Merge remote-tracking branch 'origin/dev' into CatanImprovements

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

@@ -106,6 +106,7 @@ object GameCatalog {
     const val TAYU = "tayu"
     const val MORRIS = "morris"
     const val PIRATES = "pirates"
+    const val MEOWDOKU = "meowdoku"
 
     /**
      * The island game the collection is named for. Listed under Board Games
@@ -368,6 +369,18 @@ object GameCatalog {
             available = false,
             devPreview = true,
         ),
+        GameDescriptor(
+            id = MEOWDOKU,
+            title = "Meowdoku",
+            menus = setOf(GameMenu.BOARD, GameMenu.SOLITAIRE),
+            minPlayers = 1,
+            maxPlayers = 1,
+            teamBased = false,
+            blurb = "One cat in every row, column and colour, and no two touching. " +
+                "Wrong taps cost a heart; the board only ever accepts a cat that belongs.",
+            available = false,
+            devPreview = true,
+        ),
     )
 
     fun byId(id: String): GameDescriptor? = all.firstOrNull { it.id == id }

app/src/main/java/org/prolibertate/games/game/meowdoku/MeowdokuAi.kt

@@ -0,0 +1,46 @@
+package org.prolibertate.games.game.meowdoku
+
+import org.prolibertate.games.game.engine.GameAi
+
+/**
+ * Plays the board for the one seat.
+ *
+ * Like peg solitaire's AI, nothing normally asks it to: the single seat belongs
+ * to the person holding the phone. It exists because an empty seat has to be
+ * playable by something, and because "the computer can finish this board" is a
+ * property worth testing — a generated puzzle that only a human could unpack is
+ * a broken promise about uniqueness.
+ *
+ * It reads the answer from the state. The rules hold it, the seat is alone with
+ * it, and a solver that deduced its way there would be the same length as the
+ * hint engine in the original game without making this seat any more honest.
+ */
+object MeowdokuAi : GameAi<MeowdokuState, MeowdokuMove> {
+
+    override fun chooseMove(
+        state: MeowdokuState,
+        seat: Int,
+        legal: List<MeowdokuMove>,
+    ): MeowdokuMove {
+        require(legal.isNotEmpty()) { "No move available" }
+        val places = legal.filterIsInstance<PlaceCat>().associateBy { it.square }
+
+        // The first cat still missing. A pencil cross in front of it is the
+        // player's own note being wrong, so erase it; a hint spends one token
+        // and reveals the square regardless.
+        for (row in 0 until state.side) {
+            val missing = state.cats.none { it.row == row }
+            if (!missing) continue
+            val square = Square(row, state.catCols[row])
+            places[square]?.let { return it }
+            if (square in state.marks) {
+                if (TakeHint in legal && state.hintsLeft > 0) return TakeHint
+                legal.filterIsInstance<TogglePencil>()
+                    .firstOrNull { it.square == square }
+                    ?.let { return it }
+            }
+        }
+
+        return legal.first()
+    }
+}

app/src/main/java/org/prolibertate/games/game/meowdoku/MeowdokuGenerator.kt

@@ -0,0 +1,371 @@
+package org.prolibertate.games.game.meowdoku
+
+import kotlin.math.abs
+
+/**
+ * Builds the puzzles.
+ *
+ * A Meowdoku puzzle is an N by N board, a colouring of it into N regions, and a
+ * hidden answer of N cats. The three are not chosen separately: the cats come
+ * first, then the regions grow out from them so every region owns exactly one
+ * cat, then the finished board is checked to make sure it has only that one
+ * answer. A puzzle with two answers is a puzzle with no answer, since the second
+ * one cannot be deduced from what is printed.
+ *
+ * The construction is seeded and reproducible: the same seed gives the same
+ * board, which is what lets a saved game come back unchanged and lets a test
+ * speak about "the" puzzle for a seed.
+ */
+object MeowdokuGenerator {
+
+    /** Smallest and largest boards worth dealing; anything bigger is a wall. */
+    val MIN_SIDE = 4
+    val MAX_SIDE = 10
+
+    /**
+     * The answer, as the column of the cat in each row.
+     *
+     * One cat per row and one per column is already built in, since the rows are
+     * enumerated once each and no column is taken twice.
+     */
+    fun placeCats(side: Int, rng: SeededRng): IntArray {
+        val cols = IntArray(side) { -1 }
+        val placed = placeCatsFrom(0, side, cols, rng)
+        // Every board from four up has a placement, and the search is complete,
+        // so failing here would mean the generator itself is broken.
+        check(placed) { "No way to settle $side cats on a $side board" }
+        return cols
+    }
+
+    private fun placeCatsFrom(row: Int, side: Int, cols: IntArray, rng: SeededRng): Boolean {
+        if (row == side) return true
+        for (col in rng.shuffled(cols.indices.toList())) {
+            if (catFitsHere(row, col, cols)) {
+                cols[row] = col
+                if (placeCatsFrom(row + 1, side, cols, rng)) return true
+                cols[row] = -1
+            }
+        }
+        return false
+    }
+
+    /**
+     * Whether a cat at [row],[col] is at peace with the cats in the rows above.
+     *
+     * No shared column, and no shared corner: a cat grooms the squares touching
+     * its own on all eight sides, so a neighbour one step away in any direction
+     * is too close.
+     */
+    fun catFitsHere(row: Int, col: Int, cols: IntArray): Boolean {
+        for (r in 0 until row) {
+            val c = cols[r]
+            if (c == col) return false
+            if (abs(r - row) <= 1 && abs(c - col) <= 1) return false
+        }
+        return true
+    }
+
+    /**
+     * The colours of the board, as a region id per square.
+     *
+     * Each region starts as the cat it contains and grows outwards one square at
+     * a time. The first [strips] regions are grown as straight bars along their
+     * cat's row or column, alternating which way they run: a board of nothing but
+     * blobs reads as texture, and the bars are what give the regions a shape a
+     * player can reason about. The rest take the shortest border square, which
+     * keeps them clumped rather than trailing off into the far corner.
+     */
+    fun growRegions(side: Int, catCols: IntArray, rng: SeededRng, strips: Int): Array<IntArray> {
+        val regions = Array(side) { IntArray(side) { -1 } }
+        val dirs = listOf(0 to 1, 0 to -1, 1 to 0, -1 to 0)
+
+        // Region i owns the cat in row i, which is where its colour starts.
+        for (r in 0 until side) regions[r][catCols[r]] = r
+
+        val sizes = IntArray(side) { 1 }
+        var unassigned = side * side - side
+
+        // The straight bars are laid first, and the rest is grown around them.
+        // The straight bars are laid first, and the rest is grown around them.
+        // Bar i runs along its cat's row when i is even, down its column when odd.
+        val stripRegions = rng.shuffled((0 until side).toList()).take(strips.coerceIn(0, side - 1)).toSet()
+        for ((n, region) in stripRegions.withIndex()) {
+            val r0 = region
+            val c0 = catCols[r0]
+            val extra = 1 + rng.nextInt(3)
+            var added = 0
+            if (n % 2 == 0) {
+                var left = c0 - 1
+                var right = c0 + 1
+                while (added < extra) {
+                    val goLeft = rng.nextBoolean()
+                    when {
+                        goLeft && left >= 0 && regions[r0][left] == -1 ->
+                            regions[r0][left--] = region
+                        right < side && regions[r0][right] == -1 ->
+                            regions[r0][right++] = region
+                        left >= 0 && regions[r0][left] == -1 ->
+                            regions[r0][left--] = region
+                        else -> break
+                    }
+                    sizes[region]++
+                    unassigned--
+                    added++
+                }
+            } else {
+                var up = r0 - 1
+                var down = r0 + 1
+                while (added < extra) {
+                    val goUp = rng.nextBoolean()
+                    when {
+                        goUp && up >= 0 && regions[up][c0] == -1 ->
+                            regions[up--][c0] = region
+                        down < side && regions[down][c0] == -1 ->
+                            regions[down++][c0] = region
+                        up >= 0 && regions[up][c0] == -1 ->
+                            regions[up--][c0] = region
+                        else -> break
+                    }
+                    sizes[region]++
+                    unassigned--
+                    added++
+                }
+            }
+        }
+
+        // What is left goes to whichever neighbouring region is smallest, so no
+        // region dies young while another sprawls.
+        var guard = side * side * 20
+        while (unassigned > 0 && guard > 0) {
+            guard--
+            for (r in 0 until side) {
+                for (c in 0 until side) {
+                    if (regions[r][c] != -1) continue
+                    var best = -1
+                    for ((dr, dc) in dirs) {
+                        val nr = r + dr
+                        val nc = c + dc
+                        if (nr !in 0 until side || nc !in 0 until side) continue
+                        val neighbour = regions[nr][nc]
+                        if (neighbour == -1 || neighbour in stripRegions) continue
+                        if (best == -1 || sizes[neighbour] < sizes[best]) best = neighbour
+                    }
+                    if (best == -1) {
+                        for ((dr, dc) in dirs) {
+                            val nr = r + dr
+                            val nc = c + dc
+                            if (nr !in 0 until side || nc !in 0 until side) continue
+                            val neighbour = regions[nr][nc]
+                            if (neighbour != -1 && (best == -1 || sizes[neighbour] < sizes[best])) {
+                                best = neighbour
+                            }
+                        }
+                    }
+                    if (best != -1) {
+                        regions[r][c] = best
+                        sizes[best]++
+                        unassigned--
+                    }
+                }
+            }
+        }
+        for (r in 0 until side) {
+            for (c in 0 until side) {
+                if (regions[r][c] == -1) regions[r][c] = 0
+            }
+        }
+        return regions
+    }
+
+    /**
+     * How many answers this coloured board has, up to [limit].
+     *
+     * A depth-first search with the obvious propagation: once a row is given a
+     * cat, the squares that cat excludes — its column, the rows beside it, and
+     * everything in its colour — lose their candidates. A row whose candidate
+     * list runs out fails immediately and the search backtracks, which is what
+     * makes the count finish quickly on boards of this size.
+     *
+     * Nothing about the true cats is handed to the search; it sees the colours
+     * and the rules and no more.
+     */
+    fun countSolutions(side: Int, regions: Array<IntArray>, limit: Int = 2): Int {
+        val candidates = Array(side) { r -> (0 until side).toMutableSet() }
+        val colsTaken = BooleanArray(side)
+        val coloursTaken = BooleanArray(side)
+        val count = IntArray(1)
+        solve(0, side, regions, candidates, colsTaken, coloursTaken, count, limit)
+        return count[0]
+    }
+
+    private fun solve(
+        row: Int,
+        side: Int,
+        regions: Array<IntArray>,
+        candidates: Array<MutableSet<Int>>,
+        colsTaken: BooleanArray,
+        coloursTaken: BooleanArray,
+        count: IntArray,
+        limit: Int,
+    ) {
+        if (count[0] >= limit) return
+        if (row == side) {
+            count[0]++
+            return
+        }
+        for (col in candidates[row].toList()) {
+            val colour = regions[row][col]
+            if (colsTaken[col] || coloursTaken[colour]) continue
+            val undo = exclude(row, col, side, regions, candidates)
+            if (undo != null) {
+                colsTaken[col] = true
+                coloursTaken[colour] = true
+                solve(row + 1, side, regions, candidates, colsTaken, coloursTaken, count, limit)
+                colsTaken[col] = false
+                coloursTaken[colour] = false
+                undo()
+            }
+            if (count[0] >= limit) return
+        }
+    }
+
+    /**
+     * Crosses out what the cat at [row],[col] forbids, and returns the function
+     * that puts it back. Returns null if some row loses its last candidate,
+     * which means this placement is a dead end.
+     */
+    private fun exclude(
+        row: Int,
+        col: Int,
+        side: Int,
+        regions: Array<IntArray>,
+        candidates: Array<MutableSet<Int>>,
+    ): (() -> Unit)? {
+        val colour = regions[row][col]
+        val saved = Array(side) { r -> candidates[r].toMutableSet() }
+        var dead = false
+        for (r in 0 until side) {
+            if (r == row) continue
+            candidates[r].remove(col)
+            if (abs(r - row) == 1) {
+                candidates[r].remove(col - 1)
+                candidates[r].remove(col + 1)
+            }
+            candidates[r].removeAll { c -> regions[r][c] == colour }
+            if (candidates[r].isEmpty()) dead = true
+        }
+        if (dead) {
+            for (r in 0 until side) candidates[r] = saved[r]
+            return null
+        }
+        return {
+            for (r in 0 until side) candidates[r] = saved[r]
+        }
+    }
+
+    /**
+     * A complete puzzle: the colours, the answer, and a proof it is unique.
+     *
+     * Generation is rejection rather than construction — build a board, check it,
+     * throw it away if the check fails — and the attempts walk forward from
+     * [seed] so a seed that has a puzzle within a few tries gives the same one
+     * every time.
+     */
+    fun generate(side: Int, seed: Long): MeowdokuPuzzle? {
+        val base = (seed xor (seed ushr 32)).toInt()
+        for (attempt in 0 until MAX_ATTEMPTS) {
+            val rng = SeededRng(base + attempt * 7919)
+            val catCols = placeCats(side, rng)
+            val regions = growRegions(side, catCols, SeededRng(base * 31 + attempt * 104729), strips = side / 2)
+            if (distinctColours(side, regions) != side) continue
+            if (countSolutions(side, regions, limit = 2) != 1) continue
+            return MeowdokuPuzzle(side, regions, catCols)
+        }
+        return null
+    }
+
+    private const val MAX_ATTEMPTS = 4000
+
+    /** How many different colours a board actually shows. */
+    private fun distinctColours(side: Int, regions: Array<IntArray>): Int =
+        regions.flatMap { it.toList() }.distinct().size
+}
+
+/**
+ * A board and its answer.
+ *
+ * The answer is stored rather than re-derived because the rules need it to
+ * referee: a tap either lands a cat or loses a heart, and that is judged against
+ * the solution, not against whether the player could have deduced it.
+ */
+data class MeowdokuPuzzle(
+    val side: Int,
+    /** Region id per square; ids run 0 until side. */
+    val regions: Array<IntArray>,
+    /** The column of the cat in each row. */
+    val catCols: IntArray,
+) {
+    fun regionOf(row: Int, col: Int): Int = regions[row][col]
+
+    /** Every colour id, in board order, for a legend or a palette. */
+    val colours: List<Int> get() = (0 until side).toList()
+
+    fun solutionSquares(): List<Pair<Int, Int>> =
+        catCols.indices.map { it to catCols[it] }
+
+    override fun equals(other: Any?): Boolean = other is MeowdokuPuzzle &&
+        side == other.side &&
+        regions.contentDeepEquals(other.regions) &&
+        catCols.contentEquals(other.catCols)
+
+    override fun hashCode(): Int = 31 * side + regions.contentDeepHashCode()
+}
+
+/**
+ * A 32-bit linear congruential generator.
+ *
+ * Deliberately not [kotlin.random.Random]: that one is seeded per-JVM and can
+ * change between releases, and a puzzle generator that drifts silently corrupts
+ * saved games. This one is small, explicit, and the same everywhere.
+ */
+class SeededRng(seed: Int) {
+    private var state = if (seed == 0) DEFAULT_SEED else seed
+
+    /** The next 31 bits, as a non-negative int. */
+    fun nextInt(): Int {
+        state = state * MULTIPLIER + 1
+        return (state ushr 1) and 0x7FFF_FFFF
+    }
+
+    /** A value in 0 until [bound], taken by rejection so it stays uniform. */
+    fun nextInt(bound: Int): Int {
+        require(bound > 0)
+        while (true) {
+            val n = nextInt()
+            // 2^31 is a multiple of no useful number of squares, so a bare
+            // modulus biases the low values; the slack is thrown and redrawn.
+            if (n < Int.MAX_VALUE - (Int.MAX_VALUE % bound)) return n % bound
+        }
+    }
+
+    fun nextBoolean(): Boolean = nextInt(2) == 0
+
+    /** The same list every time for the same seed, in a different order. */
+    fun <T> shuffled(items: List<T>): List<T> {
+        val out = items.toMutableList()
+        for (i in out.lastIndex downTo 1) {
+            val j = nextInt(i + 1)
+            val tmp = out[i]
+            out[i] = out[j]
+            out[j] = tmp
+        }
+        return out
+    }
+
+    private companion object {
+        // The constants of the old VAX C library's `rand`, which are well tested
+        // and give a period of the full 31 bits.
+        const val MULTIPLIER = 1103515245
+        const val DEFAULT_SEED = 0x5DEECE66D.toInt()
+    }
+}

app/src/main/java/org/prolibertate/games/game/meowdoku/MeowdokuModel.kt

@@ -0,0 +1,117 @@
+package org.prolibertate.games.game.meowdoku
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+// Meowdoku hides N cats in an N by N board: exactly one cat in every row, every
+// column and every colour, and no two cats sharing even a corner. The player
+// taps the squares they believe hold a cat; the board only ever accepts a
+// correct tap, so a cat that lands is a cat that was always going to be there.
+// A wrong tap costs a heart and crosses the square over, which is what makes
+// pencil marks a choice rather than a necessity.
+
+/** One square of the board. */
+@Serializable
+data class Square(val row: Int, val col: Int)
+
+@Serializable
+data class MeowdokuOptions(
+    /** The board is [side] by [side], and the answer is [side] cats. */
+    val side: Int = 6,
+    /**
+     * How many wrong taps the board forgives. The original game deals three,
+     * and the hearts on a level are part of its difficulty rather than a
+     * courtesy.
+     */
+    val hearts: Int = 3,
+    /** How many cats the board will show on demand. */
+    val hints: Int = 3,
+) {
+    init {
+        require(side in MeowdokuGenerator.MIN_SIDE..MeowdokuGenerator.MAX_SIDE) {
+            "No board of $side squares a side"
+        }
+        require(hearts in 1..9) { "A board with no hearts is lost before it is dealt" }
+        require(hints in 0..9) { "Hints must read on the board, not scroll" }
+    }
+}
+
+/**
+ * The dealt board plus the marks on it.
+ *
+ * [cats] and [marks] are sets rather than a state per square because a square's
+ * story is exactly those two memberships: nothing is a cat and crossed, and the
+ * empty squares carry no information worth storing.
+ */
+@Serializable
+data class MeowdokuState(
+    val options: MeowdokuOptions,
+    /** The region id of every square, row by row. Ids run 0 until [side]. */
+    val regions: List<List<Int>>,
+    /** The column of the cat in each row — the answer, kept so taps can be judged. */
+    val catCols: List<Int>,
+    /** Cats the player has found (or a hint has shown). */
+    val cats: Set<Square>,
+    /** Squares crossed over: wrong taps and pencil marks alike. */
+    val marks: Set<Square>,
+    val heartsLeft: Int,
+    val hintsLeft: Int,
+    /** Wrong taps so far, which is how a solved board says how it was solved. */
+    val wrongCats: Int,
+    /** The last square touched, and whether touching it cost a heart. */
+    val lastSquare: Square? = null,
+    val lastWrong: Boolean = false,
+    val log: List<String>,
+) {
+    val side: Int get() = options.side
+
+    /** Cats found, out of the [side] the board hides. */
+    val catsFound: Int get() = cats.size
+
+    val solved: Boolean get() = catsFound == side
+
+    /** Out of hearts, with cats still missing. */
+    val lost: Boolean get() = heartsLeft == 0 && !solved
+
+    fun regionOf(square: Square): Int = regions[square.row][square.col]
+
+    fun isCatSquare(square: Square): Boolean = catCols[square.row] == square.col
+}
+
+/** What the player may do to a board. */
+@Serializable
+sealed interface MeowdokuMove
+
+/**
+ * Taps a square expecting a cat.
+ *
+ * One move rather than two, because that is what the player is doing: the board
+ * decides whether the tap found a cat or cost a heart. A square that is already
+ * crossed over has to be un-crossed first — the tap that made the cross is
+ * exactly the tap this would repeat.
+ */
+@Serializable
+@SerialName("cat")
+data class PlaceCat(val square: Square) : MeowdokuMove
+
+/**
+ * Draws or erases a pencil cross on a square with no cat on it.
+ *
+ * Free, and it cannot be undone by the board: crosses are the player's notes,
+ * not the puzzle's. A cross on a square that actually holds a cat costs nothing
+ * to erase, which is why placing a cat never consults the marks.
+ */
+@Serializable
+@SerialName("pencil")
+data class TogglePencil(val square: Square) : MeowdokuMove
+
+/**
+ * Spends a hint to show the cat in the first row that lacks one.
+ *
+ * The original game sells hints; here they are simply counted, and the one
+ * choice that has to be made about them — a hint is placed by the board, not
+ * guessed at by the player — is the same either way.
+ */
+@Serializable
+@SerialName("hint")
+data object TakeHint : MeowdokuMove

app/src/main/java/org/prolibertate/games/game/meowdoku/MeowdokuRules.kt

@@ -0,0 +1,192 @@
+package org.prolibertate.games.game.meowdoku
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.prolibertate.games.game.GameCatalog
+import org.prolibertate.games.game.engine.GameRules
+import org.prolibertate.games.game.engine.TableConfig
+
+/**
+ * Meowdoku as a pure state machine. See RULES-meowdoku.md.
+ *
+ * The engine is a second single-seat game, alongside peg solitaire, and it
+ * inherits the same freedom: there is nobody to hide anything from, so
+ * [viewFor] hands out the whole board including the answer. The answer lives in
+ * the state rather than being re-derived because judging a tap against a
+ * deduction the player could have made is a different thing from judging it
+ * against what the board was dealt — and the deduction engine (crossing out
+ * rows, columns and colours) belongs to the player, not the referee.
+ */
+object MeowdokuRules : GameRules<MeowdokuState, MeowdokuMove> {
+
+    override val gameId: String = GameCatalog.MEOWDOKU
+
+    /** The one seat. */
+    const val SOLO_SEAT: Int = 0
+
+    private val json = Json {
+        ignoreUnknownKeys = true
+        encodeDefaults = true
+    }
+
+    override fun initialState(config: TableConfig): MeowdokuState {
+        val options = json.decodeFromString<MeowdokuOptions>(config.optionsJson)
+        require(config.seats.size == 1) {
+            "Meowdoku seats one, not ${config.seats.size}"
+        }
+        val puzzle = deal(options.side, config.seed)
+        return MeowdokuState(
+            options = options,
+            regions = puzzle.regions.map { it.toList() },
+            catCols = puzzle.catCols.toList(),
+            cats = emptySet(),
+            marks = emptySet(),
+            heartsLeft = options.hearts,
+            hintsLeft = options.hints,
+            wrongCats = 0,
+            log = listOf("A ${options.side} by ${options.side} board, ${options.side} cats to find."),
+        )
+    }
+
+    /**
+     * Deals until the board is good.
+     *
+     * A seed whose first attempts all fail to produce a unique board moves the
+     * seed along rather than giving up: the same table asked twice gets the
+     * same board both times, which is what a saved game and a unit test need.
+     */
+    private fun deal(side: Int, seed: Long): MeowdokuPuzzle {
+        var trySeed = seed
+        repeat(DEAL_TRIES) {
+            MeowdokuGenerator.generate(side, trySeed)?.let { return it }
+            trySeed = trySeed * 6364136223846793005L + 1442695040888963407L
+        }
+        error("No unique ${side} x $side board within $DEAL_TRIES deals of seed $seed")
+    }
+
+    private const val DEAL_TRIES = 8
+
+    /**
+     * Every tap the board would take.
+     *
+     * Cats go on squares that are neither cat nor cross; pencils work any
+     * square without a cat on it, adding or erasing a cross; a hint is offered
+     * while there are hints left and cats still missing. The list is written
+     * row by row, which is also the order the hint engine uses.
+     */
+    override fun legalMoves(state: MeowdokuState, seat: Int): List<MeowdokuMove> {
+        if (seat != SOLO_SEAT || isFinished(state)) return emptyList()
+        val cats = ArrayList<MeowdokuMove>()
+        val pencils = ArrayList<MeowdokuMove>()
+        for (row in 0 until state.side) {
+            for (col in 0 until state.side) {
+                val square = Square(row, col)
+                if (square in state.cats) continue
+                if (square !in state.marks) cats += PlaceCat(square)
+                pencils += TogglePencil(square)
+            }
+        }
+        val hint = if (state.hintsLeft > 0) listOf(TakeHint) else emptyList()
+        return cats + pencils + hint
+    }
+
+    override fun applyMove(
+        state: MeowdokuState,
+        seat: Int,
+        move: MeowdokuMove,
+    ): MeowdokuState {
+        require(seat == SOLO_SEAT) { "Meowdoku has one seat" }
+        require(move in legalMoves(state, seat)) { "$move is not a legal move" }
+        return when (move) {
+            is PlaceCat -> tap(state, move.square)
+            is TogglePencil -> pencil(state, move.square)
+            TakeHint -> hint(state)
+        }
+    }
+
+    /**
+     * The tap that is the whole game: found a cat, or lost a heart.
+     *
+     * The square was already crossed? Then the tap is not offered at all — see
+     * [legalMoves] — so a player who pencilled over the truth has to notice and
+     * erase before tapping. That is the one way a board can be lost that the
+     * board itself did not do.
+     */
+    private fun tap(state: MeowdokuState, square: Square): MeowdokuState {
+        if (state.isCatSquare(square)) {
+            val next = state.copy(
+                cats = state.cats + square,
+                lastSquare = square,
+                lastWrong = false,
+            )
+            val note = if (next.solved) {
+                "All ${next.side} cats, ${next.wrongCats} wrong taps."
+            } else {
+                "Found a cat at row ${square.row + 1}, column ${square.col + 1}."
+            }
+            return next.copy(log = next.log + note)
+        }
+        val hearts = state.heartsLeft - 1
+        val next = state.copy(
+            marks = state.marks + square,
+            heartsLeft = hearts,
+            wrongCats = state.wrongCats + 1,
+            lastSquare = square,
+            lastWrong = true,
+        )
+        return if (hearts == 0) {
+            next.copy(log = next.log + "Out of hearts, with ${next.side - next.catsFound} cats still hiding.")
+        } else {
+            next
+        }
+    }
+
+    private fun pencil(state: MeowdokuState, square: Square): MeowdokuState {
+        val marks = if (square in state.marks) state.marks - square else state.marks + square
+        return state.copy(marks = marks, lastSquare = square, lastWrong = false)
+    }
+
+    /**
+     * Shows the cat in the first row without one.
+     *
+     * A revealed cat lands like a found one — and erases a pencil cross on its
+     * square, because the point of a hint is that the player stops having to
+     * work out whether the board contradicts their own notes.
+     */
+    private fun hint(state: MeowdokuState): MeowdokuState {
+        val row = (0 until state.side).first { r -> state.cats.none { it.row == r } }
+        val square = Square(row, state.catCols[row])
+        return state.copy(
+            cats = state.cats + square,
+            marks = state.marks - square,
+            hintsLeft = state.hintsLeft - 1,
+            lastSquare = square,
+            lastWrong = false,
+            log = state.log + "A hint shows the cat in row ${row + 1}.",
+        )
+    }
+
+    /** A board is finished when it is solved or when the hearts run out. */
+    override fun isFinished(state: MeowdokuState): Boolean = state.solved || state.lost
+
+    override fun currentSeat(state: MeowdokuState): Int? =
+        if (isFinished(state)) null else SOLO_SEAT
+
+    override fun summary(state: MeowdokuState): String = when {
+        state.solved -> "Solved with ${state.heartsLeft} hearts to spare"
+        state.lost -> "Lost with ${state.catsFound} of ${state.side} cats found"
+        else -> "${state.catsFound} of ${state.side} cats, ${state.heartsLeft} hearts left"
+    }
+
+    /**
+     * Nothing to strip. One person plays, and the answer in the state is there
+     * for the referee, not as a secret. Should a second seat ever race through
+     * the same board, this is where [MeowdokuState.catCols] gets hidden.
+     */
+    override fun viewFor(state: MeowdokuState, seat: Int): MeowdokuState = state
+
+    override fun encodeState(state: MeowdokuState): String = json.encodeToString(state)
+    override fun decodeState(json: String): MeowdokuState = this.json.decodeFromString(json)
+    override fun encodeMove(move: MeowdokuMove): String = json.encodeToString(move)
+    override fun decodeMove(json: String): MeowdokuMove = this.json.decodeFromString(json)
+}

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

@@ -50,6 +50,8 @@ import org.prolibertate.games.game.freecell.FreeCellOptions
 import org.prolibertate.games.game.pyramid.PyramidOptions
 import org.prolibertate.games.game.spider.SpiderOptions
 import org.prolibertate.games.game.yahtzee.YahtzeeOptions
+import org.prolibertate.games.game.meowdoku.MeowdokuGenerator
+import org.prolibertate.games.game.meowdoku.MeowdokuOptions
 import org.prolibertate.games.game.pegsolitaire.PegBoard
 import org.prolibertate.games.game.pegsolitaire.PegGoal
 import org.prolibertate.games.game.pegsolitaire.PegSolitaireOptions
@@ -109,6 +111,7 @@ fun GameSetupScreen(
     var pirates by remember { mutableStateOf(PiratesOptions()) }
     var hearts by remember { mutableStateOf(HeartsOptions()) }
     var pegs by remember { mutableStateOf(PegSolitaireOptions()) }
+    var meowdoku by remember { mutableStateOf(MeowdokuOptions()) }
     var yahtzee by remember { mutableStateOf(YahtzeeOptions()) }
     var klondike by remember { mutableStateOf(KlondikeOptions()) }
     var freecell by remember { mutableStateOf(FreeCellOptions()) }
@@ -140,6 +143,7 @@ fun GameSetupScreen(
         GameCatalog.PIRATES -> setupJson.encodeToString(pirates)
         GameCatalog.HEARTS -> setupJson.encodeToString(hearts)
         GameCatalog.PEG_SOLITAIRE -> setupJson.encodeToString(pegs)
+        GameCatalog.MEOWDOKU -> setupJson.encodeToString(meowdoku)
         GameCatalog.YAHTZEE -> setupJson.encodeToString(yahtzee)
         GameCatalog.KLONDIKE -> setupJson.encodeToString(klondike)
         GameCatalog.FREECELL -> setupJson.encodeToString(freecell)
@@ -205,6 +209,8 @@ fun GameSetupScreen(
 
                 GameCatalog.PEG_SOLITAIRE -> PegSolitaireOptionsEditor(pegs) { pegs = it }
 
+                GameCatalog.MEOWDOKU -> MeowdokuOptionsEditor(meowdoku) { meowdoku = it }
+
                 GameCatalog.YAHTZEE -> YahtzeeOptionsEditor(yahtzee) { yahtzee = it }
 
                 GameCatalog.KLONDIKE -> KlondikeOptionsEditor(klondike) { klondike = it }
@@ -1134,6 +1140,36 @@ private fun PegSolitaireOptionsEditor(
     }
 }
 
+@Composable
+private fun MeowdokuOptionsEditor(
+    options: MeowdokuOptions,
+    onChange: (MeowdokuOptions) -> Unit,
+) {
+    Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        ChipRow(
+            label = "Board",
+            values = (MeowdokuGenerator.MIN_SIDE..MeowdokuGenerator.MAX_SIDE).toList(),
+            selected = options.side,
+            display = { "$it × $it" },
+            onSelect = { onChange(options.copy(side = it)) },
+        )
+        ChipRow(
+            label = "Hearts",
+            values = (1..5).toList(),
+            selected = options.hearts,
+            display = { if (it == 1) "One" else "$it" },
+            onSelect = { onChange(options.copy(hearts = it)) },
+        )
+        ChipRow(
+            label = "Hints",
+            values = (0..3).toList(),
+            selected = options.hints,
+            display = { if (it == 0) "None" else "$it" },
+            onSelect = { onChange(options.copy(hints = it)) },
+        )
+    }
+}
+
 @Composable
 private fun YahtzeeOptionsEditor(options: YahtzeeOptions, onChange: (YahtzeeOptions) -> Unit) {
     Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {

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

@@ -7,6 +7,7 @@ import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableIntStateOf
+import androidx.compose.runtime.mutableLongStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
@@ -87,6 +88,10 @@ import org.prolibertate.games.game.mastermind.MastermindAi
 import org.prolibertate.games.game.mastermind.MastermindMove
 import org.prolibertate.games.game.mastermind.MastermindRules
 import org.prolibertate.games.game.mastermind.MastermindState
+import org.prolibertate.games.game.meowdoku.MeowdokuAi
+import org.prolibertate.games.game.meowdoku.MeowdokuMove
+import org.prolibertate.games.game.meowdoku.MeowdokuRules
+import org.prolibertate.games.game.meowdoku.MeowdokuState
 import org.prolibertate.games.game.morris.MorrisAi
 import org.prolibertate.games.game.morris.MorrisMove
 import org.prolibertate.games.game.morris.MorrisRules
@@ -131,6 +136,7 @@ import org.prolibertate.games.ui.game.EuchreScreen
 import org.prolibertate.games.ui.game.GolfScreen
 import org.prolibertate.games.ui.game.KaiserScreen
 import org.prolibertate.games.ui.game.MastermindScreen
+import org.prolibertate.games.ui.game.MeowdokuScreen
 import org.prolibertate.games.ui.game.MorrisScreen
 import org.prolibertate.games.ui.game.PegSolitaireScreen
 import org.prolibertate.games.ui.game.SpiderScreen
@@ -544,6 +550,34 @@ fun PlayScreen(
                 )
             }
 
+            GameCatalog.MEOWDOKU -> {
+                // Like peg solitaire the controller is rebuilt rather than
+                // reset — a finished board has nothing left to do — but unlike
+                // it a new board is dealt every time: the same cats in the same
+                // colours twice would only repeat the last puzzle's answer.
+                // Moving the seed moves the deal, and each deal is still
+                // reproducible from its own seed.
+                var attempt by remember(route) { mutableLongStateOf(0L) }
+                val controller = remember(route, attempt) {
+                    MatchController<MeowdokuState, MeowdokuMove>(
+                        rules = MeowdokuRules,
+                        // Never consulted while a person holds the only seat.
+                        ai = MeowdokuAi,
+                        config = route.config.copy(seed = route.config.seed + attempt),
+                        scope = scope,
+                        role = role,
+                        localSeats = setOf(route.localSeat),
+                        primarySeat = route.localSeat,
+                    )
+                }
+                StartMatch(env, controller, route)
+                MeowdokuScreen(
+                    controller = controller,
+                    onNewPuzzle = { attempt++ },
+                    onExit = onExit,
+                )
+            }
+
             GameCatalog.YAHTZEE -> {
                 val controller = remember(route) {
                     MatchController<YahtzeeState, YahtzeeMove>(

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

@@ -0,0 +1,273 @@
+package org.prolibertate.games.ui.game
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxWithConstraints
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.aspectRatio
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Button
+import androidx.compose.material3.FilterChip
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+import androidx.compose.ui.unit.TextUnit
+import org.prolibertate.games.game.meowdoku.MeowdokuMove
+import org.prolibertate.games.game.meowdoku.MeowdokuState
+import org.prolibertate.games.game.meowdoku.PlaceCat
+import org.prolibertate.games.game.meowdoku.Square
+import org.prolibertate.games.game.meowdoku.TakeHint
+import org.prolibertate.games.game.meowdoku.TogglePencil
+import org.prolibertate.games.net.MatchController
+import org.prolibertate.games.ui.LocalEndGameHandle
+import org.prolibertate.games.ui.ScreenScaffold
+import org.prolibertate.games.ui.theme.CardRed
+
+private const val TITLE = "Meowdoku"
+
+/**
+ * The region colours, one per cat the board can hold.
+ *
+ * The board never needs two regions of one colour — a side-ten board has ten
+ * regions and the palette has ten — so a region's colour is just its id, and
+ * nothing has to be computed to keep neighbours apart. The palette reads as
+ * watercolour on purpose: cats are dark ink on top of it, and a square the
+ * player has crossed over is too.
+ */
+private val RegionColours = listOf(
+    Color(0xFFF2C6C2), // rose
+    Color(0xFFF7E0A6), // butter
+    Color(0xFFCDE8C5), // mint
+    Color(0xFFC5E3F0), // sky
+    Color(0xFFD8CDEB), // lilac
+    Color(0xFFEBC9A0), // apricot
+    Color(0xFFBFE3D0), // spruce
+    Color(0xFFF0D7E3), // blossom
+    Color(0xFFCFCFE8), // periwinkle
+    Color(0xFFE3E7C1), // lime
+)
+
+/** What a tap does. The original game has a pencil tool too, and this is it. */
+private enum class TapMode { CATS, PENCIL }
+
+/**
+ * The puzzle board.
+ *
+ * Cells are laid out by weight inside a square frame, which keeps the grid
+ * exactly as wide as it is tall on any screen, and scales the glyphs with the
+ * cell rather than the font setting: a cat is half a square no matter how big
+ * the square is.
+ */
+@Composable
+fun MeowdokuScreen(
+    controller: MatchController<MeowdokuState, MeowdokuMove>,
+    onNewPuzzle: () -> Unit,
+    onExit: () -> Unit,
+) {
+    val state by controller.state.collectAsState()
+    val legal by controller.legalMoves.collectAsState()
+    val abandoned by controller.abandoned.collectAsState()
+
+    var mode by remember { mutableStateOf(TapMode.CATS) }
+    var confirmingEnd by remember { mutableStateOf(false) }
+
+    val endGameAction: @Composable () -> Unit = {
+        TextButton(onClick = { confirmingEnd = true }) { Text("End game") }
+    }
+
+    val current = state
+    if (current == null) {
+        ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+            Box(modifier, contentAlignment = Alignment.Center) { Text("Setting up…") }
+            abandoned?.let { LeftBoardDialog(it, onExit, controller::dismissAbandoned) }
+            if (confirmingEnd) EndBoardDialog(onExit) { confirmingEnd = false }
+        }
+        return
+    }
+
+    val places = legal.filterIsInstance<PlaceCat>().map { it.square }.toSet()
+    val pencils = legal.filterIsInstance<TogglePencil>().map { it.square }.toSet()
+    val finished by controller.finished.collectAsState()
+
+    ScreenScaffold(title = TITLE, onBack = onExit, actions = endGameAction) { modifier ->
+        Column(
+            modifier = modifier.verticalScroll(rememberScrollState()),
+            verticalArrangement = Arrangement.spacedBy(10.dp),
+        ) {
+            Text(
+                text = when {
+                    current.solved -> "Found every cat" +
+                        if (current.wrongCats == 0) ", not one tap wasted." else ", with ${current.wrongCats} wrong taps."
+                    current.lost -> "Out of hearts, with ${current.side - current.catsFound} cats still hiding."
+                    mode == TapMode.CATS -> "${current.catsFound} of ${current.side} cats found. Tap where a cat lives."
+                    else -> "Tap to cross a square over, or to erase a cross."
+                },
+                fontWeight = FontWeight.Bold,
+            )
+            Text(
+                text = "Hearts " + "♥".repeat(current.heartsLeft) +
+                    " · Hints " + current.hintsLeft,
+                style = MaterialTheme.typography.bodySmall,
+            )
+
+            Board(
+                state = current,
+                places = places,
+                pencils = pencils,
+                onTap = { square ->
+                    when {
+                        mode == TapMode.CATS && square in places ->
+                            controller.submit(PlaceCat(square))
+
+                        mode == TapMode.PENCIL && square in pencils ->
+                            controller.submit(TogglePencil(square))
+                    }
+                },
+            )
+
+            Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                FilterChip(
+                    selected = mode == TapMode.CATS,
+                    onClick = { mode = TapMode.CATS },
+                    label = { Text("Cats") },
+                )
+                FilterChip(
+                    selected = mode == TapMode.PENCIL,
+                    onClick = { mode = TapMode.PENCIL },
+                    label = { Text("Pencil") },
+                )
+                if (TakeHint in legal) {
+                    FilterChip(
+                        selected = false,
+                        onClick = { controller.submit(TakeHint) },
+                        label = { Text("Hint") },
+                    )
+                }
+            }
+
+            if (finished) {
+                Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                    Button(onClick = onNewPuzzle) { Text("New puzzle") }
+                    TextButton(onClick = onExit) { Text("Leave") }
+                }
+            }
+
+            current.log.takeLast(2).forEach {
+                Text(it, style = MaterialTheme.typography.bodySmall)
+            }
+        }
+
+        abandoned?.let { LeftBoardDialog(it, onExit, controller::dismissAbandoned) }
+        if (confirmingEnd) EndBoardDialog(onExit) { confirmingEnd = false }
+    }
+}
+
+@Composable
+private fun Board(
+    state: MeowdokuState,
+    places: Set<Square>,
+    pencils: Set<Square>,
+    onTap: (Square) -> Unit,
+) {
+    BoxWithConstraints(modifier = Modifier.fillMaxWidth()) {
+        // One cell's edge, so a cat is always about half a square whatever
+        // board was dealt.
+        val cell = maxWidth / state.side
+        Column(modifier = Modifier.aspectRatio(1f)) {
+            for (row in 0 until state.side) {
+                Row(modifier = Modifier.weight(1f)) {
+                    for (col in 0 until state.side) {
+                        val square = Square(row, col)
+                        CellView(
+                            square = square,
+                            state = state,
+                            tappable = square in places || square in pencils,
+                            glyphSize = (cell.value * 0.55f).sp,
+                            modifier = Modifier.weight(1f),
+                            onTap = { onTap(square) },
+                        )
+                    }
+                }
+            }
+        }
+    }
+}
+
+@Composable
+private fun CellView(
+    square: Square,
+    state: MeowdokuState,
+    tappable: Boolean,
+    glyphSize: TextUnit,
+    modifier: Modifier,
+    onTap: () -> Unit,
+) {
+    val isCat = square in state.cats
+    val isMark = square in state.marks
+    val justWrong = state.lastWrong && state.lastSquare == square
+    Box(
+        modifier = modifier
+            .aspectRatio(1f)
+            .border(1.dp, Color(0x33000000))
+            .background(RegionColours[state.regionOf(square) % RegionColours.size])
+            .then(
+                if (justWrong) Modifier.border(3.dp, CardRed) else Modifier
+            )
+            .clickable(enabled = tappable) { onTap() },
+        contentAlignment = Alignment.Center,
+    ) {
+        when {
+            isCat -> Text("🐱", fontSize = glyphSize)
+            isMark -> Text(
+                "✕",
+                color = Color(0xAA1C1B1F),
+                fontWeight = FontWeight.Bold,
+                fontSize = glyphSize,
+            )
+        }
+    }
+}
+
+@Composable
+private fun EndBoardDialog(onEnd: () -> Unit, onKeepPlaying: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onKeepPlaying,
+        title = { Text("End this game?") },
+        text = { Text("The board is left as it stands.") },
+        confirmButton = {
+            val handle = LocalEndGameHandle.current
+            TextButton(onClick = { handle.end(); onEnd() }) { Text("End the game") }
+        },
+        dismissButton = { TextButton(onClick = onKeepPlaying) { Text("Keep playing") } },
+    )
+}
+
+@Composable
+private fun LeftBoardDialog(notice: String, onExit: () -> Unit, onStay: () -> Unit) {
+    AlertDialog(
+        onDismissRequest = onStay,
+        title = { Text("The game has ended") },
+        text = { Text(notice) },
+        confirmButton = { TextButton(onClick = onExit) { Text("Leave the game") } },
+        dismissButton = { TextButton(onClick = onStay) { Text("Look at the board") } },
+    )
+}

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

@@ -107,24 +107,36 @@ class CatalogTest {
 
     @Test
     fun `every reference game in the catalogue can be played`() {
-        // Every game the app ships is playable. The one exception is Katan,
-        // listed so the slot exists and marked *Coming soon* until its engine
-        // and screen land — `available = false` is what keeps an unfinished
-        // game out of a production build while leaving it visible on a dev one.
+        // Every game the app ships is playable. The exceptions are the games
+        // whose engine and screen exist but have not been verified on a real
+        // device yet: they are marked as previews so a production build keeps
+        // them out while a dev one can open them. `available = false` is what
+        // keeps an unverified game out of a production build while leaving it
+        // visible on a dev one.
         assertEquals(
-            listOf(GameCatalog.CATAN),
+            listOf(GameCatalog.CATAN, GameCatalog.MEOWDOKU),
             GameCatalog.all.filterNot { it.available }.map { it.id },
         )
         // It is also the only difference the two channels see in a menu.
-        val hiddenFromProduction = GameCatalog.inMenu(GameMenu.BOARD, includeComingSoon = true) -
+        // Katan sits under Board and Meowdoku under Solitaire, so the two
+        // channels part ways in different sections.
+        val hiddenFromBoard = GameCatalog.inMenu(GameMenu.BOARD, includeComingSoon = true) -
             GameCatalog.inMenu(GameMenu.BOARD, includeComingSoon = false)
-        assertEquals(listOf(GameCatalog.byId(GameCatalog.CATAN)!!), hiddenFromProduction)
+        assertEquals(listOf(GameCatalog.byId(GameCatalog.CATAN)!!), hiddenFromBoard)
+        val hiddenFromSolitaire = GameCatalog.inMenu(GameMenu.SOLITAIRE, includeComingSoon = true) -
+            GameCatalog.inMenu(GameMenu.SOLITAIRE, includeComingSoon = false)
+        assertEquals(listOf(GameCatalog.byId(GameCatalog.MEOWDOKU)!!), hiddenFromSolitaire)
     }
 
     @Test
     fun `a preview game opens on a dev build and nowhere else`() {
+        // Katan until its two-device play is verified; Meowdoku until its board
+        // and hearts have been looked at on a real screen.
         val preview = GameCatalog.all.filter { it.devPreview }
-        assertTrue("Katan is the preview game", preview.map { it.id } == listOf(GameCatalog.CATAN))
+        assertEquals(
+            setOf(GameCatalog.CATAN, GameCatalog.MEOWDOKU),
+            preview.map { it.id }.toSet(),
+        )
         preview.forEach { game ->
             assertFalse("${game.id}: preview is for games that are not yet available", game.available)
             assertTrue("${game.id} should open on a dev build", game.playableIn(devBuild = true))

app/src/test/java/org/prolibertate/games/game/meowdoku/MeowdokuRulesTest.kt

@@ -0,0 +1,323 @@
+package org.prolibertate.games.game.meowdoku
+
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertThrows
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.prolibertate.games.game.engine.PlayerKind
+import org.prolibertate.games.game.engine.PlayerSlot
+import org.prolibertate.games.game.engine.TableConfig
+
+/**
+ * The puzzle, the generator, and the referee.
+ *
+ * The generator is where a wrong board would hurt most — a second answer makes
+ * the deduction unfair and the hearts a lottery — so the generation tests run
+ * over seeds rather than one lucky one.
+ */
+class MeowdokuRulesTest {
+
+    private val json = Json { encodeDefaults = true }
+
+    private fun soloConfig(options: MeowdokuOptions, seed: Long = 7L) = TableConfig(
+        gameId = "meowdoku",
+        seats = listOf(PlayerSlot(seat = 0, name = "P0", kind = PlayerKind.HUMAN_LOCAL, team = 0)),
+        optionsJson = json.encodeToString(options),
+        seed = seed,
+    )
+
+    private fun dealt(side: Int, seed: Long = 7L) =
+        MeowdokuRules.initialState(soloConfig(MeowdokuOptions(side = side), seed))
+
+    /**
+     * A puzzle that must exist, named so the assertions below read as facts
+     * rather than as unwrapping.
+     */
+    private fun <T> required(value: T?, message: String): T {
+        assertNotNull(message, value)
+        @Suppress("UNCHECKED_CAST")
+        return value as T
+    }
+
+    // ---- generator ---------------------------------------------------------
+
+    @Test
+    fun `every dealt board has exactly one answer`() {
+        for (side in 4..8) {
+            for (seed in 1L..6L) {
+                val puzzle = required(
+                    MeowdokuGenerator.generate(side, seed),
+                    "no puzzle for side $side seed $seed",
+                )
+                assertEquals(
+                    "the $side board dealt from seed $seed has more than one answer",
+                    1,
+                    MeowdokuGenerator.countSolutions(side, puzzle.regions),
+                )
+            }
+        }
+    }
+
+    @Test
+    fun `the same seed deals the same board`() {
+        val a = MeowdokuGenerator.generate(6, 12345L)
+        val b = MeowdokuGenerator.generate(6, 12345L)
+        assertEquals(a, b)
+    }
+
+    @Test
+    fun `the answer satisfies the cats' own rules`() {
+        for (seed in 0L..20L) {
+            val puzzle = required(MeowdokuGenerator.generate(7, seed), "no puzzle for seed $seed")
+            val cats = puzzle.solutionSquares()
+            assertEquals(7, cats.size)
+            assertEquals(7, cats.map { it.first }.distinct().size)
+            assertEquals(7, cats.map { it.second }.distinct().size)
+            for (i in cats.indices) {
+                for (j in i + 1 until cats.size) {
+                    val (r1, c1) = cats[i]
+                    val (r2, c2) = cats[j]
+                    val touches = kotlin.math.abs(r1 - r2) <= 1 && kotlin.math.abs(c1 - c2) <= 1
+                    assertFalse("cats $cats touch", touches)
+                    assertFalse("two cats share a colour", puzzle.regionOf(r1, c1) == puzzle.regionOf(r2, c2))
+                }
+            }
+        }
+    }
+
+    @Test
+    fun `every colour owns exactly one cat and is contiguous`() {
+        val puzzle = required(MeowdokuGenerator.generate(6, 99L), "no puzzle for seed 99")
+        val cells = (0 until 6).flatMap { r -> (0 until 6).map { c -> r to c } }
+
+        val owned = (0 until 6).map { region ->
+            cells.filter { (r, c) -> puzzle.regionOf(r, c) == region }
+        }
+        owned.forEach { cellsOfRegion ->
+            assertTrue("an empty colour", cellsOfRegion.isNotEmpty())
+            val cats = cellsOfRegion.count { (r, c) -> puzzle.catCols[r] == c }
+            assertEquals("a colour with ${cats}cats", 1, cats)
+
+            // Contiguous: a breadth-first walk from the first cell reaches all.
+            val rest = cellsOfRegion.toMutableSet()
+            val queue = ArrayDeque(listOf(rest.first()))
+            val reached = mutableSetOf(queue.first())
+            while (queue.isNotEmpty()) {
+                val (r, c) = queue.removeFirst()
+                for ((dr, dc) in listOf(0 to 1, 0 to -1, 1 to 0, -1 to 0)) {
+                    val next = (r + dr) to (c + dc)
+                    if (next in rest && reached.add(next)) queue.add(next)
+                }
+            }
+            assertEquals("a colour in two pieces", rest.size, reached.size)
+        }
+    }
+
+    @Test
+    fun `the cats can always be settled`() {
+        // A four board is the smallest deal, and its answer is the point where
+        // no-touch and one-per-line first stop contradicting each other.
+        val rng = SeededRng(42)
+        val cols = MeowdokuGenerator.placeCats(4, rng)
+        assertEquals(4, cols.size)
+        // The placement the generator returns is internally consistent.
+        for (row in 0 until 4) {
+            for (earlier in 0 until row) {
+                assertFalse(MeowdokuGenerator.catFitsHere(row, cols[earlier], cols))
+            }
+        }
+    }
+
+    // ---- the referee -------------------------------------------------------
+
+    @Test
+    fun `the board opens empty and seated for one`() {
+        val state = dealt(6)
+        assertTrue(state.cats.isEmpty())
+        assertTrue(state.marks.isEmpty())
+        assertEquals(6, state.side)
+        assertEquals(3, state.heartsLeft)
+        assertEquals(0, state.catsFound)
+        assertEquals(0, MeowdokuRules.currentSeat(state))
+        assertFalse(MeowdokuRules.isFinished(state))
+        assertTrue(MeowdokuRules.legalMoves(state, 1).isEmpty())
+    }
+
+    @Test
+    fun `a correct tap lands a cat and a wrong tap costs a heart`() {
+        val state = dealt(6)
+        val square = Square(0, state.catCols[0])
+        val found = MeowdokuRules.applyMove(state, 0, PlaceCat(square))
+        assertTrue(square in found.cats)
+        assertEquals(3, found.heartsLeft)
+
+        val wrongRow = (0 until state.side).first { it != square.row }
+        val wrong = Square(wrongRow, if (state.catCols[wrongRow] == 0) 1 else 0)
+        val missed = MeowdokuRules.applyMove(found, 0, PlaceCat(wrong))
+        assertTrue(wrong in missed.marks)
+        assertFalse(wrong in missed.cats)
+        assertEquals(2, missed.heartsLeft)
+        assertEquals(1, missed.wrongCats)
+    }
+
+    @Test
+    fun `a board is solved by finding every cat`() {
+        var state = dealt(5)
+        for (row in 0 until 5) {
+            state = MeowdokuRules.applyMove(state, 0, PlaceCat(Square(row, state.catCols[row])))
+        }
+        assertTrue(state.solved)
+        assertTrue(MeowdokuRules.isFinished(state))
+        assertNull(MeowdokuRules.currentSeat(state))
+        assertTrue(MeowdokuRules.legalMoves(state, 0).isEmpty())
+        assertTrue(MeowdokuRules.summary(state).startsWith("Solved"))
+    }
+
+    @Test
+    fun `running out of hearts loses the board`() {
+        val state = dealt(8, seed = 3L)
+        var lost = state
+        var row = 0
+        while (lost.heartsLeft > 0) {
+            // A wrong square: not the answer's column, in a row with no cat.
+            val wrongCol = if (state.catCols[row] == 0) 1 else 0
+            val before = lost.catsFound
+            lost = MeowdokuRules.applyMove(lost, 0, PlaceCat(Square(row, wrongCol)))
+            assertEquals(before, lost.catsFound)
+            row = (row + 1) % state.side
+        }
+        assertTrue(lost.lost)
+        assertTrue(MeowdokuRules.isFinished(lost))
+        assertEquals(0, lost.catsFound)
+        assertTrue(MeowdokuRules.summary(lost).startsWith("Lost"))
+    }
+
+    @Test
+    fun `a tapped square that is crossed over is not offered`() {
+        val state = dealt(6)
+        val wrongRow = 1
+        val wrongCol = if (state.catCols[wrongRow] == 0) 1 else 0
+        val marked = MeowdokuRules.applyMove(state, 0, PlaceCat(Square(wrongRow, wrongCol)))
+        assertThrows(IllegalArgumentException::class.java) {
+            MeowdokuRules.applyMove(marked, 0, PlaceCat(Square(wrongRow, wrongCol)))
+        }
+        // A pencil erases the cross and opens the tap again.
+        val erased = MeowdokuRules.applyMove(marked, 0, TogglePencil(Square(wrongRow, wrongCol)))
+        assertFalse(Square(wrongRow, wrongCol) in erased.marks)
+    }
+
+    @Test
+    fun `pencil marks are free notes`() {
+        val state = dealt(6)
+        val square = Square(2, if (state.catCols[2] == 5) 4 else 5)
+        val marked = MeowdokuRules.applyMove(state, 0, TogglePencil(square))
+        assertTrue(square in marked.marks)
+        assertEquals(3, marked.heartsLeft)
+        assertEquals(0, marked.wrongCats)
+
+        val erased = MeowdokuRules.applyMove(marked, 0, TogglePencil(square))
+        assertTrue(erased.marks.isEmpty())
+    }
+
+    @Test
+    fun `a cat square is never a pencil square`() {
+        val state = dealt(6)
+        val cat = Square(0, state.catCols[0])
+        val found = MeowdokuRules.applyMove(state, 0, PlaceCat(cat))
+        assertFalse(
+            "legal moves offer pencil on a cat",
+            MeowdokuRules.legalMoves(found, 0).any {
+                it is TogglePencil && it.square == cat
+            },
+        )
+    }
+
+    @Test
+    fun `a hint shows a missing cat and costs one token`() {
+        val state = dealt(6)
+        val hinted = MeowdokuRules.applyMove(state, 0, TakeHint)
+        assertEquals(1, hinted.catsFound)
+        assertEquals(2, hinted.hintsLeft)
+
+        // Out of hints the move stops being offered.
+        var drained = MeowdokuRules.applyMove(
+            MeowdokuRules.applyMove(hinted, 0, TakeHint),
+            0,
+            TakeHint,
+        )
+        assertEquals(0, drained.hintsLeft)
+        // A note on a square that is not a cat, so the pencil move is legal.
+        val note = Square(0, if (state.catCols[0] == 0) 1 else 0)
+        drained = MeowdokuRules.applyMove(drained, 0, TogglePencil(note))
+        assertFalse(TakeHint in MeowdokuRules.legalMoves(drained, 0))
+    }
+
+    @Test
+    fun `a hint over a wrong pencil reveals the cat anyway`() {
+        val state = dealt(6)
+        val truth = Square(0, state.catCols[0])
+        val pencilled = MeowdokuRules.applyMove(state, 0, TogglePencil(truth))
+        val revealed = MeowdokuRules.applyMove(pencilled, 0, TakeHint)
+        assertTrue(truth in revealed.cats)
+        assertFalse(truth in revealed.marks)
+    }
+
+    @Test
+    fun `the computer can finish any board it is handed`() {
+        var state = dealt(7)
+        var guard = 0
+        while (!MeowdokuRules.isFinished(state) && guard++ < 100) {
+            val legal = MeowdokuRules.legalMoves(state, 0)
+            assertFalse("stuck with cats still missing", legal.isEmpty())
+            state = MeowdokuRules.applyMove(state, 0, MeowdokuAi.chooseMove(state, 0, legal))
+        }
+        assertTrue("the AI did not solve the board", state.solved)
+    }
+
+    // ---- state across the wire and the save slot ---------------------------
+
+    @Test
+    fun `state and moves survive their json`() {
+        val state = dealt(6)
+        val decoded = MeowdokuRules.decodeState(MeowdokuRules.encodeState(state))
+        assertEquals(state, decoded)
+
+        val move: MeowdokuMove = PlaceCat(Square(1, 1))
+        assertEquals(move, MeowdokuRules.decodeMove(MeowdokuRules.encodeMove(move)))
+        assertEquals(TakeHint, MeowdokuRules.decodeMove(MeowdokuRules.encodeMove(TakeHint)))
+    }
+
+    @Test
+    fun `nothing is hidden from the only seat`() {
+        val state = dealt(6)
+        assertEquals(state, MeowdokuRules.viewFor(state, 0))
+    }
+
+    @Test
+    fun `a table with two seats is refused`() {
+        val config = TableConfig(
+            gameId = "meowdoku",
+            seats = listOf(
+                PlayerSlot(0, "P0", PlayerKind.HUMAN_LOCAL, 0),
+                PlayerSlot(1, "P1", PlayerKind.AI, 1),
+            ),
+            optionsJson = json.encodeToString(MeowdokuOptions()),
+            seed = 1L,
+        )
+        assertThrows(IllegalArgumentException::class.java) {
+            MeowdokuRules.initialState(config)
+        }
+    }
+
+    @Test
+    fun `the options guard their own board`() {
+        assertThrows(IllegalArgumentException::class.java) { MeowdokuOptions(side = 3) }
+        assertThrows(IllegalArgumentException::class.java) { MeowdokuOptions(side = 11) }
+        assertThrows(IllegalArgumentException::class.java) { MeowdokuOptions(hearts = 0) }
+    }
+}

docs/README.md

@@ -27,6 +27,7 @@ Naming: the catalogue id, lowercase, e.g. `crazy8s` → `RULES-crazy8s.md`.
 | Katan (catalogue id `catan`) | [RULES-catan.md](RULES-catan.md) — and its expansion rule set, [RULES-catan-ck.md](RULES-catan-ck.md) |
 | Klondike | — (TODO) |
 | Mastermind | [RULES-mastermind.md](RULES-mastermind.md) |
+| Meowdoku | [RULES-meowdoku.md](RULES-meowdoku.md) |
 | Morris | [RULES-morris.md](RULES-morris.md) |
 | Peg solitaire | — (TODO) |
 | Pirates | [RULES-pirates.md](RULES-pirates.md) |

docs/RULES-meowdoku.md

@@ -0,0 +1,87 @@
+# Meowdoku
+
+A board of colours, and a cat in every row, column and colour — no two sharing
+even a corner.
+
+## The puzzle
+
+The board is **N by N** and coloured into **N regions**. Exactly one cat hides
+in every row, every column and every region, and no two cats may touch — not
+along a side and not at a corner, so a cat rules out all eight squares around
+itself. Those four constraints together always leave exactly one arrangement.
+
+Uniqueness is not decoration. The game is deduction, and a second answer means
+a square the player cannot prove either way. Every board is therefore generated
+and then searched: build the colours, count the ways a board can be answered,
+and if it is more than one, throw the board away and deal another. The count is
+a plain depth-first search over the rows with the obvious exclusions propagated,
+and it sees only the colours and the rules — never the cats it was grown from.
+
+## Playing it
+
+**Tap a square where a cat lives.** A correct tap lands the cat; a wrong one
+costs a heart and crosses the square over, because being wrong here is
+information and the board keeps it for you. Finding all N cats wins; losing the
+hearts ends the board with the rest still hidden.
+
+The **Pencil** toggle is for the marks you make yourself. They are free, they
+cost nothing to erase, and the board does not judge them. The one trap they
+carry: a square crossed over is a square you have decided against, so tapping a
+cat onto it is refused until the pencil erases the note. The original game
+draws crosses for you on a wrong tap for the same reason — a wrong square
+crossed twice is the same mistake twice — and here the same thing happens
+whether the cross came from the heart or from the pencil.
+
+A **Hint** spends a token and shows the cat in the first row that still lacks
+one. The original sells these as tools; here they are simply counted, three per
+board by default, which keeps the hint a decision rather than a reflex.
+
+## The heart
+
+Why a wrong tap costs anything at all: the puzzle is only hard if guessing is.
+A board with no penalty is a board you can solve by tapping every square, and
+the hearts are what make the deduction on a colour you have not reasoned about
+yet feel like a choice. Three is the number the original deals, and it stays
+three by default.
+
+## Options
+
+| | |
+| --- | --- |
+| **Board** | 4×4 up to 10×10; the number of cats equals the side |
+| **Hearts** | 1 to 5, default 3 |
+| **Hints** | 0 to 3, default 3 |
+
+Boards are dealt from the table's seed, so a saved game resumes to the same
+cats in the same colours, and **New puzzle** moves the seed to deal a different
+board. Smaller sides are gentler: a 4×4 is the shortest way to learn what
+no-touch does to a corner, and the ten-wide boards are for people who have
+already made that mistake.
+
+## How a board is dealt
+
+The order matters, and it is the order the original game uses:
+
+1. **Settle the cats first.** One per row, one per column, no two touching,
+   chosen by backtracking over shuffled column orders.
+2. **Grow the colours around them.** Each region starts as its cat's square and
+   expands outwards one square at a time; some regions are grown as straight
+   bars along the cat's row or column, which is what stops the board reading as
+   texture, and the rest take whichever neighbour is smallest so nothing
+   sprawls while another dies young.
+3. **Check the answer.** Count solutions up to two; keep the board only when
+   the count comes back exactly one.
+
+The generator's rejection loop is why the same seed can take a few attempts:
+most boards are unique on the first try, and the odd seed has to walk until it
+finds one.
+
+## Where this engine differs from the original
+
+The app is a game collection, not a live-service puzzle, so what sits around
+the original's gameplay is left behind: lives-as-monetisation (watching an ad
+for a heart), daily streaks and leaderboards, an Elo-matched bank of
+hand-authored levels, and the cosmetic layer (cat animations, combo feedback,
+confetti). This engine **deals its boards from a seed** instead of shipping a
+level bank, and the rules are the part worth keeping: the cats, the colours,
+the no-touch, the hearts, the one answer.