ruchess 0.0.8

//! # Position
//!
//! A [`Position`] is a complete chess game state: the [`Board`] of pieces,
//! the [`History`] (last move, castling rights, half-move clock, repetition
//! hashes), and the [`Color`] to move.
//!
//! Like [`Board`], [`Position`] is persistent and functional — builders and
//! [`Position::mve`] return new values rather than mutating in place.
//!
//! ## Building a position
//!
//! [`Position::new`] returns the standard starting position. Layer builders
//! to override pieces, side to move, or history:
//!
//! ```
//! # use ruchess::position::Position;
//! # use ruchess::color::Color;
//! # use ruchess::mve::Move;
//! let standard = Position::new();
//! assert_eq!(standard.color(), Color::White);
//! let moves = standard.valid_moves();
//! assert_eq!(moves.len(), 20);
//!
//! let black_to_move = Position::new().with_color(Color::Black);
//! assert_eq!(black_to_move.color(), Color::Black);
//! ```
//!
//! ## Generating moves
//!
//! [`Position::valid_moves`] returns every legal move;
//! [`Position::mve`] plays one and returns the resulting position:
//!
//! ```
//! # use ruchess::position::Position;
//! # use ruchess::square;
//! # use ruchess::color::Color;
//! let p = Position::new();
//! let next = p.mve(square::E2, square::E4, None).unwrap();
//! assert_eq!(next.color(), Color::Black);
//! assert!(next.board().is_occupied(square::E4));
//! ```
//!
//! Each move type also has a dedicated generator
//! ([`Position::pawn_moves`], [`Position::knight_moves`], …) that appends
//! into a caller owned buffer; together they partition [`Position::valid_moves`].

use crate::{
    attacks::ATTACKS,
    bitboard::Bitboard,
    board::Board,
    castles::Castles,
    color::Color,
    history::History,
    mve::Move,
    piece::Piece,
    ply::Ply,
    role::{PromotableRole, Role},
    side::Side,
    square::{self, Square},
    uci::Uci,
};

/// A complete chess game state — board, history, and side to move.
///
/// See the [module documentation](self) for an overview.
#[derive(Debug, Clone, PartialEq)]
pub struct Position {
    board: Board,
    history: History,
    color: Color,
    ply: Ply,
}

/// Token returned by [`Position::make`] that captures the prior position
/// state so [`Position::unmake`] can roll back exactly.
///
/// Stack-allocated, opaque to callers; size is dominated by the cached
/// `Board` (one `u64` per role/color bitboard).
#[derive(Debug, Clone)]
pub struct Undo {
    prior_board: Board,
    prior_castles: crate::castles::Castles,
    prior_unmoved_rooks: crate::unmoved_rooks::UnmovedRooks,
    prior_half_move_clock: crate::halfmoveclock::HalfMoveClock,
    prior_last_move: Option<Uci>,
    prior_position_hashes: crate::hash::PositionHash,
}

impl Position {
    /// Returns the standard chess starting position: pieces in their initial
    /// squares, all four castling rights, White to move, no en-passant target.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::color::Color;
    /// # use ruchess::mve::Move;
    /// let p = Position::new();
    /// assert_eq!(p.color(), Color::White);
    /// assert!(!p.is_check());
    /// let moves = p.valid_moves();
    /// assert_eq!(moves.len(), 20);
    /// ```
    pub fn new() -> Self {
        Self {
            board: Board::new(),
            history: History::new(),
            color: Color::White,
            ply: Ply::new(),
        }
    }

    /// Standard starting position with repetition tracking turned off.
    ///
    /// Use this for perft, fixed-depth search, and any workload that never
    /// queries [`History::is_threefold_repetition`] — it skips the Zobrist
    /// hash computation and the per-move `Vec<u8>` growth.
    ///
    /// See [`History::new_no_repetition`].
    pub fn new_no_repetition() -> Self {
        Self {
            board: Board::new(),
            history: History::new_no_repetition(),
            color: Color::White,
            ply: Ply::new(),
        }
    }

    /// Returns a copy of this position with repetition tracking disabled in
    /// its [`History`]. Useful after parsing a FEN to opt the resulting
    /// position chain out of Zobrist hashing.
    #[must_use]
    pub fn without_repetition(self) -> Self {
        Self {
            history: History {
                position_hashes: crate::hash::PositionHash::disabled(),
                ..self.history
            },
            ..self
        }
    }

    /// Returns a new position with the given [`Board`], preserving the
    /// existing color and history.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::board::Board;
    /// # use ruchess::square;
    /// let p = Position::new().with_board(Board::EMPTY);
    /// assert!(!p.board().is_occupied(square::E1));
    /// ```
    pub fn with_board(self, board: Board) -> Self {
        Self { board, ..self }
    }

    /// Returns a new position with the given side to move.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::color::Color;
    /// let p = Position::new().with_color(Color::Black);
    /// assert_eq!(p.color(), Color::Black);
    /// ```
    pub fn with_color(self, color: Color) -> Self {
        Self { color, ..self }
    }

    /// Returns a new position with the opposite side to move.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::color::Color;
    /// let p = Position::new();
    /// assert_eq!(p.color(), Color::White);
    /// let q = p.change_color();
    /// assert_eq!(q.color(), Color::Black);
    /// ```
    pub fn change_color(self) -> Self {
        let color = self.color;
        self.with_color(color.opponent())
    }

    /// Returns a new position with the given [`History`], preserving board
    /// and color.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::history::History;
    /// let p = Position::new().with_history(History::new());
    /// assert!(p.history().last_move.is_none());
    /// ```
    pub fn with_history(self, history: History) -> Self {
        Self { history, ..self }
    }

    /// Returns a new position with the given castling rights, leaving the
    /// rest of [`History`] (last move, half-move clock, …) unchanged.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::castles::Castles;
    /// let p = Position::new().with_castles(Castles::new(false, false, false, false));
    /// assert!(p.history().castles.is_empty());
    /// ```
    pub fn with_castles(self, castles: Castles) -> Self {
        Self {
            history: self.history.with_castles(castles),
            ..self
        }
    }

    /// Returns a new position with the ply, leaving the
    /// rest of [`Position`] unchanged.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::ply::Ply;
    /// let p = Position::new().with_ply(Ply::new());
    /// assert_eq!(p.ply().full_move_number(), 1);
    /// ```
    pub fn with_ply(self, ply: Ply) -> Self {
        Self { ply, ..self }
    }

    /// Returns a new position with `f` applied to the current [`History`].
    ///
    /// `f` receives a reference and returns a fresh value; this is a thin
    /// adapter for functional updates that aren't covered by [`Self::with_castles`]
    /// or [`Self::with_history`].
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::castles::Castles;
    /// let p = Position::new().update_history(|h| {
    ///     h.clone().with_castles(Castles::new(true, false, false, false))
    /// });
    /// assert!(p.history().castles.white_king_side());
    /// assert!(!p.history().castles.white_queen_side());
    /// ```
    pub fn update_history<F>(self, f: F) -> Self
    where
        F: FnOnce(&History) -> History,
    {
        Self {
            history: f(&self.history),
            ..self
        }
    }

    /// Plays the move from `orig` to `dest`. Returns the resulting position,
    /// or `None` if no legal move connects those squares.
    ///
    /// On success the side to move is flipped and the [`History`] is updated
    /// (last move, castling rights, half-move clock, position hashes).
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::square;
    /// # use ruchess::color::Color;
    /// let p = Position::new();
    /// let next = p.mve(square::E2, square::E4, None).unwrap();
    /// assert_eq!(next.color(), Color::Black);
    /// assert!(next.board().is_occupied(square::E4));
    /// assert!(!next.board().is_occupied(square::E2));
    ///
    /// // Illegal move → None.
    /// assert!(Position::new().mve(square::E2, square::E5, None).is_none());
    /// ```
    pub fn mve(
        mut self,
        orig: Square,
        dest: Square,
        promotion: Option<PromotableRole>,
    ) -> Option<Self> {
        let buf = self.valid_moves();
        let mve = *buf
            .iter()
            .find(|m| m.orig == orig && m.dest == dest && m.promotion == promotion)?;
        self.make(&mve);
        Some(self)
    }

    /// Plays `mve` in place and returns an [`Undo`] token. Pair with
    /// [`Self::unmake`] to step back.
    ///
    /// `mve` must be a legal move for this position. Behavior is undefined
    /// otherwise (typically: an invalid board state).
    ///
    /// The hot path used by perft and search: no allocation, no clone of
    /// the position, the move's already-built `after` board is installed
    /// directly, and the prior state is stashed in `Undo`.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::color::Color;
    /// # use ruchess::mve::Move;
    /// let mut p = Position::new();
    /// let moves = p.valid_moves();
    /// let m = moves[0];
    /// let undo = p.make(&m);
    /// assert_eq!(p.color(), Color::Black);
    /// p.unmake(undo);
    /// assert_eq!(p.color(), Color::White);
    /// ```
    pub fn make(&mut self, mve: &Move) -> Undo {
        let prior_board = self.board;
        let prior_castles = self.history.castles;
        let prior_unmoved_rooks = self.history.unmoved_rooks;
        let prior_half_move_clock = self.history.half_move_clock;
        let prior_last_move = self.history.last_move;

        // Compute the new hash trail before mutating board/color (the hash
        // depends on the prior position state). For Disabled, no work happens.
        let prior_position_hashes = std::mem::replace(
            &mut self.history.position_hashes,
            crate::hash::PositionHash::Disabled,
        );
        let new_hashes = if prior_position_hashes.is_disabled() {
            crate::hash::PositionHash::Disabled
        } else {
            let entry =
                crate::hash::PositionHash::from_hash(crate::hash::Hash::from_position(self));
            entry.combine(&prior_position_hashes)
        };
        self.history.position_hashes = new_hashes;

        // Apply the rest of the history update in place.
        self.history.last_move = Some((*mve).into());
        self.history.castles = self.history.castles.update(mve);
        self.history.unmoved_rooks = self.history.unmoved_rooks.update(mve);
        self.history.half_move_clock = if mve.piece.role == Role::Pawn || mve.capture.is_some() {
            self.history.half_move_clock.reset()
        } else {
            self.history.half_move_clock.incr()
        };

        // Apply the move's bit toggles to the board, then advance side/ply.
        apply_move(&mut self.board, mve);
        self.color = self.color.opponent();
        self.ply = self.ply.incr();

        Undo {
            prior_board,
            prior_castles,
            prior_unmoved_rooks,
            prior_half_move_clock,
            prior_last_move,
            prior_position_hashes,
        }
    }

    /// Reverses the most recent [`Self::make`], restoring the position
    /// exactly to its prior state.
    ///
    /// `undo` must be the value returned by the matching `make` call; pairing
    /// it with anything else (or calling out of order) produces an invalid
    /// position.
    pub fn unmake(&mut self, undo: Undo) {
        self.ply = self.ply.decr();
        self.color = self.color.opponent();
        self.board = undo.prior_board;
        self.history.castles = undo.prior_castles;
        self.history.unmoved_rooks = undo.prior_unmoved_rooks;
        self.history.half_move_clock = undo.prior_half_move_clock;
        self.history.last_move = undo.prior_last_move;
        self.history.position_hashes = undo.prior_position_hashes;
    }

    /// Returns a reference to the underlying [`Board`].
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::square;
    /// let p = Position::new();
    /// assert!(p.board().is_occupied(square::E1));
    /// ```
    pub fn board(&self) -> &Board {
        &self.board
    }

    /// Returns the side to move.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::color::Color;
    /// assert_eq!(Position::new().color(), Color::White);
    /// ```
    pub fn color(&self) -> Color {
        self.color
    }

    /// Returns a reference to the [`History`] — last move, castling rights,
    /// unmoved rooks, half-move clock, and position-hash trail.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// let p = Position::new();
    /// assert!(p.history().last_move.is_none());
    /// assert!(p.history().castles.white_king_side());
    /// ```
    pub fn history(&self) -> &History {
        &self.history
    }

    /// Returns the current ply (number of half-moves played).
    pub fn ply(&self) -> Ply {
        self.ply
    }

    /// Returns `true` if the side to move is in check.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// assert!(!Position::new().is_check());
    /// ```
    pub fn is_check(&self) -> bool {
        self.board.is_check(self.color)
    }

    /// Returns the en-passant target square if the previous move was a
    /// two-square pawn push, otherwise `None`.
    ///
    /// The target is the square the pushing pawn passed *over* — the square
    /// onto which a capturing pawn would land.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// // No en-passant target before any move has been played.
    /// assert_eq!(Position::new().enpassant_square(), None);
    /// ```
    pub fn enpassant_square(&self) -> Option<Square> {
        self.history
            .last_move
            .and_then(|lm| potential_enpassant_sq(lm, self.board, self.color))
    }

    /// Returns all valid moves in this position.
    ///
    /// Calls each per-piece-type generator ([`Self::pawn_moves`],
    /// [`Self::enpassant_moves`], [`Self::king_moves`], …) in a fixed order,
    /// then retains only those that satisfy check/pin legality.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// let moves = Position::new().valid_moves();
    /// assert_eq!(moves.len(), 20);
    /// ```
    pub fn valid_moves(&self) -> Vec<Move> {
        let mut buf = Vec::with_capacity(MAX_MOVES);
        self.pawn_moves(&mut buf);
        self.enpassant_moves(&mut buf);
        self.king_moves(&mut buf);
        self.knight_moves(&mut buf);
        self.bishop_moves(&mut buf);
        self.rook_moves(&mut buf);
        self.queen_moves(&mut buf);

        let ctx = LegalityContext::compute(self);
        let board = self.board;
        let color = self.color;
        buf.retain(|m| ctx.is_legal(m, board, color));
        buf
    }

    /// Returns `true` if at least one legal move exists from this position.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// assert!(Position::new().has_moves());
    /// ```
    pub fn has_moves(&self) -> bool {
        !self.valid_moves().is_empty()
    }

    /// Returns every legal move originating from `orig` to `buf`.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::square;
    /// # use ruchess::mve::Move;
    /// let p = Position::new();
    /// // The E2 pawn has two pushes (one and two squares).
    /// let moves = p.valid_moves_at(square::E2);
    /// assert_eq!(moves.len(), 2);
    ///
    /// // The A1 rook is locked in by its own pieces.
    /// let moves = p.valid_moves_at(square::A1);
    /// assert_eq!(moves.len(), 0);
    /// ```
    pub fn valid_moves_at(&self, orig: Square) -> Vec<Move> {
        let mut buf = self.valid_moves();
        buf.retain(|m| m.orig == orig);
        buf
    }

    /// Appends all pseudo-legal pawn moves to `buf`: pushes, double pushes,
    /// captures, and promotions. En-passant captures are emitted separately
    /// by [`Self::enpassant_moves`].
    ///
    /// "Pseudo-legal" — moves that leave the king in check are *not* filtered
    /// here. [`Self::valid_moves`] applies that filter once over the full
    /// pseudo-legal set.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::mve::Move;
    /// // 8 pawns × (single + double) = 16 from the starting position.
    /// let mut moves: Vec<Move> = Vec::new();
    /// Position::new().pawn_moves(&mut moves);
    /// assert_eq!(moves.len(), 16);
    /// ```
    pub fn pawn_moves(&self, buf: &mut Vec<Move>) {
        let pawns = self.board.bypiece(Piece {
            role: Role::Pawn,
            color: self.color,
        });
        let them = self.board.bycolor(self.color.opponent());

        // Captures.
        for from in pawns {
            for to in ATTACKS.pawn_attacks(self.color, from) & them {
                self.push_pawn_moves(buf, from, to, true);
            }
        }

        // Single pushes.
        let singles = !self.board.occupied()
            & (match self.color {
                Color::White => (self.board.white() & pawns) << 8,
                Color::Black => (self.board.black() & pawns) >> 8,
            });
        for to in singles {
            let from = Square(match self.color {
                Color::White => to.0 - 8,
                Color::Black => to.0 + 8,
            });
            self.push_pawn_moves(buf, from, to, false);
        }

        // Double pushes.
        let doubles = !self.board.occupied()
            & (match self.color {
                Color::White => singles << 8,
                Color::Black => singles >> 8,
            })
            & self.color.fourth_rank();
        for to in doubles {
            let from = Square(match self.color {
                Color::White => to.0 - 16,
                Color::Black => to.0 + 16,
            });
            self.push_pawn_moves(buf, from, to, false);
        }
    }

    /// Appends en-passant captures available to the side to move.
    ///
    /// Adds nothing unless the previous move was a two-square pawn push that
    /// ended next to a friendly pawn.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::mve::Move;
    /// // No en-passant on move 1 (no prior move to react to).
    /// let mut moves: Vec<Move> = Vec::new();
    /// Position::new().enpassant_moves(&mut moves);
    /// assert!(moves.is_empty());
    /// ```
    pub fn enpassant_moves(&self, buf: &mut Vec<Move>) {
        let Some(last_move) = self.history.last_move else {
            return;
        };
        let Some(target) = potential_enpassant_sq(last_move, self.board, self.color) else {
            return;
        };
        let our_pawns = self.board.bypiece(Piece {
            role: Role::Pawn,
            color: self.color,
        });
        for from in ATTACKS.pawn_attacks(self.color.opponent(), target) & our_pawns {
            if let Some(m) = self.enpassant(from, target) {
                buf.push(m);
            }
        }
    }

    /// Appends all legal king moves to `buf`, including castling.
    ///
    /// Destinations attacked by the opponent are filtered out. The check is
    /// performed against the board with our king temporarily removed, so a
    /// king cannot slide along an attacker's ray onto a "safe-looking" square
    /// that the king itself was blocking.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::mve::Move;
    /// // From the starting position the king is hemmed in by its own pieces.
    /// let mut moves: Vec<Move> = Vec::new();
    /// Position::new().king_moves(&mut moves);
    /// assert!(moves.is_empty());
    /// ```
    pub fn king_moves(&self, buf: &mut Vec<Move>) {
        let orig = self.board.king(self.color);
        // Remove our king for attack detection so sliders see through its
        // current square — otherwise the king could slide along an
        // attacker's ray.
        let board_without_king = self.board.pop(orig).0;
        for dest in ATTACKS.king_attacks(orig) {
            if board_without_king.is_attacked(dest, self.color) {
                continue;
            }
            if let Some(m) = self.normal(orig, dest, Role::King) {
                buf.push(m);
            }
        }
        self.push_castling_moves(buf);
    }

    /// Appends all pseudo-legal knight moves to `buf`.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::mve::Move;
    /// // 2 knights × 2 destinations each = 4 from the starting position.
    /// let mut moves: Vec<Move> = Vec::new();
    /// Position::new().knight_moves(&mut moves);
    /// assert_eq!(moves.len(), 4);
    /// ```
    pub fn knight_moves(&self, buf: &mut Vec<Move>) {
        let knights = self.board.bypiece(Piece {
            role: Role::Knight,
            color: self.color,
        });
        for from in knights {
            for to in ATTACKS.knight_attacks(from) {
                if let Some(m) = self.normal(from, to, Role::Knight) {
                    buf.push(m);
                }
            }
        }
    }

    /// Appends all pseudo-legal bishop moves to `buf`.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::mve::Move;
    /// // Bishops are blocked by own pawns at the start.
    /// let mut moves: Vec<Move> = Vec::new();
    /// Position::new().bishop_moves(&mut moves);
    /// assert!(moves.is_empty());
    /// ```
    pub fn bishop_moves(&self, buf: &mut Vec<Move>) {
        let bishops = self.board.bypiece(Piece {
            role: Role::Bishop,
            color: self.color,
        });
        for from in bishops {
            for to in ATTACKS.bishop_attacks(from, self.board.occupied()) {
                if let Some(m) = self.normal(from, to, Role::Bishop) {
                    buf.push(m);
                }
            }
        }
    }

    /// Appends all pseudo-legal rook moves to `buf`.
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::mve::Move;
    /// // Rooks are locked in by their own pieces at the start.
    /// let mut moves: Vec<Move> = Vec::new();
    /// Position::new().rook_moves(&mut moves);
    /// assert!(moves.is_empty());
    /// ```
    pub fn rook_moves(&self, buf: &mut Vec<Move>) {
        let rooks = self.board.bypiece(Piece {
            role: Role::Rook,
            color: self.color,
        });
        for from in rooks {
            for to in ATTACKS.rook_attacks(from, self.board.occupied()) {
                if let Some(m) = self.normal(from, to, Role::Rook) {
                    buf.push(m);
                }
            }
        }
    }

    /// Appends all pseudo-legal queen moves to `buf` (bishop-like + rook-like
    /// rays).
    ///
    /// # Example
    /// ```
    /// # use ruchess::position::Position;
    /// # use ruchess::mve::Move;
    /// // The queen has no legal moves from the starting position.
    /// let mut moves: Vec<Move> = Vec::new();
    /// Position::new().queen_moves(&mut moves);
    /// assert!(moves.is_empty());
    /// ```
    pub fn queen_moves(&self, buf: &mut Vec<Move>) {
        let queens = self.board.bypiece(Piece {
            role: Role::Queen,
            color: self.color,
        });
        let occupied = self.board.occupied();
        for from in queens {
            for to in ATTACKS.bishop_attacks(from, occupied) {
                if let Some(m) = self.normal(from, to, Role::Queen) {
                    buf.push(m);
                }
            }
            for to in ATTACKS.rook_attacks(from, occupied) {
                if let Some(m) = self.normal(from, to, Role::Queen) {
                    buf.push(m);
                }
            }
        }
    }

    /// Appends the legal castling moves for the side to move. Adds nothing
    /// if the king is currently in check.
    fn push_castling_moves(&self, buf: &mut Vec<Move>) {
        if self.board.is_check(self.color) {
            return;
        }
        for side in [Side::King, Side::Queen] {
            if let Some(m) = self.castle(side) {
                buf.push(m);
            }
        }
    }

    /// Attempts to build a single castling move on the given [`Side`].
    /// Returns `None` if rights are missing, the rook has moved, squares
    /// between king and rook are occupied, or the king would transit an
    /// attacked square.
    fn castle(&self, side: Side) -> Option<Move> {
        if !self.history.castles.can_side(self.color, side) {
            return None;
        }

        let king_from = self.board.king(self.color);
        let rook_from = self.color.castle_square(side);
        if !self.history.unmoved_rooks.contains(rook_from) {
            return None;
        }
        let (king_to, _rook_to, between, king_path) = castle_squares(self.color, side);

        if (self.board.occupied() & between).is_non_empty() {
            return None;
        }
        if king_path
            .into_iter()
            .any(|sq| self.board.is_attacked(sq, self.color))
        {
            return None;
        }

        Some(Move::castle(self.color, side, king_from, king_to))
    }

    /// Builds a non-special move (quiet push or simple capture) of `role`
    /// from `orig` to `dest`. Returns `None` if the destination holds one
    /// of our own pieces or `orig` is empty.
    fn normal(&self, orig: Square, dest: Square, role: Role) -> Option<Move> {
        let piece = Piece {
            role,
            color: self.color,
        };
        if self.board.is_occupied(dest) {
            if self.board.color_at(dest) == Some(self.color) {
                return None;
            }
            // Captured piece's color is fixed (our opponent — same-color
            // captures rejected just above). Record the role so `make` can
            // apply the capture without re-scanning the board.
            let captured_role = self.board.role_at(dest)?;
            Some(Move::capture(piece, orig, dest, dest, captured_role))
        } else {
            Some(Move::quiet(piece, orig, dest))
        }
    }

    /// Builds an en-passant [`Move`] from `orig` to `dest`. The captured
    /// pawn sits on the file of `dest` and the rank of `orig`.
    fn enpassant(&self, orig: Square, dest: Square) -> Option<Move> {
        let captured_sq = Square::from_file_and_rank(dest.file(), orig.rank());
        Some(Move::enpassant(self.color, orig, dest, captured_sq))
    }

    /// Expands a single pawn step from `from` to `to` into the appropriate
    /// move set and appends it to `buf`: four promotion moves if `from` is on
    /// the seventh rank (relative to the mover), otherwise one ordinary pawn
    /// move.
    fn push_pawn_moves(&self, buf: &mut Vec<Move>, from: Square, to: Square, is_capture: bool) {
        let is_promotion = from.rank() == self.color.seventh_rank();
        if is_promotion {
            let captured = if is_capture {
                self.board.role_at(to).map(|r| (to, r))
            } else {
                None
            };
            for r in PromotableRole::ROLES {
                buf.push(Move::promotion(self.color, from, to, r, captured));
            }
        } else if let Some(m) = self.normal(from, to, Role::Pawn) {
            buf.push(m);
        }
    }
}

/// Upper bound on legal moves in any chess position. The empirical maximum
/// is ~218; 256 is a safe round number that callers can use to preallocate
/// move buffers.
pub const MAX_MOVES: usize = 256;

/// Per-position legality info used to filter pseudo-legal moves without
/// invoking make/is_check on every candidate.
///
/// Computed once per call to [`Position::valid_moves`]. Encodes:
/// - `checkers`: bitboard of opponent pieces attacking our king.
/// - `check_mask`: squares non-king pieces may target. All squares if not
///   in check; squares between king and checker plus the checker's square
///   on single check; empty on double check.
/// - `is_double_check`: true if more than one attacker — only king moves
///   are legal.
/// - `pinned`: bitboard of our pieces pinned to the king by an opponent
///   slider. A pinned piece may only move along the line through king and
///   pinner.
struct LegalityContext {
    king_sq: Square,
    check_mask: Bitboard,
    is_double_check: bool,
    pinned: Bitboard,
}

impl LegalityContext {
    fn compute(pos: &Position) -> Self {
        let board = &pos.board;
        let us = pos.color;
        let them = us.opponent();
        let king_sq = board.king(us);

        let checkers = board.attackers(king_sq, them);
        let n_checkers = checkers.0.count_ones();

        let (check_mask, is_double_check) = if n_checkers == 0 {
            (Bitboard(!0u64), false)
        } else if n_checkers == 1 {
            // Single checker — non-king pieces must capture it or block.
            let checker_sq = Square(checkers.0.trailing_zeros() as u8);
            let between = Bitboard(ATTACKS.between[king_sq.0 as usize][checker_sq.0 as usize]);
            (between | Bitboard::from(checker_sq), false)
        } else {
            (Bitboard::EMPTY, true)
        };

        // Pin detection. For each opponent slider that lies on a ray from our
        // king (ignoring blockers), check whether exactly one of our pieces
        // sits between — that piece is pinned.
        let occupied = board.occupied();
        let our_pieces = board.bycolor(us);
        let opp_rq = board.bycolor(them) & (board.rooks() | board.queens());
        let opp_bq = board.bycolor(them) & (board.bishops() | board.queens());

        let mut pinned = Bitboard::EMPTY;

        // Rook-style pinners (rank/file rays).
        let rook_candidates = ATTACKS.rook_attacks(king_sq, Bitboard::EMPTY) & opp_rq;
        for pinner_sq in rook_candidates {
            let between = Bitboard(ATTACKS.between[king_sq.0 as usize][pinner_sq.0 as usize]);
            let blockers = between & occupied;
            if blockers.0.count_ones() == 1 && (blockers & our_pieces).is_non_empty() {
                pinned |= blockers;
            }
        }

        // Bishop-style pinners (diagonal rays).
        let bishop_candidates = ATTACKS.bishop_attacks(king_sq, Bitboard::EMPTY) & opp_bq;
        for pinner_sq in bishop_candidates {
            let between = Bitboard(ATTACKS.between[king_sq.0 as usize][pinner_sq.0 as usize]);
            let blockers = between & occupied;
            if blockers.0.count_ones() == 1 && (blockers & our_pieces).is_non_empty() {
                pinned |= blockers;
            }
        }

        Self {
            king_sq,
            check_mask,
            is_double_check,
            pinned,
        }
    }

    /// Decides legality of a pseudo-legal `mve`. Mover color is `color` and
    /// `board` is the pre-move board (used only for the en-passant fallback).
    #[inline(always)]
    fn is_legal(&self, mve: &Move, board: Board, color: Color) -> bool {
        // King moves: `king_moves` already filters its own legality (lifts
        // the king and tests attack). Castles are validated at gen time.
        if mve.piece.role == Role::King {
            return true;
        }
        // Under double check only the king can move.
        if self.is_double_check {
            return false;
        }
        // En passant: rare discovered-check via removed pawn on the same
        // rank. Cheap fallback to make/is_check.
        if mve.enpassant.is_some() {
            let mut working = board;
            apply_move(&mut working, mve);
            return !working.is_check(color);
        }
        // Must land on a check-resolving square.
        if !self.check_mask.is_set(mve.dest) {
            return false;
        }
        // Pinned pieces may only move along the king–pinner line.
        if self.pinned.is_set(mve.orig) {
            let ray = Bitboard(ATTACKS.rays[self.king_sq.0 as usize][mve.orig.0 as usize]);
            return ray.is_set(mve.dest);
        }
        true
    }
}

/// Applies `mve` to `board` in place by XOR-toggling exactly the affected
/// bits. Used as the shared make-move primitive by [`Position::make`] and
/// by the legality filter in [`Position::valid_moves`].
///
/// Symmetric: applying twice is a no-op. The move's `captured_role` field
/// (filled at generation time) supplies enough information to apply captures
/// without scanning the board.
#[inline(always)]
pub(crate) fn apply_move(board: &mut Board, mve: &Move) {
    let mover = mve.piece;

    if let Some(side) = mve.castle {
        let color = mover.color;
        let king = Piece {
            role: Role::King,
            color,
        };
        let rook = Piece {
            role: Role::Rook,
            color,
        };
        let rook_from = color.castle_square(side);
        let (king_to, rook_to, _, _) = castle_squares(color, side);
        board.toggle_piece(mve.orig, king);
        board.toggle_piece(king_to, king);
        board.toggle_piece(rook_from, rook);
        board.toggle_piece(rook_to, rook);
        return;
    }

    // Remove captured piece, if any.
    if let (Some(cap_sq), Some(cap_role)) = (mve.capture, mve.captured_role) {
        let captured = Piece {
            role: cap_role,
            color: mover.color.opponent(),
        };
        board.toggle_piece(cap_sq, captured);
    }

    // Lift the mover off its origin square.
    board.toggle_piece(mve.orig, mover);

    // Place the resulting piece at the destination: promoted role on a
    // promotion, otherwise the mover itself.
    let landed = match mve.promotion {
        Some(p) => Piece {
            role: match p {
                PromotableRole::Queen => Role::Queen,
                PromotableRole::Rook => Role::Rook,
                PromotableRole::Bishop => Role::Bishop,
                PromotableRole::Knight => Role::Knight,
            },
            color: mover.color,
        },
        None => mover,
    };
    board.toggle_piece(mve.dest, landed);
}

/// For `color` castling on `side`, returns
/// `(king_destination, rook_destination, must_be_empty, king_path_must_be_safe)`.
///
/// `must_be_empty` covers every square strictly between king and rook;
/// `king_path_must_be_safe` covers the squares the king transits to and
/// lands on (the king's starting square is already covered by the caller's
/// in-check test).
fn castle_squares(color: Color, side: Side) -> (Square, Square, Bitboard, Bitboard) {
    match (color, side) {
        (Color::White, Side::King) => (
            square::G1,
            square::F1,
            Bitboard::from(square::F1) | Bitboard::from(square::G1),
            Bitboard::from(square::F1) | Bitboard::from(square::G1),
        ),
        (Color::White, Side::Queen) => (
            square::C1,
            square::D1,
            Bitboard::from(square::B1) | Bitboard::from(square::C1) | Bitboard::from(square::D1),
            Bitboard::from(square::C1) | Bitboard::from(square::D1),
        ),
        (Color::Black, Side::King) => (
            square::G8,
            square::F8,
            Bitboard::from(square::F8) | Bitboard::from(square::G8),
            Bitboard::from(square::F8) | Bitboard::from(square::G8),
        ),
        (Color::Black, Side::Queen) => (
            square::C8,
            square::D8,
            Bitboard::from(square::B8) | Bitboard::from(square::C8) | Bitboard::from(square::D8),
            Bitboard::from(square::C8) | Bitboard::from(square::D8),
        ),
    }
}
/// Returns the en-passant target square if `last_move` was a two-square
/// pawn push by `color`'s opponent — that is, the square the pushed pawn
/// passed over. Returns `None` otherwise.
fn potential_enpassant_sq(last_move: Uci, board: Board, color: Color) -> Option<Square> {
    board.piece_at(last_move.dest).and_then(|piece| {
        if piece.color != color
            && piece.role == Role::Pawn
            && last_move.orig.ydist(last_move.dest) == 2
        {
            // The target is the square the opponent's pawn passed over —
            // one rank back from the opponent's perspective.
            last_move.dest.prev_rank(piece.color)
        } else {
            None
        }
    })
}

impl Default for Position {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::unmoved_rooks::UnmovedRooks;

    fn pc(role: Role, color: Color) -> Piece {
        Piece { role, color }
    }

    /// Position with the given board, otherwise pristine (standard castles,
    /// unmoved_rooks derived from `b`, white to move).
    fn pos_from(b: Board) -> Position {
        let history = History {
            unmoved_rooks: UnmovedRooks::from_board(b),
            ..History::new()
        };
        Position::new().with_board(b).with_history(history)
    }

    /// Test helper: collect every legal move into a fresh `Vec`.
    fn collect_valid(p: &Position) -> Vec<Move> {
        p.valid_moves()
    }

    /// Test helper: collect every legal move with `orig == s` into a fresh `Vec`.
    fn collect_at(p: &Position, s: Square) -> Vec<Move> {
        p.valid_moves_at(s)
    }

    // ── Starting position sanity ─────────────────────────────────────────

    #[test]
    fn starting_position_has_20_moves() {
        assert_eq!(collect_valid(&Position::new()).len(), 20);
    }

    #[test]
    fn starting_position_pawn_moves_count() {
        // 8 pawns × (1 single push + 1 double push) = 16
        let mut buf = Vec::new();
        Position::new().pawn_moves(&mut buf);
        assert_eq!(buf.len(), 16);
    }

    #[test]
    fn starting_position_knight_moves_count() {
        // 2 knights × 2 destinations each = 4
        let mut buf = Vec::new();
        Position::new().knight_moves(&mut buf);
        assert_eq!(buf.len(), 4);
    }

    #[test]
    fn starting_position_no_castling() {
        let castles: Vec<_> = collect_valid(&Position::new())
            .into_iter()
            .filter(|m| m.castle.is_some())
            .collect();
        assert!(
            castles.is_empty(),
            "no castles legal from start, got {castles:?}"
        );
    }

    #[test]
    fn starting_position_no_enpassant() {
        assert_eq!(Position::new().enpassant_square(), None);
        let mut buf = Vec::new();
        Position::new().enpassant_moves(&mut buf);
        assert!(buf.is_empty());
    }

    #[test]
    fn starting_position_no_promotion() {
        let proms: Vec<_> = collect_valid(&Position::new())
            .into_iter()
            .filter(|m| m.promotion.is_some())
            .collect();
        assert!(proms.is_empty());
    }

    #[test]
    fn starting_position_not_in_check() {
        assert!(!Position::new().is_check());
    }

    #[test]
    fn starting_position_has_moves() {
        assert!(Position::new().has_moves());
    }

    // ── mve / color flip / history ───────────────────────────────────────

    #[test]
    fn mve_flips_color() {
        let after = Position::new().mve(square::E2, square::E4, None).unwrap();
        assert_eq!(after.color(), Color::Black);
    }

    #[test]
    fn mve_applies_move_to_board() {
        let after = Position::new().mve(square::E2, square::E4, None).unwrap();
        assert!(!after.board().is_occupied(square::E2));
        assert_eq!(
            after.board().piece_at(square::E4),
            Some(pc(Role::Pawn, Color::White))
        );
    }

    #[test]
    fn mve_invalid_returns_none() {
        // E2 → E5 is not a legal pawn move (only 1 or 2 squares forward).
        assert!(Position::new().mve(square::E2, square::E5, None).is_none());
    }

    #[test]
    fn mve_from_empty_square_returns_none() {
        assert!(Position::new().mve(square::E4, square::E5, None).is_none());
    }

    #[test]
    fn mve_updates_history_last_move() {
        let after = Position::new().mve(square::E2, square::E4, None).unwrap();
        let lm = after.history().last_move.expect("last_move set after mve");
        assert_eq!(lm.orig, square::E2);
        assert_eq!(lm.dest, square::E4);
    }

    #[test]
    fn change_color_actually_flips() {
        let p = Position::new();
        assert_eq!(p.color(), Color::White);
        // change_color should produce a position whose color is the opponent.
        // The current implementation in position.rs:40-43 is a no-op — this
        // test surfaces the bug.
        let q = p.change_color();
        assert_eq!(q.color(), Color::Black);
    }

    // ── Pawn pushes ──────────────────────────────────────────────────────

    #[test]
    fn pawn_double_push_emits_two_destinations() {
        let moves = collect_at(&Position::new(), square::E2);
        assert_eq!(moves.len(), 2);
        let dests: Vec<_> = moves.iter().map(|m| m.dest).collect();
        assert!(dests.contains(&square::E3));
        assert!(dests.contains(&square::E4));
    }

    #[test]
    fn pawn_blocked_cannot_push() {
        // White pawn on E2 with a black knight directly in front on E3.
        let b = Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::E8, pc(Role::King, Color::Black))
            .set(square::E2, pc(Role::Pawn, Color::White))
            .set(square::E3, pc(Role::Knight, Color::Black));
        let p = pos_from(b);
        let pushes: Vec<_> = collect_at(&p, square::E2)
            .into_iter()
            .filter(|m| m.capture.is_none())
            .collect();
        assert!(
            pushes.is_empty(),
            "pawn blocked on E3 cannot push to E3 or E4, got {pushes:?}"
        );
    }

    // ── En passant ───────────────────────────────────────────────────────

    #[test]
    fn enpassant_offered_after_opposing_double_push() {
        // White pawn on E5, black just played D7→D5.
        let b = Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::E8, pc(Role::King, Color::Black))
            .set(square::E5, pc(Role::Pawn, Color::White))
            .set(square::D5, pc(Role::Pawn, Color::Black));
        let history = History {
            last_move: Some(crate::uci::Uci {
                orig: square::D7,
                dest: square::D5,
                promotion: None,
            }),
            unmoved_rooks: UnmovedRooks::from_board(b),
            ..History::new()
        };
        let p = Position::new().with_board(b).with_history(history);
        assert_eq!(
            p.enpassant_square(),
            Some(square::D6),
            "en passant target should be D6 (the square the black pawn passed)"
        );
        let mut eps: Vec<Move> = Vec::new();
        p.enpassant_moves(&mut eps);
        assert_eq!(eps.len(), 1, "exactly one en-passant move available");
        let m = eps[0];
        assert_eq!(m.orig, square::E5);
        assert_eq!(m.dest, square::D6);
        assert_eq!(m.capture, Some(square::D5));
        assert!(m.enpassant.is_some());
    }

    #[test]
    fn enpassant_not_offered_after_single_push() {
        // White pawn on E5, black played D6→D5 (single push, not double).
        let b = Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::E8, pc(Role::King, Color::Black))
            .set(square::E5, pc(Role::Pawn, Color::White))
            .set(square::D5, pc(Role::Pawn, Color::Black));
        let history = History {
            last_move: Some(crate::uci::Uci {
                orig: square::D6,
                dest: square::D5,
                promotion: None,
            }),
            unmoved_rooks: UnmovedRooks::from_board(b),
            ..History::new()
        };
        let p = Position::new().with_board(b).with_history(history);
        assert_eq!(p.enpassant_square(), None);
        let mut eps: Vec<Move> = Vec::new();
        p.enpassant_moves(&mut eps);
        assert!(eps.is_empty());
    }

    // ── Promotion ────────────────────────────────────────────────────────

    #[test]
    fn pawn_on_seventh_promotes_to_four_pieces() {
        let b = Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::H8, pc(Role::King, Color::Black))
            .set(square::A7, pc(Role::Pawn, Color::White));
        let p = pos_from(b);
        let moves = collect_at(&p, square::A7);
        assert_eq!(moves.len(), 4, "4 promotion choices; got {moves:?}");
        let promos: Vec<_> = moves.iter().filter_map(|m| m.promotion).collect();
        assert!(promos.contains(&PromotableRole::Queen));
        assert!(promos.contains(&PromotableRole::Rook));
        assert!(promos.contains(&PromotableRole::Bishop));
        assert!(promos.contains(&PromotableRole::Knight));
        for m in &moves {
            assert_eq!(m.dest, square::A8);
            assert_eq!(m.capture, None);
        }
    }

    #[test]
    fn pawn_pre_promotion_push_has_no_promotion_field() {
        let b = Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::E8, pc(Role::King, Color::Black))
            .set(square::A6, pc(Role::Pawn, Color::White));
        let p = pos_from(b);
        let moves = collect_at(&p, square::A6);
        assert!(
            moves.iter().all(|m| m.promotion.is_none()),
            "no promotion on rank-6 push, got {moves:?}"
        );
    }

    // ── Castling ─────────────────────────────────────────────────────────

    fn castling_board() -> Board {
        Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::A1, pc(Role::Rook, Color::White))
            .set(square::H1, pc(Role::Rook, Color::White))
            .set(square::E8, pc(Role::King, Color::Black))
            .set(square::A8, pc(Role::Rook, Color::Black))
            .set(square::H8, pc(Role::Rook, Color::Black))
    }

    #[test]
    fn castle_kingside_white_available() {
        let p = pos_from(castling_board());
        let castles: Vec<_> = collect_valid(&p)
            .into_iter()
            .filter(|m| m.castle == Some(Side::King))
            .collect();
        assert_eq!(castles.len(), 1);
        let m = castles[0];
        assert_eq!(m.orig, square::E1);
        assert_eq!(m.dest, square::G1);
    }

    #[test]
    fn castle_queenside_white_available() {
        let p = pos_from(castling_board());
        let castles: Vec<_> = collect_valid(&p)
            .into_iter()
            .filter(|m| m.castle == Some(Side::Queen))
            .collect();
        assert_eq!(castles.len(), 1);
        assert_eq!(castles[0].orig, square::E1);
        assert_eq!(castles[0].dest, square::C1);
    }

    #[test]
    fn cannot_castle_kingside_through_check() {
        // Black rook on F8 attacks F1 (a square the king transits).
        let b = castling_board().set(square::F8, pc(Role::Rook, Color::Black));
        let p = pos_from(b);
        let king_castle = collect_valid(&p)
            .into_iter()
            .find(|m| m.castle == Some(Side::King));
        assert!(
            king_castle.is_none(),
            "F-file rook prevents kingside castle"
        );
    }

    #[test]
    fn cannot_castle_while_in_check() {
        // Black queen on E4 attacks E1 (white king's square) along the E-file.
        let b = castling_board().set(square::E4, pc(Role::Queen, Color::Black));
        let p = pos_from(b);
        assert!(
            p.is_check(),
            "white king on E1 is in check from black Q on E4"
        );
        let any_castle = collect_valid(&p).into_iter().find(|m| m.castle.is_some());
        assert!(
            any_castle.is_none(),
            "no castles allowed while in check, got {any_castle:?}"
        );
    }

    #[test]
    fn cannot_castle_kingside_when_blocked() {
        let b = castling_board().set(square::F1, pc(Role::Bishop, Color::White));
        let p = pos_from(b);
        let king_castle = collect_valid(&p)
            .into_iter()
            .find(|m| m.castle == Some(Side::King));
        assert!(king_castle.is_none(), "bishop on F1 blocks kingside castle");
    }

    #[test]
    fn cannot_castle_without_rights() {
        let p = pos_from(castling_board()).with_castles(Castles::new(false, false, false, false));
        let any_castle = collect_valid(&p).into_iter().find(|m| m.castle.is_some());
        assert!(any_castle.is_none());
    }

    #[test]
    fn cannot_castle_with_moved_rooks() {
        let history = History {
            unmoved_rooks: UnmovedRooks::from_board(Board::EMPTY),
            ..History::new()
        };
        let p = Position::new()
            .with_board(castling_board())
            .with_history(history);
        let any_castle = collect_valid(&p).into_iter().find(|m| m.castle.is_some());
        assert!(any_castle.is_none(), "no castle when rooks have moved");
    }

    // ── is_check ─────────────────────────────────────────────────────────

    #[test]
    fn is_check_detects_back_rank_rook() {
        let b = Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::E8, pc(Role::Rook, Color::Black))
            .set(square::A8, pc(Role::King, Color::Black));
        let p = pos_from(b);
        assert!(p.is_check());
    }

    #[test]
    fn is_check_false_when_blocked() {
        let b = Board::EMPTY
            .set(square::E1, pc(Role::King, Color::White))
            .set(square::E4, pc(Role::Pawn, Color::White))
            .set(square::E8, pc(Role::Rook, Color::Black))
            .set(square::A8, pc(Role::King, Color::Black));
        let p = pos_from(b);
        assert!(!p.is_check(), "rook attack blocked by own pawn on E4");
    }

    // ── valid_moves_at consistency (small concrete check) ────────────────

    #[test]
    fn valid_moves_at_matches_filter_on_start() {
        let p = Position::new();
        let all = collect_valid(&p);
        for i in 0u8..64 {
            let s = Square(i);
            let direct = collect_at(&p, s).len();
            let filtered = all.iter().filter(|m| m.orig == s).count();
            assert_eq!(direct, filtered, "mismatch at square {s}");
        }
    }
}

#[cfg(test)]
mod proptests {
    use super::*;
    use crate::unmoved_rooks::UnmovedRooks;
    use proptest::prelude::*;

    // ── Strategies ───────────────────────────────────────────────────────

    fn sq() -> impl Strategy<Value = Square> {
        (0u8..64).prop_map(Square)
    }

    fn color() -> impl Strategy<Value = Color> {
        prop_oneof![Just(Color::White), Just(Color::Black)]
    }

    fn non_king_role() -> impl Strategy<Value = Role> {
        prop_oneof![
            Just(Role::Pawn),
            Just(Role::Knight),
            Just(Role::Bishop),
            Just(Role::Rook),
            Just(Role::Queen),
        ]
    }

    fn non_king_piece() -> impl Strategy<Value = Piece> {
        (non_king_role(), color()).prop_map(|(role, color)| Piece { role, color })
    }

    /// Builds a position with kings on `wk`/`bk`, the listed non-king pieces sprinkled
    /// on top (king squares and illegal pawn ranks skipped), and no castling rights.
    fn build_position(wk: Square, bk: Square, ops: Vec<(Square, Piece)>, c: Color) -> Position {
        let mut b = Board::EMPTY
            .set(
                wk,
                Piece {
                    role: Role::King,
                    color: Color::White,
                },
            )
            .set(
                bk,
                Piece {
                    role: Role::King,
                    color: Color::Black,
                },
            );
        for (s, p) in ops {
            if s == wk || s == bk {
                continue;
            }
            if p.role == Role::Pawn {
                let r = s.rank().as_u8();
                if r == 0 || r == 7 {
                    continue;
                }
            }
            b = b.set(s, p);
        }
        let history = History {
            castles: Castles::new(false, false, false, false),
            unmoved_rooks: UnmovedRooks::from_board(b),
            ..History::new()
        };
        Position::new()
            .with_board(b)
            .with_color(c)
            .with_history(history)
    }

    /// Random "legal-enough" position: exactly one king per color, kings non-adjacent,
    /// side-not-to-move not in check.
    fn random_position() -> impl Strategy<Value = Position> {
        (
            sq(),
            sq(),
            prop::collection::vec((sq(), non_king_piece()), 0..12),
            color(),
        )
            .prop_filter("kings distinct", |(wk, bk, _, _)| wk != bk)
            .prop_filter("kings non-adjacent", |(wk, bk, _, _)| {
                let dx = (wk.file().as_u8() as i32 - bk.file().as_u8() as i32).abs();
                let dy = (wk.rank().as_u8() as i32 - bk.rank().as_u8() as i32).abs();
                dx > 1 || dy > 1
            })
            .prop_map(|(wk, bk, ops, c)| build_position(wk, bk, ops, c))
            .prop_filter("side-not-to-move not in check", |p| {
                !p.board().is_check(p.color().opponent())
            })
    }

    fn piece_count(b: &Board, role: Role, color: Color) -> u32 {
        b.bypiece(Piece { role, color }).0.count_ones()
    }

    /// Apply `mve` to a clone of `p`'s board and return it. Replaces the
    /// pre-Phase-3 `m.after` field that move generation used to materialize.
    fn after_board(p: &Position, mve: &Move) -> Board {
        let mut b = *p.board();
        apply_move(&mut b, mve);
        b
    }

    /// Collect every legal move from `p` into a fresh `Vec`.
    fn collect_valid(p: &Position) -> Vec<Move> {
        p.valid_moves()
    }

    /// Count moves in `buf` that would not leave us in check after apply.
    fn legal_count(p: &Position, buf: &[Move]) -> usize {
        buf.iter()
            .filter(|m| !after_board(p, m).is_check(p.color))
            .count()
    }

    proptest! {
        // Every generated move is for a piece of the side to move.
        #[test]
        fn move_color_matches_side_to_move(p in random_position()) {
            for m in collect_valid(&p) {
                prop_assert_eq!(m.piece.color, p.color());
            }
        }

        // No null moves.
        #[test]
        fn move_orig_not_equal_dest(p in random_position()) {
            for m in collect_valid(&p) {
                prop_assert_ne!(m.orig, m.dest);
            }
        }

        // The origin square holds the piece claimed by the move.
        #[test]
        fn move_origin_holds_claimed_piece(p in random_position()) {
            for m in collect_valid(&p) {
                prop_assert_eq!(p.board().piece_at(m.orig), Some(m.piece));
            }
        }

        // After-board has exactly one king of each color (king cannot be captured).
        #[test]
        fn after_board_keeps_both_kings(p in random_position()) {
            for m in collect_valid(&p) {
                let after = after_board(&p, &m);
                let white_kings = piece_count(&after, Role::King, Color::White);
                let black_kings = piece_count(&after, Role::King, Color::Black);
                prop_assert_eq!(white_kings, 1);
                prop_assert_eq!(black_kings, 1);
            }
        }

        // After-board satisfies bitboard invariants.
        #[test]
        fn after_board_invariants(p in random_position()) {
            for m in collect_valid(&p) {
                let b = after_board(&p, &m);
                let white = b.bycolor(Color::White);
                let black = b.bycolor(Color::Black);
                prop_assert_eq!(b.occupied(), white | black);
                prop_assert_eq!(white & black, crate::bitboard::Bitboard::EMPTY);
            }
        }

        // Move generation is deterministic.
        #[test]
        fn deterministic(p in random_position()) {
            prop_assert_eq!(collect_valid(&p), collect_valid(&p));
        }

        // `valid_moves()` equals the disjoint union of per-piece-type generators filtered by legality.
        #[test]
        fn partition_matches_per_piece_generators(p in random_position()) {
            let total = collect_valid(&p).len();
            let mut pawns = Vec::new();
            p.pawn_moves(&mut pawns);
            let mut ep = Vec::new();
            p.enpassant_moves(&mut ep);
            let mut king = Vec::new();
            p.king_moves(&mut king);
            let mut knight = Vec::new();
            p.knight_moves(&mut knight);
            let mut bishop = Vec::new();
            p.bishop_moves(&mut bishop);
            let mut rook = Vec::new();
            p.rook_moves(&mut rook);
            let mut queen = Vec::new();
            p.queen_moves(&mut queen);
            let sum = legal_count(&p, &pawns)
                + legal_count(&p, &ep)
                + legal_count(&p, &king)
                + legal_count(&p, &knight)
                + legal_count(&p, &bishop)
                + legal_count(&p, &rook)
                + legal_count(&p, &queen);
            prop_assert_eq!(total, sum);
        }

        // `valid_moves_at(s)` is equivalent to filtering `valid_moves()` by `orig==s`.
        #[test]
        fn valid_moves_at_filter_consistent(p in random_position()) {
            let all = collect_valid(&p);
            for i in 0u8..64 {
                let s = Square(i);
                let buf = p.valid_moves_at(s);
                let filtered = all.iter().filter(|m| m.orig == s).count();
                prop_assert_eq!(buf.len(), filtered);
            }
        }

        // has_moves agrees with non-empty valid_moves.
        #[test]
        fn has_moves_iff_any(p in random_position()) {
            prop_assert_eq!(p.has_moves(), !collect_valid(&p).is_empty());
        }

        // `mve()` with a listed move succeeds and flips color.
        #[test]
        fn mve_with_listed_move_works(p in random_position()) {
            if let Some(m) = collect_valid(&p).into_iter().next() {
                let expected_after = after_board(&p, &m);
                let next = p.clone().mve(m.orig, m.dest, m.promotion).expect("listed move must succeed");
                prop_assert_eq!(next.color(), p.color().opponent());
                if m.promotion.is_none() {
                    prop_assert_eq!(*next.board(), expected_after);
                }
            }
        }

        // Promotion moves only land on the opponent's back rank.
        #[test]
        fn promotion_only_on_opponent_back_rank(p in random_position()) {
            let opp_back = p.color().opponent().back_rank();
            for m in collect_valid(&p) {
                if m.promotion.is_some() {
                    prop_assert_eq!(m.dest.rank(), opp_back);
                    prop_assert_eq!(m.piece.role, Role::Pawn);
                }
            }
        }

        // `valid_moves()` does not mutate the position.
        #[test]
        fn valid_moves_does_not_mutate(p in random_position()) {
            let before = p.clone();
            let _ = collect_valid(&p);
            prop_assert_eq!(p, before);
        }

        // make + unmake restores the exact prior position.
        #[test]
        fn make_unmake_round_trip(p in random_position()) {
            let before = p.clone();
            for m in collect_valid(&p) {
                let mut q = before.clone();
                let undo = q.make(&m);
                q.unmake(undo);
                prop_assert_eq!(&q, &before);
            }
        }

        // make produces the same end state as the persistent `mve` for every legal move.
        #[test]
        fn make_matches_persistent_mve(p in random_position()) {
            for m in collect_valid(&p) {
                let persistent = p.clone().mve(m.orig, m.dest, m.promotion).unwrap();
                let mut mutable = p.clone();
                mutable.make(&m);
                prop_assert_eq!(&mutable, &persistent);
            }
        }
    }
}