<PegSolitaireState, PegSolitaireMove>(
+                    rules = PegSolitaireRules,
+                    // Never consulted while a person holds the only seat.
+                    ai = PegSolitaireAi,
+                    config = route.config,
+                    scope = scope,
+                    role = role,
+                    localSeats = setOf(route.localSeat),
+                    primarySeat = route.localSeat,
+                )
+            }
+            StartMatch(env, controller, route)
+            PegSolitaireScreen(
+                controller = controller,
+                onRestart = { generation++ },
+                onExit = onExit,
+            )
+        }
+
+        GameCatalog.YAHTZEE -> {
+            val controller = remember(route) {
+                MatchController<YahtzeeState, YahtzeeMove>(
+                    rules = YahtzeeRules,
+                    ai = YahtzeeAi,
+                    config = route.config,
+                    scope = scope,
+                    role = role,
+                    localSeats = setOf(route.localSeat),
+                    primarySeat = route.localSeat,
+                    // Long enough to watch the dice land, short enough that a
+                    // six-handed game is not an evening of waiting.
+                    aiThinkingMillis = { settings.scaled(500L) },
+                )
+            }
+            StartMatch(env, controller, route)
+            YahtzeeScreen(controller = controller, localSeat = route.localSeat, onExit = onExit)
+        }
+
+        GameCatalog.KLONDIKE -> {
+            // Dealing again rebuilds the controller, for the same reason peg
+            // solitaire does: a blocked deal is genuinely finished, so
+            // driveIdleSeats has already stopped and advanceIdle never runs.
+            var generation by remember(route) { mutableIntStateOf(0) }
+            val controller = remember(route, generation) {
+                MatchController<KlondikeState, KlondikeMove>(
+                    rules = KlondikeRules,
+                    // Only ever asked for a hint; the seat is the player's.
+                    ai = KlondikeAi,
+                    config = route.config,
+                    scope = scope,
+                    role = role,
+                    localSeats = setOf(route.localSeat),
+                    primarySeat = route.localSeat,
+                )
+            }
+            StartMatch(env, controller, route)
+            KlondikeScreen(
+                controller = controller,
+                onRestart = { generation++ },
+                onExit = onExit,
+            )
+        }
+
+        GameCatalog.FREECELL -> {
+            var generation by remember(route) { mutableIntStateOf(0) }
+            val controller = remember(route, generation) {
+                MatchController<FreeCellState, FreeCellMove>(
+                    rules = FreeCellRules,
+                    ai = FirstLegalAi(),
+                    config = route.config,
+                    scope = scope,
+                    role = role,
+                    localSeats = setOf(route.localSeat),
+                    primarySeat = route.localSeat,
+                )
+            }
+            StartMatch(env, controller, route)
+            FreeCellScreen(controller = controller, onRestart = { generation++ }, onExit = onExit)
+        }
+
+        GameCatalog.SPIDER -> {
+            var generation by remember(route) { mutableIntStateOf(0) }
+            val controller = remember(route, generation) {
+                MatchController<SpiderState, SpiderMove>(
+                    rules = SpiderRules,
+                    ai = FirstLegalAi(),
+                    config = route.config,
+                    scope = scope,
+                    role = role,
+                    localSeats = setOf(route.localSeat),
+                    primarySeat = route.localSeat,
+                )
+            }
+            StartMatch(env, controller, route)
+            SpiderScreen(controller = controller, onRestart = { generation++ }, onExit = onExit)
+        }
+
+        GameCatalog.PYRAMID -> {
+            var generation by remember(route) { mutableIntStateOf(0) }
+            val controller = remember(route, generation) {
+                MatchController<PyramidState, PyramidMove>(
+                    rules = PyramidRules,
+                    ai = FirstLegalAi(),
+                    config = route.config,
+                    scope = scope,
+                    role = role,
+                    localSeats = setOf(route.localSeat),
+                    primarySeat = route.localSeat,
+                )
+            }
+            StartMatch(env, controller, route)
+            PyramidScreen(controller = controller, onRestart = { generation++ }, onExit = onExit)
+        }
+
         GameCatalog.MORRIS -> {
             val controller = remember(route) {
                 MatchController<MorrisState, MorrisMove>(

app/src/main/java/org/prolibertate/games/ui/game/CardTable.kt

@@ -0,0 +1,96 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.BorderStroke
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.ui.common.CARD_ASPECT
+import org.prolibertate.games.ui.theme.FeltGreenDark
+import org.prolibertate.games.ui.theme.Parchment
+import org.prolibertate.games.ui.theme.WallaceGold
+
+/**
+ * The furniture every patience table needs.
+ *
+ * FreeCell, Spider and Pyramid lay their cards out quite differently but they
+ * are all made of the same three things: a place a card sits, an empty place
+ * that can still be tapped into, and the two dialogs for leaving. Klondike keeps
+ * its own copies — it was written first and its layout constants are tuned to
+ * seven columns — but nothing new needs to repeat them.
+ */
+val TABLE_CARD_WIDTH: Dp = 44.dp
+val TABLE_CARD_HEIGHT: Dp = TABLE_CARD_WIDTH / CARD_ASPECT
+
+/** Overlapped by a third, which hides as little as a stack can afford to. */
+val TABLE_STACK_STEP: Dp = TABLE_CARD_HEIGHT * 2f / 3f
+
+@Composable
+fun TableSlot(
+    highlighted: Boolean,
+    onClick: () -> Unit,
+    modifier: Modifier = Modifier,
+    width: Dp = TABLE_CARD_WIDTH,
+    content: @Composable () -> Unit,
+) {
+    Box(
+        modifier = modifier
+            .width(width)
+            .border(
+                border = BorderStroke(if (highlighted) 3.dp else 0.dp, WallaceGold),
+                shape = RoundedCornerShape(4.dp),
+            )
+            .clickable { onClick() },
+    ) {
+        content()
+    }
+}
+
+/** A place with no card on it, drawn so it is still somewhere to aim at. */
+@Composable
+fun EmptySpace(label: String = "", width: Dp = TABLE_CARD_WIDTH) {
+    Box(
+        modifier = Modifier
+            .width(width)
+            .height(width / CARD_ASPECT)
+            .background(FeltGreenDark, RoundedCornerShape(4.dp)),
+        contentAlignment = Alignment.Center,
+    ) {
+        if (label.isNotEmpty()) {
+            Text(label, color = Parchment.copy(alpha = 0.5f))
+        }
+    }
+}
+
+@Composable
+fun PatienceEndDialog(what: String, onEnd: () -> Unit, onKeepPlaying: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onKeepPlaying,
+        title = { Text("End this game?") },
+        text = { Text(what) },
+        confirmButton = { TextButton(onClick = onEnd) { Text("End the game") } },
+        dismissButton = { TextButton(onClick = onKeepPlaying) { Text("Keep playing") } },
+    )
+}
+
+@Composable
+fun PatienceLeftDialog(notice: String, onExit: () -> Unit, onStay: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onStay,
+        title = { Text("The game has ended") },
+        text = { Text(notice) },
+        confirmButton = { TextButton(onClick = onExit) { Text("Leave the game") } },
+        dismissButton = { TextButton(onClick = onStay) { Text("Look at the table") } },
+    )
+}

app/src/main/java/org/prolibertate/games/ui/game/FreeCellScreen.kt

@@ -0,0 +1,188 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.horizontalScroll
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.Button
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.freecell.FreeCellMove
+import org.prolibertate.games.game.freecell.FreeCellState
+import org.prolibertate.games.game.freecell.MoveTo
+import org.prolibertate.games.game.freecell.Place
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.common.PlayingCardView
+
+private const val TITLE = "FreeCell"
+
+/**
+ * The FreeCell table.
+ *
+ * Cells on the left, homes on the right, eight columns below — the standard
+ * arrangement, and the one that makes the cells read as somewhere to put things
+ * rather than somewhere cards come from. Everything is face up from the first
+ * move, so the whole game is on the screen and the only thing hidden is what to
+ * do about it.
+ */
+@Composable
+fun FreeCellScreen(
+    controller: MatchController<FreeCellState, FreeCellMove>,
+    onRestart: () -> Unit,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+
+    var held by remember { mutableStateOf<Pair<Place, Int>?>(null) }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Dealing…") }
+            abandoned?.let { PatienceLeftDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) {
+                PatienceEndDialog("The deal is left as it stands.", onExit) { confirmingEnd = false }
+            }
+        }
+        return
+    }
+
+    val moves = legal.filterIsInstance<MoveTo>()
+    val finished = legal.isEmpty()
+    val targets = held?.let { (from, count) ->
+        moves.filter { it.from == from && it.count == count }.map { it.to }.toSet()
+    }.orEmpty()
+    val sources = moves.map { it.from to it.count }.toSet()
+
+    fun take(place: Place, count: Int) {
+        val inHand = held
+        held = when {
+            inHand != null && targets.contains(place) -> {
+                controller.submit(MoveTo(inHand.first, place, inHand.second))
+                null
+            }
+
+            inHand?.first == place -> null
+            sources.contains(place to count) -> place to count
+            else -> null
+        }
+    }
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.isWon -> "Out, in ${current.moves} moves."
+                    finished -> "Blocked — nothing left to try."
+                    held != null -> "Tap where it goes."
+                    else -> "${current.foundations.sumOf { it.size }} of 52 home, " +
+                        "${current.freeCellCount} cells free."
+                },
+                fontWeight = FontWeight.Bold,
+            )
+
+            Row(
+                modifier = Modifier.fillMaxWidth().horizontalScroll(rememberScrollState()),
+                horizontalArrangement = Arrangement.spacedBy(6.dp),
+            ) {
+                current.cells.forEachIndexed { index, card ->
+                    TableSlot(
+                        highlighted = held?.first == Place.cell(index) ||
+                            targets.contains(Place.cell(index)),
+                        onClick = { take(Place.cell(index), 1) },
+                    ) {
+                        card?.let { PlayingCardView(card = it, width = TABLE_CARD_WIDTH) }
+                            ?: EmptySpace(label = "·")
+                    }
+                }
+                current.foundations.forEachIndexed { index, pile ->
+                    TableSlot(
+                        highlighted = targets.contains(Place.foundation(index)),
+                        onClick = { take(Place.foundation(index), 1) },
+                    ) {
+                        pile.lastOrNull()?.let { PlayingCardView(card = it, width = TABLE_CARD_WIDTH) }
+                            ?: EmptySpace(label = Suit.entries[index].symbol)
+                    }
+                }
+            }
+
+            Row(
+                modifier = Modifier.fillMaxWidth().horizontalScroll(rememberScrollState()),
+                horizontalArrangement = Arrangement.spacedBy(6.dp),
+                verticalAlignment = Alignment.Top,
+            ) {
+                current.tableau.forEachIndexed { index, column ->
+                    if (column.isEmpty()) {
+                        TableSlot(
+                            highlighted = targets.contains(Place.column(index)),
+                            onClick = { take(Place.column(index), 1) },
+                        ) { EmptySpace() }
+                    } else {
+                        Box(
+                            modifier = Modifier
+                                .width(TABLE_CARD_WIDTH)
+                                .height(TABLE_CARD_HEIGHT + TABLE_STACK_STEP * (column.size - 1)),
+                        ) {
+                            column.forEachIndexed { depth, card ->
+                                // Taking a card takes everything resting on it.
+                                val count = column.size - depth
+                                TableSlot(
+                                    modifier = Modifier.offset(y = TABLE_STACK_STEP * depth),
+                                    highlighted = held == (Place.column(index) to count) ||
+                                        (depth == column.lastIndex && targets.contains(Place.column(index))),
+                                    onClick = { take(Place.column(index), count) },
+                                ) { PlayingCardView(card = card, width = TABLE_CARD_WIDTH) }
+                            }
+                        }
+                    }
+                }
+            }
+
+            if (finished) {
+                Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                    Button(onClick = onRestart) { Text("Deal again") }
+                    TextButton(onClick = onExit) { Text("Leave") }
+                }
+            }
+
+            current.log.takeLast(2).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { PatienceLeftDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) {
+            PatienceEndDialog("The deal is left as it stands.", onExit) { confirmingEnd = false }
+        }
+    }
+}

app/src/main/java/org/prolibertate/games/ui/game/HeartsScreen.kt

@@ -0,0 +1,249 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.ExperimentalLayoutApi
+import androidx.compose.foundation.layout.FlowRow
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Button
+import androidx.compose.material3.Divider
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.runtime.snapshots.SnapshotStateList
+import androidx.compose.runtime.mutableStateListOf
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.hearts.HeartsMove
+import org.prolibertate.games.game.hearts.HeartsPhase
+import org.prolibertate.games.game.hearts.HeartsState
+import org.prolibertate.games.game.hearts.PASS_SIZE
+import org.prolibertate.games.game.hearts.PassCards
+import org.prolibertate.games.game.hearts.PlayCard
+import org.prolibertate.games.game.hearts.pointsOf
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.common.PlayingCardView
+
+private const val TITLE = "Hearts"
+
+/**
+ * The Hearts table.
+ *
+ * Two quite different things happen at this screen, so it reads the phase and
+ * shows one or the other: choosing three cards to give away, which is a
+ * multiple selection nothing else in the app does, and then playing one card at
+ * a time, which every trick-taking screen here does the same way.
+ */
+@OptIn(ExperimentalLayoutApi::class)
+@Composable
+fun HeartsScreen(
+    controller: MatchController<HeartsState, HeartsMove>,
+    localSeat: Int,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+    val awaiting by controller.awaitingConfirmation.collectAsState()
+
+    val chosen = remember { mutableStateListOf<Card>() }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Dealing…") }
+            abandoned?.let { LeftHeartsDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) EndHeartsDialog(onExit) { confirmingEnd = false }
+        }
+        return
+    }
+
+    val hand = current.hands.getOrElse(localSeat) { emptyList() }
+    val passing = current.phase == HeartsPhase.PASSING
+    val alreadyPassed = current.passSelections.getOrElse(localSeat) { emptyList() }.isNotEmpty()
+    val playable = legal.filterIsInstance<PlayCard>().map { it.card }.toSet()
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.phase == HeartsPhase.GAME_OVER -> "Game over — lowest score wins."
+                    current.phase == HeartsPhase.ROUND_OVER -> "Round scored."
+                    passing && alreadyPassed -> "Waiting for the others to choose…"
+                    passing -> "Choose three to pass ${current.passDirection.label}"
+                    playable.isEmpty() -> "Waiting…"
+                    else -> "Your turn"
+                },
+                fontWeight = FontWeight.Bold,
+            )
+            Text(
+                text = "Every heart costs a point and the queen of spades costs thirteen. " +
+                    "Take none of them.",
+                style = MaterialTheme.typography.bodySmall,
+            )
+
+            TrickView(current)
+            ScoreRow(current, localSeat)
+            Divider()
+
+            Text(
+                text = if (passing && !alreadyPassed) {
+                    "Passing ${current.passDirection.label} — ${chosen.size} of $PASS_SIZE chosen"
+                } else {
+                    "Your hand"
+                },
+                style = MaterialTheme.typography.labelLarge,
+            )
+            FlowRow(horizontalArrangement = Arrangement.spacedBy(4.dp)) {
+                hand.forEach { card ->
+                    val selectable = passing && !alreadyPassed
+                    PlayingCardView(
+                        card = card,
+                        width = 52.dp,
+                        selected = chosen.contains(card),
+                        enabled = if (selectable) true else playable.contains(card),
+                        caption = pointsOf(card).takeIf { it > 0 }?.let { "−$it" },
+                        onClick = {
+                            if (selectable) {
+                                toggle(chosen, card)
+                            } else if (playable.contains(card)) {
+                                controller.submit(PlayCard(card))
+                            }
+                        },
+                    )
+                }
+            }
+
+            if (passing && !alreadyPassed) {
+                Button(
+                    enabled = chosen.size == PASS_SIZE,
+                    onClick = {
+                        controller.submit(PassCards(chosen.toList()))
+                        chosen.clear()
+                    },
+                ) { Text("Pass these three") }
+            }
+
+            if (awaiting) {
+                Button(onClick = controller::confirmAdvance) { Text("Next round") }
+            }
+
+            current.log.takeLast(3).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { LeftHeartsDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) EndHeartsDialog(onExit) { confirmingEnd = false }
+    }
+}
+
+/** Adds or removes [card], never letting the selection run past three. */
+private fun toggle(chosen: SnapshotStateList<Card>, card: Card) {
+    if (chosen.contains(card)) {
+        chosen.remove(card)
+    } else if (chosen.size < PASS_SIZE) {
+        chosen.add(card)
+    }
+}
+
+@OptIn(ExperimentalLayoutApi::class)
+@Composable
+private fun TrickView(state: HeartsState) {
+    // The trick just swept is held on the table for a moment, so show that when
+    // there is nothing in front of the players yet.
+    val showing = state.trick.ifEmpty { state.completedTrick }
+    Column(verticalArrangement = Arrangement.spacedBy(4.dp)) {
+        Text(
+            text = if (state.trick.isEmpty() && showing.isNotEmpty()) {
+                "Last trick — taken by seat ${state.leader}"
+            } else {
+                "Trick ${state.trickNumber + 1} of ${state.tricksPerRound}"
+            },
+            style = MaterialTheme.typography.labelLarge,
+        )
+        if (showing.isEmpty()) {
+            Text("Nothing played yet.", style = MaterialTheme.typography.bodySmall)
+        } else {
+            FlowRow(horizontalArrangement = Arrangement.spacedBy(4.dp)) {
+                showing.forEach { played ->
+                    PlayingCardView(
+                        card = played.card,
+                        width = 46.dp,
+                        caption = "seat ${played.seat}",
+                    )
+                }
+            }
+        }
+    }
+}
+
+@Composable
+private fun ScoreRow(state: HeartsState, localSeat: Int) {
+    Row(
+        modifier = Modifier.fillMaxWidth(),
+        horizontalArrangement = Arrangement.SpaceBetween,
+    ) {
+        state.scores.forEachIndexed { seat, score ->
+            Column(horizontalAlignment = Alignment.CenterHorizontally) {
+                Text(
+                    text = if (seat == localSeat) "You" else "Seat $seat",
+                    style = MaterialTheme.typography.labelSmall,
+                    fontWeight = if (seat == localSeat) FontWeight.Bold else FontWeight.Normal,
+                )
+                Text("$score", fontWeight = FontWeight.Bold)
+                // What this round has cost so far, which is the number anybody
+                // actually watches while a hand is being played.
+                Text(
+                    text = "+${state.roundScores.getOrElse(seat) { 0 }}",
+                    style = MaterialTheme.typography.labelSmall,
+                )
+            }
+        }
+    }
+}
+
+@Composable
+private fun EndHeartsDialog(onEnd: () -> Unit, onKeepPlaying: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onKeepPlaying,
+        title = { Text("End this game?") },
+        text = { Text("The game ends here. Anyone else at the table is told you have left.") },
+        confirmButton = { TextButton(onClick = onEnd) { Text("End the game") } },
+        dismissButton = { TextButton(onClick = onKeepPlaying) { Text("Keep playing") } },
+    )
+}
+
+@Composable
+private fun LeftHeartsDialog(notice: String, onExit: () -> Unit, onStay: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onStay,
+        title = { Text("The game has ended") },
+        text = { Text(notice) },
+        confirmButton = { TextButton(onClick = onExit) { Text("Leave the game") } },
+        dismissButton = { TextButton(onClick = onStay) { Text("Look at the table") } },
+    )
+}

app/src/main/java/org/prolibertate/games/ui/game/KlondikeScreen.kt

@@ -0,0 +1,359 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.BorderStroke
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.horizontalScroll
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Button
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.klondike.Draw
+import org.prolibertate.games.game.klondike.KlondikeMove
+import org.prolibertate.games.game.klondike.KlondikeState
+import org.prolibertate.games.game.klondike.MoveCards
+import org.prolibertate.games.game.klondike.Redeal
+import org.prolibertate.games.game.klondike.Spot
+import org.prolibertate.games.game.klondike.TableauPile
+import org.prolibertate.games.game.klondike.foundationSuit
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.common.CARD_ASPECT
+import org.prolibertate.games.ui.common.CardBackView
+import org.prolibertate.games.ui.common.PlayingCardView
+import org.prolibertate.games.ui.theme.FeltGreenDark
+import org.prolibertate.games.ui.theme.Parchment
+import org.prolibertate.games.ui.theme.WallaceGold
+
+private const val TITLE = "Klondike"
+
+private val CARD_WIDTH: Dp = 44.dp
+private val CARD_HEIGHT: Dp = CARD_WIDTH / CARD_ASPECT
+
+/**
+ * How far down each card in a column sits from the one above it.
+ *
+ * Two thirds of a card, so each is overlapped by a third. That is the most that
+ * can be hidden while every card still shows its rank and suit, and it is what
+ * keeps a column of thirteen on the screen instead of running off the bottom.
+ */
+private val STACK_STEP: Dp = CARD_HEIGHT * 2f / 3f
+
+/** The same overlap sideways, for the three cards turned off the stock. */
+private val FAN_STEP: Dp = CARD_WIDTH * 2f / 3f
+
+/** At most three are ever turned at once, so at most three are ever fanned. */
+private const val MAX_FANNED = 3
+
+/**
+ * The patience table.
+ *
+ * Laid out the way a real one is: the homes along the top where cards are going,
+ * the columns filling the middle, and the pack in the bottom corner under your
+ * hand. Columns overlap by a third so a long one still fits, and the cards
+ * turned off the stock fan sideways by the same third so all three can be seen
+ * rather than only the one on top.
+ *
+ * Play is tap-to-take then tap-to-put rather than dragging: across seven columns
+ * of overlapping cards on a phone, a drag is mostly a test of aim. Tapping a
+ * face-up card takes it and everything resting on it, so a run travels the way
+ * it would on a table. Which taps do anything is read off the legal moves the
+ * rules offer, so the screen never has to know why a move is allowed.
+ */
+@Composable
+fun KlondikeScreen(
+    controller: MatchController<KlondikeState, KlondikeMove>,
+    onRestart: () -> Unit,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+
+    var held by remember { mutableStateOf<Pair<Spot, Int>?>(null) }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Dealing…") }
+            abandoned?.let { LeftTableDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) EndTableDialog(onExit) { confirmingEnd = false }
+        }
+        return
+    }
+
+    val moves = legal.filterIsInstance<MoveCards>()
+    val canDraw = legal.contains(Draw)
+    val canRedeal = legal.contains(Redeal)
+    val finished = legal.isEmpty()
+
+    val targets = held?.let { (from, count) ->
+        moves.filter { it.from == from && it.count == count }.map { it.to }.toSet()
+    }.orEmpty()
+    val sources = moves.map { it.from to it.count }.toSet()
+
+    fun take(spot: Spot, count: Int) {
+        val inHand = held
+        held = when {
+            inHand != null && targets.contains(spot) -> {
+                controller.submit(MoveCards(inHand.first, spot, inHand.second))
+                null
+            }
+
+            inHand?.first == spot -> null
+            sources.contains(spot to count) -> spot to count
+            else -> null
+        }
+    }
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.isWon -> "Out, in ${current.moves} moves."
+                    finished -> "Blocked — nothing left to try."
+                    held != null -> "Tap where it goes."
+                    else -> "${current.foundations.sumOf { it.size }} of 52 home."
+                },
+                fontWeight = FontWeight.Bold,
+            )
+
+            // The homes, along the top where they are being filled towards.
+            Row(horizontalArrangement = Arrangement.spacedBy(6.dp)) {
+                current.foundations.forEachIndexed { index, pile ->
+                    Slot(
+                        highlighted = targets.contains(Spot.foundation(index)),
+                        onClick = { take(Spot.foundation(index), 1) },
+                    ) {
+                        pile.lastOrNull()
+                            ?.let { PlayingCardView(card = it, width = CARD_WIDTH) }
+                            ?: Empty(label = foundationSuit(index).symbol)
+                    }
+                }
+            }
+
+            Row(
+                modifier = Modifier.fillMaxWidth().horizontalScroll(rememberScrollState()),
+                horizontalArrangement = Arrangement.spacedBy(6.dp),
+                verticalAlignment = Alignment.Top,
+            ) {
+                current.tableau.forEachIndexed { index, pile ->
+                    ColumnOfCards(
+                        pile = pile,
+                        held = held,
+                        index = index,
+                        isTarget = targets.contains(Spot.tableau(index)),
+                        onTap = { count -> take(Spot.tableau(index), count) },
+                    )
+                }
+            }
+
+            // The pack sits at the bottom left, under the hand that turns it.
+            Row(horizontalArrangement = Arrangement.spacedBy(6.dp)) {
+                Slot(
+                    highlighted = false,
+                    onClick = {
+                        if (canDraw) {
+                            controller.submit(Draw)
+                        } else if (canRedeal) {
+                            controller.submit(Redeal)
+                        }
+                    },
+                ) {
+                    if (current.stock.isNotEmpty()) {
+                        CardBackView(width = CARD_WIDTH)
+                    } else {
+                        Empty(label = if (canRedeal) "↻" else "")
+                    }
+                }
+                WasteFan(
+                    waste = current.waste,
+                    fanned = minOf(current.options.drawCount, MAX_FANNED),
+                    highlighted = held?.first == Spot.waste,
+                    onTap = { take(Spot.waste, 1) },
+                )
+            }
+
+            if (finished) {
+                Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                    Button(onClick = onRestart) { Text("Deal again") }
+                    TextButton(onClick = onExit) { Text("Leave") }
+                }
+            }
+
+            current.log.takeLast(2).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { LeftTableDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) EndTableDialog(onExit) { confirmingEnd = false }
+    }
+}
+
+/**
+ * One column, overlapped by a third.
+ *
+ * Absolute offsets rather than negative spacing: the height has to be worked out
+ * anyway so the column reserves the right room, and once that is known placing
+ * each card is the simpler half.
+ */
+@Composable
+private fun ColumnOfCards(
+    pile: TableauPile,
+    held: Pair<Spot, Int>?,
+    index: Int,
+    isTarget: Boolean,
+    onTap: (Int) -> Unit,
+) {
+    val total = pile.faceDown.size + pile.faceUp.size
+    if (total == 0) {
+        Slot(highlighted = isTarget, onClick = { onTap(1) }) { Empty() }
+        return
+    }
+    Box(
+        modifier = Modifier
+            .width(CARD_WIDTH)
+            .height(CARD_HEIGHT + STACK_STEP * (total - 1)),
+    ) {
+        pile.faceDown.forEachIndexed { depth, _ ->
+            CardBackView(
+                width = CARD_WIDTH,
+                modifier = Modifier.offset(y = STACK_STEP * depth),
+            )
+        }
+        pile.faceUp.forEachIndexed { depth, card ->
+            // Taking a card takes everything resting on it.
+            val count = pile.faceUp.size - depth
+            val row = pile.faceDown.size + depth
+            Slot(
+                modifier = Modifier.offset(y = STACK_STEP * row),
+                highlighted = held == (Spot.tableau(index) to count) ||
+                    (depth == pile.faceUp.lastIndex && isTarget),
+                onClick = { onTap(count) },
+            ) { PlayingCardView(card = card, width = CARD_WIDTH) }
+        }
+    }
+}
+
+/**
+ * The cards turned off the stock, fanned sideways.
+ *
+ * Drawing three and showing one would hide two cards the player is entitled to
+ * see — they are face up on the table. Only the last is playable, which the
+ * rules already enforce; the other two are there to be read.
+ */
+@Composable
+private fun WasteFan(
+    waste: List<Card>,
+    fanned: Int,
+    highlighted: Boolean,
+    onTap: () -> Unit,
+) {
+    val shown = waste.takeLast(fanned)
+    if (shown.isEmpty()) {
+        Slot(highlighted = false, onClick = onTap) { Empty() }
+        return
+    }
+    Box(modifier = Modifier.width(CARD_WIDTH + FAN_STEP * (shown.size - 1))) {
+        shown.forEachIndexed { position, card ->
+            val isTop = position == shown.lastIndex
+            Slot(
+                modifier = Modifier.offset(x = FAN_STEP * position),
+                highlighted = isTop && highlighted,
+                onClick = { if (isTop) onTap() },
+            ) { PlayingCardView(card = card, width = CARD_WIDTH) }
+        }
+    }
+}
+
+@Composable
+private fun Slot(
+    highlighted: Boolean,
+    onClick: () -> Unit,
+    modifier: Modifier = Modifier,
+    content: @Composable () -> Unit,
+) {
+    Box(
+        modifier = modifier
+            .width(CARD_WIDTH)
+            .border(
+                border = BorderStroke(if (highlighted) 3.dp else 0.dp, WallaceGold),
+                shape = RoundedCornerShape(4.dp),
+            )
+            .clickable { onClick() },
+    ) {
+        content()
+    }
+}
+
+/** An empty place on the table, drawn so it can still be tapped into. */
+@Composable
+private fun Empty(label: String = "") {
+    Box(
+        modifier = Modifier
+            .width(CARD_WIDTH)
+            .height(CARD_HEIGHT)
+            .background(FeltGreenDark, RoundedCornerShape(4.dp)),
+        contentAlignment = Alignment.Center,
+    ) {
+        if (label.isNotEmpty()) {
+            Text(label, color = Parchment.copy(alpha = 0.5f))
+        }
+    }
+}
+
+@Composable
+private fun EndTableDialog(onEnd: () -> Unit, onKeepPlaying: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onKeepPlaying,
+        title = { Text("End this game?") },
+        text = { Text("The deal is left as it stands.") },
+        confirmButton = { TextButton(onClick = onEnd) { Text("End the game") } },
+        dismissButton = { TextButton(onClick = onKeepPlaying) { Text("Keep playing") } },
+    )
+}
+
+@Composable
+private fun LeftTableDialog(notice: String, onExit: () -> Unit, onStay: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onStay,
+        title = { Text("The game has ended") },
+        text = { Text(notice) },
+        confirmButton = { TextButton(onClick = onExit) { Text("Leave the game") } },
+        dismissButton = { TextButton(onClick = onStay) { Text("Look at the table") } },
+    )
+}

app/src/main/java/org/prolibertate/games/ui/game/PegSolitaireScreen.kt

@@ -0,0 +1,259 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.BorderStroke
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxWithConstraints
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Button
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.game.pegsolitaire.Hole
+import org.prolibertate.games.game.pegsolitaire.PegBoard
+import org.prolibertate.games.game.pegsolitaire.PegJump
+import org.prolibertate.games.game.pegsolitaire.PegSolitaireMove
+import org.prolibertate.games.game.pegsolitaire.PegSolitaireState
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.theme.FeltGreenDark
+import org.prolibertate.games.ui.theme.Parchment
+import org.prolibertate.games.ui.theme.WallaceGold
+import org.prolibertate.games.ui.theme.WallaceRed
+
+private const val TITLE = "Peg Solitaire"
+
+/**
+ * The peg board.
+ *
+ * One layout serves both board shapes. Each hole is placed at a position in
+ * hole-widths rather than pixels, and a triangle simply offsets its rows by half
+ * a hole so they stagger — which is the same thing that gives a triangular hole
+ * six neighbours instead of four. Scaling that grid to whatever width the phone
+ * has is then the only sizing decision left.
+ */
+@Composable
+fun PegSolitaireScreen(
+    controller: MatchController<PegSolitaireState, PegSolitaireMove>,
+    onRestart: () -> Unit,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+
+    var selected by remember { mutableStateOf<Hole?>(null) }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Setting up…") }
+            abandoned?.let { LeftBoardDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) EndBoardDialog(onExit) { confirmingEnd = false }
+        }
+        return
+    }
+
+    val jumps = legal.filterIsInstance<PegJump>()
+    val movable = jumps.map { it.from }.toSet()
+    val landings = selected?.let { from -> jumps.filter { it.from == from } }.orEmpty()
+    val destinations = landings.map { it.to }.toSet()
+    val finished = jumps.isEmpty()
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.solved -> "Solved, in ${current.jumps} jumps."
+                    finished -> "Stuck with ${current.remaining} pegs."
+                    selected != null -> "Tap a ringed hole to jump into it."
+                    else -> "Tap a peg that can jump."
+                },
+                fontWeight = FontWeight.Bold,
+            )
+            Text(
+                text = "${current.board.label}. ${current.remaining} pegs left, " +
+                    "${current.jumps} jumps made.",
+                style = MaterialTheme.typography.bodySmall,
+            )
+
+            Board(
+                state = current,
+                selected = selected,
+                movable = movable,
+                destinations = destinations,
+                onTap = { hole ->
+                    selected = when {
+                        // Completing a jump takes priority: the tapped hole is
+                        // one the selected peg was offered.
+                        selected != null && destinations.contains(hole) -> {
+                            controller.submit(PegJump(from = selected!!, to = hole))
+                            null
+                        }
+
+                        movable.contains(hole) -> hole
+                        else -> null
+                    }
+                },
+            )
+
+            if (finished) {
+                Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                    Button(onClick = onRestart) { Text("Play again") }
+                    TextButton(onClick = onExit) { Text("Leave") }
+                }
+            }
+
+            current.log.takeLast(3).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { LeftBoardDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) EndBoardDialog(onExit) { confirmingEnd = false }
+    }
+}
+
+@Composable
+private fun Board(
+    state: PegSolitaireState,
+    selected: Hole?,
+    movable: Set<Hole>,
+    destinations: Set<Hole>,
+    onTap: (Hole) -> Unit,
+) {
+    val board = state.board
+    val holes = remember(board) { board.holes().toList() }
+    val places = remember(board) { holes.associateWith { placeOf(board, it) } }
+    val xs = places.values.map { it.first }
+    val ys = places.values.map { it.second }
+    val minX = xs.min()
+    val minY = ys.min()
+    val spanX = xs.max() - minX
+    val spanY = ys.max() - minY
+
+    BoxWithConstraints(modifier = Modifier.fillMaxWidth()) {
+        // One extra hole-width so the last column has somewhere to sit.
+        val cell: Dp = maxWidth / (spanX + 1f)
+        Box(
+            modifier = Modifier
+                .fillMaxWidth()
+                .height(cell * (spanY + 1f))
+                .background(FeltGreenDark),
+        ) {
+            holes.forEach { hole ->
+                val (x, y) = places.getValue(hole)
+                HoleView(
+                    filled = state.pegs.contains(hole),
+                    selected = hole == selected,
+                    canMove = movable.contains(hole),
+                    isDestination = destinations.contains(hole),
+                    size = cell,
+                    modifier = Modifier.offset(x = cell * (x - minX), y = cell * (y - minY)),
+                    onTap = { onTap(hole) },
+                )
+            }
+        }
+    }
+}
+
+/**
+ * Where a hole sits, in hole-widths from the board's top left.
+ *
+ * The half-hole shift on a triangle is the whole difference between the two
+ * geometries as far as drawing is concerned.
+ */
+private fun placeOf(board: PegBoard, hole: Hole): Pair<Float, Float> =
+    if (board.isTriangular) {
+        (hole.col - hole.row / 2f) to hole.row.toFloat()
+    } else {
+        hole.col.toFloat() to hole.row.toFloat()
+    }
+
+@Composable
+private fun HoleView(
+    filled: Boolean,
+    selected: Boolean,
+    canMove: Boolean,
+    isDestination: Boolean,
+    size: Dp,
+    modifier: Modifier,
+    onTap: () -> Unit,
+) {
+    Box(modifier = modifier.size(size), contentAlignment = Alignment.Center) {
+        val diameter = size * 0.72f
+        val ring = when {
+            selected -> WallaceGold
+            isDestination -> WallaceGold
+            else -> Parchment.copy(alpha = 0.35f)
+        }
+        Box(
+            modifier = Modifier
+                .size(diameter)
+                .background(
+                    color = if (filled) WallaceRed else FeltGreenDark,
+                    shape = CircleShape,
+                )
+                .border(
+                    // A destination is drawn as an empty ring rather than filled,
+                    // so it never reads as a peg already sitting there.
+                    border = BorderStroke(if (selected || isDestination) 3.dp else 1.dp, ring),
+                    shape = CircleShape,
+                )
+                .clickable(enabled = filled && canMove || isDestination) { onTap() },
+        )
+    }
+}
+
+@Composable
+private fun EndBoardDialog(onEnd: () -> Unit, onKeepPlaying: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onKeepPlaying,
+        title = { Text("End this game?") },
+        text = { Text("The board is left as it stands.") },
+        confirmButton = { TextButton(onClick = onEnd) { Text("End the game") } },
+        dismissButton = { TextButton(onClick = onKeepPlaying) { Text("Keep playing") } },
+    )
+}
+
+@Composable
+private fun LeftBoardDialog(notice: String, onExit: () -> Unit, onStay: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onStay,
+        title = { Text("The game has ended") },
+        text = { Text(notice) },
+        confirmButton = { TextButton(onClick = onExit) { Text("Leave the game") } },
+        dismissButton = { TextButton(onClick = onStay) { Text("Look at the board") } },
+    )
+}

app/src/main/java/org/prolibertate/games/ui/game/PyramidScreen.kt

@@ -0,0 +1,260 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.Button
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.game.pyramid.DrawCard
+import org.prolibertate.games.game.pyramid.PYRAMID_SIZE
+import org.prolibertate.games.game.pyramid.PyramidMove
+import org.prolibertate.games.game.pyramid.PyramidSpot
+import org.prolibertate.games.game.pyramid.PyramidState
+import org.prolibertate.games.game.pyramid.PyramidZone
+import org.prolibertate.games.game.pyramid.RecycleWaste
+import org.prolibertate.games.game.pyramid.ROWS
+import org.prolibertate.games.game.pyramid.TakeKing
+import org.prolibertate.games.game.pyramid.TakePair
+import org.prolibertate.games.game.pyramid.rowStart
+import org.prolibertate.games.game.pyramid.valueOf
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.common.CardBackView
+import org.prolibertate.games.ui.common.PlayingCardView
+
+private const val TITLE = "Pyramid"
+
+/** Cards are small here: the widest row is seven of them side by side. */
+private val PYRAMID_CARD: Dp = 40.dp
+private val PYRAMID_CARD_HEIGHT: Dp = PYRAMID_CARD * 7f / 5f
+
+/** Rows overlap vertically so the whole pyramid fits above the pack. */
+private val ROW_STEP: Dp = PYRAMID_CARD_HEIGHT * 2f / 3f
+
+/**
+ * The pyramid.
+ *
+ * Drawn as it is dealt: seven rows, each one offset half a card so every card
+ * sits on the two below it, and rows overlapping by a third so all seven fit.
+ * The shape is the rules — a card is only in play once both the cards resting on
+ * it have gone, and being able to see that at a glance is most of playing it.
+ *
+ * Tapping a king takes it alone. Tapping any other card holds it, and tapping
+ * its partner takes the pair; the ones that would pair with what is held are the
+ * only cards that light up.
+ */
+@Composable
+fun PyramidScreen(
+    controller: MatchController<PyramidState, PyramidMove>,
+    onRestart: () -> Unit,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+
+    var held by remember { mutableStateOf<PyramidSpot?>(null) }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Dealing…") }
+            abandoned?.let { PatienceLeftDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) {
+                PatienceEndDialog("The pyramid is left as it stands.", onExit) { confirmingEnd = false }
+            }
+        }
+        return
+    }
+
+    val kings = legal.filterIsInstance<TakeKing>().map { it.spot }.toSet()
+    val pairs = legal.filterIsInstance<TakePair>()
+    val canDraw = legal.contains(DrawCard)
+    val canRedeal = legal.contains(RecycleWaste)
+    val finished = legal.isEmpty()
+
+    // Everything that could pair with what is in hand.
+    val partners = held?.let { spot ->
+        pairs.mapNotNull {
+            when (spot) {
+                it.first -> it.second
+                it.second -> it.first
+                else -> null
+            }
+        }.toSet()
+    }.orEmpty()
+    val pairable = pairs.flatMap { listOf(it.first, it.second) }.toSet()
+
+    fun tap(spot: PyramidSpot) {
+        held = when {
+            kings.contains(spot) && held == null -> {
+                controller.submit(TakeKing(spot))
+                null
+            }
+
+            held != null && partners.contains(spot) -> {
+                controller.submit(TakePair(held!!, spot))
+                null
+            }
+
+            held == spot -> null
+            pairable.contains(spot) -> spot
+            else -> null
+        }
+    }
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.isWon -> "Pyramid cleared, in ${current.moves} moves."
+                    finished -> "Blocked — nothing pairs to thirteen."
+                    held != null -> "Tap the card that makes it thirteen."
+                    else -> "${current.pyramid.count { it != null }} of $PYRAMID_SIZE left."
+                },
+                fontWeight = FontWeight.Bold,
+            )
+
+            PyramidView(
+                state = current,
+                held = held,
+                kings = kings,
+                partners = partners,
+                pairable = pairable,
+                onTap = ::tap,
+            )
+
+            Row(
+                horizontalArrangement = Arrangement.spacedBy(8.dp),
+                verticalAlignment = Alignment.CenterVertically,
+            ) {
+                TableSlot(
+                    highlighted = false,
+                    width = PYRAMID_CARD,
+                    onClick = {
+                        if (canDraw) {
+                            controller.submit(DrawCard)
+                        } else if (canRedeal) {
+                            controller.submit(RecycleWaste)
+                        }
+                    },
+                ) {
+                    if (current.stock.isNotEmpty()) {
+                        CardBackView(width = PYRAMID_CARD)
+                    } else {
+                        EmptySpace(label = if (canRedeal) "↻" else "", width = PYRAMID_CARD)
+                    }
+                }
+                val wasteSpot = PyramidSpot(PyramidZone.WASTE)
+                TableSlot(
+                    highlighted = held == wasteSpot ||
+                        partners.contains(wasteSpot) ||
+                        kings.contains(wasteSpot),
+                    width = PYRAMID_CARD,
+                    onClick = { tap(wasteSpot) },
+                ) {
+                    current.waste.lastOrNull()
+                        ?.let { PlayingCardView(card = it, width = PYRAMID_CARD) }
+                        ?: EmptySpace(width = PYRAMID_CARD)
+                }
+                Text(
+                    text = "${current.stock.size} in hand, " +
+                        "${current.options.redeals - current.redealsUsed} turns left",
+                    style = MaterialTheme.typography.bodySmall,
+                )
+            }
+
+            if (finished) {
+                Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                    Button(onClick = onRestart) { Text("Deal again") }
+                    TextButton(onClick = onExit) { Text("Leave") }
+                }
+            }
+
+            current.log.takeLast(2).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { PatienceLeftDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) {
+            PatienceEndDialog("The pyramid is left as it stands.", onExit) { confirmingEnd = false }
+        }
+    }
+}
+
+@Composable
+private fun PyramidView(
+    state: PyramidState,
+    held: PyramidSpot?,
+    kings: Set<PyramidSpot>,
+    partners: Set<PyramidSpot>,
+    pairable: Set<PyramidSpot>,
+    onTap: (PyramidSpot) -> Unit,
+) {
+    Box(
+        modifier = Modifier
+            .fillMaxWidth()
+            .height(PYRAMID_CARD_HEIGHT + ROW_STEP * (ROWS - 1)),
+    ) {
+        // Each row is half a card narrower on each side than the one below, so
+        // every card sits in the notch between the two under it.
+        for (row in 0 until ROWS) {
+            val cardsInRow = row + 1
+            for (column in 0 until cardsInRow) {
+                val index = rowStart(row) + column
+                val card = state.pyramid[index] ?: continue
+                val spot = PyramidSpot(PyramidZone.PYRAMID, index)
+                val playable = pairable.contains(spot) || kings.contains(spot)
+                Box(
+                    modifier = Modifier.offset(
+                        // Centred on the widest row, which is seven wide.
+                        x = PYRAMID_CARD * (ROWS - cardsInRow) / 2f + PYRAMID_CARD * column,
+                        y = ROW_STEP * row,
+                    ),
+                ) {
+                    TableSlot(
+                        highlighted = held == spot || partners.contains(spot),
+                        width = PYRAMID_CARD,
+                        onClick = { if (playable) onTap(spot) },
+                    ) {
+                        PlayingCardView(
+                            card = card,
+                            width = PYRAMID_CARD,
+                            enabled = playable,
+                            caption = valueOf(card).toString(),
+                        )
+                    }
+                }
+            }
+        }
+    }
+}

app/src/main/java/org/prolibertate/games/ui/game/SpiderScreen.kt

@@ -0,0 +1,191 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.horizontalScroll
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.Button
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.game.spider.DealRow
+import org.prolibertate.games.game.spider.MoveRun
+import org.prolibertate.games.game.spider.RUNS_TO_WIN
+import org.prolibertate.games.game.spider.SpiderMove
+import org.prolibertate.games.game.spider.SpiderState
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.common.CardBackView
+import org.prolibertate.games.ui.common.PlayingCardView
+
+private const val TITLE = "Spider"
+
+/**
+ * The Spider table.
+ *
+ * Ten columns of a hundred and four cards, so it scrolls sideways and the cards
+ * are the same third-overlapped stacks as everywhere else. What is worth showing
+ * above them is the two numbers that decide the game: how many runs have gone
+ * away, and how many rows are still in the stock waiting to bury everything.
+ */
+@Composable
+fun SpiderScreen(
+    controller: MatchController<SpiderState, SpiderMove>,
+    onRestart: () -> Unit,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+
+    var held by remember { mutableStateOf<Pair<Int, Int>?>(null) }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Dealing…") }
+            abandoned?.let { PatienceLeftDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) {
+                PatienceEndDialog("The deal is left as it stands.", onExit) { confirmingEnd = false }
+            }
+        }
+        return
+    }
+
+    val runs = legal.filterIsInstance<MoveRun>()
+    val canDeal = legal.contains(DealRow)
+    val finished = legal.isEmpty()
+    val targets = held?.let { (from, count) ->
+        runs.filter { it.from == from && it.count == count }.map { it.to }.toSet()
+    }.orEmpty()
+    val sources = runs.map { it.from to it.count }.toSet()
+
+    fun take(column: Int, count: Int) {
+        val inHand = held
+        held = when {
+            inHand != null && targets.contains(column) -> {
+                controller.submit(MoveRun(inHand.first, column, inHand.second))
+                null
+            }
+
+            inHand?.first == column && inHand.second == count -> null
+            sources.contains(column to count) -> column to count
+            else -> null
+        }
+    }
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.isWon -> "All eight runs, in ${current.moves} moves."
+                    finished -> "Blocked — nothing left to try."
+                    held != null -> "Tap the column it goes to."
+                    else -> "${current.completed} of $RUNS_TO_WIN runs away."
+                },
+                fontWeight = FontWeight.Bold,
+            )
+
+            Row(
+                horizontalArrangement = Arrangement.spacedBy(8.dp),
+                verticalAlignment = Alignment.CenterVertically,
+            ) {
+                TableSlot(
+                    highlighted = false,
+                    onClick = { if (canDeal) controller.submit(DealRow) },
+                ) {
+                    if (current.stock.isNotEmpty()) {
+                        CardBackView(width = TABLE_CARD_WIDTH)
+                    } else {
+                        EmptySpace()
+                    }
+                }
+                Text(
+                    text = "${current.stock.size / 10} rows left" +
+                        if (!canDeal && current.stock.isNotEmpty()) " — fill every column first" else "",
+                    style = MaterialTheme.typography.bodySmall,
+                )
+            }
+
+            Row(
+                modifier = Modifier.fillMaxWidth().horizontalScroll(rememberScrollState()),
+                horizontalArrangement = Arrangement.spacedBy(4.dp),
+                verticalAlignment = Alignment.Top,
+            ) {
+                current.tableau.forEachIndexed { index, pile ->
+                    val total = pile.faceDown.size + pile.faceUp.size
+                    if (total == 0) {
+                        TableSlot(
+                            highlighted = targets.contains(index),
+                            onClick = { take(index, 1) },
+                        ) { EmptySpace() }
+                    } else {
+                        Box(
+                            modifier = Modifier
+                                .width(TABLE_CARD_WIDTH)
+                                .height(TABLE_CARD_HEIGHT + TABLE_STACK_STEP * (total - 1)),
+                        ) {
+                            pile.faceDown.forEachIndexed { depth, _ ->
+                                CardBackView(
+                                    width = TABLE_CARD_WIDTH,
+                                    modifier = Modifier.offset(y = TABLE_STACK_STEP * depth),
+                                )
+                            }
+                            pile.faceUp.forEachIndexed { depth, card ->
+                                val count = pile.faceUp.size - depth
+                                val row = pile.faceDown.size + depth
+                                TableSlot(
+                                    modifier = Modifier.offset(y = TABLE_STACK_STEP * row),
+                                    highlighted = held == (index to count) ||
+                                        (depth == pile.faceUp.lastIndex && targets.contains(index)),
+                                    onClick = { take(index, count) },
+                                ) { PlayingCardView(card = card, width = TABLE_CARD_WIDTH) }
+                            }
+                        }
+                    }
+                }
+            }
+
+            if (finished) {
+                Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                    Button(onClick = onRestart) { Text("Deal again") }
+                    TextButton(onClick = onExit) { Text("Leave") }
+                }
+            }
+
+            current.log.takeLast(2).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { PatienceLeftDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) {
+            PatienceEndDialog("The deal is left as it stands.", onExit) { confirmingEnd = false }
+        }
+    }
+}

app/src/main/java/org/prolibertate/games/ui/game/YahtzeeScreen.kt

@@ -0,0 +1,304 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Button
+import androidx.compose.material3.Divider
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateListOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.game.yahtzee.DICE_COUNT
+import org.prolibertate.games.game.yahtzee.RollDice
+import org.prolibertate.games.game.yahtzee.ScoreIn
+import org.prolibertate.games.game.yahtzee.YahtzeeCategory
+import org.prolibertate.games.game.yahtzee.YahtzeeMove
+import org.prolibertate.games.game.yahtzee.YahtzeeSection
+import org.prolibertate.games.game.yahtzee.YahtzeeState
+import org.prolibertate.games.game.yahtzee.scoreOf
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.theme.Parchment
+import org.prolibertate.games.ui.theme.WallaceGold
+import org.prolibertate.games.ui.theme.WallaceRed
+
+private const val TITLE = "Yahtzee"
+
+/**
+ * The Yahtzee card.
+ *
+ * A turn is two decisions and the screen shows them together: which dice to keep
+ * before throwing again, and which box to write in when the throwing stops. Each
+ * open box shows what the dice on the table would score in it, because that
+ * comparison is the whole of the choice — and doing the arithmetic by eye is not
+ * what anyone wants from a phone.
+ */
+@Composable
+fun YahtzeeScreen(
+    controller: MatchController<YahtzeeState, YahtzeeMove>,
+    localSeat: Int,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+
+    val kept = remember { mutableStateListOf<Int>() }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Setting up…") }
+            abandoned?.let { LeftCardDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) EndCardDialog(onExit) { confirmingEnd = false }
+        }
+        return
+    }
+
+    val yours = current.turn == localSeat && !current.isOver
+    val canRoll = legal.any { it is RollDice }
+    val writable = legal.filterIsInstance<ScoreIn>().map { it.category }.toSet()
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.isOver -> "Cards full."
+                    !yours -> "Seat ${current.turn} is throwing…"
+                    !current.hasRolled -> "Your turn — throw the dice."
+                    else -> "${current.rollsLeft} throws left, or write it down."
+                },
+                fontWeight = FontWeight.Bold,
+            )
+
+            DiceRow(
+                dice = current.dice,
+                kept = kept,
+                enabled = yours && current.hasRolled && canRoll,
+                onToggle = { index ->
+                    if (kept.contains(index)) kept.remove(index) else kept.add(index)
+                },
+            )
+
+            if (yours && canRoll) {
+                Button(
+                    onClick = {
+                        controller.submit(RollDice(kept.toList()))
+                        // What was kept applied to the throw just made; the next
+                        // one starts from whatever is on the table now.
+                        kept.clear()
+                    },
+                ) {
+                    Text(if (current.hasRolled) "Throw the rest" else "Throw")
+                }
+            }
+
+            Divider()
+            ScoreCard(
+                state = current,
+                localSeat = localSeat,
+                writable = if (yours) writable else emptySet(),
+                onWrite = {
+                    controller.submit(ScoreIn(it))
+                    kept.clear()
+                },
+            )
+
+            current.log.takeLast(3).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { LeftCardDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) EndCardDialog(onExit) { confirmingEnd = false }
+    }
+}
+
+@Composable
+private fun DiceRow(
+    dice: List<Int>,
+    kept: List<Int>,
+    enabled: Boolean,
+    onToggle: (Int) -> Unit,
+) {
+    Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+        if (dice.isEmpty()) {
+            repeat(DICE_COUNT) { Die(face = null, held = false, enabled = false) {} }
+        } else {
+            dice.forEachIndexed { index, face ->
+                Die(
+                    face = face,
+                    held = kept.contains(index),
+                    enabled = enabled,
+                    onClick = { onToggle(index) },
+                )
+            }
+        }
+    }
+}
+
+@Composable
+private fun Die(face: Int?, held: Boolean, enabled: Boolean, onClick: () -> Unit) {
+    Box(
+        modifier = Modifier
+            .size(48.dp)
+            .background(if (face == null) Parchment.copy(alpha = 0.3f) else Parchment, RoundedCornerShape(8.dp))
+            .border(
+                width = if (held) 3.dp else 1.dp,
+                color = if (held) WallaceGold else WallaceRed.copy(alpha = 0.4f),
+                shape = RoundedCornerShape(8.dp),
+            )
+            .clickable(enabled = enabled) { onClick() },
+        contentAlignment = Alignment.Center,
+    ) {
+        Text(
+            text = face?.toString() ?: "·",
+            style = MaterialTheme.typography.titleLarge,
+            fontWeight = FontWeight.Bold,
+            color = WallaceRed,
+        )
+    }
+}
+
+@Composable
+private fun ScoreCard(
+    state: YahtzeeState,
+    localSeat: Int,
+    writable: Set<YahtzeeCategory>,
+    onWrite: (YahtzeeCategory) -> Unit,
+) {
+    Column(modifier = Modifier.fillMaxWidth()) {
+        Row(modifier = Modifier.fillMaxWidth()) {
+            Text("", modifier = Modifier.weight(2f), style = MaterialTheme.typography.labelSmall)
+            state.cards.indices.forEach { seat ->
+                Text(
+                    text = if (seat == localSeat) "You" else "S$seat",
+                    modifier = Modifier.weight(1f),
+                    style = MaterialTheme.typography.labelSmall,
+                    fontWeight = FontWeight.Bold,
+                )
+            }
+        }
+        YahtzeeSection.entries.forEach { section ->
+            YahtzeeCategory.entries.filter { it.section == section }.forEach { category ->
+                CardRow(state, localSeat, category, writable, onWrite)
+            }
+            if (section == YahtzeeSection.UPPER) {
+                TotalRow("Bonus at 63", state.cards.map { it.upperBonus })
+            }
+        }
+        Divider()
+        TotalRow("Total", state.cards.indices.map { state.totalFor(it) }, bold = true)
+    }
+}
+
+@Composable
+private fun CardRow(
+    state: YahtzeeState,
+    localSeat: Int,
+    category: YahtzeeCategory,
+    writable: Set<YahtzeeCategory>,
+    onWrite: (YahtzeeCategory) -> Unit,
+) {
+    val open = writable.contains(category)
+    Row(
+        modifier = Modifier
+            .fillMaxWidth()
+            .clickable(enabled = open) { onWrite(category) }
+            .padding(vertical = 2.dp),
+    ) {
+        Text(
+            text = category.label,
+            modifier = Modifier.weight(2f),
+            style = MaterialTheme.typography.bodySmall,
+            fontWeight = if (open) FontWeight.Bold else FontWeight.Normal,
+        )
+        state.cards.forEachIndexed { seat, card ->
+            val written = card[category]
+            Text(
+                text = when {
+                    written != null -> "$written"
+                    // What this throw would be worth here, which is the only
+                    // number the decision actually turns on.
+                    seat == localSeat && open -> "(${scoreOf(category, state.dice)})"
+                    else -> "—"
+                },
+                modifier = Modifier.weight(1f),
+                style = MaterialTheme.typography.bodySmall,
+                color = if (written == null && open) WallaceRed else MaterialTheme.colorScheme.onSurface,
+            )
+        }
+    }
+}
+
+@Composable
+private fun TotalRow(label: String, values: List<Int>, bold: Boolean = false) {
+    Row(modifier = Modifier.fillMaxWidth().padding(vertical = 2.dp)) {
+        Text(
+            text = label,
+            modifier = Modifier.weight(2f),
+            style = MaterialTheme.typography.bodySmall,
+            fontWeight = if (bold) FontWeight.Bold else FontWeight.Normal,
+        )
+        values.forEach {
+            Text(
+                text = "$it",
+                modifier = Modifier.weight(1f),
+                style = MaterialTheme.typography.bodySmall,
+                fontWeight = if (bold) FontWeight.Bold else FontWeight.Normal,
+            )
+        }
+    }
+}
+
+@Composable
+private fun EndCardDialog(onEnd: () -> Unit, onKeepPlaying: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onKeepPlaying,
+        title = { Text("End this game?") },
+        text = { Text("The game ends here. Anyone else at the table is told you have left.") },
+        confirmButton = { TextButton(onClick = onEnd) { Text("End the game") } },
+        dismissButton = { TextButton(onClick = onKeepPlaying) { Text("Keep playing") } },
+    )
+}
+
+@Composable
+private fun LeftCardDialog(notice: String, onExit: () -> Unit, onStay: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onStay,
+        title = { Text("The game has ended") },
+        text = { Text(notice) },
+        confirmButton = { TextButton(onClick = onExit) { Text("Leave the game") } },
+        dismissButton = { TextButton(onClick = onStay) { Text("Look at the card") } },
+    )
+}

app/src/test/java/org/prolibertate/games/game/CatalogTest.kt

@@ -16,21 +16,54 @@ class CatalogTest {
 
     @Test
     fun `a production build lists only games that can be played`() {
-        GameCategory.entries.forEach { category ->
-            val shown = GameCatalog.byCategory(category, includeComingSoon = false)
+        GameMenu.entries.forEach { menu ->
+            val shown = GameCatalog.inMenu(menu, includeComingSoon = false)
             assertTrue(
-                "$category offers an unfinished game in a release build",
+                "$menu offers an unfinished game in a release build",
                 shown.all { it.available },
             )
         }
     }
 
     @Test
-    fun `a dev build lists the whole catalogue`() {
-        val shown = GameCategory.entries.flatMap {
-            GameCatalog.byCategory(it, includeComingSoon = true)
+    fun `every game is reachable from at least one menu`() {
+        // Counting is no good now that a game can be in several menus: the
+        // union is what matters, and a game in none of them would be in the
+        // catalogue but on no screen.
+        val reachable = GameMenu.entries.flatMap {
+            GameCatalog.inMenu(it, includeComingSoon = true)
+        }.toSet()
+        assertEquals(GameCatalog.all.toSet(), reachable)
+        GameCatalog.all.forEach {
+            assertTrue("${it.id} is in no menu", it.menus.isNotEmpty())
         }
-        assertEquals(GameCatalog.all.size, shown.size)
+    }
+
+    @Test
+    fun `a game can be in more than one menu`() {
+        // The whole point of the set. A patience is solitaire and a card game;
+        // Euchre is a card game and a trick-taking one.
+        val klondike = GameCatalog.byId(GameCatalog.KLONDIKE)!!
+        assertTrue(klondike.menus.containsAll(setOf(GameMenu.SOLITAIRE, GameMenu.CARDS)))
+        assertTrue(GameCatalog.inMenu(GameMenu.SOLITAIRE).contains(klondike))
+        assertTrue(GameCatalog.inMenu(GameMenu.CARDS).contains(klondike))
+    }
+
+    @Test
+    fun `trick-taking sits under cards, and its games are card games too`() {
+        assertEquals(GameMenu.CARDS, GameMenu.TRICK_TAKING.parent)
+        assertEquals(listOf(GameMenu.TRICK_TAKING), GameMenu.CARDS.children)
+        assertTrue(GameMenu.TRICK_TAKING.isTopLevel.not())
+        assertTrue(GameMenu.SOLITAIRE.isTopLevel)
+
+        val tricks = GameCatalog.inMenu(GameMenu.TRICK_TAKING)
+        assertTrue("there should be some", tricks.isNotEmpty())
+        // A sub-menu that listed something its parent did not would be a game
+        // you could reach one way and not the other.
+        assertTrue(
+            "every trick-taking game is also a card game",
+            tricks.all { it.menus.contains(GameMenu.CARDS) },
+        )
     }
 
     @Test
@@ -42,19 +75,19 @@ class CatalogTest {
         // engine is written.
         assertTrue(GameCatalog.all.all { it.available })
         assertEquals(
-            GameCatalog.byCategory(GameCategory.BOARD, includeComingSoon = false),
-            GameCatalog.byCategory(GameCategory.BOARD, includeComingSoon = true),
+            GameCatalog.inMenu(GameMenu.BOARD, includeComingSoon = false),
+            GameCatalog.inMenu(GameMenu.BOARD, includeComingSoon = true),
         )
     }
 
     @Test
-    fun `hiding unfinished games still leaves something to play in each category`() {
-        // A category filtered down to nothing would leave a stranded heading,
-        // and a menu with no playable game at all would be a broken release.
-        GameCategory.entries.forEach { category ->
-            assertFalse(
-                "$category has nothing playable in a release build",
-                GameCatalog.byCategory(category, includeComingSoon = false).isEmpty(),
+    fun `hiding unfinished games still leaves something to play in every menu`() {
+        // A menu filtered down to nothing would leave a stranded heading, and a
+        // release where a whole section was empty would be a broken one.
+        GameMenu.entries.forEach { menu ->
+            assertTrue(
+                "$menu has nothing playable in a release build",
+                GameCatalog.hasAnything(menu, includeComingSoon = false),
             )
         }
         assertTrue(GameCatalog.playable.isNotEmpty())
@@ -71,6 +104,13 @@ class CatalogTest {
                 GameCatalog.KAISER,
                 GameCatalog.CRAZY_EIGHTS,
                 GameCatalog.CRIBBAGE,
+                GameCatalog.HEARTS,
+                GameCatalog.PEG_SOLITAIRE,
+                GameCatalog.YAHTZEE,
+                GameCatalog.KLONDIKE,
+                GameCatalog.FREECELL,
+                GameCatalog.SPIDER,
+                GameCatalog.PYRAMID,
                 GameCatalog.WIZARD,
                 GameCatalog.CHESS,
                 GameCatalog.TAYU,
@@ -87,7 +127,11 @@ class CatalogTest {
     @Test
     fun `player counts are sane`() {
         GameCatalog.all.forEach { game ->
-            assertTrue("${game.id} min players", game.minPlayers >= 2)
+            // One, not two. Peg solitaire is played alone, and the engine took
+            // that without changes — one seat, always on the clock, nothing to
+            // redact. Everything downstream reads the seat count from here, so
+            // this is the only place that had to allow it.
+            assertTrue("${game.id} min players", game.minPlayers >= 1)
             assertTrue("${game.id} max >= min", game.maxPlayers >= game.minPlayers)
         }
     }

app/src/test/java/org/prolibertate/games/game/hearts/HeartsRulesTest.kt

@@ -0,0 +1,332 @@
+package org.prolibertate.games.game.hearts
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.engine.PlayerKind
+import org.prolibertate.games.game.engine.PlayerSlot
+import org.prolibertate.games.game.engine.TableConfig
+
+class HeartsRulesTest {
+
+    private val json = Json { encodeDefaults = true }
+
+    private fun config(options: HeartsOptions = HeartsOptions(), seed: Long = 7L) = TableConfig(
+        gameId = "hearts",
+        seats = (0 until options.playerCount).map {
+            PlayerSlot(seat = it, name = "P$it", kind = PlayerKind.AI, team = it)
+        },
+        optionsJson = json.encodeToString(options),
+        seed = seed,
+    )
+
+    private fun c(rank: Rank, suit: Suit) = Card(rank, suit)
+
+    /** A state part way through a round, with passing already done. */
+    private fun playing(
+        hands: List<List<Card>>,
+        turn: Int = 0,
+        leader: Int = 0,
+        trick: List<PlayedCard> = emptyList(),
+        trickNumber: Int = 1,
+        heartsBroken: Boolean = false,
+        taken: List<List<Card>> = List(hands.size) { emptyList() },
+        options: HeartsOptions = HeartsOptions(),
+    ) = HeartsState(
+        options = options,
+        seed = 1L,
+        round = 0,
+        phase = HeartsPhase.PLAYING,
+        hands = hands,
+        handCounts = hands.map { it.size },
+        passSelections = List(hands.size) { emptyList() },
+        turn = turn,
+        leader = leader,
+        trickNumber = trickNumber,
+        trick = trick,
+        taken = taken,
+        heartsBroken = heartsBroken,
+        scores = List(hands.size) { 0 },
+        roundScores = List(hands.size) { 0 },
+        log = emptyList(),
+    )
+
+    @Test
+    fun `a deal gives four hands of thirteen from one pack`() {
+        val state = HeartsRules.initialState(config())
+        assertEquals(4, state.hands.size)
+        state.hands.forEach { assertEquals(13, it.size) }
+        val all = state.hands.flatten()
+        assertEquals(52, all.size)
+        assertEquals("no card is dealt twice", 52, all.distinct().size)
+    }
+
+    @Test
+    fun `the pass direction cycles and the fourth round holds`() {
+        assertEquals(PassDirection.LEFT, PassDirection.forRound(0))
+        assertEquals(PassDirection.RIGHT, PassDirection.forRound(1))
+        assertEquals(PassDirection.ACROSS, PassDirection.forRound(2))
+        assertEquals(PassDirection.HOLD, PassDirection.forRound(3))
+        assertEquals(PassDirection.LEFT, PassDirection.forRound(4))
+    }
+
+    @Test
+    fun `a hold round opens straight onto the lead`() {
+        // Round three holds, so there is nothing to swap and play begins at once
+        // with whoever holds the two of clubs.
+        var state = HeartsRules.initialState(config())
+        // Walk to round three through the public entry point rather than
+        // reaching into the private deal.
+        repeat(3) {
+            state = HeartsRules.startNextRound(state.copy(phase = HeartsPhase.ROUND_OVER))
+        }
+        assertEquals(3, state.round)
+        assertEquals(PassDirection.HOLD, state.passDirection)
+        assertEquals(HeartsPhase.PLAYING, state.phase)
+        assertTrue(state.hands[state.turn].contains(TWO_OF_CLUBS))
+    }
+
+    @Test
+    fun `passing left moves three cards to the next seat`() {
+        var state = HeartsRules.initialState(config())
+        assertEquals(HeartsPhase.PASSING, state.phase)
+        val given = (0 until 4).map { state.hands[it].take(PASS_SIZE) }
+        for (seat in 0 until 4) {
+            assertEquals(seat, HeartsRules.currentSeat(state))
+            state = HeartsRules.applyMove(state, seat, PassCards(given[seat]))
+        }
+        assertEquals(HeartsPhase.PLAYING, state.phase)
+        for (seat in 0 until 4) {
+            val receiver = (seat + 1) % 4
+            assertTrue(
+                "seat $receiver should hold what seat $seat passed",
+                state.hands[receiver].containsAll(given[seat]),
+            )
+            assertFalse(
+                "seat $seat should not still hold what it passed",
+                state.hands[seat].any { given[seat].contains(it) },
+            )
+        }
+        state.hands.forEach { assertEquals(13, it.size) }
+    }
+
+    @Test
+    fun `a pass must be three distinct cards from the hand`() {
+        val state = HeartsRules.initialState(config())
+        val hand = state.hands[0]
+        assertThrows { HeartsRules.applyMove(state, 0, PassCards(hand.take(2))) }
+        assertThrows { HeartsRules.applyMove(state, 0, PassCards(listOf(hand[0], hand[0], hand[1]))) }
+        // A card this seat does not hold — it is in somebody else's thirteen.
+        val notHeld = state.hands[1].first()
+        assertThrows { HeartsRules.applyMove(state, 0, PassCards(listOf(notHeld) + hand.take(2))) }
+    }
+
+    @Test
+    fun `the two of clubs opens the round`() {
+        val hands = listOf(
+            listOf(c(Rank.TWO, Suit.CLUBS), c(Rank.KING, Suit.SPADES)),
+            listOf(c(Rank.THREE, Suit.CLUBS), c(Rank.FOUR, Suit.HEARTS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS), c(Rank.SIX, Suit.DIAMONDS)),
+            listOf(c(Rank.SEVEN, Suit.CLUBS), c(Rank.EIGHT, Suit.DIAMONDS)),
+        )
+        val state = playing(hands, trickNumber = 0)
+        assertEquals(listOf(TWO_OF_CLUBS), HeartsRules.playableCards(state, 0))
+        assertThrows { HeartsRules.applyMove(state, 0, PlayCard(c(Rank.KING, Suit.SPADES))) }
+    }
+
+    @Test
+    fun `a seat holding the led suit must follow it`() {
+        val hands = listOf(
+            listOf(c(Rank.TWO, Suit.CLUBS)),
+            listOf(c(Rank.THREE, Suit.CLUBS), c(Rank.ACE, Suit.HEARTS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS)),
+            listOf(c(Rank.SEVEN, Suit.CLUBS)),
+        )
+        val state = playing(
+            hands,
+            turn = 1,
+            trick = listOf(PlayedCard(0, c(Rank.TWO, Suit.CLUBS))),
+        )
+        assertEquals(listOf(c(Rank.THREE, Suit.CLUBS)), HeartsRules.playableCards(state, 1))
+    }
+
+    @Test
+    fun `no point card may be discarded on the first trick`() {
+        val hands = listOf(
+            listOf(c(Rank.TWO, Suit.CLUBS)),
+            // Void in clubs, holding the queen, a heart, and one harmless card.
+            listOf(QUEEN_OF_SPADES, c(Rank.ACE, Suit.HEARTS), c(Rank.NINE, Suit.DIAMONDS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS)),
+            listOf(c(Rank.SEVEN, Suit.CLUBS)),
+        )
+        val state = playing(
+            hands,
+            turn = 1,
+            trickNumber = 0,
+            trick = listOf(PlayedCard(0, c(Rank.TWO, Suit.CLUBS))),
+        )
+        assertEquals(listOf(c(Rank.NINE, Suit.DIAMONDS)), HeartsRules.playableCards(state, 1))
+    }
+
+    @Test
+    fun `the first trick ban lifts when a seat holds nothing but points`() {
+        val hands = listOf(
+            listOf(c(Rank.TWO, Suit.CLUBS)),
+            listOf(QUEEN_OF_SPADES, c(Rank.ACE, Suit.HEARTS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS)),
+            listOf(c(Rank.SEVEN, Suit.CLUBS)),
+        )
+        val state = playing(
+            hands,
+            turn = 1,
+            trickNumber = 0,
+            trick = listOf(PlayedCard(0, c(Rank.TWO, Suit.CLUBS))),
+        )
+        assertEquals(
+            setOf(QUEEN_OF_SPADES, c(Rank.ACE, Suit.HEARTS)),
+            HeartsRules.playableCards(state, 1).toSet(),
+        )
+    }
+
+    @Test
+    fun `hearts cannot be led until one has been played`() {
+        val hands = listOf(
+            listOf(c(Rank.ACE, Suit.HEARTS), c(Rank.THREE, Suit.CLUBS)),
+            listOf(c(Rank.FOUR, Suit.CLUBS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS)),
+            listOf(c(Rank.SIX, Suit.CLUBS)),
+        )
+        val unbroken = playing(hands, heartsBroken = false)
+        assertEquals(listOf(c(Rank.THREE, Suit.CLUBS)), HeartsRules.playableCards(unbroken, 0))
+
+        val broken = playing(hands, heartsBroken = true)
+        assertEquals(2, HeartsRules.playableCards(broken, 0).size)
+    }
+
+    @Test
+    fun `hearts may be led when they are all that is left`() {
+        val hands = listOf(
+            listOf(c(Rank.ACE, Suit.HEARTS), c(Rank.TWO, Suit.HEARTS)),
+            listOf(c(Rank.FOUR, Suit.CLUBS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS)),
+            listOf(c(Rank.SIX, Suit.CLUBS)),
+        )
+        val state = playing(hands, heartsBroken = false)
+        assertEquals(2, HeartsRules.playableCards(state, 0).size)
+    }
+
+    @Test
+    fun `the highest card of the led suit takes the trick`() {
+        val hands = listOf(
+            listOf(c(Rank.TWO, Suit.CLUBS)),
+            listOf(c(Rank.KING, Suit.CLUBS)),
+            // An ace off suit cannot win, however high.
+            listOf(c(Rank.ACE, Suit.DIAMONDS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS)),
+        )
+        var state = playing(hands)
+        state = HeartsRules.applyMove(state, 0, PlayCard(c(Rank.TWO, Suit.CLUBS)))
+        state = HeartsRules.applyMove(state, 1, PlayCard(c(Rank.KING, Suit.CLUBS)))
+        state = HeartsRules.applyMove(state, 2, PlayCard(c(Rank.ACE, Suit.DIAMONDS)))
+        state = HeartsRules.applyMove(state, 3, PlayCard(c(Rank.FIVE, Suit.CLUBS)))
+        assertEquals("king of clubs takes it", 1, state.leader)
+        assertEquals(4, state.taken[1].size)
+    }
+
+    @Test
+    fun `playing a heart breaks them`() {
+        val hands = listOf(
+            listOf(c(Rank.TWO, Suit.CLUBS)),
+            listOf(c(Rank.ACE, Suit.HEARTS)),
+            listOf(c(Rank.FIVE, Suit.CLUBS)),
+            listOf(c(Rank.SEVEN, Suit.CLUBS)),
+        )
+        var state = playing(hands)
+        assertFalse(state.heartsBroken)
+        state = HeartsRules.applyMove(state, 0, PlayCard(c(Rank.TWO, Suit.CLUBS)))
+        state = HeartsRules.applyMove(state, 1, PlayCard(c(Rank.ACE, Suit.HEARTS)))
+        assertTrue(state.heartsBroken)
+    }
+
+    @Test
+    fun `hearts cost a point each and the queen costs thirteen`() {
+        assertEquals(1, pointsOf(c(Rank.TWO, Suit.HEARTS)))
+        assertEquals(1, pointsOf(c(Rank.ACE, Suit.HEARTS)))
+        assertEquals(13, pointsOf(QUEEN_OF_SPADES))
+        assertEquals(0, pointsOf(c(Rank.KING, Suit.SPADES)))
+        assertEquals(0, pointsOf(c(Rank.ACE, Suit.DIAMONDS)))
+        // The whole pack is worth twenty-six and nothing more.
+        assertEquals(ALL_POINTS, org.prolibertate.games.game.cards.Decks.standard52().sumOf { pointsOf(it) })
+    }
+
+    @Test
+    fun `an ordinary round shares out twenty-six points`() {
+        val hearts = Rank.standard.map { Card(it, Suit.HEARTS) }
+        val taken = listOf(
+            hearts.take(6),
+            hearts.drop(6) + QUEEN_OF_SPADES,
+            emptyList(),
+            emptyList(),
+        )
+        val scored = scoreRound(taken, allowShootTheMoon = true)
+        assertEquals(listOf(6, 20, 0, 0), scored)
+        assertEquals(ALL_POINTS, scored.sum())
+    }
+
+    @Test
+    fun `shooting the moon scores everyone else instead`() {
+        val everything = Rank.standard.map { Card(it, Suit.HEARTS) } + QUEEN_OF_SPADES
+        val taken = listOf(emptyList(), everything, emptyList<Card>(), emptyList())
+        assertEquals(listOf(26, 0, 26, 26), scoreRound(taken, allowShootTheMoon = true))
+        // Switched off, a moon is simply the worst hand at the table.
+        assertEquals(listOf(0, 26, 0, 0), scoreRound(taken, allowShootTheMoon = false))
+    }
+
+    @Test
+    fun `a client sees neither other hands nor what they are passing`() {
+        var state = HeartsRules.initialState(config())
+        state = HeartsRules.applyMove(state, 0, PassCards(state.hands[0].take(PASS_SIZE)))
+        val view = HeartsRules.viewFor(state, 0)
+        assertEquals(state.hands[0], view.hands[0])
+        assertTrue("other hands are stripped", view.hands.drop(1).all { it.isEmpty() })
+        assertEquals("counts survive so backs can be drawn", state.handCounts, view.handCounts)
+        assertEquals(state.passSelections[0], view.passSelections[0])
+        assertTrue(view.passSelections.drop(1).all { it.isEmpty() })
+    }
+
+    @Test
+    fun `state and moves survive a round trip`() {
+        val state = HeartsRules.initialState(config())
+        val decoded = HeartsRules.decodeState(HeartsRules.encodeState(state))
+        assertEquals(state, decoded)
+
+        val pass: HeartsMove = PassCards(state.hands[0].take(PASS_SIZE))
+        assertEquals(pass, HeartsRules.decodeMove(HeartsRules.encodeMove(pass)))
+        val play: HeartsMove = PlayCard(TWO_OF_CLUBS)
+        assertEquals(play, HeartsRules.decodeMove(HeartsRules.encodeMove(play)))
+    }
+
+    @Test
+    fun `nobody is on the clock once the game is over`() {
+        val state = HeartsRules.initialState(config()).copy(phase = HeartsPhase.GAME_OVER)
+        assertNull(HeartsRules.currentSeat(state))
+        assertTrue(HeartsRules.legalMoves(state, 0).isEmpty())
+        assertTrue(HeartsRules.isFinished(state))
+    }
+
+    private fun assertThrows(block: () -> Unit) {
+        try {
+            block()
+            throw AssertionError("expected the move to be rejected")
+        } catch (expected: IllegalArgumentException) {
+            // The rules rejected it, which is the point.
+        }
+    }
+}

app/src/test/java/org/prolibertate/games/game/klondike/KlondikeRulesTest.kt

@@ -0,0 +1,334 @@
+package org.prolibertate.games.game.klondike
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.engine.PlayerKind
+import org.prolibertate.games.game.engine.PlayerSlot
+import org.prolibertate.games.game.engine.TableConfig
+import org.prolibertate.games.game.solitaire.acceptsOnFoundation
+import org.prolibertate.games.game.solitaire.patienceOrder
+
+class KlondikeRulesTest {
+
+    private val json = Json { encodeDefaults = true }
+
+    private fun config(options: KlondikeOptions = KlondikeOptions(), seed: Long = 5L) = TableConfig(
+        gameId = "klondike",
+        seats = listOf(PlayerSlot(seat = 0, name = "P0", kind = PlayerKind.HUMAN_LOCAL, team = 0)),
+        optionsJson = json.encodeToString(options),
+        seed = seed,
+    )
+
+    private fun c(rank: Rank, suit: Suit) = Card(rank, suit)
+
+    // ---- the deal ----------------------------------------------------------
+
+    @Test
+    fun `the deal is seven columns and a stock of twenty-four`() {
+        val state = KlondikeRules.initialState(config())
+        assertEquals(TABLEAU_PILES, state.tableau.size)
+        state.tableau.forEachIndexed { index, pile ->
+            assertEquals("column ${index + 1}", index + 1, pile.faceDown.size + pile.faceUp.size)
+            assertEquals("only the last card is turned up", 1, pile.faceUp.size)
+        }
+        assertEquals(24, state.stock.size)
+        assertTrue(state.waste.isEmpty())
+        assertTrue(state.foundations.all { it.isEmpty() })
+    }
+
+    @Test
+    fun `the pack is dealt whole and no card is dealt twice`() {
+        val state = KlondikeRules.initialState(config())
+        val all = state.stock + state.waste + state.tableau.flatMap { it.faceDown + it.faceUp }
+        assertEquals(52, all.size)
+        assertEquals(52, all.distinct().size)
+    }
+
+    @Test
+    fun `a table for more than one seat is refused`() {
+        val two = TableConfig(
+            gameId = "klondike",
+            seats = (0..1).map { PlayerSlot(seat = it, name = "P$it", kind = PlayerKind.AI, team = it) },
+            optionsJson = json.encodeToString(KlondikeOptions()),
+            seed = 1L,
+        )
+        assertThrows { KlondikeRules.initialState(two) }
+    }
+
+    // ---- stock and waste ---------------------------------------------------
+
+    @Test
+    fun `drawing one turns one card up`() {
+        val state = KlondikeRules.initialState(config(KlondikeOptions(drawCount = 1)))
+        val drawn = KlondikeRules.applyMove(state, 0, Draw)
+        assertEquals(23, drawn.stock.size)
+        assertEquals(1, drawn.waste.size)
+        assertEquals("the top of the stock is the card turned", state.stock.last(), drawn.waste.last())
+    }
+
+    @Test
+    fun `drawing three leaves the deepest of them showing`() {
+        // Turning a packet of three over reverses it, so the card that was
+        // furthest down is the one in play.
+        val state = KlondikeRules.initialState(config(KlondikeOptions(drawCount = 3)))
+        val drawn = KlondikeRules.applyMove(state, 0, Draw)
+        assertEquals(21, drawn.stock.size)
+        assertEquals(3, drawn.waste.size)
+        val top3 = state.stock.takeLast(3)
+        assertEquals(top3.reversed(), drawn.waste)
+        assertEquals(top3.first(), drawn.waste.last())
+    }
+
+    @Test
+    fun `the waste turns back over into a new stock`() {
+        var state = KlondikeRules.initialState(config(KlondikeOptions(drawCount = 3)))
+        repeat(8) { state = KlondikeRules.applyMove(state, 0, Draw) }
+        assertTrue(state.stock.isEmpty())
+        assertEquals(24, state.waste.size)
+        assertTrue(state.canRedeal)
+
+        val wasteBefore = state.waste
+        val turned = KlondikeRules.applyMove(state, 0, Redeal)
+        assertEquals(wasteBefore.reversed(), turned.stock)
+        assertTrue(turned.waste.isEmpty())
+        assertEquals(1, turned.redealsUsed)
+    }
+
+    @Test
+    fun `a redeal limit of zero means one pass through the pack`() {
+        var state = KlondikeRules.initialState(
+            config(KlondikeOptions(drawCount = 3, redealLimit = 0)),
+        )
+        repeat(8) { state = KlondikeRules.applyMove(state, 0, Draw) }
+        assertFalse("no turning it back over", state.canRedeal)
+        assertFalse(KlondikeRules.legalMoves(state, 0).contains(Redeal))
+    }
+
+    // ---- building ----------------------------------------------------------
+
+    @Test
+    fun `the ace is low here, whatever the rest of the deck thinks`() {
+        // Rank.order puts the ace at fourteen, because the deck was built for
+        // trick-taking games where it is the highest card there is. Patience
+        // counts it as one, and every comparison in this game has to agree —
+        // otherwise a two can never follow an ace home and the game is
+        // unwinnable in a way nothing else would show.
+        assertEquals(14, Rank.ACE.order)
+        assertEquals(1, Rank.ACE.patienceOrder)
+        assertEquals(13, Rank.KING.patienceOrder)
+
+        var foundation = emptyList<Card>()
+        Rank.standard.sortedBy { it.patienceOrder }.forEach { rank ->
+            val card = c(rank, Suit.SPADES)
+            assertTrue("$rank should go home next", acceptsOnFoundation(foundation, card))
+            foundation = foundation + card
+        }
+        assertEquals(13, foundation.size)
+
+        // And a column runs the other way, down to an ace.
+        assertTrue(buildsDown(c(Rank.ACE, Suit.HEARTS), c(Rank.TWO, Suit.SPADES)))
+        assertTrue(buildsDown(c(Rank.QUEEN, Suit.HEARTS), c(Rank.KING, Suit.SPADES)))
+    }
+
+    @Test
+    fun `foundations run ace upwards in one suit`() {
+        assertTrue(acceptsOnFoundation(emptyList(), c(Rank.ACE, Suit.SPADES)))
+        assertFalse(acceptsOnFoundation(emptyList(), c(Rank.TWO, Suit.SPADES)))
+
+        val ace = listOf(c(Rank.ACE, Suit.SPADES))
+        assertTrue(acceptsOnFoundation(ace, c(Rank.TWO, Suit.SPADES)))
+        assertFalse("wrong suit", acceptsOnFoundation(ace, c(Rank.TWO, Suit.HEARTS)))
+        assertFalse("skips a rank", acceptsOnFoundation(ace, c(Rank.THREE, Suit.SPADES)))
+    }
+
+    @Test
+    fun `columns build down and in the other colour`() {
+        assertTrue(buildsDown(c(Rank.NINE, Suit.HEARTS), c(Rank.TEN, Suit.SPADES)))
+        assertTrue(buildsDown(c(Rank.NINE, Suit.DIAMONDS), c(Rank.TEN, Suit.CLUBS)))
+        assertFalse("same colour", buildsDown(c(Rank.NINE, Suit.HEARTS), c(Rank.TEN, Suit.DIAMONDS)))
+        assertFalse("wrong way up", buildsDown(c(Rank.JACK, Suit.HEARTS), c(Rank.TEN, Suit.SPADES)))
+    }
+
+    @Test
+    fun `a run travels only if it is a run`() {
+        assertTrue(
+            isMovableRun(
+                listOf(
+                    c(Rank.TEN, Suit.SPADES),
+                    c(Rank.NINE, Suit.HEARTS),
+                    c(Rank.EIGHT, Suit.CLUBS),
+                ),
+            ),
+        )
+        assertFalse(
+            "colours repeat",
+            isMovableRun(listOf(c(Rank.TEN, Suit.SPADES), c(Rank.NINE, Suit.CLUBS))),
+        )
+        assertTrue("one card is a run", isMovableRun(listOf(c(Rank.TEN, Suit.SPADES))))
+        assertFalse(isMovableRun(emptyList()))
+    }
+
+    @Test
+    fun `an empty column takes a king, or anything if the table says so`() {
+        assertTrue(acceptsInSpace(c(Rank.KING, Suit.SPADES), kingsOnly = true))
+        assertFalse(acceptsInSpace(c(Rank.QUEEN, Suit.SPADES), kingsOnly = true))
+        assertTrue(acceptsInSpace(c(Rank.TWO, Suit.SPADES), kingsOnly = false))
+    }
+
+    // ---- moving ------------------------------------------------------------
+
+    @Test
+    fun `emptying a column turns the card underneath up`() {
+        val base = KlondikeRules.initialState(config())
+        // Column 1 holds one buried card and a red six; column 2's black seven
+        // will take it.
+        val state = base.copy(
+            tableau = base.tableau.toMutableList().also {
+                it[1] = TableauPile(
+                    faceDown = listOf(c(Rank.ACE, Suit.CLUBS)),
+                    faceUp = listOf(c(Rank.SIX, Suit.HEARTS)),
+                )
+                it[2] = TableauPile(faceUp = listOf(c(Rank.SEVEN, Suit.SPADES)))
+            },
+        )
+        val move = MoveCards(Spot.tableau(1), Spot.tableau(2))
+        assertTrue(KlondikeRules.legalMoves(state, 0).contains(move))
+
+        val after = KlondikeRules.applyMove(state, 0, move)
+        assertEquals(listOf(c(Rank.ACE, Suit.CLUBS)), after.tableau[1].faceUp)
+        assertTrue("nothing left buried", after.tableau[1].faceDown.isEmpty())
+        assertEquals(
+            listOf(c(Rank.SEVEN, Suit.SPADES), c(Rank.SIX, Suit.HEARTS)),
+            after.tableau[2].faceUp,
+        )
+    }
+
+    @Test
+    fun `shifting a whole column into an empty one is not offered`() {
+        // It gains nothing and would let a game shuffle back and forth for ever.
+        val base = KlondikeRules.initialState(config())
+        val state = base.copy(
+            tableau = base.tableau.toMutableList().also {
+                it[0] = TableauPile(faceUp = listOf(c(Rank.KING, Suit.SPADES)))
+                it[1] = TableauPile()
+            },
+        )
+        assertFalse(
+            KlondikeRules.legalMoves(state, 0)
+                .contains(MoveCards(Spot.tableau(0), Spot.tableau(1))),
+        )
+    }
+
+    @Test
+    fun `an illegal move is refused rather than applied`() {
+        val state = KlondikeRules.initialState(config())
+        // Nothing is ever taken off the stock by hand, and an empty foundation
+        // has nothing to give — neither depends on how the pack fell.
+        assertThrows {
+            KlondikeRules.applyMove(state, 0, MoveCards(Spot.stock, Spot.tableau(0)))
+        }
+        assertThrows {
+            KlondikeRules.applyMove(state, 0, MoveCards(Spot.foundation(0), Spot.tableau(0)))
+        }
+        assertThrows { KlondikeRules.applyMove(state, 1, Draw) }
+    }
+
+    // ---- finishing ---------------------------------------------------------
+
+    @Test
+    fun `four full foundations is out`() {
+        val base = KlondikeRules.initialState(config())
+        val full = Suit.entries.map { suit -> Rank.standard.map { Card(it, suit) } }
+        val won = base.copy(foundations = full, tableau = List(TABLEAU_PILES) { TableauPile() })
+        assertTrue(won.isWon)
+        assertTrue(KlondikeRules.isFinished(won))
+        assertNull(KlondikeRules.currentSeat(won))
+        assertTrue(KlondikeRules.summary(won).contains("Out"))
+    }
+
+    @Test
+    fun `a deal with nothing left to try is finished too`() {
+        val base = KlondikeRules.initialState(config())
+        // No stock, no waste, and two columns that cannot touch each other.
+        val stuck = base.copy(
+            stock = emptyList(),
+            waste = emptyList(),
+            tableau = List(TABLEAU_PILES) { index ->
+                when (index) {
+                    0 -> TableauPile(faceUp = listOf(c(Rank.FIVE, Suit.SPADES)))
+                    1 -> TableauPile(faceUp = listOf(c(Rank.NINE, Suit.CLUBS)))
+                    else -> TableauPile(faceUp = listOf(c(Rank.SEVEN, Suit.CLUBS)))
+                }
+            },
+        )
+        assertFalse(stuck.isWon)
+        assertTrue(KlondikeRules.isFinished(stuck))
+        assertTrue(KlondikeRules.summary(stuck).contains("Blocked"))
+    }
+
+    @Test
+    fun `a deal can be played a long way without the rules complaining`() {
+        var state = KlondikeRules.initialState(config())
+        var guard = 400
+        while (!KlondikeRules.isFinished(state) && guard-- > 0) {
+            val legal = KlondikeRules.legalMoves(state, 0)
+            state = KlondikeRules.applyMove(state, 0, KlondikeAi.chooseMove(state, 0, legal))
+            // Every card is somewhere, and only ever in one place.
+            val all = state.stock + state.waste +
+                state.foundations.flatten() +
+                state.tableau.flatMap { it.faceDown + it.faceUp }
+            assertEquals("no card lost or copied", 52, all.size)
+            assertEquals(52, all.distinct().size)
+        }
+    }
+
+    // ---- housekeeping ------------------------------------------------------
+
+    @Test
+    fun `restart deals the same pack again`() {
+        val state = KlondikeRules.initialState(config())
+        val played = KlondikeRules.applyMove(state, 0, Draw)
+        val again = KlondikeRules.restart(played)
+        assertEquals(state.tableau, again.tableau)
+        assertEquals(state.stock, again.stock)
+        assertEquals(0, again.moves)
+    }
+
+    @Test
+    fun `nothing is hidden from the only player`() {
+        val state = KlondikeRules.initialState(config())
+        assertEquals(state, KlondikeRules.viewFor(state, 0))
+    }
+
+    @Test
+    fun `state and moves survive a round trip`() {
+        val state = KlondikeRules.applyMove(KlondikeRules.initialState(config()), 0, Draw)
+        assertEquals(state, KlondikeRules.decodeState(KlondikeRules.encodeState(state)))
+
+        listOf<KlondikeMove>(
+            Draw,
+            Redeal,
+            MoveCards(Spot.waste, Spot.foundation(2)),
+            MoveCards(Spot.tableau(3), Spot.tableau(5), 4),
+        ).forEach {
+            assertEquals(it, KlondikeRules.decodeMove(KlondikeRules.encodeMove(it)))
+        }
+    }
+
+    private fun assertThrows(block: () -> Unit) {
+        try {
+            block()
+            throw AssertionError("expected the rules to reject that")
+        } catch (expected: IllegalArgumentException) {
+            // Rejected, which is the point.
+        }
+    }
+}

app/src/test/java/org/prolibertate/games/game/pegsolitaire/PegSolitaireRulesTest.kt

@@ -0,0 +1,240 @@
+package org.prolibertate.games.game.pegsolitaire
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.prolibertate.games.game.engine.PlayerKind
+import org.prolibertate.games.game.engine.PlayerSlot
+import org.prolibertate.games.game.engine.TableConfig
+
+class PegSolitaireRulesTest {
+
+    private val json = Json { encodeDefaults = true }
+
+    private fun config(options: PegSolitaireOptions = PegSolitaireOptions()) = TableConfig(
+        gameId = "pegsolitaire",
+        seats = listOf(PlayerSlot(seat = 0, name = "P0", kind = PlayerKind.HUMAN_LOCAL, team = 0)),
+        optionsJson = json.encodeToString(options),
+        seed = 1L,
+    )
+
+    @Test
+    fun `every board has the number of holes it claims`() {
+        PegBoard.entries.forEach { board ->
+            assertEquals(board.label, board.holeCount, board.holes().size)
+        }
+    }
+
+    @Test
+    fun `a triangle staggers its rows and a cross does not`() {
+        // Six ways off a hole on a triangle, four on a square grid. This is the
+        // only thing that differs between the two shapes.
+        assertEquals(6, PegBoard.TRIANGLE_15.directions().size)
+        assertEquals(6, PegBoard.TRIANGLE_21.directions().size)
+        assertEquals(4, PegBoard.ENGLISH_CROSS.directions().size)
+        assertEquals(4, PegBoard.FRENCH_CROSS.directions().size)
+        assertEquals(4, PegBoard.DIAMOND_41.directions().size)
+        // No diagonal creeps into a cross board.
+        assertTrue(PegBoard.ENGLISH_CROSS.directions().all { (dr, dc) -> dr == 0 || dc == 0 })
+    }
+
+    @Test
+    fun `the cross boards are cut where they should be`() {
+        val english = PegBoard.ENGLISH_CROSS.holes()
+        assertFalse("a two-by-two corner is gone", english.contains(Hole(0, 0)))
+        assertFalse(english.contains(Hole(1, 1)))
+        assertTrue(english.contains(Hole(0, 3)))
+        assertTrue(english.contains(Hole(3, 3)))
+
+        val french = PegBoard.FRENCH_CROSS.holes()
+        // The shallower cut takes three holes per corner, so (1,1) survives.
+        assertFalse(french.contains(Hole(0, 0)))
+        assertFalse(french.contains(Hole(0, 1)))
+        assertFalse(french.contains(Hole(1, 0)))
+        assertTrue("the French cut leaves this one", french.contains(Hole(1, 1)))
+
+        val diamond = PegBoard.DIAMOND_41.holes()
+        assertTrue(diamond.contains(Hole(0, 4)))
+        assertTrue(diamond.contains(Hole(4, 0)))
+        assertFalse(diamond.contains(Hole(0, 0)))
+    }
+
+    @Test
+    fun `a board opens one peg short`() {
+        val state = PegSolitaireRules.initialState(config())
+        assertEquals(PegBoard.TRIANGLE_15.holeCount - 1, state.remaining)
+        assertFalse("the opening hole is empty", state.pegs.contains(Hole(0, 0)))
+        assertEquals(0, state.jumps)
+        assertNull(state.lastJump)
+    }
+
+    @Test
+    fun `the tee game opens with exactly two jumps, both into the apex`() {
+        val state = PegSolitaireRules.initialState(config())
+        val jumps = PegSolitaireRules.legalJumps(state)
+        assertEquals(2, jumps.size)
+        assertTrue("every opening jump lands in the empty hole", jumps.all { it.to == Hole(0, 0) })
+        assertEquals(setOf(Hole(2, 0), Hole(2, 2)), jumps.map { it.from }.toSet())
+        // The jumped peg is the one between the ends, and it is never invented.
+        assertEquals(setOf(Hole(1, 0), Hole(1, 1)), jumps.map { it.over }.toSet())
+    }
+
+    @Test
+    fun `a jump moves one peg and takes the one it passed`() {
+        val state = PegSolitaireRules.initialState(config())
+        val jump = PegJump(from = Hole(2, 0), to = Hole(0, 0))
+        val after = PegSolitaireRules.applyMove(state, 0, jump)
+        assertFalse("the mover left", after.pegs.contains(Hole(2, 0)))
+        assertFalse("the jumped peg was taken", after.pegs.contains(Hole(1, 0)))
+        assertTrue("the mover arrived", after.pegs.contains(Hole(0, 0)))
+        assertEquals(state.remaining - 1, after.remaining)
+        assertEquals(1, after.jumps)
+        assertEquals(jump, after.lastJump)
+    }
+
+    @Test
+    fun `a jump needs a peg to pass and an empty hole to land in`() {
+        val state = PegSolitaireRules.initialState(config())
+        // Landing hole already occupied.
+        assertThrows { PegSolitaireRules.applyMove(state, 0, PegJump(Hole(4, 0), Hole(2, 0))) }
+        // Nothing to jump over: (1,0) is empty only after a jump, but (0,0) is
+        // the opening hole, so a jump starting there has no peg to move.
+        assertThrows { PegSolitaireRules.applyMove(state, 0, PegJump(Hole(0, 0), Hole(2, 0))) }
+        // Straight off the board.
+        assertThrows { PegSolitaireRules.applyMove(state, 0, PegJump(Hole(2, 0), Hole(2, 4))) }
+    }
+
+    @Test
+    fun `only the one seat may play`() {
+        val state = PegSolitaireRules.initialState(config())
+        assertEquals(0, PegSolitaireRules.currentSeat(state))
+        assertTrue(PegSolitaireRules.legalMoves(state, 1).isEmpty())
+        assertThrows { PegSolitaireRules.applyMove(state, 1, PegJump(Hole(2, 0), Hole(0, 0))) }
+    }
+
+    @Test
+    fun `a table for anything but one seat is refused`() {
+        val twoSeats = TableConfig(
+            gameId = "pegsolitaire",
+            seats = (0..1).map {
+                PlayerSlot(seat = it, name = "P$it", kind = PlayerKind.AI, team = it)
+            },
+            optionsJson = json.encodeToString(PegSolitaireOptions()),
+            seed = 1L,
+        )
+        assertThrows { PegSolitaireRules.initialState(twoSeats) }
+    }
+
+    @Test
+    fun `one peg left is solved, and the stricter goal asks where`() {
+        val loose = PegSolitaireOptions(goal = PegGoal.ONE_PEG)
+        val strict = PegSolitaireOptions(goal = PegGoal.ONE_PEG_IN_START)
+        val base = PegSolitaireRules.initialState(config(loose))
+
+        val elsewhere = base.copy(pegs = setOf(Hole(4, 2)))
+        assertTrue(elsewhere.solved)
+        assertFalse("the strict goal wants it back where it started", elsewhere.copy(options = strict).solved)
+
+        val atStart = base.copy(options = strict, pegs = setOf(Hole(0, 0)))
+        assertTrue("the survivor is in the opening hole", atStart.solved)
+    }
+
+    @Test
+    fun `a board with no jumps left is finished, solved or not`() {
+        val state = PegSolitaireRules.initialState(config())
+        // Two pegs far apart cannot reach each other, so nothing can jump.
+        val stuck = state.copy(pegs = setOf(Hole(4, 0), Hole(4, 4)))
+        assertTrue(PegSolitaireRules.isFinished(stuck))
+        assertFalse(stuck.solved)
+        assertNull(PegSolitaireRules.currentSeat(stuck))
+        assertTrue(PegSolitaireRules.summary(stuck).contains("Stuck"))
+
+        val won = state.copy(pegs = setOf(Hole(2, 1)))
+        assertTrue(PegSolitaireRules.isFinished(won))
+        assertTrue(won.solved)
+    }
+
+    @Test
+    fun `an opening hole has to be a hole on the board`() {
+        assertThrows { PegSolitaireOptions(board = PegBoard.TRIANGLE_15, startEmpty = Hole(9, 9)) }
+        // A hole that exists on one board but not another.
+        assertThrows {
+            PegSolitaireOptions(board = PegBoard.ENGLISH_CROSS, startEmpty = Hole(0, 0))
+        }
+    }
+
+    @Test
+    fun `every board can be opened and played`() {
+        // Each board deals, offers jumps, and runs to a finish without the rules
+        // throwing. This is what catches a geometry that produces a board nobody
+        // can move on at all.
+        PegBoard.entries.forEach { board ->
+            val state = PegSolitaireRules.initialState(
+                config(PegSolitaireOptions(board = board)),
+            )
+            assertEquals(board.holeCount - 1, state.remaining)
+            assertTrue(
+                "${board.label} should have an opening jump",
+                PegSolitaireRules.legalJumps(state).isNotEmpty(),
+            )
+            val end = playOut(state)
+            assertTrue(PegSolitaireRules.isFinished(end))
+            assertTrue("${board.label} should take pegs off", end.remaining < state.remaining)
+        }
+    }
+
+    @Test
+    fun `restart puts every peg back`() {
+        val state = PegSolitaireRules.initialState(config())
+        val played = PegSolitaireRules.applyMove(state, 0, PegJump(Hole(2, 0), Hole(0, 0)))
+        val again = PegSolitaireRules.restart(played)
+        assertEquals(state.pegs, again.pegs)
+        assertEquals(0, again.jumps)
+        assertNull(again.lastJump)
+    }
+
+    @Test
+    fun `nothing is hidden from the only player`() {
+        val state = PegSolitaireRules.initialState(config())
+        assertEquals(state, PegSolitaireRules.viewFor(state, 0))
+    }
+
+    @Test
+    fun `state and moves survive a round trip`() {
+        PegBoard.entries.forEach { board ->
+            val state = PegSolitaireRules.initialState(
+                config(PegSolitaireOptions(board = board, goal = PegGoal.ONE_PEG_IN_START)),
+            )
+            assertEquals(state, PegSolitaireRules.decodeState(PegSolitaireRules.encodeState(state)))
+        }
+        val move: PegSolitaireMove = PegJump(Hole(2, 0), Hole(0, 0))
+        assertEquals(move, PegSolitaireRules.decodeMove(PegSolitaireRules.encodeMove(move)))
+    }
+
+    /** Runs a board to a standstill using the AI, which never has to guess. */
+    private fun playOut(from: PegSolitaireState): PegSolitaireState {
+        var state = from
+        // Each jump takes exactly one peg off, so the board cannot outlast its
+        // own peg count — a loop that did would be a bug in applyMove.
+        var guard = from.remaining + 1
+        while (!PegSolitaireRules.isFinished(state) && guard-- > 0) {
+            val legal = PegSolitaireRules.legalMoves(state, 0)
+            state = PegSolitaireRules.applyMove(state, 0, PegSolitaireAi.chooseMove(state, 0, legal))
+        }
+        assertTrue("playing out should terminate", guard > 0)
+        return state
+    }
+
+    private fun assertThrows(block: () -> Unit) {
+        try {
+            block()
+            throw AssertionError("expected the rules to reject that")
+        } catch (expected: IllegalArgumentException) {
+            // Rejected, which is the point.
+        }
+    }
+}

app/src/test/java/org/prolibertate/games/game/solitaire/SolitaireRulesTest.kt

@@ -0,0 +1,368 @@
+package org.prolibertate.games.game.solitaire
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.prolibertate.games.game.cards.Card
+import org.prolibertate.games.game.cards.Rank
+import org.prolibertate.games.game.cards.Suit
+import org.prolibertate.games.game.engine.PlayerKind
+import org.prolibertate.games.game.engine.PlayerSlot
+import org.prolibertate.games.game.engine.TableConfig
+import org.prolibertate.games.game.freecell.COLUMNS as FREECELL_COLUMNS
+import org.prolibertate.games.game.freecell.FreeCellOptions
+import org.prolibertate.games.game.freecell.FreeCellRules
+import org.prolibertate.games.game.freecell.MoveTo
+import org.prolibertate.games.game.freecell.Place
+import org.prolibertate.games.game.freecell.maxRunLength
+import org.prolibertate.games.game.pyramid.PYRAMID_SIZE
+import org.prolibertate.games.game.pyramid.PyramidOptions
+import org.prolibertate.games.game.pyramid.PyramidRules
+import org.prolibertate.games.game.pyramid.PyramidZone
+import org.prolibertate.games.game.pyramid.TARGET
+import org.prolibertate.games.game.pyramid.coveredBy
+import org.prolibertate.games.game.pyramid.isExposed
+import org.prolibertate.games.game.pyramid.rowOf
+import org.prolibertate.games.game.pyramid.valueOf
+import org.prolibertate.games.game.spider.COLUMNS as SPIDER_COLUMNS
+import org.prolibertate.games.game.spider.DealRow
+import org.prolibertate.games.game.spider.RUN_LENGTH
+import org.prolibertate.games.game.spider.SpiderOptions
+import org.prolibertate.games.game.spider.SpiderRules
+import org.prolibertate.games.game.spider.isCompleteRun
+import org.prolibertate.games.game.spider.landsOn
+import org.prolibertate.games.game.spider.sameSuitRun
+import org.prolibertate.games.game.spider.spiderDeck
+
+/**
+ * The three patiences added together, tested together.
+ *
+ * They share the ace-low ordering and little else, so what is worth pinning is
+ * each one's own shape: how many cards it deals, what it will let travel, and
+ * what counts as finished.
+ */
+class SolitaireRulesTest {
+
+    private val json = Json { encodeDefaults = true }
+
+    private fun soloConfig(gameId: String, optionsJson: String, seed: Long = 3L) = TableConfig(
+        gameId = gameId,
+        seats = listOf(PlayerSlot(seat = 0, name = "P0", kind = PlayerKind.HUMAN_LOCAL, team = 0)),
+        optionsJson = optionsJson,
+        seed = seed,
+    )
+
+    private fun c(rank: Rank, suit: Suit) = Card(rank, suit)
+
+    // ---- shared ------------------------------------------------------------
+
+    @Test
+    fun `the ace is low for all of them`() {
+        assertEquals(1, Rank.ACE.patienceOrder)
+        assertEquals(11, Rank.JACK.patienceOrder)
+        assertEquals(13, Rank.KING.patienceOrder)
+        assertTrue(buildsDownAlternating(c(Rank.ACE, Suit.HEARTS), c(Rank.TWO, Suit.SPADES)))
+        assertTrue(buildsDownInSuit(c(Rank.ACE, Suit.SPADES), c(Rank.TWO, Suit.SPADES)))
+        assertFalse(buildsDownInSuit(c(Rank.ACE, Suit.HEARTS), c(Rank.TWO, Suit.SPADES)))
+    }
+
+    // ---- FreeCell ----------------------------------------------------------
+
+    @Test
+    fun `FreeCell deals the whole pack face up`() {
+        val state = FreeCellRules.initialState(
+            soloConfig("freecell", json.encodeToString(FreeCellOptions())),
+        )
+        assertEquals(FREECELL_COLUMNS, state.tableau.size)
+        // Fifty-two into eight is four sevens and four sixes.
+        assertEquals(listOf(7, 7, 7, 7, 6, 6, 6, 6), state.tableau.map { it.size })
+        val all = state.tableau.flatten()
+        assertEquals(52, all.size)
+        assertEquals(52, all.distinct().size)
+        assertEquals(4, state.freeCellCount)
+        assertEquals(0, state.emptyColumns)
+    }
+
+    @Test
+    fun `how much can travel depends on cells and spaces`() {
+        // One card per free cell plus the card itself, doubled for each empty
+        // column that can be borrowed as a staging pile.
+        assertEquals(5, maxRunLength(freeCells = 4, emptyColumns = 0, intoEmptyColumn = false))
+        assertEquals(10, maxRunLength(freeCells = 4, emptyColumns = 1, intoEmptyColumn = false))
+        assertEquals(20, maxRunLength(freeCells = 4, emptyColumns = 2, intoEmptyColumn = false))
+        // Moving into an empty column cannot also stage through it.
+        assertEquals(5, maxRunLength(freeCells = 4, emptyColumns = 1, intoEmptyColumn = true))
+        assertEquals(1, maxRunLength(freeCells = 0, emptyColumns = 0, intoEmptyColumn = false))
+    }
+
+    @Test
+    fun `FreeCell refuses a table for more than one`() {
+        val two = TableConfig(
+            gameId = "freecell",
+            seats = (0..1).map { PlayerSlot(seat = it, name = "P$it", kind = PlayerKind.AI, team = it) },
+            optionsJson = json.encodeToString(FreeCellOptions()),
+            seed = 1L,
+        )
+        assertThrows { FreeCellRules.initialState(two) }
+    }
+
+    @Test
+    fun `FreeCell keeps every card wherever it is put`() {
+        var state = FreeCellRules.initialState(
+            soloConfig("freecell", json.encodeToString(FreeCellOptions())),
+        )
+        val ai = FirstLegalAi<org.prolibertate.games.game.freecell.FreeCellState,
+            org.prolibertate.games.game.freecell.FreeCellMove>()
+        var guard = 120
+        while (!FreeCellRules.isFinished(state) && guard-- > 0) {
+            val legal = FreeCellRules.legalMoves(state, 0)
+            state = FreeCellRules.applyMove(state, 0, ai.chooseMove(state, 0, legal))
+            val all = state.tableau.flatten() + state.foundations.flatten() + state.cells.filterNotNull()
+            assertEquals("no card lost or copied", 52, all.size)
+            assertEquals(52, all.distinct().size)
+        }
+    }
+
+    @Test
+    fun `parking a card takes it off the column and into the cell`() {
+        val state = FreeCellRules.initialState(
+            soloConfig("freecell", json.encodeToString(FreeCellOptions())),
+        )
+        val toCell = FreeCellRules.legalMoves(state, 0)
+            .filterIsInstance<MoveTo>()
+            .first { it.to.kind == org.prolibertate.games.game.freecell.CellKind.CELL }
+        val column = toCell.from.index
+        val card = state.tableau[column].last()
+
+        val parked = FreeCellRules.applyMove(state, 0, toCell)
+        assertEquals(3, parked.freeCellCount)
+        assertEquals(card, parked.cells[toCell.to.index])
+        assertEquals(state.tableau[column].size - 1, parked.tableau[column].size)
+        // Deliberately not asserting the card can come back out. A parked card
+        // that nothing will take is stranded, and that is the whole danger of
+        // the cells rather than a fault in them.
+    }
+
+    @Test
+    fun `an ace in a cell can always go home`() {
+        val base = FreeCellRules.initialState(
+            soloConfig("freecell", json.encodeToString(FreeCellOptions())),
+        )
+        val ace = c(Rank.ACE, Suit.SPADES)
+        val held = base.copy(cells = listOf(ace, null, null, null))
+        assertTrue(
+            "an ace is the one card a cell can never strand",
+            FreeCellRules.legalMoves(held, 0).contains(
+                MoveTo(Place.cell(0), Place.foundation(Suit.SPADES.ordinal), 1),
+            ),
+        )
+    }
+
+    // ---- Spider ------------------------------------------------------------
+
+    @Test
+    fun `a Spider pack is always a hundred and four`() {
+        listOf(1, 2, 4).forEach { suits ->
+            val deck = spiderDeck(suits)
+            assertEquals("$suits suits", 104, deck.size)
+            assertEquals("$suits suits", suits, deck.map { it.suit }.distinct().size)
+            // Eight of every rank, however the suits were shared out.
+            assertEquals(8, deck.count { it.rank == Rank.KING })
+        }
+    }
+
+    @Test
+    fun `Spider deals fifty-four and keeps fifty back`() {
+        val state = SpiderRules.initialState(
+            soloConfig("spider", json.encodeToString(SpiderOptions())),
+        )
+        assertEquals(SPIDER_COLUMNS, state.tableau.size)
+        assertEquals(listOf(6, 6, 6, 6, 5, 5, 5, 5, 5, 5), state.tableau.map { it.faceDown.size + it.faceUp.size })
+        state.tableau.forEach { assertEquals("only the last is turned up", 1, it.faceUp.size) }
+        assertEquals(50, state.stock.size)
+        assertEquals(0, state.completed)
+    }
+
+    @Test
+    fun `only a same-suit run travels, but anything lands on a higher card`() {
+        val spades = listOf(c(Rank.TEN, Suit.SPADES), c(Rank.NINE, Suit.SPADES))
+        val mixed = listOf(c(Rank.TEN, Suit.SPADES), c(Rank.NINE, Suit.HEARTS))
+        assertTrue(sameSuitRun(spades))
+        assertFalse("a mixed run cannot travel as one", sameSuitRun(mixed))
+        // Landing is looser than travelling: any suit, one rank higher.
+        assertTrue(landsOn(c(Rank.NINE, Suit.HEARTS), c(Rank.TEN, Suit.SPADES)))
+        assertFalse(landsOn(c(Rank.NINE, Suit.HEARTS), c(Rank.JACK, Suit.SPADES)))
+    }
+
+    @Test
+    fun `a complete run is king down to ace in one suit`() {
+        val run = Rank.standard.sortedByDescending { it.patienceOrder }.map { c(it, Suit.SPADES) }
+        assertEquals(RUN_LENGTH, run.size)
+        assertTrue(isCompleteRun(run))
+        // One card of the wrong suit and it is not going anywhere.
+        val broken = run.toMutableList().also { it[5] = c(it[5].rank, Suit.HEARTS) }
+        assertFalse(isCompleteRun(broken))
+        assertFalse(isCompleteRun(run.drop(1)))
+    }
+
+    @Test
+    fun `a row cannot be dealt onto an empty column`() {
+        val state = SpiderRules.initialState(
+            soloConfig("spider", json.encodeToString(SpiderOptions())),
+        )
+        assertTrue(state.canDeal)
+        assertTrue(SpiderRules.legalMoves(state, 0).contains(DealRow))
+
+        val emptied = state.copy(
+            tableau = state.tableau.toMutableList().also {
+                it[0] = org.prolibertate.games.game.spider.SpiderPile()
+            },
+        )
+        assertFalse(emptied.canDeal)
+        assertFalse(SpiderRules.legalMoves(emptied, 0).contains(DealRow))
+    }
+
+    @Test
+    fun `dealing a row puts one card on every column`() {
+        val state = SpiderRules.initialState(
+            soloConfig("spider", json.encodeToString(SpiderOptions())),
+        )
+        val before = state.tableau.map { it.faceUp.size }
+        val dealt = SpiderRules.applyMove(state, 0, DealRow)
+        assertEquals(40, dealt.stock.size)
+        dealt.tableau.forEachIndexed { index, pile ->
+            assertEquals(before[index] + 1, pile.faceUp.size)
+        }
+    }
+
+    // ---- Pyramid -----------------------------------------------------------
+
+    @Test
+    fun `the pyramid is twenty-eight cards in seven rows`() {
+        assertEquals(28, PYRAMID_SIZE)
+        assertEquals(0, rowOf(0))
+        assertEquals(1, rowOf(1))
+        assertEquals(1, rowOf(2))
+        assertEquals(2, rowOf(3))
+        assertEquals(6, rowOf(27))
+
+        val state = PyramidRules.initialState(
+            soloConfig("pyramid", json.encodeToString(PyramidOptions())),
+        )
+        assertEquals(PYRAMID_SIZE, state.pyramid.size)
+        assertEquals(24, state.stock.size)
+        assertTrue(state.pyramid.all { it != null })
+    }
+
+    @Test
+    fun `a card is buried until both the cards on it are gone`() {
+        // The apex rests on the two cards of the second row.
+        assertEquals(listOf(1, 2), coveredBy(0))
+        // The bottom row rests on nothing.
+        assertTrue(coveredBy(27).isEmpty())
+
+        val pyramid = MutableList<Card?>(PYRAMID_SIZE) { c(Rank.FIVE, Suit.SPADES) }
+        assertFalse("the apex starts buried", isExposed(pyramid, 0))
+        pyramid[1] = null
+        assertFalse("one of the two is not enough", isExposed(pyramid, 0))
+        pyramid[2] = null
+        assertTrue(isExposed(pyramid, 0))
+        // A place already taken is not exposed, it is gone.
+        pyramid[0] = null
+        assertFalse(isExposed(pyramid, 0))
+    }
+
+    @Test
+    fun `pairs make thirteen, and a king makes it alone`() {
+        assertEquals(1, valueOf(c(Rank.ACE, Suit.SPADES)))
+        assertEquals(11, valueOf(c(Rank.JACK, Suit.SPADES)))
+        assertEquals(TARGET, valueOf(c(Rank.KING, Suit.SPADES)))
+        assertEquals(TARGET, valueOf(c(Rank.ACE, Suit.SPADES)) + valueOf(c(Rank.QUEEN, Suit.HEARTS)))
+        assertEquals(TARGET, valueOf(c(Rank.SIX, Suit.CLUBS)) + valueOf(c(Rank.SEVEN, Suit.DIAMONDS)))
+    }
+
+    @Test
+    fun `only the bottom row is in play at the start`() {
+        val state = PyramidRules.initialState(
+            soloConfig("pyramid", json.encodeToString(PyramidOptions())),
+        )
+        val reachable = PyramidRules.available(state)
+        val fromPyramid = reachable.filter { it.first.zone == PyramidZone.PYRAMID }
+        assertEquals("the seven of the bottom row", 7, fromPyramid.size)
+        assertTrue(fromPyramid.all { rowOf(it.first.index) == 6 })
+        // Nothing has been turned yet, so the waste offers nothing.
+        assertTrue(reachable.none { it.first.zone == PyramidZone.WASTE })
+    }
+
+    @Test
+    fun `drawing turns a card and the waste can be turned back`() {
+        var state = PyramidRules.initialState(
+            soloConfig("pyramid", json.encodeToString(PyramidOptions(redeals = 1))),
+        )
+        val stock = state.stock.size
+        state = PyramidRules.applyMove(state, 0, org.prolibertate.games.game.pyramid.DrawCard)
+        assertEquals(stock - 1, state.stock.size)
+        assertEquals(1, state.waste.size)
+
+        var guard = 40
+        while (state.stock.isNotEmpty() && guard-- > 0) {
+            state = PyramidRules.applyMove(state, 0, org.prolibertate.games.game.pyramid.DrawCard)
+        }
+        assertTrue(state.canRedeal)
+        val turned = PyramidRules.applyMove(state, 0, org.prolibertate.games.game.pyramid.RecycleWaste)
+        assertTrue(turned.waste.isEmpty())
+        assertEquals(1, turned.redealsUsed)
+        assertFalse("only the one redeal was allowed", turned.canRedeal)
+    }
+
+    @Test
+    fun `a cleared pyramid is won`() {
+        val state = PyramidRules.initialState(
+            soloConfig("pyramid", json.encodeToString(PyramidOptions())),
+        )
+        val cleared = state.copy(pyramid = List(PYRAMID_SIZE) { null })
+        assertTrue(cleared.isWon)
+        assertTrue(PyramidRules.isFinished(cleared))
+        assertNull(PyramidRules.currentSeat(cleared))
+        assertTrue(PyramidRules.summary(cleared).contains("Cleared"))
+    }
+
+    // ---- codecs ------------------------------------------------------------
+
+    @Test
+    fun `each of them survives a round trip`() {
+        val freecell = FreeCellRules.initialState(
+            soloConfig("freecell", json.encodeToString(FreeCellOptions())),
+        )
+        assertEquals(freecell, FreeCellRules.decodeState(FreeCellRules.encodeState(freecell)))
+
+        val spider = SpiderRules.initialState(
+            soloConfig("spider", json.encodeToString(SpiderOptions(suits = 4))),
+        )
+        assertEquals(spider, SpiderRules.decodeState(SpiderRules.encodeState(spider)))
+        assertEquals(DealRow, SpiderRules.decodeMove(SpiderRules.encodeMove(DealRow)))
+
+        val pyramid = PyramidRules.initialState(
+            soloConfig("pyramid", json.encodeToString(PyramidOptions())),
+        )
+        assertEquals(pyramid, PyramidRules.decodeState(PyramidRules.encodeState(pyramid)))
+        val king = org.prolibertate.games.game.pyramid.TakeKing(PyramidSpotAt(3))
+        assertEquals(king, PyramidRules.decodeMove(PyramidRules.encodeMove(king)))
+    }
+
+    private fun PyramidSpotAt(index: Int) =
+        org.prolibertate.games.game.pyramid.PyramidSpot(PyramidZone.PYRAMID, index)
+
+    private fun assertThrows(block: () -> Unit) {
+        try {
+            block()
+            throw AssertionError("expected the rules to reject that")
+        } catch (expected: IllegalArgumentException) {
+            // Rejected, which is the point.
+        }
+    }
+}

app/src/test/java/org/prolibertate/games/game/yahtzee/YahtzeeRulesTest.kt

@@ -0,0 +1,294 @@
+package org.prolibertate.games.game.yahtzee
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.prolibertate.games.game.engine.PlayerKind
+import org.prolibertate.games.game.engine.PlayerSlot
+import org.prolibertate.games.game.engine.TableConfig
+
+class YahtzeeRulesTest {
+
+    private val json = Json { encodeDefaults = true }
+
+    private fun config(options: YahtzeeOptions = YahtzeeOptions(), seed: Long = 11L) = TableConfig(
+        gameId = "yahtzee",
+        seats = (0 until options.playerCount).map {
+            PlayerSlot(seat = it, name = "P$it", kind = PlayerKind.AI, team = it)
+        },
+        optionsJson = json.encodeToString(options),
+        seed = seed,
+    )
+
+    // ---- scoring -----------------------------------------------------------
+
+    @Test
+    fun `the upper six count only the die they name`() {
+        val dice = listOf(3, 3, 5, 1, 3)
+        assertEquals(1, scoreOf(YahtzeeCategory.ONES, dice))
+        assertEquals(0, scoreOf(YahtzeeCategory.TWOS, dice))
+        assertEquals(9, scoreOf(YahtzeeCategory.THREES, dice))
+        assertEquals(5, scoreOf(YahtzeeCategory.FIVES, dice))
+        assertEquals(0, scoreOf(YahtzeeCategory.SIXES, dice))
+    }
+
+    @Test
+    fun `three and four of a kind pay the whole throw`() {
+        val three = listOf(4, 4, 4, 2, 6)
+        assertEquals(20, scoreOf(YahtzeeCategory.THREE_OF_A_KIND, three))
+        assertEquals(0, scoreOf(YahtzeeCategory.FOUR_OF_A_KIND, three))
+
+        val four = listOf(4, 4, 4, 4, 6)
+        assertEquals(22, scoreOf(YahtzeeCategory.THREE_OF_A_KIND, four))
+        assertEquals(22, scoreOf(YahtzeeCategory.FOUR_OF_A_KIND, four))
+
+        assertEquals(0, scoreOf(YahtzeeCategory.THREE_OF_A_KIND, listOf(1, 2, 3, 4, 5)))
+    }
+
+    @Test
+    fun `a full house is three of one and two of another, and nothing else`() {
+        assertEquals(25, scoreOf(YahtzeeCategory.FULL_HOUSE, listOf(2, 2, 5, 5, 5)))
+        assertEquals(0, scoreOf(YahtzeeCategory.FULL_HOUSE, listOf(2, 2, 2, 5, 6)))
+        // Five of a kind is not a full house on the printed card, whatever the
+        // joker variations do with it.
+        assertEquals(0, scoreOf(YahtzeeCategory.FULL_HOUSE, listOf(4, 4, 4, 4, 4)))
+    }
+
+    @Test
+    fun `straights want consecutive faces, and a repeat does not break them`() {
+        assertEquals(4, longestRun(listOf(1, 2, 3, 4, 4)))
+        assertEquals(5, longestRun(listOf(2, 3, 4, 5, 6)))
+        assertEquals(1, longestRun(listOf(3, 3, 3, 3, 3)))
+
+        assertEquals(30, scoreOf(YahtzeeCategory.SMALL_STRAIGHT, listOf(1, 2, 3, 4, 4)))
+        assertEquals(0, scoreOf(YahtzeeCategory.LARGE_STRAIGHT, listOf(1, 2, 3, 4, 4)))
+        assertEquals(40, scoreOf(YahtzeeCategory.LARGE_STRAIGHT, listOf(3, 4, 5, 6, 2)))
+        // A large straight is a small one as well.
+        assertEquals(30, scoreOf(YahtzeeCategory.SMALL_STRAIGHT, listOf(3, 4, 5, 6, 2)))
+    }
+
+    @Test
+    fun `five of a kind is fifty, and chance is whatever is showing`() {
+        assertEquals(50, scoreOf(YahtzeeCategory.YAHTZEE, listOf(6, 6, 6, 6, 6)))
+        assertEquals(0, scoreOf(YahtzeeCategory.YAHTZEE, listOf(6, 6, 6, 6, 1)))
+        assertEquals(19, scoreOf(YahtzeeCategory.CHANCE, listOf(6, 6, 4, 2, 1)))
+        assertTrue(isYahtzee(listOf(2, 2, 2, 2, 2)))
+        assertFalse(isYahtzee(listOf(2, 2, 2, 2, 3)))
+    }
+
+    @Test
+    fun `the upper bonus arrives at sixty-three`() {
+        var card = YahtzeeCard()
+        // Exactly three of each is sixty-three, which is what the bonus is for.
+        YahtzeeCategory.entries.filter { it.section == YahtzeeSection.UPPER }.forEach {
+            card = card.with(it, it.face!! * 3)
+        }
+        assertEquals(63, card.upperSubtotal)
+        assertEquals(35, card.upperBonus)
+
+        var short = YahtzeeCard()
+        YahtzeeCategory.entries.filter { it.section == YahtzeeSection.UPPER }.forEach {
+            short = short.with(it, it.face!! * 2)
+        }
+        assertEquals(42, short.upperSubtotal)
+        assertEquals(0, short.upperBonus)
+    }
+
+    @Test
+    fun `a box can only be written in once`() {
+        val card = YahtzeeCard().with(YahtzeeCategory.CHANCE, 20)
+        assertTrue(card.isFilled(YahtzeeCategory.CHANCE))
+        assertFalse(card.isFilled(YahtzeeCategory.ONES))
+        try {
+            card.with(YahtzeeCategory.CHANCE, 5)
+            throw AssertionError("expected a rejection")
+        } catch (expected: IllegalArgumentException) {
+            // As intended.
+        }
+    }
+
+    // ---- the state machine -------------------------------------------------
+
+    @Test
+    fun `a turn opens with one thing to do, and it is throwing`() {
+        val state = YahtzeeRules.initialState(config())
+        assertEquals(0, YahtzeeRules.currentSeat(state))
+        val legal = YahtzeeRules.legalMoves(state, 0)
+        assertEquals(listOf(RollDice(emptyList())), legal)
+        // Nothing may be written in before the dice have been thrown.
+        assertThrows { YahtzeeRules.applyMove(state, 0, ScoreIn(YahtzeeCategory.CHANCE)) }
+    }
+
+    @Test
+    fun `a throw puts five dice on the table`() {
+        val state = YahtzeeRules.initialState(config())
+        val rolled = YahtzeeRules.applyMove(state, 0, RollDice())
+        assertEquals(DICE_COUNT, rolled.dice.size)
+        assertTrue(rolled.dice.all { it in 1..6 })
+        assertEquals(1, rolled.rollsUsed)
+        assertEquals(2, rolled.rollsLeft)
+    }
+
+    @Test
+    fun `the same state and move always throw the same dice`() {
+        // The host and every client run applyMove separately; a throw that
+        // differed between them would split the game in two.
+        val state = YahtzeeRules.initialState(config())
+        val once = YahtzeeRules.applyMove(state, 0, RollDice())
+        val again = YahtzeeRules.applyMove(state, 0, RollDice())
+        assertEquals(once.dice, again.dice)
+
+        val keptOnce = YahtzeeRules.applyMove(once, 0, RollDice(listOf(0, 1)))
+        val keptAgain = YahtzeeRules.applyMove(once, 0, RollDice(listOf(0, 1)))
+        assertEquals(keptOnce.dice, keptAgain.dice)
+    }
+
+    @Test
+    fun `kept dice stay put and the rest are thrown again`() {
+        val state = YahtzeeRules.applyMove(YahtzeeRules.initialState(config()), 0, RollDice())
+        val kept = YahtzeeRules.applyMove(state, 0, RollDice(listOf(0, 2, 4)))
+        assertEquals(state.dice[0], kept.dice[0])
+        assertEquals(state.dice[2], kept.dice[2])
+        assertEquals(state.dice[4], kept.dice[4])
+        assertEquals(2, kept.rollsUsed)
+    }
+
+    @Test
+    fun `there are three throws in a turn and no more`() {
+        var state = YahtzeeRules.applyMove(YahtzeeRules.initialState(config()), 0, RollDice())
+        state = YahtzeeRules.applyMove(state, 0, RollDice())
+        state = YahtzeeRules.applyMove(state, 0, RollDice())
+        assertEquals(ROLLS_PER_TURN, state.rollsUsed)
+        assertEquals(0, state.rollsLeft)
+        assertThrows { YahtzeeRules.applyMove(state, 0, RollDice()) }
+        // Only writing it down is left.
+        assertTrue(YahtzeeRules.legalMoves(state, 0).all { it is ScoreIn })
+    }
+
+    @Test
+    fun `nothing may be kept before the first throw`() {
+        val state = YahtzeeRules.initialState(config())
+        assertThrows { YahtzeeRules.applyMove(state, 0, RollDice(listOf(0))) }
+    }
+
+    @Test
+    fun `writing a box passes the dice on and the round turns over`() {
+        val state = YahtzeeRules.applyMove(YahtzeeRules.initialState(config()), 0, RollDice())
+        val written = YahtzeeRules.applyMove(state, 0, ScoreIn(YahtzeeCategory.CHANCE))
+        assertEquals("the dice go back in the cup", emptyList<Int>(), written.dice)
+        assertEquals(0, written.rollsUsed)
+        assertEquals(1, written.turn)
+        assertEquals("still the first round", 0, written.round)
+        assertEquals(state.dice.sum(), written.cards[0][YahtzeeCategory.CHANCE])
+
+        val second = YahtzeeRules.applyMove(written, 1, RollDice())
+        val wrapped = YahtzeeRules.applyMove(second, 1, ScoreIn(YahtzeeCategory.CHANCE))
+        assertEquals(0, wrapped.turn)
+        assertEquals("round turns over once everyone has written", 1, wrapped.round)
+    }
+
+    @Test
+    fun `a seat that is not on the clock cannot move`() {
+        val state = YahtzeeRules.initialState(config())
+        assertTrue(YahtzeeRules.legalMoves(state, 1).isEmpty())
+        assertThrows { YahtzeeRules.applyMove(state, 1, RollDice()) }
+    }
+
+    @Test
+    fun `a second Yahtzee pays a hundred, but only over a fifty`() {
+        val base = YahtzeeRules.initialState(config())
+        val fives = listOf(5, 5, 5, 5, 5)
+
+        // The box already holds the fifty, so the bonus is earned.
+        val earned = base.copy(
+            dice = fives,
+            rollsUsed = 1,
+            cards = listOf(
+                YahtzeeCard().with(YahtzeeCategory.YAHTZEE, YAHTZEE_SCORE),
+                YahtzeeCard(),
+            ),
+        )
+        val after = YahtzeeRules.applyMove(earned, 0, ScoreIn(YahtzeeCategory.FIVES))
+        assertEquals(YAHTZEE_BONUS, after.yahtzeeBonuses[0])
+        assertEquals(25, after.cards[0][YahtzeeCategory.FIVES])
+
+        // A zero written there earlier forfeits it.
+        val forfeited = base.copy(
+            dice = fives,
+            rollsUsed = 1,
+            cards = listOf(YahtzeeCard().with(YahtzeeCategory.YAHTZEE, 0), YahtzeeCard()),
+        )
+        assertEquals(
+            0,
+            YahtzeeRules.applyMove(forfeited, 0, ScoreIn(YahtzeeCategory.FIVES)).yahtzeeBonuses[0],
+        )
+
+        // And it can be switched off at the table.
+        val off = earned.copy(options = YahtzeeOptions(playerCount = 2, yahtzeeBonus = false))
+        assertEquals(
+            0,
+            YahtzeeRules.applyMove(off, 0, ScoreIn(YahtzeeCategory.FIVES)).yahtzeeBonuses[0],
+        )
+    }
+
+    @Test
+    fun `a full card ends the game`() {
+        var state = YahtzeeRules.initialState(config(YahtzeeOptions(playerCount = 1)))
+        // Thirteen turns, each filling one box.
+        var guard = 200
+        while (!YahtzeeRules.isFinished(state) && guard-- > 0) {
+            val legal = YahtzeeRules.legalMoves(state, state.turn)
+            state = YahtzeeRules.applyMove(state, state.turn, YahtzeeAi.chooseMove(state, state.turn, legal))
+        }
+        assertTrue("the game should end on its own", guard > 0)
+        assertTrue(YahtzeeRules.isFinished(state))
+        assertTrue(state.cards[0].isComplete)
+        assertEquals(YahtzeeCategory.entries.size, state.round)
+        assertNull(YahtzeeRules.currentSeat(state))
+        assertTrue(YahtzeeRules.summary(state).contains("Winner"))
+    }
+
+    @Test
+    fun `two can play it through as well`() {
+        var state = YahtzeeRules.initialState(config(YahtzeeOptions(playerCount = 2)))
+        var guard = 400
+        while (!YahtzeeRules.isFinished(state) && guard-- > 0) {
+            val legal = YahtzeeRules.legalMoves(state, state.turn)
+            state = YahtzeeRules.applyMove(state, state.turn, YahtzeeAi.chooseMove(state, state.turn, legal))
+        }
+        assertTrue(guard > 0)
+        assertTrue(state.cards.all { it.isComplete })
+        state.cards.indices.forEach { assertTrue(state.totalFor(it) >= 0) }
+    }
+
+    @Test
+    fun `nothing is hidden from anyone`() {
+        val state = YahtzeeRules.initialState(config())
+        assertEquals(state, YahtzeeRules.viewFor(state, 0))
+        assertEquals(state, YahtzeeRules.viewFor(state, 1))
+    }
+
+    @Test
+    fun `state and moves survive a round trip`() {
+        val state = YahtzeeRules.applyMove(YahtzeeRules.initialState(config()), 0, RollDice())
+        assertEquals(state, YahtzeeRules.decodeState(YahtzeeRules.encodeState(state)))
+
+        val roll: YahtzeeMove = RollDice(listOf(0, 3))
+        assertEquals(roll, YahtzeeRules.decodeMove(YahtzeeRules.encodeMove(roll)))
+        val write: YahtzeeMove = ScoreIn(YahtzeeCategory.LARGE_STRAIGHT)
+        assertEquals(write, YahtzeeRules.decodeMove(YahtzeeRules.encodeMove(write)))
+    }
+
+    private fun assertThrows(block: () -> Unit) {
+        try {
+            block()
+            throw AssertionError("expected the rules to reject that")
+        } catch (expected: IllegalArgumentException) {
+            // Rejected, which is the point.
+        }
+    }
+}