Katan: expansions as toggles, scenario chosen in options

Replace the single CatanRuleSet with a set of expansions and add a
scenario field; saves with the old ruleSet still decode. Setup shows
a toggle per expansion and a map picker for Seafarers. Combining
expansions is rejected for now (lifted in a later phase).

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TWwLCrXa9aFVg8dZsCdLBZ

CLAUDE.md

@@ -51,8 +51,8 @@ integration:
 
 ## Katan
 
-The island game is catalogue id `catan`, shown as **Katan**. It has two rule
-sets chosen at setup: the base game (`docs/RULES-catan.md`), Cities & Knights
+The island game is catalogue id `catan`, shown as **Katan**. It has two expansions
+(toggles, `CatanOptions.expansions`; none on is the base game): the base game (`docs/RULES-catan.md`), Cities & Knights
 (`docs/RULES-catan-ck.md`, code in `game/catan/CatanCk*.kt`, state behind
 `CatanState.ck`, which is null in the base game) and Seafarers
 (`docs/RULES-catan-seafarers.md`, code in `game/catan/CatanSea*.kt`, state behind

app/src/main/java/org/prolibertate/games/game/catan/CatanModel.kt

@@ -1,7 +1,16 @@
 package org.prolibertate.games.game.catan
 
+import kotlinx.serialization.KSerializer
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonDecoder
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.jsonObject
 
 /** Sentinel stored in [CatanState.roads] for an unclaimed road position. */
 const val NO_PLAYER = -1
@@ -151,12 +160,9 @@ enum class CatanPhase {
     CHORE,
 }
 
-/** Which rules a table plays. The expansion is a whole rule set, not a toggle. */
+/** An expansion a table can switch on. The base game is what is left with none selected. */
 @Serializable
-enum class CatanRuleSet {
-    /** The base game: development cards, Largest Army, a robber from the start. */
-    BASE,
-
+enum class CatanExpansion {
     /** Cities & Knights: commodities, knights, barbarians, city improvements. */
     CITIES_AND_KNIGHTS,
 
@@ -164,15 +170,58 @@ enum class CatanRuleSet {
     SEAFARERS,
 }
 
-@Serializable
+/**
+ * What a table plays: the expansions switched on, and the Seafarers map when that one is.
+ *
+ * Saves written before expansions became toggles carry `"ruleSet"` instead of `"expansions"`;
+ * [CatanOptionsSerializer] reads either.
+ */
+@Serializable(with = CatanOptionsSerializer::class)
 data class CatanOptions(
     val playerCount: Int = 3,
     val pointsToWin: Int = 10,
-    val ruleSet: CatanRuleSet = CatanRuleSet.BASE,
+    val expansions: Set<CatanExpansion> = emptySet(),
+    val scenario: SeaScenario = SeaScenario.NEW_SHORES,
 ) {
     init {
         require(playerCount in 3..4) { "The island seats three or four players" }
         require(pointsToWin >= 2) { "pointsToWin must be at least 2" }
+        // Lifted once Cities & Knights runs on a Seafarers map.
+        require(expansions.size <= 1) { "Combining expansions is not supported yet" }
+    }
+
+    val seafarers: Boolean get() = CatanExpansion.SEAFARERS in expansions
+    val citiesAndKnights: Boolean get() = CatanExpansion.CITIES_AND_KNIGHTS in expansions
+}
+
+/** The wire shape of [CatanOptions]. */
+@Serializable
+private class CatanOptionsWire(
+    val playerCount: Int = 3,
+    val pointsToWin: Int = 10,
+    val expansions: Set<CatanExpansion> = emptySet(),
+    val scenario: SeaScenario = SeaScenario.NEW_SHORES,
+)
+
+/** Reads the legacy single `ruleSet` field as an expansion set. */
+object CatanOptionsSerializer : KSerializer<CatanOptions> {
+    override val descriptor: SerialDescriptor = CatanOptionsWire.serializer().descriptor
+
+    override fun serialize(encoder: Encoder, value: CatanOptions) = encoder.encodeSerializableValue(
+        CatanOptionsWire.serializer(),
+        CatanOptionsWire(value.playerCount, value.pointsToWin, value.expansions, value.scenario),
+    )
+
+    override fun deserialize(decoder: Decoder): CatanOptions {
+        val input = decoder as JsonDecoder
+        var obj = input.decodeJsonElement().jsonObject
+        val legacy = (obj["ruleSet"] as? JsonPrimitive)?.content
+        if (legacy != null) {
+            val expansions = if (legacy == "BASE") emptyList() else listOf(JsonPrimitive(legacy))
+            obj = JsonObject(obj - "ruleSet" + ("expansions" to JsonArray(expansions)))
+        }
+        val wire = input.json.decodeFromJsonElement(CatanOptionsWire.serializer(), obj)
+        return CatanOptions(wire.playerCount, wire.pointsToWin, wire.expansions, wire.scenario)
     }
 }
 

app/src/main/java/org/prolibertate/games/game/catan/CatanRules.kt

@@ -54,15 +54,15 @@ object CatanRules : GameRules<CatanState, CatanMove> {
             "Seat count ${config.seats.size} does not match ${options.playerCount} players"
         }
 
-        val sea = options.ruleSet == CatanRuleSet.SEAFARERS
-        val scenario = SeaScenario.NEW_SHORES
+        val sea = options.seafarers
+        val scenario = options.scenario
         val seaBoard = if (sea) SeaBoards.generate(scenario, config.seed) else null
         val topo = if (seaBoard != null) SeaBoards.topology(scenario) else CatanBoard.base
         val board = seaBoard?.let { CatanBoard.Generated(it.terrains, it.numbers, it.robber, it.ports) }
             ?: CatanBoard.generate(config.seed)
         // A different random stream than the island, so the same seed still
         // produces both a fixed island and a fixed deck.
-        val ck = options.ruleSet == CatanRuleSet.CITIES_AND_KNIGHTS
+        val ck = options.citiesAndKnights
         // Cities & Knights has no development cards: knights and progress
         // cards take their place.
         val devDeck = if (ck) {

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

@@ -59,7 +59,8 @@ import org.prolibertate.games.game.morris.MorrisOptions
 import org.prolibertate.games.game.pirates.PiratesOptions
 import org.prolibertate.games.game.president.PresidentOptions
 import org.prolibertate.games.game.catan.CatanOptions
-import org.prolibertate.games.game.catan.CatanRuleSet
+import org.prolibertate.games.game.catan.CatanExpansion
+import org.prolibertate.games.game.catan.SeaScenario
 import org.prolibertate.games.game.sequence.SequenceOptions
 import org.prolibertate.games.game.tayu.TayuLevel
 import org.prolibertate.games.game.tayu.TayuOptions
@@ -373,47 +374,48 @@ fun teamForSeat(gameId: String, seat: Int, seatCount: Int): Int = when (gameId)
     else -> seat
 }
 
-/** The targets the setup screen offers for a Katan rule set. */
-internal fun catanPointsChoices(ruleSet: CatanRuleSet): List<Int> = when (ruleSet) {
-    // Five is a short game for trying things out; ten is the published target.
-    CatanRuleSet.BASE -> listOf(5, 8, 10, 12)
-    // Cities & Knights is played to thirteen, which is also why it runs longer.
-    CatanRuleSet.CITIES_AND_KNIGHTS -> listOf(8, 10, 13, 15)
+/** The targets the setup screen offers for a Katan game with [expansions] on. */
+internal fun catanPointsChoices(expansions: Set<CatanExpansion>): List<Int> = when {
     // Seafarers adds two points for each new island, so the target is higher.
-    CatanRuleSet.SEAFARERS -> listOf(8, 10, 12, 14)
+    CatanExpansion.SEAFARERS in expansions -> listOf(8, 10, 12, 14)
+    // Cities & Knights is played to thirteen, which is also why it runs longer.
+    CatanExpansion.CITIES_AND_KNIGHTS in expansions -> listOf(8, 10, 13, 15)
+    // Five is a short game for trying things out; ten is the published target.
+    else -> listOf(5, 8, 10, 12)
 }
 
-/** The published target for a rule set. */
-internal fun catanDefaultPoints(ruleSet: CatanRuleSet): Int = when (ruleSet) {
-    CatanRuleSet.BASE -> 10
-    CatanRuleSet.CITIES_AND_KNIGHTS -> 13
-    CatanRuleSet.SEAFARERS -> 12
+/** The published target for a game with [expansions] on. */
+internal fun catanDefaultPoints(expansions: Set<CatanExpansion>): Int = when {
+    CatanExpansion.SEAFARERS in expansions -> 12
+    CatanExpansion.CITIES_AND_KNIGHTS in expansions -> 13
+    else -> 10
 }
 
 /**
- * [options] switched to [ruleSet]. The target moves to the new rule set's own
- * default unless the one already chosen is on its menu too.
+ * [options] with [expansion] switched on or off. The target moves to the new combination's own
+ * default unless the one already chosen is on its menu too. Expansions cannot be combined yet,
+ * so switching one on switches the other off.
  */
-internal fun catanSwitchRules(options: CatanOptions, ruleSet: CatanRuleSet): CatanOptions =
-    options.copy(
-        ruleSet = ruleSet,
-        pointsToWin = options.pointsToWin.takeIf { it in catanPointsChoices(ruleSet) && it != catanDefaultPoints(options.ruleSet) }
-            ?: catanDefaultPoints(ruleSet),
+internal fun catanToggleExpansion(options: CatanOptions, expansion: CatanExpansion, on: Boolean): CatanOptions {
+    val expansions = if (on) setOf(expansion) else options.expansions - expansion
+    return options.copy(
+        expansions = expansions,
+        pointsToWin = options.pointsToWin.takeIf {
+            it in catanPointsChoices(expansions) && it != catanDefaultPoints(options.expansions)
+        } ?: catanDefaultPoints(expansions),
     )
-
-private fun catanRuleSetName(ruleSet: CatanRuleSet): String = when (ruleSet) {
-    CatanRuleSet.BASE -> "Base game"
-    CatanRuleSet.CITIES_AND_KNIGHTS -> "Cities & Knights"
-    CatanRuleSet.SEAFARERS -> "Seafarers"
 }
 
-private fun catanRuleSetBlurb(ruleSet: CatanRuleSet): String = when (ruleSet) {
-    CatanRuleSet.BASE ->
-        "Settle the island, trade, and play development cards. Longest Road and Largest Army score."
-    CatanRuleSet.CITIES_AND_KNIGHTS ->
+private fun catanExpansionName(expansion: CatanExpansion): String = when (expansion) {
+    CatanExpansion.CITIES_AND_KNIGHTS -> "Cities & Knights"
+    CatanExpansion.SEAFARERS -> "Seafarers"
+}
+
+private fun catanExpansionBlurb(expansion: CatanExpansion): String = when (expansion) {
+    CatanExpansion.CITIES_AND_KNIGHTS ->
         "Cities make commodities, knights hold off the barbarians, and improvements earn progress " +
             "cards and metropolises. No development cards; the robber sleeps until the first attack."
-    CatanRuleSet.SEAFARERS ->
+    CatanExpansion.SEAFARERS ->
         "Build ships to reach new islands. Gold fields pay any resource, a pirate guards the sea, " +
             "and the first settlement on each new island scores two extra points."
 }
@@ -421,14 +423,24 @@ private fun catanRuleSetBlurb(ruleSet: CatanRuleSet): String = when (ruleSet) {
 @Composable
 private fun CatanOptionsEditor(options: CatanOptions, onChange: (CatanOptions) -> Unit) {
     Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
-        ChipRow(
-            label = "Rules",
-            values = CatanRuleSet.entries,
-            selected = options.ruleSet,
-            display = ::catanRuleSetName,
-            onSelect = { onChange(catanSwitchRules(options, it)) },
-        )
-        Text(catanRuleSetBlurb(options.ruleSet), style = MaterialTheme.typography.bodySmall)
+        CatanExpansion.entries.forEach { expansion ->
+            ToggleRow(
+                title = catanExpansionName(expansion),
+                subtitle = catanExpansionBlurb(expansion),
+                checked = expansion in options.expansions,
+                onChange = { onChange(catanToggleExpansion(options, expansion, it)) },
+            )
+        }
+        if (options.seafarers) {
+            ChipRow(
+                label = "Map",
+                values = SeaScenario.entries,
+                selected = options.scenario,
+                display = { it.title },
+                onSelect = { onChange(options.copy(scenario = it)) },
+            )
+            Text(options.scenario.blurb, style = MaterialTheme.typography.bodySmall)
+        }
         ChipRow(
             label = "Players",
             values = listOf(3, 4),
@@ -438,7 +450,7 @@ private fun CatanOptionsEditor(options: CatanOptions, onChange: (CatanOptions) -
         )
         ChipRow(
             label = "Game to",
-            values = catanPointsChoices(options.ruleSet),
+            values = catanPointsChoices(options.expansions),
             selected = options.pointsToWin,
             display = { "$it points" },
             onSelect = { onChange(options.copy(pointsToWin = it)) },

app/src/test/java/org/prolibertate/games/game/catan/CatanAiProfilesTest.kt

@@ -55,15 +55,15 @@ class CatanAiProfilesTest {
             (0 until 3).any { p.of(it).style == style }
         }
 
-    private fun forEveryProfile(rules: CatanRuleSet, cap: Int) {
+    private fun forEveryProfile(rules: Set<CatanExpansion>, cap: Int) {
         val options = CatanOptions(
             playerCount = 3,
-            pointsToWin = when (rules) {
-                CatanRuleSet.BASE -> 10
-                CatanRuleSet.CITIES_AND_KNIGHTS -> 13
-                else -> 12
+            pointsToWin = when {
+                CatanExpansion.CITIES_AND_KNIGHTS in rules -> 13
+                CatanExpansion.SEAFARERS in rules -> 12
+                else -> 10
             },
-            ruleSet = rules,
+            expansions = rules,
         )
         for (difficulty in listOf(AiDifficulty.EASY, AiDifficulty.HARD)) {
             for (style in AiStyle.entries) {
@@ -78,14 +78,14 @@ class CatanAiProfilesTest {
     }
 
     @Test
-    fun `easy and hard seats of every style play the base game`() = forEveryProfile(CatanRuleSet.BASE, 1_500)
+    fun `easy and hard seats of every style play the base game`() = forEveryProfile(emptySet(), 1_500)
 
     @Test
     fun `easy and hard seats of every style play Cities and Knights`() =
-        forEveryProfile(CatanRuleSet.CITIES_AND_KNIGHTS, 1_500)
+        forEveryProfile(setOf(CatanExpansion.CITIES_AND_KNIGHTS), 1_500)
 
     @Test
-    fun `easy and hard seats of every style play Seafarers`() = forEveryProfile(CatanRuleSet.SEAFARERS, 1_500)
+    fun `easy and hard seats of every style play Seafarers`() = forEveryProfile(setOf(CatanExpansion.SEAFARERS), 1_500)
 
     @Test
     fun `a mixed table of four finishes a base game`() {
@@ -95,8 +95,8 @@ class CatanAiProfilesTest {
 
     @Test
     fun `the default profile plays exactly as an explicit medium table does`() {
-        for (rules in listOf(CatanRuleSet.BASE, CatanRuleSet.CITIES_AND_KNIGHTS)) {
-            val options = CatanOptions(playerCount = 3, pointsToWin = 10, ruleSet = rules)
+        for (rules in listOf(emptySet(), setOf(CatanExpansion.CITIES_AND_KNIGHTS))) {
+            val options = CatanOptions(playerCount = 3, pointsToWin = 10, expansions = rules)
             val plain = play(options, 9L, AiProfiles.DEFAULT, 600)
             val medium = play(options, 9L, AiProfiles(AiDifficulty.MEDIUM, 1234L), 600)
             assertEquals("$rules", plain, medium)

app/src/test/java/org/prolibertate/games/game/catan/CatanCkRulesTest.kt

@@ -25,7 +25,7 @@ class CatanCkRulesTest {
             CatanOptions(
                 playerCount = players,
                 pointsToWin = 13,
-                ruleSet = if (ck) CatanRuleSet.CITIES_AND_KNIGHTS else CatanRuleSet.BASE,
+                expansions = if (ck) setOf(CatanExpansion.CITIES_AND_KNIGHTS) else emptySet(),
             ),
         ),
         seed = seed,
@@ -138,7 +138,7 @@ class CatanCkRulesTest {
         val base = CatanRules.initialState(config(ck = false))
         assertNull(base.ck)
         assertEquals(25, base.devDeck.size)
-        assertEquals(CatanRuleSet.BASE, base.options.ruleSet)
+        assertEquals(emptySet<CatanExpansion>(), base.options.expansions)
     }
 
     @Test
@@ -157,7 +157,21 @@ class CatanCkRulesTest {
     fun `the options without a rule set still read as the base game`() {
         val old = """{"playerCount":4,"pointsToWin":10}"""
         val options = Json { ignoreUnknownKeys = true }.decodeFromString<CatanOptions>(old)
-        assertEquals(CatanRuleSet.BASE, options.ruleSet)
+        assertEquals(emptySet<CatanExpansion>(), options.expansions)
+    }
+
+    @Test
+    fun `options saved with a rule set still read as the expansion they named`() {
+        val read = { text: String -> Json { ignoreUnknownKeys = true }.decodeFromString<CatanOptions>(text) }
+        assertEquals(
+            setOf(CatanExpansion.CITIES_AND_KNIGHTS),
+            read("""{"playerCount":3,"pointsToWin":13,"ruleSet":"CITIES_AND_KNIGHTS"}""").expansions,
+        )
+        assertEquals(
+            setOf(CatanExpansion.SEAFARERS),
+            read("""{"playerCount":3,"pointsToWin":12,"ruleSet":"SEAFARERS"}""").expansions,
+        )
+        assertEquals(emptySet<CatanExpansion>(), read("""{"ruleSet":"BASE"}""").expansions)
     }
 
     @Test

app/src/test/java/org/prolibertate/games/game/catan/CatanCkSelfPlayTest.kt

@@ -152,7 +152,7 @@ class CatanCkSelfPlayTest {
     fun `three engines play Cities and Knights to a finish`() {
         val reports = (1L..12L).map { seed ->
             val (state, report) = playOut(
-                CatanOptions(playerCount = 3, pointsToWin = 13, ruleSet = CatanRuleSet.CITIES_AND_KNIGHTS),
+                CatanOptions(playerCount = 3, pointsToWin = 13, expansions = setOf(CatanExpansion.CITIES_AND_KNIGHTS)),
                 seed,
             )
             assertEquals("seed $seed", CatanPhase.GAME_OVER, state.phase)
@@ -171,7 +171,7 @@ class CatanCkSelfPlayTest {
     fun `four engines play Cities and Knights to a finish`() {
         val reports = (101L..108L).map { seed ->
             val (state, report) = playOut(
-                CatanOptions(playerCount = 4, pointsToWin = 13, ruleSet = CatanRuleSet.CITIES_AND_KNIGHTS),
+                CatanOptions(playerCount = 4, pointsToWin = 13, expansions = setOf(CatanExpansion.CITIES_AND_KNIGHTS)),
                 seed,
             )
             assertEquals("seed $seed", CatanPhase.GAME_OVER, state.phase)

app/src/test/java/org/prolibertate/games/game/catan/CatanSeafarersTest.kt

@@ -17,7 +17,7 @@ class CatanSeafarersTest {
     private val json = Json { encodeDefaults = true }
 
     private fun options(players: Int = 3) =
-        CatanOptions(playerCount = players, pointsToWin = 12, ruleSet = CatanRuleSet.SEAFARERS)
+        CatanOptions(playerCount = players, pointsToWin = 12, expansions = setOf(CatanExpansion.SEAFARERS))
 
     private fun config(options: CatanOptions, seed: Long) = TableConfig(
         gameId = GameCatalog.CATAN,

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

@@ -171,7 +171,7 @@ class SaveRoundTripAllGamesTest {
         GameCatalog.CATAN,
         CatanRules,
         CatanAi(),
-        opts(CatanOptions(3, 13, org.prolibertate.games.game.catan.CatanRuleSet.CITIES_AND_KNIGHTS)),
+        opts(CatanOptions(3, 13, setOf(org.prolibertate.games.game.catan.CatanExpansion.CITIES_AND_KNIGHTS))),
         steps = 600,
     )
 }

app/src/test/java/org/prolibertate/games/ui/game/catan/CatanCkInteractionTest.kt

@@ -21,7 +21,7 @@ import org.prolibertate.games.game.catan.CatanBoard
 import org.prolibertate.games.game.catan.CatanMove
 import org.prolibertate.games.game.catan.CatanOptions
 import org.prolibertate.games.game.catan.CatanPhase
-import org.prolibertate.games.game.catan.CatanRuleSet
+import org.prolibertate.games.game.catan.CatanExpansion
 import org.prolibertate.games.game.catan.CatanRules
 import org.prolibertate.games.game.catan.CatanState
 import org.prolibertate.games.game.catan.ChaseRobber
@@ -69,7 +69,7 @@ class CatanCkInteractionTest {
         gameId = GameCatalog.CATAN,
         seats = (0 until players).map { PlayerSlot(seat = it, name = "P$it", kind = PlayerKind.AI, team = it) },
         optionsJson = json.encodeToString(
-            CatanOptions(players, pointsToWin = 13, ruleSet = CatanRuleSet.CITIES_AND_KNIGHTS),
+            CatanOptions(players, pointsToWin = 13, expansions = setOf(CatanExpansion.CITIES_AND_KNIGHTS)),
         ),
         seed = seed,
     )

app/src/test/java/org/prolibertate/games/ui/game/catan/CatanCkTextTest.kt

@@ -8,7 +8,7 @@ import org.junit.Test
 import org.prolibertate.games.game.GameCatalog
 import org.prolibertate.games.game.catan.CatanAi
 import org.prolibertate.games.game.catan.CatanOptions
-import org.prolibertate.games.game.catan.CatanRuleSet
+import org.prolibertate.games.game.catan.CatanExpansion
 import org.prolibertate.games.game.catan.CatanRules
 import org.prolibertate.games.game.catan.ProgressType
 import org.prolibertate.games.game.engine.PlayerKind
@@ -16,7 +16,7 @@ import org.prolibertate.games.game.engine.PlayerSlot
 import org.prolibertate.games.game.engine.TableConfig
 import org.prolibertate.games.ui.catanDefaultPoints
 import org.prolibertate.games.ui.catanPointsChoices
-import org.prolibertate.games.ui.catanSwitchRules
+import org.prolibertate.games.ui.catanToggleExpansion
 
 class CatanCkTextTest {
 
@@ -28,7 +28,7 @@ class CatanCkTextTest {
             gameId = GameCatalog.CATAN,
             seats = (0 until 3).map { PlayerSlot(seat = it, name = "P$it", kind = PlayerKind.AI, team = it) },
             optionsJson = json.encodeToString(
-                CatanOptions(3, 13, CatanRuleSet.CITIES_AND_KNIGHTS),
+                CatanOptions(3, 13, setOf(CatanExpansion.CITIES_AND_KNIGHTS)),
             ),
             seed = 31L,
         )
@@ -56,20 +56,24 @@ class CatanCkTextTest {
 
     @Test
     fun `the setup screen offers each rule set its own targets and switches sensibly`() {
-        assertEquals(listOf(5, 8, 10, 12), catanPointsChoices(CatanRuleSet.BASE))
-        assertEquals(listOf(8, 10, 13, 15), catanPointsChoices(CatanRuleSet.CITIES_AND_KNIGHTS))
-        assertEquals(10, catanDefaultPoints(CatanRuleSet.BASE))
-        assertEquals(13, catanDefaultPoints(CatanRuleSet.CITIES_AND_KNIGHTS))
+        assertEquals(listOf(5, 8, 10, 12), catanPointsChoices(emptySet()))
+        assertEquals(listOf(8, 10, 13, 15), catanPointsChoices(setOf(CatanExpansion.CITIES_AND_KNIGHTS)))
+        assertEquals(10, catanDefaultPoints(emptySet()))
+        assertEquals(13, catanDefaultPoints(setOf(CatanExpansion.CITIES_AND_KNIGHTS)))
 
         val base = CatanOptions()
-        val ck = catanSwitchRules(base, CatanRuleSet.CITIES_AND_KNIGHTS)
-        assertEquals(CatanRuleSet.CITIES_AND_KNIGHTS, ck.ruleSet)
+        val ck = catanToggleExpansion(base, CatanExpansion.CITIES_AND_KNIGHTS, true)
+        assertEquals(setOf(CatanExpansion.CITIES_AND_KNIGHTS), ck.expansions)
         assertEquals("the published target for the new rules", 13, ck.pointsToWin)
-        assertEquals(10, catanSwitchRules(ck, CatanRuleSet.BASE).pointsToWin)
+        assertEquals(10, catanToggleExpansion(ck, CatanExpansion.CITIES_AND_KNIGHTS, false).pointsToWin)
 
         // A short game picked on purpose is kept when both menus have it.
-        val short = catanSwitchRules(base.copy(pointsToWin = 8), CatanRuleSet.CITIES_AND_KNIGHTS)
+        val short = catanToggleExpansion(base.copy(pointsToWin = 8), CatanExpansion.CITIES_AND_KNIGHTS, true)
         assertEquals(8, short.pointsToWin)
         assertEquals(3, ck.playerCount)
+
+        // Expansions cannot be combined yet: switching one on switches the other off.
+        val sea = catanToggleExpansion(ck, CatanExpansion.SEAFARERS, true)
+        assertEquals(setOf(CatanExpansion.SEAFARERS), sea.expansions)
     }
 }

app/src/test/java/org/prolibertate/games/ui/game/catan/CatanSeafarersInteractionTest.kt

@@ -12,7 +12,7 @@ import org.prolibertate.games.game.catan.CatanAi
 import org.prolibertate.games.game.catan.CatanBuilding
 import org.prolibertate.games.game.catan.CatanOptions
 import org.prolibertate.games.game.catan.CatanPhase
-import org.prolibertate.games.game.catan.CatanRuleSet
+import org.prolibertate.games.game.catan.CatanExpansion
 import org.prolibertate.games.game.catan.CatanRules
 import org.prolibertate.games.game.catan.CatanState
 import org.prolibertate.games.game.catan.MovePirate
@@ -29,7 +29,7 @@ import org.prolibertate.games.game.engine.TableConfig
 class CatanSeafarersInteractionTest {
 
     private fun fresh(): CatanState {
-        val options = CatanOptions(playerCount = 3, pointsToWin = 12, ruleSet = CatanRuleSet.SEAFARERS)
+        val options = CatanOptions(playerCount = 3, pointsToWin = 12, expansions = setOf(CatanExpansion.SEAFARERS))
         return CatanRules.initialState(
             TableConfig(
                 gameId = GameCatalog.CATAN,

docs/RULES-catan-seafarers.md

@@ -1,10 +1,10 @@
 # Katan: Seafarers
 
-The Seafarers rule set is chosen at setup next to *Base game* and *Cities & Knights*. It is the base
+Seafarers is an expansion switched on at setup (`CatanOptions.expansions`; the old single `ruleSet` in saves is still read) and its map is chosen beside it (`CatanOptions.scenario`). It is the base
 game ([RULES-catan.md](RULES-catan.md)) with a sea, ships, a pirate, gold fields and islands to
 settle. Code: `game/catan/CatanSeaRules.kt` (the additions), `CatanSeaBoard.kt` (the map),
 `CatanBoard.kt` (`Topology`, the geometry shared by every map), state behind `CatanState.sea`
-(null in the other rule sets). It cannot be combined with Cities & Knights.
+(null in the other rule sets). It cannot yet be combined with Cities & Knights; the setup screen switches one off when the other is turned on.
 
 Where the published rules leave room, or where this game differs, it says so below.