feat(core): Spider rules as a second card_game::Game implementation

Two-deck (104-card) Spider on the upstream Stack/Pile containers,
exercising the multi-deck Card encoding for the first time:

- SpiderGame: 10-pile deal (4x6 + 6x5), build-down-any-suit,
  same-suit-run pickup, deal-10 gated on no empty pile, automatic
  K->A run removal, win at 8 runs
- SpiderSuits difficulty (1/2/4 suits over the same 104 cards);
  1- and 2-suit games contain identical Card values by construction
  (documented — engine entity mapping will need positional keys)
- Seeded deals via inline SplitMix64 + Fisher-Yates (core has no
  rand dep; Spider's seed space is deliberately self-contained)
- SpiderGameState session wrapper mirroring GameState conventions:
  Result<_, MoveError> mutations, upstream undo/score bookkeeping,
  Microsoft-style scoring (500 base, -1/move, +100/run, -1/undo)
- 17 unit tests + card-conservation/validity proptest over random
  legal walks; stacked-deal win-path test included

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_core/src/lib.rs

@@ -3,6 +3,7 @@ pub mod error;
 pub mod game_state;
 pub mod klondike_adapter;
 pub mod scoring;
+pub mod spider;
 
 // Re-export the upstream types that cross the solitaire_core API boundary so
 // downstream crates (engine, wasm) can import from one place without a direct
@@ -21,6 +22,13 @@ pub use klondike::{
 // former `solitaire_data::solver` wrapper module.
 pub use game_state::{DEFAULT_SOLVE_MOVES_BUDGET, DEFAULT_SOLVE_STATES_BUDGET, SolveOutcome};
 
+// Spider rules (second `card_game::Game` implementation; engine UI is a
+// later phase — nothing outside solitaire_core consumes these yet).
+pub use spider::{
+    SpiderConfig, SpiderGame, SpiderGameState, SpiderInstruction, SpiderScoring, SpiderStats,
+    SpiderSuits,
+};
+
 /// All four foundation slots, in slot order.
 ///
 /// Canonical iteration source for `Foundation` — upstream `klondike` has no

solitaire_core/src/spider.rs

@@ -0,0 +1,952 @@
+//! Spider solitaire rules — a second [`card_game::Game`] implementation
+//! alongside the upstream Klondike.
+//!
+//! Built directly on the upstream `card_game` containers: two decks
+//! ([`Deck::Deck1`] + [`Deck::Deck2`], 104 cards) dealt into ten
+//! [`Pile`]s, exercising the multi-deck [`Card`] encoding and the
+//! `Stack`/`Pile` public API that `klondike` uses internally.
+//!
+//! ## Rules implemented
+//!
+//! - 10 tableau piles: the first 4 receive 6 cards, the last 6 receive
+//!   5 (top card face-up) — 54 dealt, 50 in stock.
+//! - Build down regardless of suit; only same-suit descending runs may
+//!   be picked up and moved.
+//! - Empty piles accept any card or movable run.
+//! - The stock deals one card to every pile (10 total), only while no
+//!   pile is empty.
+//! - A completed K→A same-suit run is removed automatically; the game
+//!   is won when all 8 runs are removed.
+//! - Difficulty via suit count ([`SpiderSuits`]): 1, 2, or 4 suits
+//!   spread over the same 104 cards.
+//!
+//! ## Determinism
+//!
+//! Deals are seeded with an inline SplitMix64 + Fisher–Yates shuffle:
+//! `solitaire_core` has no `rand` dependency (and adding one needs
+//! explicit approval), so Spider's seed space is deliberately
+//! self-contained rather than shared with Klondike's `StdRng` seeds.
+//! The same seed + suit count always produces the same deal.
+//!
+//! ## Card identity caveat (engine integration, later phase)
+//!
+//! [`Card`] packs deck/suit/rank into one byte, and 1- and 2-suit
+//! games need more than four copies of a suit, so *identical* `Card`
+//! values legitimately coexist (e.g. eight `♠A` in a 1-suit game,
+//! spread over `Deck1..Deck4` twice). Rules only compare suit/rank so
+//! core is unaffected, but the engine's `Card → Entity` mapping is
+//! keyed by card value and will need positional keys before a Spider
+//! UI lands.
+
+use card_game::{Card, Deck, Game, Pile, Rank, Session, SessionConfig, Stack, Suit};
+use serde::{Deserialize, Serialize};
+
+use crate::error::MoveError;
+
+/// Number of tableau piles.
+pub const SPIDER_TABLEAUS: usize = 10;
+/// Total cards in play (two decks).
+pub const SPIDER_DECK_SIZE: usize = 104;
+/// Cards left in the stock after the opening deal (5 deals × 10).
+const STOCK_SIZE: usize = 50;
+/// Cards in one completed run (K → A).
+const RUN_LEN: usize = 13;
+/// Runs required to win.
+const TOTAL_RUNS: u8 = 8;
+/// Face-down cards in the deepest opening pile.
+const MAX_FACE_DOWN: usize = 5;
+
+// ---------------------------------------------------------------------------
+// Config
+// ---------------------------------------------------------------------------
+
+/// Spider difficulty: how many distinct suits the 104 cards span.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize, Default)]
+pub enum SpiderSuits {
+    /// All spades — the beginner layout (default).
+    #[default]
+    One,
+    /// Spades + hearts.
+    Two,
+    /// Full two-deck spread, the classic four-suit game.
+    Four,
+}
+
+impl SpiderSuits {
+    /// Suits used at this difficulty.
+    fn suits(self) -> &'static [Suit] {
+        match self {
+            Self::One => &[Suit::Spades],
+            Self::Two => &[Suit::Spades, Suit::Hearts],
+            Self::Four => &[Suit::Spades, Suit::Hearts, Suit::Clubs, Suit::Diamonds],
+        }
+    }
+}
+
+/// Scoring knobs, mirroring the shape of `klondike::ScoringConfig`.
+///
+/// Defaults follow the familiar Microsoft formula: start at 500, −1
+/// per move, +100 per completed run (the upstream session adds
+/// `undos × undo_penalty` on top).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct SpiderScoring {
+    /// Score a fresh deal starts from.
+    pub base: i32,
+    /// Added per move (conventionally negative).
+    pub move_penalty: i32,
+    /// Added per completed K→A run.
+    pub run_bonus: i32,
+}
+
+impl Default for SpiderScoring {
+    fn default() -> Self {
+        Self {
+            base: 500,
+            move_penalty: -1,
+            run_bonus: 100,
+        }
+    }
+}
+
+/// Full Spider rules configuration (the `Game::Config` type).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, Default)]
+pub struct SpiderConfig {
+    /// Suit-count difficulty.
+    pub suits: SpiderSuits,
+    /// Scoring knobs.
+    pub scoring: SpiderScoring,
+}
+
+// ---------------------------------------------------------------------------
+// Stats
+// ---------------------------------------------------------------------------
+
+/// Per-game counters (the `Game::Stats` type). Cumulative — they are
+/// not rolled back by undo, matching upstream `KlondikeStats`.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, Default)]
+pub struct SpiderStats {
+    moves: u32,
+    deals: u32,
+    runs_completed: u32,
+}
+
+impl SpiderStats {
+    /// Total instructions processed (moves + deals).
+    pub const fn moves(&self) -> u32 {
+        self.moves
+    }
+    /// Stock deals performed.
+    pub const fn deals(&self) -> u32 {
+        self.deals
+    }
+    /// K→A runs completed over the whole game (not undone-adjusted).
+    pub const fn runs_completed(&self) -> u32 {
+        self.runs_completed
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Instruction
+// ---------------------------------------------------------------------------
+
+/// One atomic Spider action (the `Game::Instruction` type).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum SpiderInstruction {
+    /// Deal one card from the stock onto every tableau pile.
+    Deal,
+    /// Move the top `count` face-up cards (a same-suit descending run)
+    /// from pile `from` to pile `to`. Pile indices are `0..10`.
+    Move {
+        /// Source pile index.
+        from: u8,
+        /// Destination pile index.
+        to: u8,
+        /// Number of cards in the moved run (≥ 1).
+        count: u8,
+    },
+}
+
+// ---------------------------------------------------------------------------
+// Game state
+// ---------------------------------------------------------------------------
+
+/// Pure Spider game position: ten tableaus, the stock, and the count
+/// of completed runs. Everything else (undo, score bookkeeping) lives
+/// in the wrapping [`Session`].
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub struct SpiderGame {
+    tableaus: [Pile<MAX_FACE_DOWN, SPIDER_DECK_SIZE>; SPIDER_TABLEAUS],
+    stock: Stack<STOCK_SIZE>,
+    completed_runs: u8,
+}
+
+/// SplitMix64 step — small, well-known, and dependency-free. Spider
+/// only needs a deterministic shuffle, not cryptographic quality.
+const fn splitmix64(state: u64) -> (u64, u64) {
+    let state = state.wrapping_add(0x9E37_79B9_7F4A_7C15);
+    let mut z = state;
+    z = (z ^ (z >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
+    z = (z ^ (z >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
+    (state, z ^ (z >> 31))
+}
+
+/// In-place Fisher–Yates driven by SplitMix64. The modulo bias is
+/// astronomically small for n ≤ 104 and irrelevant for gameplay.
+fn shuffle(cards: &mut [Card], seed: u64) {
+    let mut state = seed;
+    for i in (1..cards.len()).rev() {
+        let (next_state, r) = splitmix64(state);
+        state = next_state;
+        #[allow(clippy::cast_possible_truncation)]
+        let j = (r % (i as u64 + 1)) as usize;
+        cards.swap(i, j);
+    }
+}
+
+/// Builds the 104-card Spider deck for a suit-count difficulty.
+///
+/// Deck ids spread copies apart where possible (`Deck1..Deck4`), but
+/// 1- and 2-suit games necessarily contain identical `Card` values —
+/// see the module docs.
+fn build_deck(suits: SpiderSuits) -> Vec<Card> {
+    let suit_set = suits.suits();
+    let copies = SPIDER_DECK_SIZE / (suit_set.len() * RUN_LEN);
+    let decks = [Deck::Deck1, Deck::Deck2, Deck::Deck3, Deck::Deck4];
+    let mut cards = Vec::with_capacity(SPIDER_DECK_SIZE);
+    for copy in 0..copies {
+        for &suit in suit_set {
+            for rank in Rank::RANKS {
+                cards.push(Card::new(decks[copy % decks.len()], suit, rank));
+            }
+        }
+    }
+    cards
+}
+
+impl SpiderGame {
+    /// Deals a new seeded game at the given suit difficulty.
+    pub fn with_seed(seed: u64, suits: SpiderSuits) -> Self {
+        let mut deck = build_deck(suits);
+        shuffle(&mut deck, seed);
+        let mut cards = deck.into_iter();
+
+        let tableaus = core::array::from_fn(|index| {
+            // Piles 0–3 open with 5 face-down cards, piles 4–9 with 4;
+            // one face-up card lands on each afterwards.
+            let down_count = if index < 4 { 5 } else { 4 };
+            let stack: Stack<MAX_FACE_DOWN> = cards.by_ref().take(down_count).collect();
+            let mut pile = Pile::new_face_down(stack);
+            if let Some(card) = cards.next() {
+                pile.push(card);
+            }
+            pile
+        });
+        let stock: Stack<STOCK_SIZE> = cards.collect();
+
+        Self {
+            tableaus,
+            stock,
+            completed_runs: 0,
+        }
+    }
+
+    /// Face-up cards of pile `index` (bottom → top). Empty slice for
+    /// out-of-range indices.
+    pub fn tableau_face_up(&self, index: usize) -> &[Card] {
+        self.tableaus.get(index).map_or(&[], |pile| pile.face_up())
+    }
+
+    /// Face-down cards of pile `index` (bottom → top).
+    pub fn tableau_face_down(&self, index: usize) -> &[Card] {
+        self.tableaus
+            .get(index)
+            .map_or(&[], |pile| pile.face_down())
+    }
+
+    /// Cards remaining in the stock.
+    pub fn stock_len(&self) -> usize {
+        self.stock.len()
+    }
+
+    /// Completed K→A runs removed from play so far.
+    pub const fn completed_runs(&self) -> u8 {
+        self.completed_runs
+    }
+
+    /// Length of the longest movable run on top of pile `index`: the
+    /// maximal same-suit, strictly-descending face-up suffix.
+    fn movable_run_len(&self, index: usize) -> usize {
+        let Some(pile) = self.tableaus.get(index) else {
+            return 0;
+        };
+        let up = pile.face_up();
+        let mut len = usize::from(!up.is_empty());
+        while len < up.len() {
+            let above = &up[up.len() - len];
+            let below = &up[up.len() - len - 1];
+            let descends =
+                below.suit() == above.suit() && below.rank() as u8 == above.rank() as u8 + 1;
+            if !descends {
+                break;
+            }
+            len += 1;
+        }
+        len
+    }
+
+    /// Whether `Move { from, to, count }` is legal in this position.
+    fn is_move_valid(&self, from: u8, to: u8, count: u8) -> bool {
+        let (from, to, count) = (from as usize, to as usize, count as usize);
+        if from == to || from >= SPIDER_TABLEAUS || to >= SPIDER_TABLEAUS || count == 0 {
+            return false;
+        }
+        if count > self.movable_run_len(from) {
+            return false;
+        }
+        let src_up = self.tableaus[from].face_up();
+        // Bottom card of the moved run; `count <= movable_run_len <=
+        // src_up.len()` guarantees the index is in range.
+        let Some(moved_bottom) = src_up.get(src_up.len() - count) else {
+            return false;
+        };
+        match self.tableaus[to].face_up().last() {
+            // Build down regardless of suit.
+            Some(dest_top) => dest_top.rank() as u8 == moved_bottom.rank() as u8 + 1,
+            // Empty pile accepts anything (face-down remnant can't
+            // exist without a face-up card — Pile flips eagerly).
+            None => self.tableaus[to].is_empty(),
+        }
+    }
+
+    /// Whether the stock may deal: cards remain and no pile is empty.
+    fn is_deal_valid(&self) -> bool {
+        !self.stock.is_empty() && self.tableaus.iter().all(|pile| !pile.is_empty())
+    }
+
+    /// Removes a completed K→A same-suit run from the top of pile
+    /// `index`, if one is present. Returns `true` when a run was
+    /// removed (and the next face-down card, if any, was flipped).
+    fn sweep_completed_run(&mut self, index: usize) -> bool {
+        let Some(pile) = self.tableaus.get_mut(index) else {
+            return false;
+        };
+        let up = pile.face_up();
+        if up.len() < RUN_LEN {
+            return false;
+        }
+        let run = &up[up.len() - RUN_LEN..];
+        let suit = run[0].suit();
+        let is_complete = run.iter().enumerate().all(|(offset, card)| {
+            card.suit() == suit && card.rank() as u8 == (RUN_LEN - offset) as u8
+        });
+        if !is_complete {
+            return false;
+        }
+        let start = up.len() - RUN_LEN;
+        let (_removed, _flipped) = pile.take_range_flip_up(start..);
+        true
+    }
+}
+
+impl Game for SpiderGame {
+    type Score = i32;
+    type Stats = SpiderStats;
+    type Config = SpiderConfig;
+    type Instruction = SpiderInstruction;
+
+    fn score(&self, stats: &Self::Stats, config: &Self::Config) -> Self::Score {
+        let scoring = &config.scoring;
+        let moves = i32::try_from(stats.moves).unwrap_or(i32::MAX);
+        let runs = i32::from(self.completed_runs);
+        scoring.base + moves.saturating_mul(scoring.move_penalty) + runs * scoring.run_bonus
+    }
+
+    fn possible_instructions(
+        &self,
+        config: &Self::Config,
+    ) -> impl Iterator<Item = Self::Instruction> + use<> {
+        let mut out = Vec::new();
+        if self.is_deal_valid() {
+            out.push(SpiderInstruction::Deal);
+        }
+        for from in 0..SPIDER_TABLEAUS {
+            let max_run = self.movable_run_len(from);
+            for count in 1..=max_run {
+                for to in 0..SPIDER_TABLEAUS {
+                    #[allow(clippy::cast_possible_truncation)]
+                    let instruction = SpiderInstruction::Move {
+                        from: from as u8,
+                        to: to as u8,
+                        count: count as u8,
+                    };
+                    if self.is_move_valid(from as u8, to as u8, count as u8) {
+                        out.push(instruction);
+                    }
+                }
+            }
+        }
+        let _ = config;
+        out.into_iter()
+    }
+
+    fn is_instruction_valid(&self, _config: &Self::Config, instruction: Self::Instruction) -> bool {
+        match instruction {
+            SpiderInstruction::Deal => self.is_deal_valid(),
+            SpiderInstruction::Move { from, to, count } => self.is_move_valid(from, to, count),
+        }
+    }
+
+    fn process_instruction(
+        &mut self,
+        stats: &mut Self::Stats,
+        config: &Self::Config,
+        instruction: Self::Instruction,
+    ) {
+        // The trait offers no error channel; an invalid instruction is
+        // a no-op rather than a panic (error policy: no panics in game
+        // logic). Callers route through `SpiderGameState`, which
+        // validates first and surfaces `MoveError`.
+        if !self.is_instruction_valid(config, instruction) {
+            return;
+        }
+        stats.moves += 1;
+        match instruction {
+            SpiderInstruction::Deal => {
+                stats.deals += 1;
+                for index in 0..SPIDER_TABLEAUS {
+                    match self.stock.pop() {
+                        Some(card) => self.tableaus[index].push(card),
+                        None => break,
+                    }
+                }
+                for index in 0..SPIDER_TABLEAUS {
+                    if self.sweep_completed_run(index) {
+                        self.completed_runs += 1;
+                        stats.runs_completed += 1;
+                    }
+                }
+            }
+            SpiderInstruction::Move { from, to, count } => {
+                let (from, to, count) = (from as usize, to as usize, count as usize);
+                let src_len = self.tableaus[from].face_up().len();
+                let (cards, _flipped) = self.tableaus[from].take_range_flip_up(src_len - count..);
+                self.tableaus[to].extend(cards);
+                if self.sweep_completed_run(to) {
+                    self.completed_runs += 1;
+                    stats.runs_completed += 1;
+                }
+            }
+        }
+    }
+
+    fn is_win(&self) -> bool {
+        self.completed_runs >= TOTAL_RUNS
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Session wrapper
+// ---------------------------------------------------------------------------
+
+/// Spider counterpart of [`crate::game_state::GameState`]: owns the
+/// upstream [`Session`] (undo history + score/undo bookkeeping) and
+/// exposes `Result<_, MoveError>` mutations, mirroring the Klondike
+/// wrapper's conventions.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SpiderGameState {
+    seed: u64,
+    session: Session<SpiderGame>,
+}
+
+impl SpiderGameState {
+    /// New seeded game with the default (1-suit) difficulty.
+    pub fn new(seed: u64) -> Self {
+        Self::new_with_suits(seed, SpiderSuits::default())
+    }
+
+    /// New seeded game at an explicit suit difficulty.
+    pub fn new_with_suits(seed: u64, suits: SpiderSuits) -> Self {
+        let config = SessionConfig {
+            inner: SpiderConfig {
+                suits,
+                scoring: SpiderScoring::default(),
+            },
+            // Undoing costs one move, matching the familiar Spider
+            // scoring; the upstream formula applies `undos × penalty`.
+            undo_penalty: -1,
+            ..SessionConfig::default()
+        };
+        Self {
+            seed,
+            session: Session::new(SpiderGame::with_seed(seed, suits), config),
+        }
+    }
+
+    /// The deal seed this game was created from.
+    pub const fn seed(&self) -> u64 {
+        self.seed
+    }
+
+    /// Suit difficulty of this game.
+    pub fn suits(&self) -> SpiderSuits {
+        self.session.config().inner.suits
+    }
+
+    /// Current position (read-only).
+    pub fn game(&self) -> &SpiderGame {
+        self.session.state().state()
+    }
+
+    /// In-play score, clamped at 0 like the Klondike wrapper.
+    pub fn score(&self) -> i32 {
+        self.session
+            .state()
+            .score(self.session.stats(), self.session.config())
+            .max(0)
+    }
+
+    /// Whether all 8 runs are complete.
+    pub fn is_won(&self) -> bool {
+        self.session.is_win()
+    }
+
+    /// Total instructions applied (deals + moves), from history.
+    pub fn move_count(&self) -> u32 {
+        u32::try_from(self.session.history().len()).unwrap_or(u32::MAX)
+    }
+
+    /// Successful undos this session.
+    pub fn undo_count(&self) -> u32 {
+        self.session.stats().undos()
+    }
+
+    /// Legal instructions in the current position.
+    pub fn possible_instructions(&self) -> Vec<SpiderInstruction> {
+        self.session.possible_instructions().collect()
+    }
+
+    /// Applies one instruction. `Err` on illegal instructions and
+    /// finished games; the session records the undo snapshot.
+    pub fn apply_instruction(&mut self, instruction: SpiderInstruction) -> Result<(), MoveError> {
+        if self.is_won() {
+            return Err(MoveError::GameAlreadyWon);
+        }
+        let config = &self.session.config().inner;
+        if !self
+            .session
+            .state()
+            .state()
+            .is_instruction_valid(config, instruction)
+        {
+            return Err(MoveError::RuleViolation("move violates rules".into()));
+        }
+        self.session.process_instruction(instruction);
+        Ok(())
+    }
+
+    /// Restores the previous snapshot. Mirrors the Klondike wrapper's
+    /// guards; the upstream session counts the undo for scoring.
+    pub fn undo(&mut self) -> Result<(), MoveError> {
+        if self.is_won() {
+            return Err(MoveError::GameAlreadyWon);
+        }
+        if self.session.history().is_empty() {
+            return Err(MoveError::UndoStackEmpty);
+        }
+        self.session.undo();
+        Ok(())
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Test support
+// ---------------------------------------------------------------------------
+
+#[cfg(any(test, feature = "test-support"))]
+impl SpiderGame {
+    /// Builds an arbitrary position for tests: per-pile
+    /// `(face_down, face_up)` card lists plus stock and completed-run
+    /// count. No card-count invariants are enforced — stacked
+    /// positions for rule tests routinely use partial layouts.
+    pub fn from_test_layout(
+        piles: [(Vec<Card>, Vec<Card>); SPIDER_TABLEAUS],
+        stock: Vec<Card>,
+        completed_runs: u8,
+    ) -> Self {
+        let mut iter = piles.into_iter();
+        let tableaus = core::array::from_fn(|_| {
+            // `piles` has exactly SPIDER_TABLEAUS entries.
+            let (down, up) = iter.next().unwrap_or_default();
+            let mut pile = Pile::new_face_down(down.into_iter().collect());
+            pile.extend(up);
+            pile
+        });
+        Self {
+            tableaus,
+            stock: stock.into_iter().collect(),
+            completed_runs,
+        }
+    }
+}
+
+#[cfg(any(test, feature = "test-support"))]
+impl SpiderGameState {
+    /// Wraps an arbitrary position in a fresh session (empty history).
+    pub fn from_test_game(game: SpiderGame, suits: SpiderSuits) -> Self {
+        let config = SessionConfig {
+            inner: SpiderConfig {
+                suits,
+                scoring: SpiderScoring::default(),
+            },
+            undo_penalty: -1,
+            ..SessionConfig::default()
+        };
+        Self {
+            seed: 0,
+            session: Session::new(game, config),
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// `Card::new` shorthand for stacked positions.
+    fn card(suit: Suit, rank: Rank) -> Card {
+        Card::new(Deck::Deck1, suit, rank)
+    }
+
+    /// K→A same-suit run, bottom (King) first — the face-up order a
+    /// completed run occupies on a pile.
+    fn full_run(suit: Suit) -> Vec<Card> {
+        let mut ranks: Vec<Rank> = Rank::RANKS.to_vec();
+        ranks.reverse();
+        ranks.into_iter().map(|rank| card(suit, rank)).collect()
+    }
+
+    fn empty_layout() -> [(Vec<Card>, Vec<Card>); SPIDER_TABLEAUS] {
+        core::array::from_fn(|_| (Vec::new(), Vec::new()))
+    }
+
+    /// A layout where every pile is non-empty (single junk card), so
+    /// deal-validity tests can toggle exactly one condition.
+    fn junk_layout() -> [(Vec<Card>, Vec<Card>); SPIDER_TABLEAUS] {
+        core::array::from_fn(|_| (Vec::new(), vec![card(Suit::Clubs, Rank::King)]))
+    }
+
+    // -- dealing ----------------------------------------------------------
+
+    #[test]
+    fn opening_deal_shape_is_4x6_6x5_with_50_in_stock() {
+        let game = SpiderGame::with_seed(42, SpiderSuits::Four);
+        for index in 0..SPIDER_TABLEAUS {
+            let expected_down = if index < 4 { 5 } else { 4 };
+            assert_eq!(game.tableau_face_down(index).len(), expected_down);
+            assert_eq!(game.tableau_face_up(index).len(), 1, "one card face-up");
+        }
+        assert_eq!(game.stock_len(), STOCK_SIZE);
+        assert_eq!(game.completed_runs(), 0);
+    }
+
+    #[test]
+    fn same_seed_same_deal_different_seed_different_deal() {
+        let a = SpiderGame::with_seed(7, SpiderSuits::Four);
+        let b = SpiderGame::with_seed(7, SpiderSuits::Four);
+        let c = SpiderGame::with_seed(8, SpiderSuits::Four);
+        assert_eq!(a, b);
+        assert_ne!(a, c);
+    }
+
+    #[test]
+    fn suit_difficulty_controls_suit_spread() {
+        let one = build_deck(SpiderSuits::One);
+        let two = build_deck(SpiderSuits::Two);
+        let four = build_deck(SpiderSuits::Four);
+        for deck in [&one, &two, &four] {
+            assert_eq!(deck.len(), SPIDER_DECK_SIZE);
+        }
+        assert!(one.iter().all(|c| c.suit() == Suit::Spades));
+        assert!(
+            two.iter()
+                .all(|c| matches!(c.suit(), Suit::Spades | Suit::Hearts))
+        );
+        for suit in Suit::SUITS {
+            assert_eq!(
+                four.iter().filter(|c| c.suit() == suit).count(),
+                2 * RUN_LEN,
+                "four-suit deck has exactly two copies per suit"
+            );
+        }
+    }
+
+    // -- move rules -------------------------------------------------------
+
+    #[test]
+    fn single_card_builds_down_regardless_of_suit() {
+        let mut layout = empty_layout();
+        layout[0].1 = vec![card(Suit::Hearts, Rank::Five)];
+        layout[1].1 = vec![card(Suit::Spades, Rank::Six)];
+        let game = SpiderGame::from_test_layout(layout, Vec::new(), 0);
+        assert!(game.is_move_valid(0, 1, 1), "5♥ onto 6♠ is legal");
+        assert!(!game.is_move_valid(1, 0, 1), "6♠ onto 5♥ is not");
+    }
+
+    #[test]
+    fn only_same_suit_runs_are_movable_as_a_group() {
+        let mut layout = empty_layout();
+        // Pile 0: 7♠ 6♠ (movable run of 2). Pile 1: 7♥ 6♠ (mixed).
+        layout[0].1 = vec![
+            card(Suit::Spades, Rank::Seven),
+            card(Suit::Spades, Rank::Six),
+        ];
+        layout[1].1 = vec![
+            card(Suit::Hearts, Rank::Seven),
+            card(Suit::Spades, Rank::Six),
+        ];
+        // Destinations: 8♣ (for the pair), 7♦ (for a lone six).
+        layout[2].1 = vec![card(Suit::Clubs, Rank::Eight)];
+        layout[3].1 = vec![card(Suit::Diamonds, Rank::Seven)];
+        let game = SpiderGame::from_test_layout(layout, Vec::new(), 0);
+        assert!(game.is_move_valid(0, 2, 2), "same-suit pair moves");
+        assert!(!game.is_move_valid(1, 2, 2), "mixed-suit pair does not");
+        assert!(game.is_move_valid(1, 3, 1), "its top card alone does");
+    }
+
+    #[test]
+    fn empty_pile_accepts_any_run_and_occupied_gap_rules_hold() {
+        let mut layout = empty_layout();
+        layout[0].1 = vec![
+            card(Suit::Spades, Rank::Nine),
+            card(Suit::Spades, Rank::Eight),
+        ];
+        // Pile 1 deliberately left empty.
+        layout[2].1 = vec![card(Suit::Hearts, Rank::Nine)];
+        let game = SpiderGame::from_test_layout(layout, Vec::new(), 0);
+        assert!(game.is_move_valid(0, 1, 2), "run onto empty pile");
+        assert!(game.is_move_valid(2, 1, 1), "single onto empty pile");
+        assert!(
+            !game.is_move_valid(0, 2, 2),
+            "9♠8♠ cannot land on 9♥ (needs a 10)"
+        );
+    }
+
+    #[test]
+    fn invalid_moves_rejected_out_of_range_zero_count_self_move() {
+        let mut layout = empty_layout();
+        layout[0].1 = vec![card(Suit::Spades, Rank::Five)];
+        let game = SpiderGame::from_test_layout(layout, Vec::new(), 0);
+        assert!(!game.is_move_valid(0, 0, 1), "self-move");
+        assert!(!game.is_move_valid(0, 1, 0), "zero count");
+        assert!(!game.is_move_valid(0, 10, 1), "destination out of range");
+        assert!(!game.is_move_valid(10, 0, 1), "source out of range");
+        assert!(!game.is_move_valid(0, 1, 2), "count exceeds run");
+    }
+
+    // -- dealing from stock -------------------------------------------------
+
+    #[test]
+    fn deal_requires_stock_and_no_empty_pile() {
+        let stock = vec![card(Suit::Spades, Rank::Ace); 10];
+        let game = SpiderGame::from_test_layout(junk_layout(), stock.clone(), 0);
+        assert!(game.is_deal_valid());
+
+        let mut with_gap = junk_layout();
+        with_gap[3].1.clear();
+        let game = SpiderGame::from_test_layout(with_gap, stock, 0);
+        assert!(!game.is_deal_valid(), "empty pile blocks the deal");
+
+        let game = SpiderGame::from_test_layout(junk_layout(), Vec::new(), 0);
+        assert!(!game.is_deal_valid(), "empty stock blocks the deal");
+    }
+
+    #[test]
+    fn deal_puts_one_card_on_every_pile() {
+        let mut state = SpiderGameState::new_with_suits(3, SpiderSuits::Two);
+        let before: Vec<usize> = (0..SPIDER_TABLEAUS)
+            .map(|i| state.game().tableau_face_up(i).len())
+            .collect();
+        state
+            .apply_instruction(SpiderInstruction::Deal)
+            .expect("deal is legal on a fresh game");
+        for (index, previous) in before.iter().enumerate() {
+            assert_eq!(state.game().tableau_face_up(index).len(), previous + 1);
+        }
+        assert_eq!(state.game().stock_len(), STOCK_SIZE - SPIDER_TABLEAUS);
+    }
+
+    #[test]
+    fn stock_supports_exactly_five_deals() {
+        let mut state = SpiderGameState::new_with_suits(11, SpiderSuits::One);
+        for _ in 0..5 {
+            state
+                .apply_instruction(SpiderInstruction::Deal)
+                .expect("five deals must all be legal on untouched piles");
+        }
+        assert_eq!(state.game().stock_len(), 0);
+        assert!(matches!(
+            state.apply_instruction(SpiderInstruction::Deal),
+            Err(MoveError::RuleViolation(_))
+        ));
+    }
+
+    // -- run completion & win ----------------------------------------------
+
+    #[test]
+    fn completing_a_run_removes_it_and_flips_the_card_beneath() {
+        let mut layout = empty_layout();
+        // Pile 0: one face-down card under K..2 of spades; the ace
+        // arrives from pile 1.
+        let mut run = full_run(Suit::Spades);
+        let ace = run.pop().unwrap_or(card(Suit::Spades, Rank::Ace));
+        layout[0] = (vec![card(Suit::Hearts, Rank::Nine)], run);
+        layout[1].1 = vec![ace];
+        let game = SpiderGame::from_test_layout(layout, Vec::new(), 0);
+        let mut state = SpiderGameState::from_test_game(game, SpiderSuits::One);
+
+        state
+            .apply_instruction(SpiderInstruction::Move {
+                from: 1,
+                to: 0,
+                count: 1,
+            })
+            .expect("ace onto two completes the run");
+
+        assert_eq!(state.game().completed_runs(), 1);
+        assert_eq!(
+            state.game().tableau_face_up(0),
+            &[card(Suit::Hearts, Rank::Nine)],
+            "run removed and the buried card flipped face-up"
+        );
+        assert!(state.game().tableau_face_up(1).is_empty());
+    }
+
+    #[test]
+    fn eighth_run_wins_the_game() {
+        let mut layout = empty_layout();
+        let mut run = full_run(Suit::Spades);
+        let ace = run.pop().unwrap_or(card(Suit::Spades, Rank::Ace));
+        layout[0].1 = run;
+        layout[1].1 = vec![ace];
+        let game = SpiderGame::from_test_layout(layout, Vec::new(), TOTAL_RUNS - 1);
+        let mut state = SpiderGameState::from_test_game(game, SpiderSuits::One);
+
+        assert!(!state.is_won());
+        state
+            .apply_instruction(SpiderInstruction::Move {
+                from: 1,
+                to: 0,
+                count: 1,
+            })
+            .expect("winning move is legal");
+        assert!(state.is_won());
+        assert!(matches!(
+            state.apply_instruction(SpiderInstruction::Deal),
+            Err(MoveError::GameAlreadyWon)
+        ));
+    }
+
+    // -- session wrapper -----------------------------------------------------
+
+    #[test]
+    fn undo_restores_position_and_counts() {
+        let mut state = SpiderGameState::new_with_suits(21, SpiderSuits::Two);
+        let fresh = state.game().clone();
+
+        assert!(matches!(state.undo(), Err(MoveError::UndoStackEmpty)));
+        state
+            .apply_instruction(SpiderInstruction::Deal)
+            .expect("deal");
+        assert_ne!(*state.game(), fresh);
+
+        state.undo().expect("one snapshot to restore");
+        assert_eq!(*state.game(), fresh);
+        assert_eq!(state.undo_count(), 1);
+        assert_eq!(state.move_count(), 0, "undo pops the history entry");
+    }
+
+    #[test]
+    fn score_follows_base_move_penalty_and_run_bonus() {
+        let mut state = SpiderGameState::new_with_suits(5, SpiderSuits::One);
+        assert_eq!(state.score(), 500, "fresh game starts at base");
+        state
+            .apply_instruction(SpiderInstruction::Deal)
+            .expect("deal");
+        assert_eq!(state.score(), 499, "one move costs one point");
+    }
+
+    #[test]
+    fn rule_violation_surfaces_move_error() {
+        let mut state = SpiderGameState::new_with_suits(9, SpiderSuits::One);
+        let result = state.apply_instruction(SpiderInstruction::Move {
+            from: 0,
+            to: 0,
+            count: 1,
+        });
+        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
+    }
+
+    // -- generated instructions ----------------------------------------------
+
+    #[test]
+    fn possible_instructions_are_all_valid_and_include_deal() {
+        let state = SpiderGameState::new_with_suits(13, SpiderSuits::Four);
+        let config = SpiderConfig::default();
+        let instructions = state.possible_instructions();
+        assert!(
+            instructions.contains(&SpiderInstruction::Deal),
+            "fresh game has no empty pile, so Deal must be offered"
+        );
+        for instruction in instructions {
+            assert!(state.game().is_instruction_valid(&config, instruction));
+        }
+    }
+}
+
+#[cfg(test)]
+mod proptests {
+    use super::*;
+    use proptest::prelude::*;
+
+    /// Total cards across tableaus + stock + removed runs must always
+    /// equal 104, and every generated instruction must validate — for
+    /// any seed, difficulty, and random walk through legal moves.
+    fn card_conservation(game: &SpiderGame) -> usize {
+        let on_piles: usize = (0..SPIDER_TABLEAUS)
+            .map(|i| game.tableau_face_up(i).len() + game.tableau_face_down(i).len())
+            .sum();
+        on_piles + game.stock_len() + usize::from(game.completed_runs()) * RUN_LEN
+    }
+
+    proptest! {
+        #[test]
+        fn random_walks_conserve_cards_and_stay_valid(
+            seed in any::<u64>(),
+            suit_pick in 0u8..3,
+            steps in 0usize..40,
+            choices in proptest::collection::vec(any::<u32>(), 40),
+        ) {
+            let suits = match suit_pick {
+                0 => SpiderSuits::One,
+                1 => SpiderSuits::Two,
+                _ => SpiderSuits::Four,
+            };
+            let mut state = SpiderGameState::new_with_suits(seed, suits);
+            prop_assert_eq!(card_conservation(state.game()), SPIDER_DECK_SIZE);
+
+            for choice in choices.iter().take(steps) {
+                let legal = state.possible_instructions();
+                if legal.is_empty() {
+                    break;
+                }
+                let instruction = legal[*choice as usize % legal.len()];
+                prop_assert!(state.apply_instruction(instruction).is_ok());
+                prop_assert_eq!(card_conservation(state.game()), SPIDER_DECK_SIZE);
+            }
+        }
+    }
+}