Scorekeeper: share a sheet between devices

A host can share its score sheet and others nearby can join it, found the
same way games are, or by typed address over a hotspot.

- A joining guest is not let in on its own say-so. The host is prompted with
  the guest's name and answers Allow or Deny. A device already let in that
  drops out (locked phone) is let straight back; the host can remove anyone.
- The host's sheet is authoritative. What travels is what was done (SheetOp:
  add round, rename, move column, ...), not whole sheets, so changes made at
  the same moment all land. A guest's own changes show at once and are laid
  over the host's sheet until confirmed (SheetReplica).
- Sync asks the host for the sheet as it stands, and runs on its own when
  the app returns to the front, as it does for a game table.
- Hello carries a purpose and hosts advertise one, so a game join screen does
  not list score sheets and a game guest typing a sheet's address is told so.
- The Scorekeeper screen now expresses every edit as a SheetOp, so one grid
  serves a local sheet, a hosted one and a joined one.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01CBG7URddsUZjQ6KE1Kqf6G

README.md

@@ -79,6 +79,26 @@ Points are taken away by typing a minus, or with the **±** button, which flips
 the last number typed. Not every keyboard offers a minus on its number pad, and
 subtracting has to work on all of them.
 
+### Sharing a sheet between devices
+
+Tap **Share** and this phone becomes the host of its sheet; others choose
+**Scorekeeper → Join a shared sheet nearby**. Discovery and joining by address
+work exactly as they do for a game, hotspot included. Nobody is let in on their
+own say-so: the host gets a prompt with the guest's name and has to **Allow** or
+**Deny** it. A guest who was let in and drops out — a locked phone, say — is let
+straight back in rather than asked again, and the host can **Remove** anybody
+from the Sharing panel.
+
+Everyone on the sheet can add rounds, correct them, rename and move columns. The
+host's copy is the truth, and what travels is what was *done* ("add a round
+with these points"), not whole sheets, so two people entering a round at once
+get two rounds rather than one overwriting the other. A guest's own change shows
+immediately and is laid over whatever the host sends until the host confirms it.
+**Sync** asks the host for the sheet as it stands, and happens by itself when the
+app comes back to the front. The host keeps the sheet as its own saved one;
+a guest's view is not saved, and leaving puts them back on their own sheet. Only
+the host can start a new game.
+
 The sheet is saved as it is edited, so leaving the app or taking a phone call
 does not lose the game. It is kept in its own DataStore rather than with the
 settings, because it is a game in progress rather than a preference.

app/src/main/java/org/prolibertate/games/MainActivity.kt

@@ -5,7 +5,9 @@ import android.provider.Settings as AndroidSettings
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.lifecycle.lifecycleScope
+import org.prolibertate.games.net.LanTransport
 import org.prolibertate.games.net.LobbyController
+import org.prolibertate.games.net.ScorekeeperSession
 import org.prolibertate.games.score.ScorekeeperRepository
 import org.prolibertate.games.settings.SettingsRepository
 import org.prolibertate.games.ui.AppEnv
@@ -21,12 +23,23 @@ class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
 
+        val peerId = stablePeerId()
+        // Its own transport rather than the lobby's: both listen on the same
+        // port, and a screen at a time is only ever using one of them.
+        val scoreLan = LanTransport(applicationContext)
+
         env = AppEnv(
             settingsRepository = SettingsRepository(applicationContext),
             updater = Updater(this),
             lobby = LobbyController(applicationContext, lifecycleScope),
             scorekeeper = ScorekeeperRepository(applicationContext),
-            peerId = stablePeerId(),
+            scoreSession = ScorekeeperSession(
+                transport = scoreLan,
+                endpoint = scoreLan.endpoint,
+                scope = lifecycleScope,
+                deviceId = peerId,
+            ),
+            peerId = peerId,
         )
 
         setContent {
@@ -50,6 +63,9 @@ class MainActivity : ComponentActivity() {
 
     override fun onDestroy() {
         super.onDestroy()
-        if (::env.isInitialized) env.lobby.stop()
+        if (::env.isInitialized) {
+            env.lobby.stop()
+            env.scoreSession.stop()
+        }
     }
 }

app/src/main/java/org/prolibertate/games/net/LanTransport.kt

@@ -83,7 +83,11 @@ class LanTransport(private val context: Context) : Transport {
     // Hosting
     // -----------------------------------------------------------------------
 
-    override fun host(displayName: String, scope: CoroutineScope): Flow<Connection> = callbackFlow {
+    override fun host(
+        displayName: String,
+        scope: CoroutineScope,
+        purpose: HostPurpose,
+    ): Flow<Connection> = callbackFlow {
         holdRadio()
 
         // The known port when it can be had, so that joining by hand needs an
@@ -97,7 +101,7 @@ class LanTransport(private val context: Context) : Transport {
         multicast.acquire()
 
         val serviceInfo = NsdServiceInfo().apply {
-            serviceName = "$SERVICE_NAME @ $displayName"
+            serviceName = "${serviceLabel(purpose)} @ $displayName"
             serviceType = SERVICE_TYPE
             port = server.localPort
         }
@@ -175,6 +179,7 @@ class LanTransport(private val context: Context) : Transport {
                             kind = TransportKind.LAN,
                             address = address,
                             port = info.port,
+                            purpose = purposeOf(info.serviceName),
                         )
                         synchronized(found) { found[host.id] = host }
                         publish()
@@ -278,6 +283,22 @@ class LanTransport(private val context: Context) : Transport {
     }
 
     private companion object {
+        /**
+         * The service name is the only thing a host says about itself before
+         * anyone has connected, so what it is hosting goes in it.
+         */
+        const val SCOREKEEPER_LABEL = "$SERVICE_NAME Scorekeeper"
+
+        fun serviceLabel(purpose: HostPurpose): String =
+            if (purpose == HostPurpose.SCOREKEEPER) SCOREKEEPER_LABEL else SERVICE_NAME
+
+        fun purposeOf(serviceName: String?): HostPurpose =
+            if (serviceName?.startsWith("$SCOREKEEPER_LABEL @") == true) {
+                HostPurpose.SCOREKEEPER
+            } else {
+                HostPurpose.GAME
+            }
+
         const val CONNECT_TIMEOUT_MS = 8_000
         const val WIFI_LOCK_TAG = "ProLibertateGames:lan"
     }

app/src/main/java/org/prolibertate/games/net/LobbyController.kt

@@ -129,6 +129,18 @@ class LobbyController(
         jobs += scope.launch {
             connection.incoming.collect { message ->
                 if (message !is Hello) return@collect
+                if (message.purpose != PURPOSE_GAME) {
+                    connection.send(
+                        Welcome(
+                            hostName = _state.value.hostName,
+                            gameId = _state.value.gameId.orEmpty(),
+                            accepted = false,
+                            reason = "That is a game, not a score sheet.",
+                        )
+                    )
+                    connection.close()
+                    return@collect
+                }
                 if (message.protocol != PROTOCOL_VERSION) {
                     connection.send(
                         Welcome(
@@ -257,7 +269,9 @@ class LobbyController(
         _state.value = State(joining = true, message = "Looking for games…")
         for (transport in availableTransports()) {
             jobs += scope.launch {
-                transport.discover(scope).collect { hosts ->
+                transport.discover(scope).collect { found ->
+                    // Score sheets are joined from the Scorekeeper, not from here.
+                    val hosts = found.filter { it.purpose == HostPurpose.GAME }
                     val others = _state.value.discovered.filter { it.kind != transport.kind }
                     _state.value = _state.value.copy(discovered = others + hosts)
                 }

app/src/main/java/org/prolibertate/games/net/Protocol.kt

@@ -6,6 +6,8 @@ import kotlinx.serialization.json.Json
 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.score.ScoreSheet
+import org.prolibertate.games.score.SheetOp
 
 /**
  * The wire protocol, shared by every transport.
@@ -25,6 +27,10 @@ import org.prolibertate.games.game.engine.TableConfig
  */
 const val PROTOCOL_VERSION = 2
 
+/** What a [Hello] is asking to join. Absent means a game, which is all there was before. */
+const val PURPOSE_GAME = "game"
+const val PURPOSE_SCOREKEEPER = "scorekeeper"
+
 /** Service type and name advertised over mDNS. */
 const val SERVICE_TYPE = "_plgames._tcp"
 const val SERVICE_NAME = "ProLibertateGames"
@@ -63,6 +69,13 @@ data class Hello(
     val peerId: String,
     val displayName: String,
     val protocol: Int = PROTOCOL_VERSION,
+    /**
+     * Whether this is somebody joining a game or a shared score sheet. Both
+     * listen the same way, so a phone pointed at the wrong sort of host by a typed
+     * address needs a way to be told so rather than waiting for a table that will
+     * never start.
+     */
+    val purpose: String = PURPOSE_GAME,
 ) : NetMessage
 
 /** Host's reply, or a refusal if the lobby is full or the versions differ. */
@@ -150,3 +163,26 @@ data object Pong : NetMessage
 @Serializable
 @SerialName("bye")
 data object Bye : NetMessage
+
+/**
+ * The host's score sheet, sent to a guest on joining, whenever it changes, and
+ * whenever the guest asks with [Resync].
+ *
+ * The whole sheet rather than the change: a sheet is a few hundred bytes, and a
+ * guest that has missed a message is put right by the next one instead of
+ * needing to be told what it missed. [acknowledged] is the last [SheetEdit] from
+ * *this* guest that the sheet already includes, which is how the guest knows
+ * which of its own changes it may stop holding on top of it.
+ */
+@Serializable
+@SerialName("sheet")
+data class SheetState(
+    val revision: Int,
+    val sheet: ScoreSheet,
+    val acknowledged: Int = 0,
+) : NetMessage
+
+/** A guest's change to the shared sheet. The host applies it, then answers with [SheetState]. */
+@Serializable
+@SerialName("sheet-edit")
+data class SheetEdit(val seq: Int, val op: SheetOp) : NetMessage

app/src/main/java/org/prolibertate/games/net/ScorekeeperSession.kt

@@ -0,0 +1,564 @@
+package org.prolibertate.games.net
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.onSubscription
+import kotlinx.coroutines.flow.update
+import kotlinx.coroutines.isActive
+import kotlinx.coroutines.launch
+import org.prolibertate.games.score.ScoreSheet
+import org.prolibertate.games.score.SheetOp
+import org.prolibertate.games.score.SheetReplica
+import java.util.concurrent.CopyOnWriteArrayList
+import java.util.concurrent.atomic.AtomicBoolean
+
+/**
+ * One score sheet shared between several devices, in either of two roles.
+ *
+ * As HOST this device owns the sheet. Everybody's changes — its own and its
+ * guests' — are applied to it here, in the order they arrive, and the result is
+ * sent back out. As GUEST it holds what the host last sent, with its own
+ * unacknowledged changes laid on top (see [SheetReplica]), and sends changes
+ * rather than sheets.
+ *
+ * A guest is not let in on its own say-so. Its [Hello] lands in [State.requests]
+ * and waits there until somebody on the host answers it with [allow] or [deny];
+ * nothing the guest sends in the meantime is looked at. A device that has already
+ * been let in is let back in without asking again, because a phone that was
+ * locked for a minute is not a new person.
+ *
+ * All the state is changed synchronously, under one lock, and the messages that
+ * follow from it are queued and sent in order by one coroutine. The first half
+ * is what lets a keystroke show on screen the moment it is typed; the second is
+ * what stops a guest being sent a sheet before it has been sent its welcome.
+ */
+class ScorekeeperSession(
+    private val transport: Transport,
+    private val endpoint: StateFlow<HostEndpoint?>,
+    private val scope: CoroutineScope,
+    /** Identifies this device across reconnects, so a returning guest is recognised. */
+    private val deviceId: String,
+) {
+
+    enum class Role { NONE, HOST, GUEST }
+
+    /** A guest the host has let in. [online] is false while its link is down. */
+    data class Guest(val id: String, val name: String, val online: Boolean)
+
+    /** Somebody asking to be let in. */
+    data class Request(val id: String, val name: String)
+
+    data class State(
+        val role: Role = Role.NONE,
+        val hostName: String = "",
+        /** The sheet being shared: the host's own, or a guest's view of it. */
+        val sheet: ScoreSheet = ScoreSheet(),
+        /** Guest: let in, and the link to the host is up. */
+        val connected: Boolean = false,
+        /** Guest: asked to join and not yet answered. */
+        val awaiting: Boolean = false,
+        /** Guest: turned away, or removed. Nothing will be retried. */
+        val refused: Boolean = false,
+        val guests: List<Guest> = emptyList(),
+        val requests: List<Request> = emptyList(),
+        val searching: Boolean = false,
+        val discovered: List<DiscoveredHost> = emptyList(),
+        /** Host: where this device is listening, for reading out. */
+        val endpoint: HostEndpoint? = null,
+        val message: String? = null,
+    )
+
+    private val _state = MutableStateFlow(State())
+    val state: StateFlow<State> = _state.asStateFlow()
+
+    private val lock = Any()
+    private val jobs = CopyOnWriteArrayList<Job>()
+    private var discoveryJob: Job? = null
+
+    /** Everything that goes out, in order. See the class comment. */
+    private val outbox = Channel<suspend () -> Unit>(Channel.UNLIMITED)
+
+    init {
+        scope.launch {
+            for (task in outbox) runCatching { task() }
+        }
+    }
+
+    private fun enqueue(connection: Connection, message: NetMessage) {
+        outbox.trySend { connection.send(message) }
+    }
+
+    private fun enqueueClose(connection: Connection) {
+        outbox.trySend { connection.close() }
+    }
+
+    // -----------------------------------------------------------------------
+    // Host
+    // -----------------------------------------------------------------------
+
+    /** A guest that has been let in. Its [connection] is null while the link is down. */
+    private class HostLink(
+        val deviceId: String,
+        var name: String,
+        var connection: Connection?,
+        /** The last change from this guest, on this link, that has been applied. */
+        var lastSeq: Int = 0,
+    )
+
+    private val admitted = linkedMapOf<String, HostLink>()
+    private val waiting = linkedMapOf<String, Pair<String, Connection>>()
+    private var hostSheet = ScoreSheet()
+    private var revision = 0
+    private var hostName = ""
+
+    /** Starts sharing [sheet] with whoever is allowed to join. */
+    fun startHosting(name: String, sheet: ScoreSheet) {
+        stop()
+        synchronized(lock) {
+            hostName = name
+            hostSheet = sheet
+            revision = 0
+            _state.value = State(
+                role = Role.HOST,
+                hostName = name,
+                sheet = sheet,
+                endpoint = endpoint.value,
+                message = "Waiting for others to join…",
+            )
+        }
+        jobs += scope.launch {
+            transport.host(name, scope, HostPurpose.SCOREKEEPER).collect { connection ->
+                listenToGuest(connection)
+            }
+        }
+        jobs += scope.launch {
+            endpoint.collect { where -> _state.update { it.copy(endpoint = where) } }
+        }
+    }
+
+    private fun listenToGuest(connection: Connection) {
+        jobs += scope.launch {
+            connection.incoming.collect { message ->
+                synchronized(lock) { onHostMessage(connection, message) }
+            }
+        }
+    }
+
+    private fun linkFor(connection: Connection): HostLink? =
+        admitted.values.firstOrNull { it.connection === connection }
+
+    private fun onHostMessage(connection: Connection, message: NetMessage) {
+        if (_state.value.role != Role.HOST) return
+        when (message) {
+            is Hello -> onHello(connection, message)
+
+            is SheetEdit -> {
+                val link = linkFor(connection) ?: return
+                // A change already applied is answered rather than applied twice.
+                if (message.seq > link.lastSeq) {
+                    link.lastSeq = message.seq
+                    val next = message.op.applyTo(hostSheet)
+                    if (next != hostSheet) {
+                        hostSheet = next
+                        revision++
+                    }
+                }
+                broadcast()
+            }
+
+            is Resync -> linkFor(connection)?.let { sendSheet(it) }
+
+            is Bye -> {
+                val link = linkFor(connection)
+                if (link != null) {
+                    link.connection = null
+                    publishHost("${link.name} lost their connection.")
+                } else {
+                    val gone = waiting.filterValues { it.second === connection }.keys
+                    if (gone.isNotEmpty()) {
+                        gone.toList().forEach { waiting.remove(it) }
+                        publishHost(null)
+                    }
+                }
+            }
+
+            else -> Unit
+        }
+    }
+
+    private fun onHello(connection: Connection, hello: Hello) {
+        val refusal = when {
+            hello.purpose != PURPOSE_SCOREKEEPER -> "That is a score sheet, not a game."
+            hello.protocol != PROTOCOL_VERSION -> "Different app version — update both devices."
+            else -> null
+        }
+        if (refusal != null) {
+            enqueue(connection, Welcome(hostName, "", accepted = false, reason = refusal))
+            enqueueClose(connection)
+            return
+        }
+
+        val known = admitted[hello.peerId]
+        if (known != null) {
+            // Somebody already let in, back on a new link — or saying hello a
+            // second time on the one it has, which is what a guest does until it
+            // hears something. Either way it is put straight back.
+            val stale = known.connection
+            if (stale != null && stale !== connection) enqueueClose(stale)
+            if (known.connection !== connection) known.lastSeq = 0
+            known.connection = connection
+            known.name = hello.displayName
+            enqueue(connection, Welcome(hostName, "", accepted = true))
+            sendSheet(known)
+            publishHost("${known.name} is back.")
+            return
+        }
+
+        if (admitted.size >= MAX_GUESTS) {
+            enqueue(connection, Welcome(hostName, "", accepted = false, reason = "The sheet is full."))
+            enqueueClose(connection)
+            return
+        }
+
+        val earlier = waiting[hello.peerId]?.second
+        if (earlier != null && earlier !== connection) enqueueClose(earlier)
+        waiting[hello.peerId] = hello.displayName to connection
+        publishHost("${hello.displayName} wants to join.")
+    }
+
+    /** Host: lets a waiting guest in. */
+    fun allow(requestId: String) = synchronized(lock) {
+        val (name, connection) = waiting.remove(requestId) ?: return@synchronized
+        val link = HostLink(requestId, name, connection)
+        admitted[requestId] = link
+        enqueue(connection, Welcome(hostName, "", accepted = true))
+        sendSheet(link)
+        publishHost("$name joined.")
+    }
+
+    /** Host: turns a waiting guest away. */
+    fun deny(requestId: String) = synchronized(lock) {
+        val (name, connection) = waiting.remove(requestId) ?: return@synchronized
+        enqueue(connection, Welcome(hostName, "", accepted = false, reason = "The host declined."))
+        enqueueClose(connection)
+        publishHost("Turned $name away.")
+    }
+
+    /** Host: takes a guest who was let in back out. They are asked again if they return. */
+    fun remove(guestId: String) = synchronized(lock) {
+        val link = admitted.remove(guestId) ?: return@synchronized
+        link.connection?.let {
+            enqueue(it, Welcome(hostName, "", accepted = false, reason = "The host removed you."))
+            enqueueClose(it)
+        }
+        publishHost("${link.name} was removed.")
+    }
+
+    private fun sendSheet(link: HostLink) {
+        link.connection?.let { enqueue(it, SheetState(revision, hostSheet, link.lastSeq)) }
+    }
+
+    private fun broadcast() {
+        admitted.values.forEach { sendSheet(it) }
+        publishHost(_state.value.message)
+    }
+
+    private fun publishHost(message: String?) {
+        _state.update {
+            it.copy(
+                sheet = hostSheet,
+                guests = admitted.values.map { g -> Guest(g.deviceId, g.name, g.connection != null) },
+                requests = waiting.map { (id, entry) -> Request(id, entry.first) },
+                message = message,
+            )
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Guest
+    // -----------------------------------------------------------------------
+
+    private val replica = SheetReplica()
+    private var link: Connection? = null
+    private var linkJob: Job? = null
+    private var joined: DiscoveredHost? = null
+    private var joinedName = ""
+    @Volatile
+    private var heardBack = false
+    private var refused = false
+    private val reconnecting = AtomicBoolean(false)
+
+    /** Starts looking for sheets being shared nearby. */
+    fun startDiscovery() {
+        stop()
+        _state.value = State(searching = true, message = "Looking for shared sheets…")
+        discoveryJob = scope.launch {
+            transport.discover(scope).collect { found ->
+                val sheets = found.filter { it.purpose == HostPurpose.SCOREKEEPER }
+                _state.update { it.copy(discovered = sheets) }
+            }
+        }
+    }
+
+    /** Joins a host at an address somebody typed in, as [LobbyController.joinAt] does. */
+    fun joinAt(typed: String, name: String) {
+        val parsed = parseManualAddress(typed)
+        if (parsed == null) {
+            _state.update { it.copy(message = "That doesn't look like an address. Try 192.168.43.1") }
+            return
+        }
+        join(
+            DiscoveredHost(
+                id = "lan:${parsed.address}:${parsed.port}",
+                name = parsed.address,
+                kind = TransportKind.LAN,
+                address = parsed.address,
+                port = parsed.port,
+                purpose = HostPurpose.SCOREKEEPER,
+            ),
+            name,
+        )
+    }
+
+    fun join(host: DiscoveredHost, name: String) {
+        discoveryJob?.cancel()
+        discoveryJob = null
+        synchronized(lock) {
+            joined = host
+            joinedName = name
+            heardBack = false
+            refused = false
+            replica.reset()
+            _state.value = State(
+                role = Role.GUEST,
+                hostName = host.name,
+                awaiting = true,
+                message = "Connecting to ${host.name}…",
+            )
+        }
+        jobs += scope.launch {
+            val connection = dial(host)
+            if (connection == null) {
+                // Back to the list, with the reason on it.
+                val why = _state.value.message
+                startDiscovery()
+                _state.update { it.copy(message = why) }
+                return@launch
+            }
+            attach(connection)
+        }
+    }
+
+    private suspend fun dial(host: DiscoveredHost): Connection? =
+        runCatching { transport.join(host, scope) }.getOrElse { error ->
+            _state.update { it.copy(message = "Couldn't connect: ${error.message}") }
+            null
+        }
+
+    /**
+     * Starts reading [connection] as the way to the host, and introduces this
+     * device on it.
+     *
+     * The hello is sent from inside the subscription, so it cannot leave before
+     * anything is listening for the answer. It is then repeated until the host
+     * has said something, because a host that has only just accepted the link
+     * may not be listening yet either, and a hello nobody heard is a guest
+     * waiting for an answer to a question that was never asked.
+     */
+    private fun attach(connection: Connection) {
+        synchronized(lock) {
+            linkJob?.cancel()
+            link?.let { old -> if (old !== connection) enqueueClose(old) }
+            link = connection
+            heardBack = false
+            replica.reset()
+        }
+        val hello = Hello(peerId = deviceId, displayName = joinedName, purpose = PURPOSE_SCOREKEEPER)
+        linkJob = scope.launch {
+            launch {
+                var tries = 0
+                while (isActive && !heardBack && connection.isOpen && tries++ < HELLO_TRIES) {
+                    delay(HELLO_RETRY_MILLIS)
+                    if (!heardBack && connection.isOpen) connection.send(hello)
+                }
+            }
+            connection.incoming
+                .onSubscription { connection.send(hello) }
+                .collect { message -> synchronized(lock) { onGuestMessage(connection, message) } }
+        }
+    }
+
+    private fun onGuestMessage(connection: Connection, message: NetMessage) {
+        if (connection !== link || _state.value.role != Role.GUEST) return
+        when (message) {
+            is Welcome -> {
+                heardBack = true
+                if (message.accepted) {
+                    _state.update {
+                        it.copy(
+                            hostName = message.hostName.ifBlank { it.hostName },
+                            awaiting = false,
+                            connected = true,
+                            message = "Joined ${message.hostName.ifBlank { it.hostName }}.",
+                        )
+                    }
+                } else {
+                    refused = true
+                    enqueueClose(connection)
+                    _state.update {
+                        it.copy(
+                            awaiting = false,
+                            connected = false,
+                            refused = true,
+                            message = message.reason ?: "The host declined.",
+                        )
+                    }
+                }
+            }
+
+            is SheetState -> {
+                if (refused) return
+                heardBack = true
+                replica.receive(message.sheet, message.revision, message.acknowledged)
+                _state.update { it.copy(sheet = replica.view) }
+            }
+
+            is Bye -> {
+                // A refusal closes the link too, and the Bye that follows it
+                // must not turn into an attempt to get back in.
+                if (refused) return
+                _state.update {
+                    it.copy(connected = false, message = "Lost the connection to the host.")
+                }
+                reconnectLater()
+            }
+
+            else -> Unit
+        }
+    }
+
+    /**
+     * Guest: asks the host for the sheet as it stands, or gets back to the host
+     * if the link has gone.
+     *
+     * Always safe, and cheap when nothing is wrong — a live link is sent one
+     * [Resync] and answered with the sheet it already has.
+     */
+    fun sync() {
+        val live = synchronized(lock) {
+            if (_state.value.role != Role.GUEST || refused) return
+            link
+        }
+        if (live != null && live.isOpen) {
+            enqueue(live, Resync)
+        } else {
+            reconnectLater()
+        }
+    }
+
+    private fun reconnectLater() {
+        jobs += scope.launch { reconnect() }
+    }
+
+    private suspend fun reconnect() {
+        val host = synchronized(lock) { if (refused) null else joined } ?: return
+        if (!reconnecting.compareAndSet(false, true)) return
+        try {
+            repeat(RECONNECT_ATTEMPTS) { attempt ->
+                if (attempt > 0) delay(RECONNECT_BACKOFF_MILLIS * attempt)
+                if (synchronized(lock) { refused || _state.value.role != Role.GUEST }) return
+                _state.update { it.copy(message = "Reconnecting to ${host.name}…") }
+                val connection = dial(host)
+                if (connection != null) {
+                    attach(connection)
+                    return
+                }
+            }
+            _state.update { it.copy(message = "Couldn't get back to ${host.name}.") }
+        } finally {
+            reconnecting.set(false)
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Either role
+    // -----------------------------------------------------------------------
+
+    /**
+     * Makes a change to the sheet being shared.
+     *
+     * Applied here before it returns, so the screen draws it at once. As host it
+     * is then sent to everyone; as guest it is sent to the host, which may
+     * disagree — the sheet it sends back is the answer.
+     */
+    fun edit(op: SheetOp) {
+        synchronized(lock) {
+            when (_state.value.role) {
+                Role.HOST -> {
+                    val next = op.applyTo(hostSheet)
+                    if (next != hostSheet) {
+                        hostSheet = next
+                        revision++
+                        broadcast()
+                    }
+                }
+
+                Role.GUEST -> {
+                    val live = link
+                    if (!_state.value.connected || live == null || !live.isOpen) {
+                        _state.update {
+                            it.copy(message = "Not connected to the host. Tap Sync to reconnect.")
+                        }
+                        return
+                    }
+                    val seq = replica.local(op)
+                    _state.update { it.copy(sheet = replica.view) }
+                    enqueue(live, SheetEdit(seq, op))
+                }
+
+                Role.NONE -> Unit
+            }
+        }
+    }
+
+    /** Stops hosting, joining or looking, and lets go of every link. */
+    fun stop() {
+        discoveryJob?.cancel()
+        discoveryJob = null
+        jobs.forEach { it.cancel() }
+        jobs.clear()
+        linkJob?.cancel()
+        linkJob = null
+        val open = synchronized(lock) {
+            val all = admitted.values.mapNotNull { it.connection } +
+                waiting.values.map { it.second } + listOfNotNull(link)
+            admitted.clear()
+            waiting.clear()
+            link = null
+            joined = null
+            refused = false
+            heardBack = false
+            replica.reset()
+            hostSheet = ScoreSheet()
+            revision = 0
+            all
+        }
+        open.forEach { runCatching { it.close() } }
+        runCatching { transport.stop() }
+        _state.value = State()
+    }
+
+    private companion object {
+        /** A table of friends, not a room. */
+        const val MAX_GUESTS = 8
+        const val HELLO_RETRY_MILLIS = 2_000L
+        const val HELLO_TRIES = 15
+        const val RECONNECT_ATTEMPTS = 4
+        const val RECONNECT_BACKOFF_MILLIS = 750L
+    }
+}

app/src/main/java/org/prolibertate/games/net/Transport.kt

@@ -24,6 +24,9 @@ enum class TransportKind(val label: String) {
     LAN("Wi-Fi"),
 }
 
+/** What a host is hosting, so a phone looking for one sort is not shown the other. */
+enum class HostPurpose { GAME, SCOREKEEPER }
+
 /**
  * A host someone could join, found by one of the transports. [address] is
  * transport-specific: an IP for LAN.
@@ -34,6 +37,7 @@ data class DiscoveredHost(
     val kind: TransportKind,
     val address: String,
     val port: Int = 0,
+    val purpose: HostPurpose = HostPurpose.GAME,
 )
 
 /**
@@ -292,7 +296,11 @@ interface Transport {
     fun isAvailable(): Boolean
 
     /** Starts advertising and emits each client that connects. */
-    fun host(displayName: String, scope: CoroutineScope): Flow<Connection>
+    fun host(
+        displayName: String,
+        scope: CoroutineScope,
+        purpose: HostPurpose = HostPurpose.GAME,
+    ): Flow<Connection>
 
     /** Emits the current list of visible hosts as it changes. */
     fun discover(scope: CoroutineScope): Flow<List<DiscoveredHost>>

app/src/main/java/org/prolibertate/games/score/SheetOp.kt

@@ -0,0 +1,89 @@
+package org.prolibertate.games.score
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * One change to a score sheet, as something that can be sent.
+ *
+ * A shared sheet cannot work by guests posting whole sheets at the host: two
+ * people entering a round at the same moment would each send a sheet missing the
+ * other's, and one of the rounds would quietly vanish. Sending what was *done*
+ * instead means the host can apply everybody's changes in turn, and two rounds
+ * entered together are simply two rounds.
+ *
+ * Players are named by id wherever they are named at all, for the same reason
+ * [ScorePlayer.id] exists: a position is only right until somebody else moves
+ * a column. Rounds have no id, so [EditRound] and [DeleteRound] go by index; if
+ * two people delete rounds in the same instant the second can land on the wrong
+ * one. Both of them are in the room looking at the same table, and the sheet
+ * shows what happened.
+ *
+ * Applying an op to a sheet it no longer makes sense for — editing a round that
+ * has since been deleted, renaming a player who has been removed — does nothing,
+ * as the sheet's own methods already do.
+ */
+@Serializable
+sealed interface SheetOp {
+    fun applyTo(sheet: ScoreSheet): ScoreSheet
+}
+
+/** Sets a fresh sheet up for [count] players. */
+@Serializable
+@SerialName("start")
+data class StartSheet(val count: Int) : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = ScoreSheet.of(count)
+}
+
+/** Rubs everything out. */
+@Serializable
+@SerialName("clear")
+data object ClearSheet : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = ScoreSheet()
+}
+
+@Serializable
+@SerialName("add-player")
+data object AddPlayer : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = sheet.withPlayerAdded()
+}
+
+@Serializable
+@SerialName("remove-player")
+data class RemovePlayer(val playerId: Int) : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = sheet.withPlayerRemoved(playerId)
+}
+
+@Serializable
+@SerialName("rename")
+data class RenamePlayer(val playerId: Int, val name: String) : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = sheet.renamed(playerId, name)
+}
+
+/** Moves a player's column so that it sits at position [to]. */
+@Serializable
+@SerialName("move-player")
+data class MovePlayer(val playerId: Int, val to: Int) : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet {
+        val from = sheet.players.indexOfFirst { it.id == playerId }
+        return if (from < 0) sheet else sheet.moved(from, to)
+    }
+}
+
+@Serializable
+@SerialName("add-round")
+data class AddRound(val deltas: Map<Int, Int>) : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = sheet.withRound(deltas)
+}
+
+@Serializable
+@SerialName("edit-round")
+data class EditRound(val index: Int, val deltas: Map<Int, Int>) : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = sheet.withRoundAt(index, deltas)
+}
+
+@Serializable
+@SerialName("delete-round")
+data class DeleteRound(val index: Int) : SheetOp {
+    override fun applyTo(sheet: ScoreSheet): ScoreSheet = sheet.withoutRound(index)
+}

app/src/main/java/org/prolibertate/games/score/SheetReplica.kt

@@ -0,0 +1,71 @@
+package org.prolibertate.games.score
+
+/**
+ * A guest's copy of somebody else's score sheet.
+ *
+ * The host's sheet is the truth. What a guest *shows* is that sheet with the
+ * guest's own not-yet-acknowledged changes laid on top, so a number typed into a
+ * cell appears at once instead of after a round trip — and, more to the point,
+ * so that a sheet pushed by the host a moment before the guest's change has
+ * landed does not wipe out what the guest has just done. Each change is tagged
+ * with a sequence number; the host reports the last one it has applied, and
+ * everything up to it is dropped from the pile because it is by then part of the
+ * sheet the host sent.
+ *
+ * Not thread-safe. The session that owns one serialises access to it.
+ */
+class SheetReplica {
+
+    private var confirmed = ScoreSheet()
+    private var nextSeq = 1
+    private val pending = ArrayDeque<Pair<Int, SheetOp>>()
+
+    /** The revision of the host's sheet this was last brought up to. */
+    var revision: Int = 0
+        private set
+
+    /** What to draw: the host's sheet plus this guest's own unacknowledged changes. */
+    var view: ScoreSheet = confirmed
+        private set
+
+    /** How many of this guest's changes the host has not yet confirmed. */
+    val unconfirmed: Int get() = pending.size
+
+    /** Records a change made here, and returns the sequence number to send it under. */
+    fun local(op: SheetOp): Int {
+        val seq = nextSeq++
+        pending.addLast(seq to op)
+        view = op.applyTo(view)
+        return seq
+    }
+
+    /**
+     * Takes the host's sheet at [revision], which includes every change of this
+     * guest's up to and including [acknowledged].
+     *
+     * A sheet older than the one already held is ignored; links deliver in
+     * order, but a resync request and a broadcast can cross.
+     */
+    fun receive(sheet: ScoreSheet, revision: Int, acknowledged: Int) {
+        if (revision < this.revision) return
+        this.revision = revision
+        confirmed = sheet
+        while (pending.isNotEmpty() && pending.first().first <= acknowledged) pending.removeFirst()
+        view = pending.fold(confirmed) { acc, (_, op) -> op.applyTo(acc) }
+    }
+
+    /**
+     * Forgets everything, for a fresh link to the host.
+     *
+     * Changes still waiting on the old link are gone with it: nothing says
+     * whether the host got them, and replaying them could apply them twice. The
+     * sheet the host sends next is the answer either way.
+     */
+    fun reset() {
+        confirmed = ScoreSheet()
+        view = confirmed
+        pending.clear()
+        revision = 0
+        nextSeq = 1
+    }
+}

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

@@ -43,6 +43,7 @@ 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
+import org.prolibertate.games.net.ScorekeeperSession
 import org.prolibertate.games.score.ScorekeeperRepository
 import org.prolibertate.games.settings.Settings
 import org.prolibertate.games.settings.SettingsRepository
@@ -56,6 +57,8 @@ class AppEnv(
     val updater: Updater,
     val lobby: LobbyController,
     val scorekeeper: ScorekeeperRepository,
+    /** The shared score sheet, for the Scorekeeper to host or join. */
+    val scoreSession: ScorekeeperSession,
     val peerId: String,
 )
 
@@ -186,6 +189,8 @@ private fun AppContent(
 
         is Route.Scorekeeper -> ScorekeeperScreen(
             repository = env.scorekeeper,
+            session = env.scoreSession,
+            playerName = settings.displayName,
             onBack = { pop() },
         )
 

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

@@ -1,5 +1,6 @@
 package org.prolibertate.games.ui
 
+import androidx.activity.compose.BackHandler
 import androidx.compose.foundation.ScrollState
 import androidx.compose.foundation.background
 import androidx.compose.foundation.border
@@ -30,7 +31,9 @@ import androidx.compose.material3.OutlinedTextField
 import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateMapOf
 import androidx.compose.runtime.mutableStateOf
@@ -47,6 +50,7 @@ import androidx.compose.ui.focus.focusRequester
 import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.platform.LocalLifecycleOwner
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.input.KeyboardType
 import androidx.compose.ui.text.style.TextAlign
@@ -54,11 +58,25 @@ import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.zIndex
+import androidx.lifecycle.Lifecycle
+import androidx.lifecycle.LifecycleEventObserver
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.launch
+import org.prolibertate.games.net.ScorekeeperSession
+import org.prolibertate.games.net.ScorekeeperSession.Role
+import org.prolibertate.games.score.AddPlayer
+import org.prolibertate.games.score.AddRound
+import org.prolibertate.games.score.ClearSheet
+import org.prolibertate.games.score.DeleteRound
+import org.prolibertate.games.score.EditRound
+import org.prolibertate.games.score.MovePlayer
+import org.prolibertate.games.score.RemovePlayer
+import org.prolibertate.games.score.RenamePlayer
 import org.prolibertate.games.score.ScorePlayer
 import org.prolibertate.games.score.ScoreSheet
 import org.prolibertate.games.score.ScorekeeperRepository
+import org.prolibertate.games.score.SheetOp
+import org.prolibertate.games.score.StartSheet
 import kotlin.math.roundToInt
 
 /**
@@ -68,10 +86,18 @@ import kotlin.math.roundToInt
  *
  * The bottom row is always the one being filled in. Finish it and it is written
  * down, and a fresh empty row opens above the totals for the next round.
+ *
+ * The sheet can be shared. Sharing makes this device the host of a
+ * [ScorekeeperSession]; joining makes it a guest of somebody else's. Either way
+ * every change is a [SheetOp] — applied here when the sheet is only this
+ * device's, and handed to the session when it is not — so the grid below does
+ * not need to know which of the three it is drawing.
  */
 @Composable
 fun ScorekeeperScreen(
     repository: ScorekeeperRepository,
+    session: ScorekeeperSession,
+    playerName: String,
     onBack: () -> Unit,
 ) {
     val scope = rememberCoroutineScope()
@@ -80,36 +106,121 @@ fun ScorekeeperScreen(
     // of storage on every change would put a DataStore round trip between a
     // keystroke and the digit appearing, which is how a field loses characters.
     var loaded by remember { mutableStateOf(false) }
-    var sheet by remember { mutableStateOf(ScoreSheet()) }
+    var local by remember { mutableStateOf(ScoreSheet()) }
     LaunchedEffect(Unit) {
-        sheet = repository.sheet.first()
+        local = repository.sheet.first()
         loaded = true
     }
 
-    val update: (ScoreSheet) -> Unit = { next ->
-        sheet = next
-        scope.launch { repository.save(next) }
-    }
-
+    val shared by session.state.collectAsState()
+    val role = shared.role
+    var joining by remember { mutableStateOf(false) }
+    var showShare by remember { mutableStateOf(false) }
     var confirmNewGame by remember { mutableStateOf(false) }
 
+    // A session lives exactly as long as this screen does: leaving it ends the
+    // sharing, and with it the links.
+    DisposableEffect(Unit) { onDispose { session.stop() } }
+
+    // What the host shares is the host's own sheet, so it is the one kept. A
+    // phone killed mid-game comes back to the sheet as the guests last saw it.
+    LaunchedEffect(shared.sheet, role) {
+        if (role == Role.HOST) repository.save(shared.sheet)
+    }
+
+    // Coming back to the screen after the phone was put down. A guest has missed
+    // whatever the host did in the meantime, and may have lost its link while the
+    // radio slept, so it asks again — the same thing a game table does.
+    val lifecycleOwner = LocalLifecycleOwner.current
+    DisposableEffect(lifecycleOwner) {
+        val observer = LifecycleEventObserver { _, event ->
+            if (event == Lifecycle.Event.ON_START) session.sync()
+        }
+        lifecycleOwner.lifecycle.addObserver(observer)
+        onDispose { lifecycleOwner.lifecycle.removeObserver(observer) }
+    }
+
+    val sheet = if (role == Role.NONE) local else shared.sheet
+
+    val change: (SheetOp) -> Unit = { op ->
+        if (role == Role.NONE) {
+            val next = op.applyTo(local)
+            local = next
+            scope.launch { repository.save(next) }
+        } else {
+            session.edit(op)
+        }
+    }
+
+    // Stops whatever is going on with the network and goes back to this
+    // device's own sheet — which, for a host, is the one it has been keeping.
+    val leaveSharing: () -> Unit = {
+        if (role == Role.HOST) local = shared.sheet
+        session.stop()
+        joining = false
+        showShare = false
+    }
+
+    BackHandler(enabled = role == Role.GUEST || joining) { leaveSharing() }
+
     ScreenScaffold(
-        title = "Scorekeeper",
-        onBack = onBack,
+        title = when (role) {
+            Role.GUEST -> "Scorekeeper · ${shared.hostName}"
+            Role.HOST -> "Scorekeeper · sharing"
+            Role.NONE -> "Scorekeeper"
+        },
+        onBack = { if (role == Role.GUEST || joining) leaveSharing() else onBack() },
         actions = {
+            when (role) {
+                Role.NONE -> if (sheet.started && !joining) {
+                    TextButton(onClick = {
+                        session.startHosting(playerName, local)
+                        showShare = true
+                    }) { Text("Share") }
+                    TextButton(onClick = { confirmNewGame = true }) { Text("New game") }
+                }
+
+                Role.HOST -> {
+                    TextButton(onClick = { showShare = true }) {
+                        Text(if (shared.guests.isEmpty()) "Sharing" else "Sharing (${shared.guests.size})")
+                    }
                     if (sheet.started) {
                         TextButton(onClick = { confirmNewGame = true }) { Text("New game") }
                     }
+                }
+
+                // Rubbing out the sheet is the host's to do; a guest can leave
+                // and nothing more drastic.
+                Role.GUEST -> {
+                    TextButton(onClick = { session.sync() }) { Text("Sync") }
+                    TextButton(onClick = leaveSharing) { Text("Leave") }
+                }
+            }
         },
     ) { modifier ->
         // Nothing is drawn until the stored sheet has been read, so a game in
         // progress does not flash the "how many are playing" pane first.
-        if (!loaded) {
-            Box(modifier = modifier)
-        } else if (!sheet.started) {
-            SetupPane(modifier = modifier) { count -> update(ScoreSheet.of(count)) }
-        } else {
-            Sheet(modifier = modifier, sheet = sheet, onChange = update)
+        when {
+            !loaded -> Box(modifier = modifier)
+
+            role == Role.GUEST && (shared.awaiting || shared.refused || !sheet.started) ->
+                WaitingPane(modifier = modifier, shared = shared, onLeave = leaveSharing)
+
+            role == Role.GUEST -> Column(modifier = modifier) {
+                ConnectionBanner(shared = shared, onSync = { session.sync() })
+                Sheet(modifier = Modifier.weight(1f), sheet = sheet, onOp = change)
+            }
+
+            joining && role == Role.NONE ->
+                JoinPane(modifier = modifier, session = session, shared = shared, playerName = playerName)
+
+            !sheet.started -> SetupPane(
+                modifier = modifier,
+                onStart = { count -> change(StartSheet(count)) },
+                onJoin = { joining = true },
+            )
+
+            else -> Sheet(modifier = modifier, sheet = sheet, onOp = change)
         }
     }
 
@@ -117,11 +228,20 @@ fun ScorekeeperScreen(
         AlertDialog(
             onDismissRequest = { confirmNewGame = false },
             title = { Text("Start a new game?") },
-            text = { Text("The players and every round scored so far are rubbed out.") },
+            text = {
+                Text(
+                    if (role == Role.HOST) {
+                        "The players and every round scored so far are rubbed out, " +
+                            "for everyone on the sheet."
+                    } else {
+                        "The players and every round scored so far are rubbed out."
+                    }
+                )
+            },
             confirmButton = {
                 TextButton(onClick = {
                     confirmNewGame = false
-                    update(ScoreSheet())
+                    change(ClearSheet)
                 }) { Text("Rub it out") }
             },
             dismissButton = {
@@ -129,6 +249,25 @@ fun ScorekeeperScreen(
             },
         )
     }
+
+    // Asked over whatever else is on screen, one at a time.
+    if (role == Role.HOST) {
+        shared.requests.firstOrNull()?.let { request ->
+            JoinRequestDialog(
+                request = request,
+                onAllow = { session.allow(request.id) },
+                onDeny = { session.deny(request.id) },
+            )
+        }
+        if (showShare) {
+            ShareDialog(
+                shared = shared,
+                onRemove = { session.remove(it) },
+                onStop = leaveSharing,
+                onClose = { showShare = false },
+            )
+        }
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -137,7 +276,7 @@ fun ScorekeeperScreen(
 
 @OptIn(ExperimentalLayoutApi::class)
 @Composable
-private fun SetupPane(modifier: Modifier, onStart: (Int) -> Unit) {
+private fun SetupPane(modifier: Modifier, onStart: (Int) -> Unit, onJoin: () -> Unit) {
     var count by remember { mutableStateOf(ScoreSheet.DEFAULT_PLAYERS) }
 
     Column(
@@ -170,6 +309,9 @@ private fun SetupPane(modifier: Modifier, onStart: (Int) -> Unit) {
             style = MaterialTheme.typography.bodySmall,
         )
         PrimaryAction(text = "Start scoring") { onStart(count) }
+        OutlinedButton(onClick = onJoin, modifier = Modifier.fillMaxWidth()) {
+            Text("Join a shared sheet nearby")
+        }
     }
 }
 
@@ -178,7 +320,7 @@ private fun SetupPane(modifier: Modifier, onStart: (Int) -> Unit) {
 // ---------------------------------------------------------------------------
 
 @Composable
-private fun Sheet(modifier: Modifier, sheet: ScoreSheet, onChange: (ScoreSheet) -> Unit) {
+private fun Sheet(modifier: Modifier, sheet: ScoreSheet, onOp: (SheetOp) -> Unit) {
     // One scroll state shared by the head, the rows and the totals, so all three
     // move together: a column and the name over it must never come apart. Every
     // row is built the same way and is therefore exactly the same width, which
@@ -213,7 +355,16 @@ private fun Sheet(modifier: Modifier, sheet: ScoreSheet, onChange: (ScoreSheet)
     // Keep the row being filled in in view when a round is written down.
     LaunchedEffect(sheet.rounds.size) { rows.animateScrollTo(rows.maxValue) }
 
-    val drag = rememberColumnDrag(sheet = sheet, onChange = onChange)
+    // On a shared sheet somebody else can delete the round being corrected from
+    // under it. There is then nothing to correct, and the row it was in is gone.
+    LaunchedEffect(sheet.rounds.size) {
+        if (correcting >= sheet.rounds.size) {
+            correcting = -1
+            correction.clear()
+        }
+    }
+
+    val drag = rememberColumnDrag(sheet = sheet, onOp = onOp)
 
     Column(modifier = modifier) {
         Text(
@@ -234,7 +385,7 @@ private fun Sheet(modifier: Modifier, sheet: ScoreSheet, onChange: (ScoreSheet)
             trailing = {
                 AddColumnCell(
                     enabled = sheet.players.size < ScoreSheet.MAX_PLAYERS,
-                    onClick = { onChange(sheet.withPlayerAdded()) },
+                    onClick = { onOp(AddPlayer) },
                 )
             },
         ) { index, player ->
@@ -360,18 +511,18 @@ private fun Sheet(modifier: Modifier, sheet: ScoreSheet, onChange: (ScoreSheet)
             onFlip = { editing[lastTyped] = flipSign(editing[lastTyped].orEmpty()) },
             onFinish = {
                 if (correcting >= 0) {
-                    onChange(sheet.withRoundAt(correcting, amountsAsPoints(correction)))
+                    onOp(EditRound(correcting, amountsAsPoints(correction)))
                     correcting = -1
                     correction.clear()
                 } else {
-                    onChange(sheet.withRound(amountsAsPoints(pending)))
+                    onOp(AddRound(amountsAsPoints(pending)))
                     pending.clear()
                 }
                 lastTyped = -1
                 focusPlayer = -1
             },
             onDelete = {
-                onChange(sheet.withoutRound(correcting))
+                onOp(DeleteRound(correcting))
                 correcting = -1
                 correction.clear()
                 lastTyped = -1
@@ -391,9 +542,9 @@ private fun Sheet(modifier: Modifier, sheet: ScoreSheet, onChange: (ScoreSheet)
         RenameDialog(
             sheet = sheet,
             player = renamed,
-            onName = { onChange(sheet.renamed(renamed.id, it)) },
+            onName = { onOp(RenamePlayer(renamed.id, it)) },
             onRemove = {
-                onChange(sheet.withPlayerRemoved(renamed.id))
+                onOp(RemovePlayer(renamed.id))
                 renaming = -1
             },
             onDismiss = { renaming = -1 },
@@ -417,17 +568,17 @@ private fun Sheet(modifier: Modifier, sheet: ScoreSheet, onChange: (ScoreSheet)
  * since.
  */
 @Composable
-private fun rememberColumnDrag(sheet: ScoreSheet, onChange: (ScoreSheet) -> Unit): ColumnDrag {
+private fun rememberColumnDrag(sheet: ScoreSheet, onOp: (SheetOp) -> Unit): ColumnDrag {
     val widthPx = with(LocalDensity.current) { PLAYER_COLUMN.toPx() }
     val currentSheet by rememberUpdatedState(sheet)
-    val commit by rememberUpdatedState(onChange)
+    val commit by rememberUpdatedState(onOp)
     return remember { ColumnDrag(widthPx, { currentSheet }, { commit(it) }) }
 }
 
 private class ColumnDrag(
     private val widthPx: Float,
     private val sheet: () -> ScoreSheet,
-    private val commit: (ScoreSheet) -> Unit,
+    private val commit: (SheetOp) -> Unit,
 ) {
     /** Which column is under the finger, or -1 when nothing is being dragged. */
     var index by mutableStateOf(-1)
@@ -454,7 +605,8 @@ private class ColumnDrag(
         val to = landing()
         index = -1
         offset = 0f
-        if (from >= 0 && to >= 0) commit(sheet().moved(from, to))
+        val moved = sheet().players.getOrNull(from)
+        if (moved != null && to >= 0 && to != from) commit(MovePlayer(moved.id, to))
     }
 
     fun cancel() {

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

@@ -0,0 +1,239 @@
+package org.prolibertate.games.ui
+
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Arrangement
+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.rememberScrollState
+import androidx.compose.foundation.text.KeyboardOptions
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Button
+import androidx.compose.material3.Card
+import androidx.compose.material3.Divider
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedButton
+import androidx.compose.material3.OutlinedTextField
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+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.text.input.KeyboardType
+import androidx.compose.ui.unit.dp
+import org.prolibertate.games.net.ScorekeeperSession
+
+/*
+ * The parts of the Scorekeeper that only exist while a sheet is being shared:
+ * finding one to join, waiting to be let in, and — on the host — letting people
+ * in and seeing who is there.
+ */
+
+/** Guest: the sheets nearby, and a way in by address when they cannot be found. */
+@Composable
+internal fun JoinPane(
+    modifier: Modifier,
+    session: ScorekeeperSession,
+    shared: ScorekeeperSession.State,
+    playerName: String,
+) {
+    LaunchedEffect(Unit) { session.startDiscovery() }
+
+    var typed by remember { mutableStateOf("") }
+
+    Column(
+        modifier = modifier.verticalScroll(rememberScrollState()),
+        verticalArrangement = Arrangement.spacedBy(12.dp),
+    ) {
+        Text(
+            text = "Join a sheet somebody else is keeping. The host is asked to let " +
+                "you in, and once you are, you see the same sheet they do.",
+            style = MaterialTheme.typography.bodyMedium,
+        )
+        shared.message?.let { Text(it, style = MaterialTheme.typography.bodyMedium) }
+
+        if (shared.discovered.isEmpty()) {
+            Text("No shared sheets found yet. Make sure the host has tapped Share.")
+        } else {
+            shared.discovered.forEach { host ->
+                Card(
+                    modifier = Modifier
+                        .fillMaxWidth()
+                        .clickable { session.join(host, playerName) },
+                ) {
+                    Column(modifier = Modifier.padding(12.dp)) {
+                        Text(host.name, fontWeight = FontWeight.Bold)
+                        Text(
+                            text = "over ${host.kind.label}",
+                            style = MaterialTheme.typography.bodySmall,
+                        )
+                    }
+                }
+            }
+        }
+
+        Divider()
+
+        // Always offered, for the same reason the game lobby always offers it:
+        // over a phone's hotspot nothing is found, and a list that only says
+        // "no sheets" leaves nowhere to go.
+        Text("Join by address", fontWeight = FontWeight.Bold)
+        Text(
+            text = "The host's screen shows where it is, under Sharing.",
+            style = MaterialTheme.typography.bodySmall,
+        )
+        Row(
+            modifier = Modifier.fillMaxWidth(),
+            horizontalArrangement = Arrangement.spacedBy(8.dp),
+            verticalAlignment = Alignment.CenterVertically,
+        ) {
+            OutlinedTextField(
+                value = typed,
+                onValueChange = { typed = it },
+                singleLine = true,
+                label = { Text("192.168.43.1") },
+                keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Uri),
+                modifier = Modifier.weight(1f),
+            )
+            Button(
+                onClick = { session.joinAt(typed, playerName) },
+                enabled = typed.isNotBlank(),
+            ) { Text("Connect") }
+        }
+    }
+}
+
+/** Guest: asked to join, and nobody has answered yet — or somebody has said no. */
+@Composable
+internal fun WaitingPane(
+    modifier: Modifier,
+    shared: ScorekeeperSession.State,
+    onLeave: () -> Unit,
+) {
+    Column(
+        modifier = modifier,
+        verticalArrangement = Arrangement.spacedBy(12.dp),
+    ) {
+        Text(
+            text = when {
+                shared.refused -> shared.message ?: "The host declined."
+                shared.awaiting -> "Waiting for ${shared.hostName} to let you in…"
+                else -> "Waiting for ${shared.hostName} to set up the sheet…"
+            },
+            style = MaterialTheme.typography.titleMedium,
+        )
+        if (shared.awaiting) {
+            Text(
+                text = "They will see a request with your name on it.",
+                style = MaterialTheme.typography.bodySmall,
+            )
+        }
+        OutlinedButton(onClick = onLeave) { Text(if (shared.refused) "Back" else "Cancel") }
+    }
+}
+
+/** Guest: the link is down, or was never up. Shown over a sheet that can still be read. */
+@Composable
+internal fun ConnectionBanner(shared: ScorekeeperSession.State, onSync: () -> Unit) {
+    if (shared.connected) return
+    Row(
+        modifier = Modifier.fillMaxWidth().padding(bottom = 6.dp),
+        verticalAlignment = Alignment.CenterVertically,
+        horizontalArrangement = Arrangement.SpaceBetween,
+    ) {
+        Text(
+            text = shared.message ?: "Not connected to the host.",
+            style = MaterialTheme.typography.bodySmall,
+            color = MaterialTheme.colorScheme.error,
+            modifier = Modifier.weight(1f),
+        )
+        TextButton(onClick = onSync) { Text("Reconnect") }
+    }
+}
+
+/**
+ * Host: somebody is asking to join.
+ *
+ * Not dismissible by tapping outside it. A request that goes away on a stray
+ * touch is a guest told nothing, and a request answered by accident is a stranger
+ * with a pencil; either way it should take a button.
+ */
+@Composable
+internal fun JoinRequestDialog(
+    request: ScorekeeperSession.Request,
+    onAllow: () -> Unit,
+    onDeny: () -> Unit,
+) {
+    AlertDialog(
+        onDismissRequest = {},
+        title = { Text("${request.name} wants to join") },
+        text = { Text("Let them see this sheet and add scores to it?") },
+        confirmButton = { TextButton(onClick = onAllow) { Text("Allow") } },
+        dismissButton = { TextButton(onClick = onDeny) { Text("Deny") } },
+    )
+}
+
+/** Host: where this phone is, who is on the sheet, and the way to stop sharing it. */
+@Composable
+internal fun ShareDialog(
+    shared: ScorekeeperSession.State,
+    onRemove: (String) -> Unit,
+    onStop: () -> Unit,
+    onClose: () -> Unit,
+) {
+    AlertDialog(
+        onDismissRequest = onClose,
+        title = { Text("Sharing this sheet") },
+        text = {
+            Column(
+                modifier = Modifier.verticalScroll(rememberScrollState()),
+                verticalArrangement = Arrangement.spacedBy(8.dp),
+            ) {
+                Text(
+                    text = "Others nearby can find it under Scorekeeper → Join a shared sheet.",
+                    style = MaterialTheme.typography.bodySmall,
+                )
+                shared.endpoint?.primary?.let { where ->
+                    Text("This phone is at", style = MaterialTheme.typography.labelSmall)
+                    Text(where, fontWeight = FontWeight.Bold)
+                    Text(
+                        text = "If the other phone doesn't find it on its own — which happens " +
+                            "over a phone's hotspot — type that into its Join screen.",
+                        style = MaterialTheme.typography.bodySmall,
+                    )
+                }
+                Divider()
+                Text("On the sheet", fontWeight = FontWeight.Bold)
+                if (shared.guests.isEmpty()) {
+                    Text("Nobody else yet.", style = MaterialTheme.typography.bodySmall)
+                }
+                shared.guests.forEach { guest ->
+                    Row(
+                        modifier = Modifier.fillMaxWidth(),
+                        verticalAlignment = Alignment.CenterVertically,
+                        horizontalArrangement = Arrangement.SpaceBetween,
+                    ) {
+                        Column(modifier = Modifier.weight(1f)) {
+                            Text(guest.name)
+                            Text(
+                                text = if (guest.online) "Connected" else "Connection lost",
+                                style = MaterialTheme.typography.bodySmall,
+                            )
+                        }
+                        TextButton(onClick = { onRemove(guest.id) }) { Text("Remove") }
+                    }
+                }
+            }
+        },
+        confirmButton = { TextButton(onClick = onClose) { Text("Done") } },
+        dismissButton = { TextButton(onClick = onStop) { Text("Stop sharing") } },
+    )
+}

app/src/test/java/org/prolibertate/games/net/ScorekeeperSessionTest.kt

@@ -0,0 +1,358 @@
+package org.prolibertate.games.net
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.cancel
+import kotlinx.coroutines.channels.awaitClose
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.callbackFlow
+import kotlinx.coroutines.flow.flowOf
+import kotlinx.coroutines.flow.onSubscription
+import kotlinx.coroutines.isActive
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Assert.fail
+import org.junit.Test
+import org.prolibertate.games.net.ScorekeeperSession.Role
+import org.prolibertate.games.score.AddRound
+import org.prolibertate.games.score.RenamePlayer
+import org.prolibertate.games.score.ScoreSheet
+import java.net.ServerSocket
+import java.net.Socket
+import java.util.concurrent.atomic.AtomicInteger
+
+/**
+ * A shared score sheet between real sessions, over real sockets on loopback.
+ *
+ * The sheet logic is covered on its own; what is worth testing here is the part
+ * that only exists with a link in it — being asked before being let in, being
+ * let back in without asking, and everybody ending up looking at one sheet.
+ */
+class ScorekeeperSessionTest {
+
+    private val scopes = mutableListOf<CoroutineScope>()
+    private val sessions = mutableListOf<ScorekeeperSession>()
+    private val transports = mutableListOf<LoopbackTransport>()
+
+    @After
+    fun tearDown() {
+        sessions.forEach { runCatching { it.stop() } }
+        transports.forEach { runCatching { it.stop() } }
+        scopes.forEach { it.cancel() }
+    }
+
+    /** TCP on loopback, with the discovery a test does not need left out. */
+    private class LoopbackTransport : Transport {
+        override val kind = TransportKind.LAN
+
+        @Volatile
+        var server: ServerSocket? = null
+        val port: Int get() = server!!.localPort
+
+        override fun isAvailable() = true
+
+        override fun host(
+            displayName: String,
+            scope: CoroutineScope,
+            purpose: HostPurpose,
+        ): Flow<Connection> = callbackFlow {
+            val listening = ServerSocket(0)
+            server = listening
+            val accepting = scope.launch(Dispatchers.IO) {
+                while (isActive && !listening.isClosed) {
+                    val socket = runCatching { listening.accept() }.getOrNull() ?: break
+                    trySend(
+                        StreamConnection(
+                            peerId = "guest:${socket.port}",
+                            kind = TransportKind.LAN,
+                            input = socket.getInputStream(),
+                            output = socket.getOutputStream(),
+                            scope = scope,
+                            onClosed = { runCatching { socket.close() } },
+                        )
+                    )
+                }
+            }
+            awaitClose {
+                accepting.cancel()
+                runCatching { listening.close() }
+            }
+        }
+
+        override fun discover(scope: CoroutineScope): Flow<List<DiscoveredHost>> = flowOf(emptyList())
+
+        override suspend fun join(host: DiscoveredHost, scope: CoroutineScope): Connection {
+            val socket = Socket("127.0.0.1", host.port)
+            return StreamConnection(
+                peerId = host.id,
+                kind = TransportKind.LAN,
+                input = socket.getInputStream(),
+                output = socket.getOutputStream(),
+                scope = scope,
+                onClosed = { runCatching { socket.close() } },
+            )
+        }
+
+        override fun stop() {
+            runCatching { server?.close() }
+        }
+    }
+
+    private fun newSession(deviceId: String): Pair<ScorekeeperSession, LoopbackTransport> {
+        val scope = CoroutineScope(SupervisorJob() + Dispatchers.IO).also { scopes += it }
+        val transport = LoopbackTransport().also { transports += it }
+        val session = ScorekeeperSession(transport, MutableStateFlow(null), scope, deviceId)
+            .also { sessions += it }
+        return session to transport
+    }
+
+    private class Host(val session: ScorekeeperSession, val transport: LoopbackTransport) {
+        val state get() = session.state.value
+    }
+
+    private fun startHost(sheet: ScoreSheet = ScoreSheet.of(2)): Host {
+        val (session, transport) = newSession("host-device")
+        session.startHosting("Hosty", sheet)
+        await("the host to be listening") { transport.server != null }
+        return Host(session, transport)
+    }
+
+    private fun joinOf(host: Host) = DiscoveredHost(
+        id = "lan:127.0.0.1:${host.transport.port}",
+        name = "Hosty",
+        kind = TransportKind.LAN,
+        address = "127.0.0.1",
+        port = host.transport.port,
+        purpose = HostPurpose.SCOREKEEPER,
+    )
+
+    private fun guestOf(host: Host, deviceId: String, name: String): ScorekeeperSession {
+        val (session, _) = newSession(deviceId)
+        session.join(joinOf(host), name)
+        return session
+    }
+
+    /** A guest that has asked, been let in, and is looking at the host's sheet. */
+    private fun admittedGuest(host: Host, deviceId: String, name: String): ScorekeeperSession {
+        val guest = guestOf(host, deviceId, name)
+        await("$name to be asking") { host.state.requests.any { it.id == deviceId } }
+        host.session.allow(deviceId)
+        await("$name to be let in") { guest.state.value.connected }
+        return guest
+    }
+
+    @Test
+    fun `a guest is asked about, and sees nothing, until the host allows it`() {
+        val host = startHost()
+        val guest = guestOf(host, "g1", "Gwen")
+
+        await("the request to reach the host") { host.state.requests.isNotEmpty() }
+        assertEquals("Gwen", host.state.requests.single().name)
+        assertTrue(guest.state.value.awaiting)
+        assertFalse(guest.state.value.connected)
+        assertFalse(guest.state.value.sheet.started)
+        assertTrue(host.state.guests.isEmpty())
+
+        host.session.allow("g1")
+
+        await("the guest to be let in") { guest.state.value.connected }
+        assertFalse(guest.state.value.awaiting)
+        assertEquals(host.state.sheet, guest.state.value.sheet)
+        assertTrue(guest.state.value.sheet.started)
+        assertTrue(host.state.requests.isEmpty())
+        assertEquals(listOf("Gwen"), host.state.guests.map { it.name })
+    }
+
+    @Test
+    fun `a guest that is turned away stays turned away`() {
+        val host = startHost()
+        val guest = guestOf(host, "g1", "Gwen")
+        await("the request to reach the host") { host.state.requests.isNotEmpty() }
+
+        host.session.deny("g1")
+
+        await("the refusal to arrive") { guest.state.value.refused }
+        assertEquals("The host declined.", guest.state.value.message)
+        assertFalse(guest.state.value.connected)
+        assertFalse(guest.state.value.sheet.started)
+        assertTrue(host.state.guests.isEmpty())
+        assertTrue(host.state.requests.isEmpty())
+    }
+
+    @Test
+    fun `a guest's change reaches the host and the other guests`() {
+        val host = startHost()
+        val first = admittedGuest(host, "g1", "Gwen")
+        val second = admittedGuest(host, "g2", "Gus")
+        val player = host.state.sheet.players[0].id
+
+        first.edit(AddRound(mapOf(player to 7)))
+
+        // Shown at once on the guest that made it, without waiting for the host.
+        assertEquals(7, first.state.value.sheet.total(player))
+        await("the host to apply it") { host.state.sheet.total(player) == 7 }
+        await("the other guest to see it") { second.state.value.sheet.total(player) == 7 }
+        assertEquals(1, host.state.sheet.rounds.size)
+    }
+
+    @Test
+    fun `the host's own change reaches the guests`() {
+        val host = startHost()
+        val guest = admittedGuest(host, "g1", "Gwen")
+        val player = host.state.sheet.players[1].id
+
+        host.session.edit(RenamePlayer(player, "Hosty"))
+
+        await("the guest to see the new name") {
+            guest.state.value.sheet.players[1].name == "Hosty"
+        }
+    }
+
+    @Test
+    fun `changes made at the same moment all land`() {
+        val host = startHost()
+        val first = admittedGuest(host, "g1", "Gwen")
+        val second = admittedGuest(host, "g2", "Gus")
+        val player = host.state.sheet.players[0].id
+
+        first.edit(AddRound(mapOf(player to 1)))
+        second.edit(AddRound(mapOf(player to 10)))
+        host.session.edit(AddRound(mapOf(player to 100)))
+
+        await("every round to land everywhere") {
+            listOf(host.state.sheet, first.state.value.sheet, second.state.value.sheet)
+                .all { it.rounds.size == 3 && it.total(player) == 111 }
+        }
+    }
+
+    @Test
+    fun `a guest asking to sync is sent the sheet as it stands`() {
+        val host = startHost()
+        val guest = admittedGuest(host, "g1", "Gwen")
+        val player = host.state.sheet.players[0].id
+        host.session.edit(AddRound(mapOf(player to 4)))
+
+        guest.sync()
+
+        await("the guest to be in step") { guest.state.value.sheet.total(player) == 4 }
+        assertEquals(host.state.sheet, guest.state.value.sheet)
+    }
+
+    @Test
+    fun `a guest that was let in is let back in without being asked again`() {
+        val host = startHost()
+        val first = admittedGuest(host, "g1", "Gwen")
+        // The phone is put down and its link goes with it.
+        first.stop()
+        await("the host to notice") { host.state.guests.singleOrNull()?.online == false }
+
+        val back = guestOf(host, "g1", "Gwen")
+
+        await("the guest to be back on the sheet") { back.state.value.connected }
+        assertTrue(host.state.requests.isEmpty())
+        assertEquals(1, host.state.guests.size)
+        assertTrue(host.state.guests.single().online)
+        assertEquals(host.state.sheet, back.state.value.sheet)
+    }
+
+    @Test
+    fun `a guest the host removes has to ask again`() {
+        val host = startHost()
+        val guest = admittedGuest(host, "g1", "Gwen")
+
+        host.session.remove("g1")
+
+        await("the removal to arrive") { guest.state.value.refused }
+        assertEquals("The host removed you.", guest.state.value.message)
+        assertTrue(host.state.guests.isEmpty())
+
+        val again = guestOf(host, "g1", "Gwen")
+        await("the new request") { host.state.requests.any { it.id == "g1" } }
+        assertFalse(again.state.value.connected)
+    }
+
+    @Test
+    fun `somebody who is not let in cannot change the sheet`() {
+        val host = startHost()
+        val (_, transport) = newSession("stranger")
+        val before = host.state.sheet
+        val player = before.players[0].id
+
+        runBlocking {
+            val raw = transport.join(joinOf(host), CoroutineScope(Dispatchers.IO).also { scopes += it })
+            val hello = Hello("stranger", "Stan", purpose = PURPOSE_SCOREKEEPER)
+            // Said more than once, as a real guest does: the host may not be
+            // listening yet the first time. Saying it twice is harmless.
+            repeat(3) {
+                raw.send(hello)
+                delay(150)
+            }
+            raw.send(SheetEdit(1, AddRound(mapOf(player to 99))))
+        }
+
+        await("the request to reach the host") { host.state.requests.any { it.id == "stranger" } }
+        Thread.sleep(300)
+        assertEquals(before, host.state.sheet)
+    }
+
+    @Test
+    fun `a phone looking for a game is told this is a score sheet`() {
+        val host = startHost()
+        val (_, transport) = newSession("gamer")
+        val answer = AtomicInteger(0)
+
+        runBlocking {
+            val scope = CoroutineScope(Dispatchers.IO).also { scopes += it }
+            val raw = transport.join(joinOf(host), scope)
+            val reply = scope.launch {
+                raw.incoming
+                    .onSubscription { raw.send(Hello("gamer", "Gail", purpose = PURPOSE_GAME)) }
+                    .collect { message ->
+                        if (message is Welcome && !message.accepted) {
+                            assertEquals("That is a score sheet, not a game.", message.reason)
+                            answer.set(1)
+                        }
+                    }
+            }
+            val hello = Hello("gamer", "Gail", purpose = PURPOSE_GAME)
+            val end = System.currentTimeMillis() + 5_000
+            while (answer.get() == 0 && System.currentTimeMillis() < end) {
+                delay(300)
+                if (answer.get() == 0) raw.send(hello)
+            }
+            reply.cancel()
+        }
+
+        assertEquals(1, answer.get())
+        assertTrue(host.state.requests.isEmpty())
+        assertEquals(Role.HOST, host.state.role)
+    }
+
+    @Test
+    fun `stopping hosting lets go of everybody`() {
+        val host = startHost()
+        val guest = admittedGuest(host, "g1", "Gwen")
+
+        host.session.stop()
+
+        assertEquals(Role.NONE, host.state.role)
+        assertTrue(host.state.guests.isEmpty())
+        await("the guest to notice") { !guest.state.value.connected }
+    }
+
+    private fun await(what: String, timeoutMillis: Long = 5_000, condition: () -> Boolean) {
+        val end = System.currentTimeMillis() + timeoutMillis
+        while (System.currentTimeMillis() < end) {
+            if (condition()) return
+            Thread.sleep(25)
+        }
+        fail("Timed out waiting for $what")
+    }
+}

app/src/test/java/org/prolibertate/games/score/SheetSyncTest.kt

@@ -0,0 +1,176 @@
+package org.prolibertate.games.score
+
+import kotlinx.serialization.encodeToString
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.prolibertate.games.net.Hello
+import org.prolibertate.games.net.NetMessage
+import org.prolibertate.games.net.PURPOSE_GAME
+import org.prolibertate.games.net.SheetEdit
+import org.prolibertate.games.net.SheetState
+import org.prolibertate.games.net.protocolJson
+
+class SheetOpTest {
+
+    private val table = ScoreSheet.of(3)
+    private val ids = table.players.map { it.id }
+
+    @Test
+    fun `every op does what the sheet method of the same name does`() {
+        assertEquals(table.withPlayerAdded(), AddPlayer.applyTo(table))
+        assertEquals(table.renamed(ids[1], "Bob"), RenamePlayer(ids[1], "Bob").applyTo(table))
+        assertEquals(table.withPlayerRemoved(ids[2]), RemovePlayer(ids[2]).applyTo(table))
+        assertEquals(table.moved(0, 2), MovePlayer(ids[0], 2).applyTo(table))
+        assertEquals(ScoreSheet.of(5), StartSheet(5).applyTo(table))
+        assertEquals(ScoreSheet(), ClearSheet.applyTo(table))
+
+        val scored = AddRound(mapOf(ids[0] to 4)).applyTo(table)
+        assertEquals(table.withRound(mapOf(ids[0] to 4)), scored)
+        assertEquals(4, EditRound(0, mapOf(ids[0] to 4)).applyTo(AddRound(mapOf(ids[0] to 1)).applyTo(table)).total(ids[0]))
+        assertTrue(DeleteRound(0).applyTo(scored).rounds.isEmpty())
+    }
+
+    @Test
+    fun `an op that no longer makes sense is a no-op`() {
+        val scored = AddRound(mapOf(ids[0] to 4)).applyTo(table)
+
+        assertEquals(scored, RenamePlayer(999, "Nobody").applyTo(scored))
+        assertEquals(scored, MovePlayer(999, 0).applyTo(scored))
+        assertEquals(scored, RemovePlayer(999).applyTo(scored))
+        assertEquals(scored, EditRound(7, mapOf(ids[0] to 1)).applyTo(scored))
+        assertEquals(scored, DeleteRound(7).applyTo(scored))
+    }
+
+    @Test
+    fun `moving a column goes by who it is, not where it was`() {
+        // Somebody else has already moved the column this guest was looking at.
+        val shuffled = table.moved(0, 2)
+        val moved = MovePlayer(ids[0], 0).applyTo(shuffled)
+        assertEquals(ids, moved.players.map { it.id })
+    }
+
+    @Test
+    fun `two rounds entered at once are two rounds`() {
+        val both = listOf(
+            AddRound(mapOf(ids[0] to 3)),
+            AddRound(mapOf(ids[1] to 5)),
+        ).fold(table) { sheet, op -> op.applyTo(sheet) }
+
+        assertEquals(2, both.rounds.size)
+        assertEquals(3, both.total(ids[0]))
+        assertEquals(5, both.total(ids[1]))
+    }
+
+    @Test
+    fun `every op survives the wire`() {
+        val ops = listOf(
+            StartSheet(4), ClearSheet, AddPlayer, RemovePlayer(2), RenamePlayer(1, "Ann"),
+            MovePlayer(1, 3), AddRound(mapOf(1 to 5, 2 to -3, 3 to 0)),
+            EditRound(0, mapOf(1 to 9)), DeleteRound(2),
+        )
+        ops.forEach { op ->
+            val message: NetMessage = SheetEdit(seq = 7, op = op)
+            val decoded = protocolJson.decodeFromString<NetMessage>(protocolJson.encodeToString(message))
+            assertEquals(message, decoded)
+        }
+    }
+
+    @Test
+    fun `a sheet survives the wire with its rounds`() {
+        val sheet = AddRound(mapOf(ids[0] to 3, ids[2] to -2)).applyTo(table)
+        val message: NetMessage = SheetState(revision = 4, sheet = sheet, acknowledged = 2)
+        assertEquals(message, protocolJson.decodeFromString<NetMessage>(protocolJson.encodeToString(message)))
+    }
+
+    @Test
+    fun `a hello from before there were score sheets is a hello for a game`() {
+        val old = """{"t":"hello","peerId":"p","displayName":"Ann","protocol":2}"""
+        val hello = protocolJson.decodeFromString<NetMessage>(old) as Hello
+        assertEquals(PURPOSE_GAME, hello.purpose)
+    }
+}
+
+class SheetReplicaTest {
+
+    private val base = ScoreSheet.of(2)
+    private val a = base.players[0].id
+    private val b = base.players[1].id
+
+    @Test
+    fun `a change shows at once`() {
+        val replica = SheetReplica()
+        replica.receive(base, revision = 1, acknowledged = 0)
+
+        replica.local(AddRound(mapOf(a to 5)))
+
+        assertEquals(5, replica.view.total(a))
+        assertEquals(1, replica.unconfirmed)
+    }
+
+    @Test
+    fun `a sheet that has not seen my change does not wipe it out`() {
+        val replica = SheetReplica()
+        replica.receive(base, 1, 0)
+        replica.local(AddRound(mapOf(a to 5)))
+
+        // The host pushes a rename it made before it had heard about the round.
+        replica.receive(base.renamed(b, "Bob"), 2, 0)
+
+        assertEquals("Bob", replica.view.players[1].name)
+        assertEquals(5, replica.view.total(a))
+    }
+
+    @Test
+    fun `a change the host has applied is not applied again`() {
+        val replica = SheetReplica()
+        replica.receive(base, 1, 0)
+        val seq = replica.local(AddRound(mapOf(a to 5)))
+
+        // The host's sheet now includes the round, and says so.
+        replica.receive(base.withRound(mapOf(a to 5)), 2, seq)
+
+        assertEquals(5, replica.view.total(a))
+        assertEquals(1, replica.view.rounds.size)
+        assertEquals(0, replica.unconfirmed)
+    }
+
+    @Test
+    fun `only the acknowledged changes are dropped`() {
+        val replica = SheetReplica()
+        replica.receive(base, 1, 0)
+        val first = replica.local(AddRound(mapOf(a to 1)))
+        replica.local(AddRound(mapOf(a to 10)))
+
+        replica.receive(base.withRound(mapOf(a to 1)), 2, first)
+
+        assertEquals(1, replica.unconfirmed)
+        assertEquals(11, replica.view.total(a))
+        assertEquals(2, replica.view.rounds.size)
+    }
+
+    @Test
+    fun `an older sheet that arrives late is ignored`() {
+        val replica = SheetReplica()
+        replica.receive(base.withRound(mapOf(a to 3)), 5, 0)
+        replica.receive(base, 4, 0)
+
+        assertEquals(3, replica.view.total(a))
+        assertEquals(5, replica.revision)
+    }
+
+    @Test
+    fun `a fresh link starts from nothing`() {
+        val replica = SheetReplica()
+        replica.receive(base, 3, 0)
+        replica.local(AddRound(mapOf(a to 5)))
+
+        replica.reset()
+
+        assertEquals(ScoreSheet(), replica.view)
+        assertEquals(0, replica.unconfirmed)
+        // And a sheet at an early revision is taken rather than thought stale.
+        replica.receive(base, 1, 0)
+        assertEquals(base, replica.view)
+    }
+}