use std::collections::{HashMap, VecDeque};
use serde::{Deserialize, Serialize};
use crate::card::Card;
use crate::deck::{deal_klondike, Deck};
use crate::error::MoveError;
use crate::pile::{Pile, PileType};
use crate::rules::{can_place_on_foundation, can_place_on_tableau, is_valid_tableau_sequence};
use crate::scoring::{compute_time_bonus as scoring_time_bonus, score_move, score_undo as scoring_undo};

const MAX_UNDO_STACK: usize = 64;

/// Save-file schema version for `GameState`. Increment when the on-disk
/// representation changes incompatibly so `load_game_state_from` can refuse
/// older formats and start the player on a fresh game.
///
/// History:
/// - v1: `Foundation(Suit)` keys.
/// - v2 (current): `Foundation(u8)` slot keys; claimed suit derived from the
///   bottom card of the pile.
pub const GAME_STATE_SCHEMA_VERSION: u32 = 2;

/// Default value for `GameState::schema_version` when deserialising older
/// save files that pre-date the field.
fn schema_v1() -> u32 { 1 }

/// Serialize `HashMap<PileType, Pile>` as a `Vec` of `(key, value)` pairs so
/// that JSON (which requires string map keys) round-trips correctly.
mod pile_map_serde {
    use std::collections::HashMap;
    use serde::{Deserialize, Deserializer, Serialize, Serializer};
    use crate::pile::{Pile, PileType};

    pub fn serialize<S: Serializer>(map: &HashMap<PileType, Pile>, s: S) -> Result<S::Ok, S::Error> {
        let mut entries: Vec<(&PileType, &Pile)> = map.iter().collect();
        entries.sort_by_key(|(k, _)| *k);
        entries.serialize(s)
    }

    pub fn deserialize<'de, D: Deserializer<'de>>(d: D) -> Result<HashMap<PileType, Pile>, D::Error> {
        let entries: Vec<(PileType, Pile)> = Vec::deserialize(d)?;
        Ok(entries.into_iter().collect())
    }
}

/// Whether cards are drawn one at a time or three at a time from the stock.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum DrawMode {
    /// Draw one card from stock per turn.
    DrawOne,
    /// Draw three cards from stock per turn; only the top is playable.
    DrawThree,
}

/// Difficulty tier for `GameMode::Difficulty`. Controls which pre-verified seed
/// catalog is drawn from. `Random` skips verification entirely and uses a
/// system-time seed — deals may or may not be winnable.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize, Default)]
pub enum DifficultyLevel {
    #[default]
    Easy,
    Medium,
    Hard,
    Expert,
    Grandmaster,
    /// Unverified system-time seed — may or may not be winnable.
    Random,
}

impl DifficultyLevel {
    /// Short human-readable label shown in the HUD and win summary.
    pub fn label(self) -> &'static str {
        match self {
            Self::Easy => "Easy",
            Self::Medium => "Medium",
            Self::Hard => "Hard",
            Self::Expert => "Expert",
            Self::Grandmaster => "Grandmaster",
            Self::Random => "Random",
        }
    }
}

/// Top-level game mode. Affects scoring, undo, and (eventually) timer behaviour.
///
/// - `Classic`: standard Klondike scoring, undo allowed.
/// - `Zen`: scoring suppressed (stays at 0); undo allowed; intended for relaxed play.
/// - `Challenge`: standard scoring, **undo disabled** (returns
///   `MoveError::RuleViolation`).
/// - `TimeAttack`: standard scoring + undo; the engine wraps a 10-minute
///   countdown around the session and auto-deals a fresh game on every win
///   (see `solitaire_engine::TimeAttackPlugin`).
/// - `Difficulty(DifficultyLevel)`: seed drawn from a pre-verified per-tier catalog
///   (or system-time for `Random`). Rules identical to Classic.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
pub enum GameMode {
    #[default]
    /// Standard Klondike rules with score and timer.
    Classic,
    /// No timer, no score display, ambient audio only.
    Zen,
    /// Fixed hard seeds, no undo, must win to advance.
    Challenge,
    /// Play as many games as possible within 10 minutes.
    TimeAttack,
    /// Seed drawn from a difficulty-tiered catalog; rules identical to Classic.
    Difficulty(DifficultyLevel),
}

/// Snapshot of game state used for undo.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
struct StateSnapshot {
    #[serde(with = "pile_map_serde")]
    piles: HashMap<PileType, Pile>,
    score: i32,
    move_count: u32,
}

/// Full state of an in-progress Klondike Solitaire game.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct GameState {
    /// All card piles keyed by pile type. Contains Stock, Waste, 4 Foundations, and 7 Tableau piles.
    #[serde(with = "pile_map_serde")]
    pub piles: HashMap<PileType, Pile>,
    /// Whether the player draws one or three cards from the stock per turn.
    pub draw_mode: DrawMode,
    /// Top-level mode (Classic / Zen). Defaults to Classic for backwards
    /// compatibility with older save files via `#[serde(default)]`.
    #[serde(default)]
    pub mode: GameMode,
    /// Current game score. Can be negative (undo penalties subtract from score).
    pub score: i32,
    /// Total moves made this game, including draws and stock recycles.
    pub move_count: u32,
    /// Seconds elapsed since the game started, used for time-bonus scoring.
    pub elapsed_seconds: u64,
    /// RNG seed used to deal this game. Same seed always produces the same layout.
    pub seed: u64,
    /// True once all 52 cards are on the foundations. No further moves are accepted.
    pub is_won: bool,
    /// True when the game can be completed without further input (all remaining cards are face-up and in order).
    pub is_auto_completable: bool,
    /// Number of times `undo()` has been successfully invoked this game.
    /// Used by achievement conditions like `no_undo`.
    pub undo_count: u32,
    /// Number of times the waste pile has been recycled back to stock this game.
    /// Used by the `comeback` achievement condition.
    #[serde(default)]
    pub recycle_count: u32,
    /// When `true`, the player may move the top card of a foundation pile back
    /// onto a compatible tableau column. Off by default — non-standard house rule.
    #[serde(default)]
    pub take_from_foundation: bool,
    /// Save-file schema version. Defaults to `1` for older files that pre-date
    /// the field. The loader refuses any value other than
    /// [`GAME_STATE_SCHEMA_VERSION`].
    #[serde(default = "schema_v1")]
    pub schema_version: u32,
    #[serde(skip)]
    undo_stack: VecDeque<StateSnapshot>,
}

impl GameState {
    /// Creates a new Classic-mode game dealt from the given seed and draw mode.
    pub fn new(seed: u64, draw_mode: DrawMode) -> Self {
        Self::new_with_mode(seed, draw_mode, GameMode::Classic)
    }

    /// Creates a new game with an explicit `GameMode`.
    pub fn new_with_mode(seed: u64, draw_mode: DrawMode, mode: GameMode) -> Self {
        let mut deck = Deck::new();
        deck.shuffle(seed);
        let (tableau, stock) = deal_klondike(deck);

        let mut piles: HashMap<PileType, Pile> = HashMap::new();
        piles.insert(PileType::Stock, stock);
        piles.insert(PileType::Waste, Pile::new(PileType::Waste));
        for slot in 0..4_u8 {
            piles.insert(PileType::Foundation(slot), Pile::new(PileType::Foundation(slot)));
        }
        for (i, pile) in tableau.into_iter().enumerate() {
            piles.insert(PileType::Tableau(i), pile);
        }

        Self {
            piles,
            draw_mode,
            mode,
            score: 0,
            move_count: 0,
            elapsed_seconds: 0,
            seed,
            is_won: false,
            is_auto_completable: false,
            undo_count: 0,
            recycle_count: 0,
            take_from_foundation: true,
            schema_version: GAME_STATE_SCHEMA_VERSION,
            undo_stack: VecDeque::new(),
        }
    }

    /// Number of snapshots currently on the undo stack.
    pub fn undo_stack_len(&self) -> usize {
        self.undo_stack.len()
    }

    fn take_snapshot(&self) -> StateSnapshot {
        StateSnapshot {
            piles: self.piles.clone(),
            score: self.score,
            move_count: self.move_count,
        }
    }

    fn push_snapshot(&mut self) {
        if self.undo_stack.len() >= MAX_UNDO_STACK {
            self.undo_stack.pop_front();  // O(1)
        }
        self.undo_stack.push_back(self.take_snapshot());
    }

    /// Draw cards from stock to waste. When stock is empty, recycles waste back to stock.
    /// Recycling is unlimited: `StockEmpty` is only returned when both stock and waste are empty.
    pub fn draw(&mut self) -> Result<(), MoveError> {
        if self.is_won {
            return Err(MoveError::GameAlreadyWon);
        }

        let stock_len = self.piles.get(&PileType::Stock).ok_or(MoveError::InvalidSource)?.cards.len();

        if stock_len == 0 {
            let waste_len = self.piles.get(&PileType::Waste).ok_or(MoveError::InvalidSource)?.cards.len();
            if waste_len == 0 {
                return Err(MoveError::StockEmpty);
            }
            // Recycle: snapshot so undo can reverse it, then move waste back to stock face-down
            self.push_snapshot();
            let waste_cards: Vec<Card> = self.piles
                .get_mut(&PileType::Waste)
                .ok_or(MoveError::InvalidSource)?
                .cards
                .drain(..)
                .collect();
            let stock = self.piles.get_mut(&PileType::Stock).ok_or(MoveError::InvalidDestination)?;
            for mut card in waste_cards.into_iter().rev() {
                card.face_up = false;
                stock.cards.push(card);
            }
            self.recycle_count = self.recycle_count.saturating_add(1);
            self.move_count = self.move_count.saturating_add(1);
            return Ok(());
        }

        self.push_snapshot();

        let draw_count = match self.draw_mode {
            DrawMode::DrawOne => 1,
            DrawMode::DrawThree => 3,
        };
        let available = stock_len.min(draw_count);
        let drain_start = stock_len - available;

        let drawn: Vec<Card> = self.piles
            .get_mut(&PileType::Stock)
            .ok_or(MoveError::InvalidSource)?
            .cards
            .drain(drain_start..)
            .collect();

        let waste = self.piles.get_mut(&PileType::Waste).ok_or(MoveError::InvalidDestination)?;
        for mut card in drawn {
            card.face_up = true;
            waste.cards.push(card);
        }

        self.move_count = self.move_count.saturating_add(1);
        Ok(())
    }

    /// Move `count` cards from pile `from` to pile `to`.
    ///
    /// Returns `Err(MoveError)` if the move is illegal. On success, updates score,
    /// flips the newly exposed source card if needed, and checks win/auto-complete.
    pub fn move_cards(&mut self, from: PileType, to: PileType, count: usize) -> Result<(), MoveError> {
        if self.is_won {
            return Err(MoveError::GameAlreadyWon);
        }
        if from == to {
            return Err(MoveError::RuleViolation("source and destination must differ".into()));
        }

        // Validate via scoped immutable borrows
        let move_start = {
            let from_pile = self.piles.get(&from).ok_or(MoveError::InvalidSource)?;
            if from_pile.cards.is_empty() {
                return Err(MoveError::EmptySource);
            }
            if count == 0 || count > from_pile.cards.len() {
                return Err(MoveError::RuleViolation("invalid card count".into()));
            }
            let start = from_pile.cards.len() - count;
            for card in &from_pile.cards[start..] {
                if !card.face_up {
                    return Err(MoveError::RuleViolation("cannot move face-down card".into()));
                }
            }
            let bottom_card = from_pile.cards[start].clone();

            match &to {
                PileType::Foundation(_) => {
                    if count != 1 {
                        return Err(MoveError::RuleViolation(
                            "only one card can move to foundation at a time".into(),
                        ));
                    }
                    let dest = self.piles.get(&to).ok_or(MoveError::InvalidDestination)?;
                    if !can_place_on_foundation(&bottom_card, dest) {
                        return Err(MoveError::RuleViolation("invalid foundation placement".into()));
                    }
                }
                PileType::Tableau(_) => {
                    if matches!(&from, PileType::Foundation(_)) {
                        if !self.take_from_foundation {
                            return Err(MoveError::RuleViolation(
                                "take-from-foundation rule is disabled".into(),
                            ));
                        }
                        if count != 1 {
                            return Err(MoveError::RuleViolation(
                                "only one card can return from foundation at a time".into(),
                            ));
                        }
                    }
                    let dest = self.piles.get(&to).ok_or(MoveError::InvalidDestination)?;
                    if !can_place_on_tableau(&bottom_card, dest) {
                        return Err(MoveError::RuleViolation("invalid tableau placement".into()));
                    }
                    // The previous check only validates that the *bottom* of the
                    // moved stack lands on the destination's top card. Without
                    // this guard, a player could lift an arbitrary multi-card
                    // selection from one column and drop it onto another whenever
                    // the bottom card happens to match — even if the cards
                    // above the bottom don't form a legal descending
                    // alternating-colour run.
                    if !is_valid_tableau_sequence(&from_pile.cards[start..]) {
                        return Err(MoveError::RuleViolation(
                            "moved cards must form a valid tableau run".into(),
                        ));
                    }
                }
                _ => return Err(MoveError::InvalidDestination),
            }
            start
        };

        let score_delta = if self.mode == GameMode::Zen {
            0
        } else {
            score_move(&from, &to)
        };
        self.push_snapshot();

        // Execute move
        let mut moved: Vec<Card> = self.piles
            .get_mut(&from)
            .ok_or(MoveError::InvalidSource)?
            .cards
            .split_off(move_start);

        // Flip the newly exposed top card of the source pile
        if let Some(top) = self.piles
            .get_mut(&from)
            .ok_or(MoveError::InvalidSource)?
            .cards
            .last_mut()
            && !top.face_up
        {
            top.face_up = true;
        }

        self.piles.get_mut(&to).ok_or(MoveError::InvalidDestination)?.cards.append(&mut moved);

        self.score = (self.score + score_delta).max(0);
        self.move_count = self.move_count.saturating_add(1);

        self.is_won = self.check_win();
        if !self.is_won {
            self.is_auto_completable = self.check_auto_complete();
        }

        Ok(())
    }

    /// Restore the most recent undo snapshot and apply the undo score penalty (-15).
    /// Disabled in `GameMode::Challenge` — returns `MoveError::RuleViolation`.
    pub fn undo(&mut self) -> Result<(), MoveError> {
        if self.is_won {
            return Err(MoveError::GameAlreadyWon);
        }
        if self.mode == GameMode::Challenge {
            return Err(MoveError::RuleViolation(
                "undo is disabled in Challenge mode".into(),
            ));
        }
        let snapshot = self.undo_stack.pop_back().ok_or(MoveError::UndoStackEmpty)?;
        self.piles = snapshot.piles;
        self.score = if self.mode == GameMode::Zen {
            0
        } else {
            (self.score + scoring_undo()).max(0)
        };
        self.move_count = snapshot.move_count;
        self.is_won = false;
        self.is_auto_completable = false;
        self.undo_count = self.undo_count.saturating_add(1);
        Ok(())
    }

    /// Returns `true` when all four foundation slots each contain a valid A→K
    /// sequence of a single suit.
    ///
    /// Counting 13 cards is not sufficient — a corrupt save could produce 13
    /// arbitrary cards per pile and permanently lock the game via `GameAlreadyWon`.
    pub fn check_win(&self) -> bool {
        (0..4_u8).all(|slot| self.is_valid_foundation_pile(slot))
    }

    fn is_valid_foundation_pile(&self, slot: u8) -> bool {
        let Some(pile) = self.piles.get(&PileType::Foundation(slot)) else {
            return false;
        };
        if pile.cards.len() != 13 {
            return false;
        }
        let suit = pile.cards[0].suit;
        pile.cards.iter().enumerate().all(|(i, card)| {
            card.suit == suit && card.rank.value() == (i as u8 + 1)
        })
    }

    /// Returns `true` when stock and waste are empty and all tableau cards are face-up.
    /// At that point the game can be completed without further player input.
    pub fn check_auto_complete(&self) -> bool {
        // Stock must be empty; waste may still have cards (they are resolved
        // by draw() calls inside next_auto_complete_move / auto_complete_step).
        if self.piles.get(&PileType::Stock).is_none_or(|p| !p.cards.is_empty()) {
            return false;
        }
        (0..7).all(|i| {
            self.piles
                .get(&PileType::Tableau(i))
                .is_some_and(|p| p.cards.iter().all(|c| c.face_up))
        })
    }

    /// Returns all currently valid `move_cards` calls as `(from, to, count)` triples.
    ///
    /// Does not include stock draws — callers check `piles[&PileType::Stock]` directly.
    /// Every returned triple is guaranteed to succeed when passed to `move_cards`.
    pub fn possible_instructions(&self) -> Vec<(PileType, PileType, usize)> {
        if self.is_won {
            return Vec::new();
        }

        let mut moves = Vec::new();

        // Waste top card → foundation or tableau
        if let Some(waste_top) = self.piles.get(&PileType::Waste).and_then(|p| p.cards.last()) {
            for slot in 0..4_u8 {
                if let Some(f) = self.piles.get(&PileType::Foundation(slot))
                    && can_place_on_foundation(waste_top, f)
                {
                    moves.push((PileType::Waste, PileType::Foundation(slot), 1));
                }
            }
            for dst in 0..7_usize {
                if let Some(t) = self.piles.get(&PileType::Tableau(dst))
                    && can_place_on_tableau(waste_top, t)
                {
                    moves.push((PileType::Waste, PileType::Tableau(dst), 1));
                }
            }
        }

        // Tableau sources
        for src in 0..7_usize {
            let Some(src_pile) = self.piles.get(&PileType::Tableau(src)) else { continue };
            if src_pile.cards.is_empty() {
                continue;
            }
            let run_len = src_pile.cards.iter().rev().take_while(|c| c.face_up).count();
            if run_len == 0 {
                continue;
            }
            for count in 1..=run_len {
                let seq_start = src_pile.cards.len() - count;
                if !is_valid_tableau_sequence(&src_pile.cards[seq_start..]) {
                    continue;
                }
                let bottom = &src_pile.cards[seq_start];
                if count == 1 {
                    for slot in 0..4_u8 {
                        if let Some(f) = self.piles.get(&PileType::Foundation(slot))
                            && can_place_on_foundation(bottom, f)
                        {
                            moves.push((PileType::Tableau(src), PileType::Foundation(slot), 1));
                        }
                    }
                }
                for dst in 0..7_usize {
                    if dst == src {
                        continue;
                    }
                    if let Some(t) = self.piles.get(&PileType::Tableau(dst))
                        && can_place_on_tableau(bottom, t)
                    {
                        moves.push((PileType::Tableau(src), PileType::Tableau(dst), count));
                    }
                }
            }
        }

        // Foundation top → tableau (only when house rule is enabled)
        if self.take_from_foundation {
            for slot in 0..4_u8 {
                let Some(f) = self.piles.get(&PileType::Foundation(slot)) else { continue };
                let Some(top) = f.cards.last() else { continue };
                for dst in 0..7_usize {
                    if let Some(t) = self.piles.get(&PileType::Tableau(dst))
                        && can_place_on_tableau(top, t)
                    {
                        moves.push((PileType::Foundation(slot), PileType::Tableau(dst), 1));
                    }
                }
            }
        }

        moves
    }

    /// Returns the next `(from, to)` move that advances auto-complete, or
    /// `None` if no such move exists (or `is_auto_completable` is not set).
    ///
    /// Scans tableau piles 0–6 in order, returning the first top card that
    /// can be placed on any foundation pile. The scan order ensures Aces are
    /// resolved before higher ranks that depend on them.
    ///
    /// # Precondition
    ///
    /// This function is only called when `is_auto_completable` is `true`.
    /// Auto-completability requires the waste pile to be empty, as enforced by
    /// [`check_auto_complete`](Self::check_auto_complete) — it returns `false`
    /// whenever `piles[Waste]` is non-empty. Therefore, skipping the waste pile
    /// in this scan is intentional and correct: by the time this function is
    /// reached, there are guaranteed to be no cards there to move.
    pub fn next_auto_complete_move(&self) -> Option<(PileType, PileType)> {
        if !self.is_auto_completable || self.is_won {
            return None;
        }
        // Check waste top first — when stock is exhausted the waste may still
        // contain cards that can go directly to a foundation.
        let waste = PileType::Waste;
        if let Some((card, slot)) = self.piles.get(&waste)
            .and_then(|p| p.cards.last())
            .and_then(|c| self.foundation_slot_for(c).map(|s| (c, s)))
        {
            let _ = card; // borrow ends here
            return Some((waste, PileType::Foundation(slot)));
        }
        for i in 0..7 {
            let tableau = PileType::Tableau(i);
            if let Some(slot) = self.piles.get(&tableau)
                .and_then(|p| p.cards.last())
                .and_then(|c| self.foundation_slot_for(c))
            {
                return Some((tableau, PileType::Foundation(slot)));
            }
        }
        None
    }

    /// Return the foundation slot index that `card` can legally move to, or
    /// `None` if no such slot exists.
    ///
    /// Prefers the slot already claiming this card's suit so Aces always land
    /// in a consistent column. Falls back to an empty slot only for Aces.
    fn foundation_slot_for(&self, card: &crate::card::Card) -> Option<u8> {
        let mut candidate: Option<u8> = None;
        let mut empty_slot: Option<u8> = None;
        for slot in 0..4_u8 {
            let Some(pile) = self.piles.get(&PileType::Foundation(slot)) else { continue };
            if pile.cards.is_empty() {
                if empty_slot.is_none() {
                    empty_slot = Some(slot);
                }
            } else if pile.claimed_suit() == Some(card.suit) {
                candidate = Some(slot);
                break;
            }
        }
        let target = candidate.or_else(|| {
            if card.rank.value() == 1 { empty_slot } else { None }
        });
        target.filter(|&slot| {
            self.piles.get(&PileType::Foundation(slot))
                .is_some_and(|p| can_place_on_foundation(card, p))
        })
    }

    /// Time bonus added to score on win: `700_000 / elapsed_seconds` (0 if elapsed is 0).
    pub fn compute_time_bonus(&self) -> i32 {
        scoring_time_bonus(self.elapsed_seconds)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::card::{Card, Rank, Suit};

    fn new_game() -> GameState {
        GameState::new(42, DrawMode::DrawOne)
    }

    // --- Initial state ---

    #[test]
    fn new_game_has_correct_tableau_sizes() {
        let g = new_game();
        let total: usize = (0..7).map(|i| g.piles[&PileType::Tableau(i)].cards.len()).sum();
        assert_eq!(total, 28);
        for i in 0..7 {
            assert_eq!(g.piles[&PileType::Tableau(i)].cards.len(), i + 1);
        }
    }

    #[test]
    fn new_game_stock_has_24_cards() {
        assert_eq!(new_game().piles[&PileType::Stock].cards.len(), 24);
    }

    #[test]
    fn new_game_waste_is_empty() {
        assert!(new_game().piles[&PileType::Waste].cards.is_empty());
    }

    #[test]
    fn new_game_foundations_are_empty() {
        let g = new_game();
        for slot in 0..4_u8 {
            assert!(g.piles[&PileType::Foundation(slot)].cards.is_empty());
        }
    }

    #[test]
    fn new_game_is_not_won() {
        assert!(!new_game().is_won);
    }

    // --- Seeded reproducibility ---

    #[test]
    fn same_seed_produces_identical_layout() {
        let g1 = GameState::new(12345, DrawMode::DrawOne);
        let g2 = GameState::new(12345, DrawMode::DrawOne);
        for i in 0..7 {
            assert_eq!(
                g1.piles[&PileType::Tableau(i)].cards,
                g2.piles[&PileType::Tableau(i)].cards
            );
        }
        assert_eq!(
            g1.piles[&PileType::Stock].cards,
            g2.piles[&PileType::Stock].cards
        );
    }

    #[test]
    fn different_seeds_produce_different_layouts() {
        let g1 = GameState::new(1, DrawMode::DrawOne);
        let g2 = GameState::new(2, DrawMode::DrawOne);
        let t1: Vec<u32> = g1.piles[&PileType::Tableau(0)].cards.iter().map(|c| c.id).collect();
        let t2: Vec<u32> = g2.piles[&PileType::Tableau(0)].cards.iter().map(|c| c.id).collect();
        assert_ne!(t1, t2);
    }

    // --- Draw ---

    #[test]
    fn draw_one_moves_one_card_to_waste() {
        let mut g = new_game();
        let stock_before = g.piles[&PileType::Stock].cards.len();
        g.draw().unwrap();
        assert_eq!(g.piles[&PileType::Stock].cards.len(), stock_before - 1);
        assert_eq!(g.piles[&PileType::Waste].cards.len(), 1);
    }

    #[test]
    fn drawn_card_is_face_up() {
        let mut g = new_game();
        g.draw().unwrap();
        assert!(g.piles[&PileType::Waste].cards.last().unwrap().face_up);
    }

    #[test]
    fn draw_three_moves_up_to_three_cards() {
        let mut g = GameState::new(42, DrawMode::DrawThree);
        g.draw().unwrap();
        assert_eq!(g.piles[&PileType::Waste].cards.len(), 3);
        assert_eq!(g.piles[&PileType::Stock].cards.len(), 21);
    }

    #[test]
    fn draw_three_partial_draw_when_fewer_than_three_remain() {
        let mut g = GameState::new(42, DrawMode::DrawThree);
        // Replace the stock with exactly 2 cards so the draw is a partial batch.
        let two_cards: Vec<Card> = g.piles[&PileType::Stock].cards[..2].to_vec();
        g.piles.get_mut(&PileType::Stock).unwrap().cards = two_cards;
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();

        g.draw().unwrap();

        assert_eq!(g.piles[&PileType::Waste].cards.len(), 2, "only 2 cards should move when stock has 2");
        assert!(g.piles[&PileType::Stock].cards.is_empty());
    }

    #[test]
    fn draw_three_all_drawn_cards_are_face_up() {
        let mut g = GameState::new(42, DrawMode::DrawThree);
        g.draw().unwrap();
        assert!(
            g.piles[&PileType::Waste].cards.iter().all(|c| c.face_up),
            "all drawn cards must be face-up in waste"
        );
    }

    #[test]
    fn draw_three_undo_returns_all_cards_to_stock() {
        let mut g = GameState::new(42, DrawMode::DrawThree);
        let stock_before = g.piles[&PileType::Stock].cards.len();

        g.draw().unwrap();
        assert_eq!(g.piles[&PileType::Waste].cards.len(), 3);

        g.undo().unwrap();
        assert_eq!(g.piles[&PileType::Stock].cards.len(), stock_before);
        assert!(g.piles[&PileType::Waste].cards.is_empty());
    }

    #[test]
    fn draw_three_recycle_restores_waste_to_stock_face_down() {
        let mut g = GameState::new(42, DrawMode::DrawThree);
        // Drain all 24 stock cards into waste via repeated draws.
        while !g.piles[&PileType::Stock].cards.is_empty() {
            g.draw().unwrap();
        }
        let waste_count = g.piles[&PileType::Waste].cards.len();
        assert!(waste_count > 0);

        // Recycle: drawing when stock is empty returns all waste cards to stock.
        g.draw().unwrap();
        assert_eq!(g.piles[&PileType::Stock].cards.len(), waste_count);
        assert!(g.piles[&PileType::Waste].cards.is_empty());
        assert!(
            g.piles[&PileType::Stock].cards.iter().all(|c| !c.face_up),
            "recycled cards must be face-down"
        );
    }

    #[test]
    fn draw_from_empty_stock_recycles_waste() {
        let mut g = new_game();
        while !g.piles[&PileType::Stock].cards.is_empty() {
            g.draw().unwrap();
        }
        let waste_count = g.piles[&PileType::Waste].cards.len();
        assert!(waste_count > 0);
        g.draw().unwrap(); // recycle
        assert_eq!(g.piles[&PileType::Stock].cards.len(), waste_count);
        assert!(g.piles[&PileType::Waste].cards.is_empty());
    }

    #[test]
    fn recycle_count_increments_on_each_waste_recycle() {
        let mut g = new_game();
        assert_eq!(g.recycle_count, 0);
        // Drain entire stock to waste.
        while !g.piles[&PileType::Stock].cards.is_empty() {
            g.draw().unwrap();
        }
        g.draw().unwrap(); // first recycle
        assert_eq!(g.recycle_count, 1);
        // Drain again and recycle a second time.
        while !g.piles[&PileType::Stock].cards.is_empty() {
            g.draw().unwrap();
        }
        g.draw().unwrap(); // second recycle
        assert_eq!(g.recycle_count, 2);
    }

    #[test]
    fn move_count_increments_on_recycle() {
        let mut g = new_game();
        // Drain stock to waste, recording how many draws it took.
        let mut draws: u32 = 0;
        while !g.piles[&PileType::Stock].cards.is_empty() {
            g.draw().unwrap();
            draws += 1;
        }
        let before = g.move_count;
        g.draw().unwrap(); // recycle
        assert_eq!(
            g.move_count,
            before + 1,
            "recycling waste back to stock must increment move_count (was {before}, draws={draws})"
        );
    }

    #[test]
    fn draw_from_empty_stock_and_waste_returns_error() {
        // The only stop condition for draw() is: both stock AND waste are
        // simultaneously empty. Manually empty both, then verify the error.
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        assert_eq!(g.draw(), Err(MoveError::StockEmpty));
    }

    // --- Move validation ---

    #[test]
    fn move_zero_cards_returns_rule_violation() {
        let mut g = new_game();
        let result = g.move_cards(PileType::Tableau(0), PileType::Tableau(1), 0);
        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
    }

    #[test]
    fn move_to_stock_returns_invalid_destination() {
        let mut g = new_game();
        let result = g.move_cards(PileType::Tableau(0), PileType::Stock, 1);
        assert_eq!(result, Err(MoveError::InvalidDestination));
    }

    #[test]
    fn move_to_waste_returns_invalid_destination() {
        let mut g = new_game();
        let result = g.move_cards(PileType::Tableau(0), PileType::Waste, 1);
        assert_eq!(result, Err(MoveError::InvalidDestination));
    }

    #[test]
    fn move_same_source_and_dest_returns_rule_violation() {
        let mut g = new_game();
        let result = g.move_cards(PileType::Tableau(0), PileType::Tableau(0), 1);
        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
    }

    #[test]
    fn move_face_down_card_returns_rule_violation() {
        let mut g = new_game();
        // Tableau(6) has 7 cards; card 0 is always face-down.
        // Attempt to move 7 cards (the whole pile including face-down ones).
        let result = g.move_cards(PileType::Tableau(6), PileType::Tableau(5), 7);
        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
    }

    #[test]
    fn move_multiple_cards_to_foundation_returns_rule_violation() {
        let mut g = new_game();
        // Inject two face-up cards into tableau(0) so count=2 is a valid count.
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards = vec![
            Card { id: 1, suit: Suit::Clubs, rank: Rank::Ace, face_up: true },
            Card { id: 2, suit: Suit::Clubs, rank: Rank::Two, face_up: true },
        ];
        let result = g.move_cards(
            PileType::Tableau(0),
            PileType::Foundation(0),
            2,
        );
        assert!(
            matches!(result, Err(MoveError::RuleViolation(_))),
            "moving 2 cards to foundation must be rejected"
        );
    }

    #[test]
    fn move_count_exceeding_pile_size_returns_rule_violation() {
        let mut g = new_game();
        // Tableau(0) has exactly 1 card; asking for 2 should fail.
        let result = g.move_cards(PileType::Tableau(0), PileType::Tableau(1), 2);
        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
    }

    #[test]
    fn move_multi_card_sequence_tableau_to_tableau_succeeds() {
        let mut g = new_game();
        // Clear both piles and construct a known valid sequence.
        let t0 = g.piles.get_mut(&PileType::Tableau(0)).unwrap();
        t0.cards = vec![
            Card { id: 10, suit: Suit::Spades,  rank: Rank::King,  face_up: true },
            Card { id: 11, suit: Suit::Hearts,  rank: Rank::Queen, face_up: true },
            Card { id: 12, suit: Suit::Spades,  rank: Rank::Jack,  face_up: true },
        ];
        // Tableau(1) needs an Ace so we can check empty pile correctly — use a red King target.
        let t1 = g.piles.get_mut(&PileType::Tableau(1)).unwrap();
        t1.cards.clear(); // empty accepts a King

        // Move the whole 3-card sequence to the empty pile.
        let result = g.move_cards(PileType::Tableau(0), PileType::Tableau(1), 3);
        assert!(result.is_ok(), "valid multi-card move must succeed: {result:?}");
        assert!(g.piles[&PileType::Tableau(0)].cards.is_empty());
        assert_eq!(g.piles[&PileType::Tableau(1)].cards.len(), 3);
        assert_eq!(g.move_count, 1);
    }

    // --- Win detection ---

    #[test]
    fn win_detection_all_foundations_complete() {
        let mut g = new_game();
        let suits = [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades];
        for (slot, suit) in suits.into_iter().enumerate() {
            let f = g.piles.get_mut(&PileType::Foundation(slot as u8)).unwrap();
            f.cards.clear();
            for rank in [
                Rank::Ace, Rank::Two, Rank::Three, Rank::Four, Rank::Five,
                Rank::Six, Rank::Seven, Rank::Eight, Rank::Nine, Rank::Ten,
                Rank::Jack, Rank::Queen, Rank::King,
            ] {
                f.cards.push(Card { id: 0, suit, rank, face_up: true });
            }
        }
        assert!(g.check_win());
    }

    #[test]
    fn win_detection_incomplete_is_false() {
        assert!(!new_game().check_win());
    }

    // --- Undo ---

    #[test]
    fn undo_empty_stack_returns_error() {
        let mut g = new_game();
        assert_eq!(g.undo(), Err(MoveError::UndoStackEmpty));
    }

    #[test]
    fn undo_after_draw_restores_pile_sizes() {
        let mut g = new_game();
        let stock_before = g.piles[&PileType::Stock].cards.len();
        let waste_before = g.piles[&PileType::Waste].cards.len();
        g.draw().unwrap();
        g.undo().unwrap();
        assert_eq!(g.piles[&PileType::Stock].cards.len(), stock_before);
        assert_eq!(g.piles[&PileType::Waste].cards.len(), waste_before);
    }

    #[test]
    fn undo_applies_score_penalty() {
        let mut g = new_game();
        let score_before = g.score;
        g.draw().unwrap();
        g.undo().unwrap();
        let expected = (score_before + scoring_undo()).max(0);
        assert_eq!(g.score, expected);
    }

    #[test]
    fn undo_stack_capped_at_64() {
        let mut g = new_game();
        for _ in 0..70 {
            let _ = g.draw();
        }
        assert!(g.undo_stack_len() <= 64);
    }

    #[test]
    fn undo_count_increments_on_each_undo() {
        let mut g = new_game();
        g.draw().unwrap();
        assert_eq!(g.undo_count, 0, "undo_count unchanged before calling undo");
        g.undo().unwrap();
        assert_eq!(g.undo_count, 1);
        g.draw().unwrap();
        g.undo().unwrap();
        assert_eq!(g.undo_count, 2);
    }

    #[test]
    fn undo_count_saturates_at_max() {
        let mut g = new_game();
        g.undo_count = u32::MAX;
        g.draw().unwrap();
        g.undo().unwrap();
        assert_eq!(g.undo_count, u32::MAX, "undo_count must saturate at u32::MAX");
    }

    // --- Fields excluded from undo snapshot ---

    #[test]
    fn undo_does_not_roll_back_elapsed_seconds() {
        // elapsed_seconds tracks wall time and must be monotonic; undo must never
        // reduce it, otherwise the time-bonus calculation would be gamed.
        let mut g = new_game();
        g.elapsed_seconds = 120;
        g.draw().unwrap();
        g.undo().unwrap();
        assert_eq!(g.elapsed_seconds, 120, "undo must leave elapsed_seconds unchanged");
    }

    #[test]
    fn undo_does_not_roll_back_recycle_count() {
        // recycle_count is a lifetime counter used for the 'comeback' achievement;
        // rolling it back on undo would make the condition unachievable after recycling.
        let mut g = new_game();
        // Drain stock and recycle to increment recycle_count.
        while !g.piles[&PileType::Stock].cards.is_empty() {
            g.draw().unwrap();
        }
        g.draw().unwrap(); // recycle
        assert_eq!(g.recycle_count, 1);
        // Now draw one more card and undo it.
        g.draw().unwrap();
        g.undo().unwrap();
        assert_eq!(g.recycle_count, 1, "undo must leave recycle_count unchanged");
    }

    #[test]
    fn undo_after_win_returns_game_already_won() {
        let mut g = new_game();
        g.is_won = true;
        assert_eq!(g.undo(), Err(MoveError::GameAlreadyWon));
    }

    // --- Scoring ---

    #[test]
    fn score_never_goes_below_zero() {
        let mut g = new_game();
        for _ in 0..5 {
            g.draw().unwrap();
            g.undo().unwrap();
        }
        assert!(g.score >= 0);
    }

    // --- GameMode: Zen ---

    #[test]
    fn zen_mode_score_stays_zero_after_undo() {
        let mut g = GameState::new_with_mode(42, DrawMode::DrawOne, GameMode::Zen);
        g.draw().unwrap();
        g.undo().unwrap();
        assert_eq!(g.score, 0);
    }

    #[test]
    fn zen_mode_field_persists_through_construction() {
        let g = GameState::new_with_mode(1, DrawMode::DrawThree, GameMode::Zen);
        assert_eq!(g.mode, GameMode::Zen);
        assert_eq!(g.draw_mode, DrawMode::DrawThree);
    }

    // --- GameMode: Challenge ---

    #[test]
    fn challenge_mode_disables_undo() {
        let mut g = GameState::new_with_mode(42, DrawMode::DrawOne, GameMode::Challenge);
        g.draw().unwrap();
        let result = g.undo();
        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
    }

    #[test]
    fn challenge_mode_still_allows_normal_moves() {
        let g = GameState::new_with_mode(42, DrawMode::DrawOne, GameMode::Challenge);
        // Just verify the game initialises cleanly with Challenge mode.
        assert_eq!(g.mode, GameMode::Challenge);
        assert_eq!(g.score, 0);
    }

    #[test]
    fn challenge_mode_scoring_applies_normally() {
        // Challenge uses Classic scoring; only undo is disabled.
        let g = GameState::new_with_mode(42, DrawMode::DrawOne, GameMode::Challenge);
        assert_eq!(g.score, 0);
        // Note: Verifying score increases on actual moves would require
        // hand-crafting a legal move from the dealt state. We rely on the
        // fact that move_cards' score path is identical to Classic.
    }

    // --- GameMode: TimeAttack ---

    #[test]
    fn time_attack_mode_field_persists() {
        let g = GameState::new_with_mode(1, DrawMode::DrawOne, GameMode::TimeAttack);
        assert_eq!(g.mode, GameMode::TimeAttack);
    }

    #[test]
    fn time_attack_allows_undo() {
        let mut g = GameState::new_with_mode(42, DrawMode::DrawOne, GameMode::TimeAttack);
        g.draw().unwrap();
        // TimeAttack does not disable undo — only Challenge does.
        assert!(g.undo().is_ok(), "undo must be permitted in TimeAttack mode");
    }

    #[test]
    fn time_attack_draw_three_combination() {
        // TimeAttack + DrawThree is a valid combination; verify construction.
        let g = GameState::new_with_mode(7, DrawMode::DrawThree, GameMode::TimeAttack);
        assert_eq!(g.mode, GameMode::TimeAttack);
        assert_eq!(g.draw_mode, DrawMode::DrawThree);
        assert_eq!(g.piles[&PileType::Stock].cards.len(), 24);
    }

    // --- Auto-complete ---

    #[test]
    fn auto_complete_false_when_stock_not_empty() {
        assert!(!new_game().check_auto_complete());
    }

    #[test]
    fn auto_complete_false_when_face_down_cards_remain() {
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        // Tableau(1) has a face-down card at index 0
        assert!(!g.check_auto_complete());
    }

    #[test]
    fn auto_complete_true_when_stock_empty_waste_has_cards() {
        // Waste no longer blocks auto-complete — draw() drains it during
        // auto-complete steps. Only stock-not-empty and face-down tableau
        // cards block the flag.
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.push(Card {
            id: 99,
            suit: Suit::Clubs,
            rank: Rank::Ace,
            face_up: true,
        });
        for i in 0..7 {
            for c in g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.iter_mut() {
                c.face_up = true;
            }
        }
        assert!(g.check_auto_complete());
    }

    #[test]
    fn auto_complete_true_when_all_prerequisites_met() {
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        // Clear all tableau and put a single face-up card — all face-up guard passes.
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 1,
            suit: Suit::Clubs,
            rank: Rank::Ace,
            face_up: true,
        });
        assert!(g.check_auto_complete());
    }

    // --- Time bonus ---

    #[test]
    fn time_bonus_zero_when_elapsed_is_zero() {
        let mut g = new_game();
        g.elapsed_seconds = 0;
        assert_eq!(g.compute_time_bonus(), 0);
    }

    #[test]
    fn time_bonus_at_100_seconds() {
        let mut g = new_game();
        g.elapsed_seconds = 100;
        assert_eq!(g.compute_time_bonus(), 7000);
    }

    // --- EmptySource error path ---

    #[test]
    fn move_from_empty_pile_returns_empty_source() {
        // Build a game state, clear a tableau pile entirely, then attempt to
        // move from it. The source pile exists in `piles` (key is present) but
        // contains no cards — exactly the code path that returns EmptySource.
        let mut g = new_game();
        // Tableau(0) starts with exactly 1 card; clear it to make the pile empty.
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.clear();
        let result = g.move_cards(PileType::Tableau(0), PileType::Tableau(1), 1);
        assert_eq!(
            result,
            Err(MoveError::EmptySource),
            "moving from an empty pile must return EmptySource"
        );
    }

    // --- next_auto_complete_move ---

    #[test]
    fn next_auto_complete_move_returns_none_on_fresh_game() {
        // A fresh game has stock and face-down cards — not auto-completable.
        assert!(new_game().next_auto_complete_move().is_none());
    }

    #[test]
    fn next_auto_complete_move_finds_ace_on_auto_completable_board() {
        use crate::card::{Card, Rank};

        let mut g = new_game();
        // Clear stock and waste to satisfy auto-complete precondition.
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        // Clear all tableau piles and put a single face-up Ace of Clubs
        // into Tableau(0); all other piles empty.
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 99,
            suit: Suit::Clubs,
            rank: Rank::Ace,
            face_up: true,
        });
        g.is_auto_completable = true;

        let mv = g.next_auto_complete_move().expect("should find a move");
        assert_eq!(mv.0, PileType::Tableau(0));
        // Slot 0 is the first empty foundation; the Ace lands there.
        assert_eq!(mv.1, PileType::Foundation(0));
    }

    #[test]
    fn next_auto_complete_move_returns_none_when_already_won() {
        let mut g = new_game();
        g.is_auto_completable = true;
        g.is_won = true;
        assert!(g.next_auto_complete_move().is_none());
    }

    // --- Slot-based foundation behaviour (refactor coverage) ---

    /// Aces land in the first empty slot regardless of suit, and successive
    /// Aces fan out across slots 0, 1, 2, 3 in deterministic order.
    #[test]
    fn any_ace_lands_in_first_empty_foundation() {
        let mut g = new_game();
        // Clear stock/waste/tableau so we can hand-construct moves directly.
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        // Place an Ace of Clubs on tableau 0; move it to slot 0.
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 1, suit: Suit::Clubs, rank: Rank::Ace, face_up: true,
        });
        g.move_cards(PileType::Tableau(0), PileType::Foundation(0), 1).unwrap();
        // Now place an Ace of Spades on tableau 0 and move it to slot 1.
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 2, suit: Suit::Spades, rank: Rank::Ace, face_up: true,
        });
        g.move_cards(PileType::Tableau(0), PileType::Foundation(1), 1).unwrap();

        assert_eq!(g.piles[&PileType::Foundation(0)].claimed_suit(), Some(Suit::Clubs));
        assert_eq!(g.piles[&PileType::Foundation(1)].claimed_suit(), Some(Suit::Spades));
    }

    /// `Pile::claimed_suit` reads the bottom card's suit on a populated
    /// foundation slot, regardless of which slot index the pile occupies.
    #[test]
    fn claimed_suit_is_derived_from_bottom_card() {
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 50, suit: Suit::Hearts, rank: Rank::Ace, face_up: true,
        });
        g.move_cards(PileType::Tableau(0), PileType::Foundation(2), 1).unwrap();

        assert_eq!(
            g.piles[&PileType::Foundation(2)].claimed_suit(),
            Some(Suit::Hearts)
        );
    }

    /// Undoing the only card from a foundation slot drops the claimed suit;
    /// the slot then accepts a different Ace.
    #[test]
    fn foundation_claim_drops_when_emptied_via_undo() {
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 1, suit: Suit::Hearts, rank: Rank::Ace, face_up: true,
        });
        g.move_cards(PileType::Tableau(0), PileType::Foundation(0), 1).unwrap();
        assert_eq!(g.piles[&PileType::Foundation(0)].claimed_suit(), Some(Suit::Hearts));

        g.undo().unwrap();
        assert!(g.piles[&PileType::Foundation(0)].cards.is_empty());
        assert!(g.piles[&PileType::Foundation(0)].claimed_suit().is_none());

        // A different Ace can now claim slot 0.
        let t0 = g.piles.get_mut(&PileType::Tableau(0)).unwrap();
        t0.cards.clear();
        t0.cards.push(Card { id: 2, suit: Suit::Spades, rank: Rank::Ace, face_up: true });
        g.move_cards(PileType::Tableau(0), PileType::Foundation(0), 1).unwrap();
        assert_eq!(g.piles[&PileType::Foundation(0)].claimed_suit(), Some(Suit::Spades));
    }

    /// Successive Aces from the waste pile distribute across slots 0..=3 in
    /// order — the player picks the slot, but `move_cards` accepts any
    /// empty-slot placement for an Ace.
    #[test]
    fn multiple_aces_distribute_across_slots() {
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        let aces = [
            (Suit::Clubs, 10),
            (Suit::Diamonds, 11),
            (Suit::Hearts, 12),
            (Suit::Spades, 13),
        ];
        for (slot, (suit, id)) in aces.iter().enumerate() {
            g.piles.get_mut(&PileType::Waste).unwrap().cards.push(Card {
                id: *id, suit: *suit, rank: Rank::Ace, face_up: true,
            });
            g.move_cards(PileType::Waste, PileType::Foundation(slot as u8), 1).unwrap();
        }
        for (slot, (suit, _)) in aces.iter().enumerate() {
            assert_eq!(
                g.piles[&PileType::Foundation(slot as u8)].claimed_suit(),
                Some(*suit),
                "slot {slot} should claim {suit:?}",
            );
        }
    }

    /// Auto-complete prefers the foundation slot whose claimed suit matches
    /// the candidate card's suit, even if an empty slot exists at a lower
    /// index.
    #[test]
    fn next_auto_complete_move_picks_slot_with_matching_claim() {
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        // Slot 0 is empty; slot 1 already claims Hearts via Ace of Hearts.
        g.piles.get_mut(&PileType::Foundation(1)).unwrap().cards.push(Card {
            id: 1, suit: Suit::Hearts, rank: Rank::Ace, face_up: true,
        });
        // Tableau 0 holds the 2 of Hearts to play.
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 2, suit: Suit::Hearts, rank: Rank::Two, face_up: true,
        });
        g.is_auto_completable = true;

        let mv = g.next_auto_complete_move().expect("auto-complete must find slot 1");
        assert_eq!(mv.0, PileType::Tableau(0));
        assert_eq!(
            mv.1,
            PileType::Foundation(1),
            "must target the Hearts-claimed slot, not the empty slot 0",
        );
    }

    fn setup_take_from_foundation_game() -> GameState {
        let mut g = new_game();
        // Clear the board so we control the layout exactly.
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        // Foundation slot 0: A♠, 2♠ (top = 2♠)
        let f = g.piles.get_mut(&PileType::Foundation(0)).unwrap();
        f.cards.push(Card { id: 1, suit: Suit::Spades, rank: Rank::Ace,  face_up: true });
        f.cards.push(Card { id: 2, suit: Suit::Spades, rank: Rank::Two,  face_up: true });
        // Tableau 0: 3♥ face-up (2♠ can go on 3♥ — different colour, rank-1)
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 3, suit: Suit::Hearts, rank: Rank::Three, face_up: true,
        });
        g
    }

    #[test]
    fn take_from_foundation_enabled_by_default() {
        let g = setup_take_from_foundation_game();
        assert!(g.take_from_foundation, "take_from_foundation is on by default (standard Klondike rule)");
    }

    #[test]
    fn take_from_foundation_blocked_when_disabled() {
        let mut g = setup_take_from_foundation_game();
        g.take_from_foundation = false;
        let err = g
            .move_cards(PileType::Foundation(0), PileType::Tableau(0), 1)
            .unwrap_err();
        assert!(
            matches!(err, MoveError::RuleViolation(_)),
            "expected RuleViolation, got {err:?}",
        );
    }

    #[test]
    fn take_from_foundation_allowed_when_enabled() {
        let mut g = setup_take_from_foundation_game();
        // already true by default; explicit set confirms the behaviour holds
        g.take_from_foundation = true;
        g.move_cards(PileType::Foundation(0), PileType::Tableau(0), 1).unwrap();
        // Foundation slot 0 should now hold only the Ace.
        assert_eq!(g.piles[&PileType::Foundation(0)].cards.len(), 1);
        assert_eq!(g.piles[&PileType::Foundation(0)].cards[0].rank, Rank::Ace);
        // The 2♠ should be on top of tableau 0 above the 3♥.
        let t0 = &g.piles[&PileType::Tableau(0)].cards;
        assert_eq!(t0.len(), 2);
        assert_eq!(t0[1].rank, Rank::Two);
    }

    #[test]
    fn take_from_foundation_rejects_illegal_tableau_placement() {
        let mut g = setup_take_from_foundation_game();
        g.take_from_foundation = true;
        // Tableau 1 is empty — only a King can go there; 2♠ is not a King.
        let err = g
            .move_cards(PileType::Foundation(0), PileType::Tableau(1), 1)
            .unwrap_err();
        assert!(matches!(err, MoveError::RuleViolation(_)));
    }

    #[test]
    fn take_from_foundation_rejects_count_gt_1() {
        let mut g = setup_take_from_foundation_game();
        g.take_from_foundation = true;
        let err = g
            .move_cards(PileType::Foundation(0), PileType::Tableau(0), 2)
            .unwrap_err();
        assert!(matches!(err, MoveError::RuleViolation(_)));
    }

    // --- possible_instructions ---

    #[test]
    fn possible_instructions_empty_when_won() {
        let mut g = new_game();
        g.is_won = true;
        assert!(g.possible_instructions().is_empty());
    }

    #[test]
    fn possible_instructions_includes_ace_to_foundation() {
        let mut g = new_game();
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        g.piles.get_mut(&PileType::Tableau(0)).unwrap().cards.push(Card {
            id: 1, suit: Suit::Clubs, rank: Rank::Ace, face_up: true,
        });
        let moves = g.possible_instructions();
        assert!(
            moves.contains(&(PileType::Tableau(0), PileType::Foundation(0), 1)),
            "Ace must be moveable to empty foundation slot 0; got {moves:?}"
        );
    }

    #[test]
    fn possible_instructions_all_valid_on_fresh_game() {
        // Every triple returned must actually succeed when applied to a clone of the state.
        let g = new_game();
        for (from, to, count) in g.possible_instructions() {
            let mut clone = g.clone();
            assert!(
                clone.move_cards(from.clone(), to.clone(), count).is_ok(),
                "instruction ({from:?}, {to:?}, {count}) from possible_instructions must succeed"
            );
        }
    }

    #[test]
    fn possible_instructions_no_face_down_sources() {
        let g = new_game();
        for (from, _, count) in g.possible_instructions() {
            if let PileType::Tableau(i) = from {
                let pile = &g.piles[&PileType::Tableau(i)];
                let run_len = pile.cards.iter().rev().take_while(|c| c.face_up).count();
                assert!(
                    count <= run_len,
                    "count {count} exceeds face-up run {run_len} for Tableau({i})"
                );
            }
        }
    }

    #[test]
    fn possible_instructions_waste_top_included() {
        let mut g = new_game();
        // Clear board, put a King on waste, and an empty tableau pile — waste→tableau must appear.
        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
        for i in 0..7 {
            g.piles.get_mut(&PileType::Tableau(i)).unwrap().cards.clear();
        }
        g.piles.get_mut(&PileType::Waste).unwrap().cards.push(Card {
            id: 99, suit: Suit::Spades, rank: Rank::King, face_up: true,
        });
        let moves = g.possible_instructions();
        // King goes on any of the 7 empty tableau piles
        assert!(
            (0..7).any(|dst| moves.contains(&(PileType::Waste, PileType::Tableau(dst), 1))),
            "King on waste must be moveable to an empty tableau column"
        );
    }
}