chessfriend/board/src/board.rs

// Eryn Wells <eryn@erynwells.me>

use crate::{
    castle, display::DiagramFormatter, piece_sets::PlacePieceError, EnPassant, MoveCounter,
    PieceSet,
};
use chessfriend_bitboard::BitBoard;
use chessfriend_core::{Color, Piece, PlacedPiece, Shape, Square};
use std::iter::Iterator;

#[derive(Clone, Debug, Eq)]
pub struct Board {
    pieces: PieceSet,
    en_passant: Option<EnPassant>,
    pub move_counter: MoveCounter,
    pub castling_rights: castle::Rights,
}

impl Board {
    /// An empty board
    #[must_use]
    pub fn empty() -> Self {
        Board::default()
    }

    /// The starting position
    #[must_use]
    pub fn starting() -> Self {
        const BLACK_PIECES: [BitBoard; Shape::NUM] = [
            BitBoard::new(0b0000_0000_1111_1111 << 48),
            BitBoard::new(0b0100_0010_0000_0000 << 48),
            BitBoard::new(0b0010_0100_0000_0000 << 48),
            BitBoard::new(0b1000_0001_0000_0000 << 48),
            BitBoard::new(0b0000_1000_0000_0000 << 48),
            BitBoard::new(0b0001_0000_0000_0000 << 48),
        ];

        const WHITE_PIECES: [BitBoard; Shape::NUM] = [
            BitBoard::new(0b1111_1111_0000_0000),
            BitBoard::new(0b0000_0000_0100_0010),
            BitBoard::new(0b0000_0000_0010_0100),
            BitBoard::new(0b0000_0000_1000_0001),
            BitBoard::new(0b0000_0000_0000_1000),
            BitBoard::new(0b0000_0000_0001_0000),
        ];

        Self {
            pieces: PieceSet::new([WHITE_PIECES, BLACK_PIECES]),
            ..Default::default()
        }
    }

    #[must_use]
    pub fn player_to_move(&self) -> Color {
        self.move_counter.active_color
    }

    /// Returns `true` if the player has the right to castle on the given side
    /// of the board.
    ///
    /// A player retains the right to castle on a particular side of the board
    /// as long as they have not moved their king, or the rook on that side of
    /// the board.
    #[must_use]
    pub fn player_has_right_to_castle(&self, color: Color, castle: Castle) -> bool {
        self.flags.player_has_right_to_castle(color, castle)
    }

    /// The rook to use for a castling move.
    #[must_use]
    pub fn rook_for_castle(&self, player: Color, castle: Castle) -> Option<PlacedPiece> {
        let square = castle.parameters(player).rook_origin_square();
        self.piece_on_square(square)
    }

    /// Returns `true` if the player is able to castle on the given side of the board.
    ///
    /// The following requirements must be met:
    ///
    /// 1. The player must still have the right to castle on that side of the
    ///    board. The king and rook involved in the castle must not have moved.
    /// 1. The spaces between the king and rook must be clear
    /// 2. The king must not be in check
    /// 3. In the course of castling on that side, the king must not pass
    ///    through a square that an enemy piece can see
    pub fn player_can_castle(&self, player: Color, castle: Castle) -> bool {
        if !self
            .castling_rights
            .player_has_right_to_castle(player, castle.into())
        {
            return false;
        }

        let castling_parameters = castle.parameters(player);

        let all_pieces = self.all_pieces_bitboard();
        if !(all_pieces & castling_parameters.clear_squares()).is_empty() {
            return false;
        }

        let danger_squares = self.king_danger(player);
        if !(danger_squares & castling_parameters.check_squares()).is_empty() {
            return false;
        }

        true
    }

    /// A [`BitBoard`] representing the set of squares containing a piece. This
    /// set is the inverse of [`Board::empty_squares`].
    #[must_use]
    pub fn occupied_squares(&self) -> BitBoard {
        self.pieces.all_pieces()
    }

    #[must_use]
    pub fn friendly_pieces_bitboard(&self) -> BitBoard {
        self.pieces.all_pieces_of_color(self.player_to_move)
    }

    #[must_use]
    pub fn opposing_pieces_bitboard(&self) -> BitBoard {
        self.pieces.all_pieces_of_color(self.player_to_move.other())
    }

    #[must_use]
    pub fn all_pieces(&self) -> (BitBoard, BitBoard) {
        (
            self.friendly_pieces_bitboard(),
            self.opposing_pieces_bitboard(),
        )
    }

    /// A [BitBoard] representing the set of squares containing a piece. This
    /// set is the inverse of [`Board::occupied_squares`].
    #[must_use]
    pub fn empty_squares(&self) -> BitBoard {
        !self.occupied_squares()
    }

    #[must_use]
    pub fn piece_on_square(&self, square: Square) -> Option<PlacedPiece> {
        self.pieces
            .get(square)
            .map(|piece| PlacedPiece::new(piece, square))
    }

    #[must_use]
    pub fn iter_all_pieces(&self) -> impl Iterator<Item = PlacedPiece> + '_ {
        self.pieces.iter()
    }

    #[must_use]
    pub fn iter_pieces_of_color(&self, color: Color) -> impl Iterator<Item = PlacedPiece> + '_ {
        self.pieces
            .iter()
            .filter(move |piece| piece.color() == color)
    }

    #[must_use]
    pub fn has_en_passant_square(&self) -> bool {
        self.en_passant.is_some()
    }

    #[must_use]
    pub fn en_passant(&self) -> Option<EnPassant> {
        self.en_passant
    }

    fn king_bitboard(&self, player: Color) -> BitBoard {
        self.pieces.bitboard_for_piece(Piece::king(player))
    }

    pub(crate) fn king_square(&self, player: Color) -> Square {
        self.king_bitboard(player)
            .occupied_squares()
            .next()
            .unwrap()
    }

    #[must_use]
    pub fn display(&self) -> DiagramFormatter<'_> {
        DiagramFormatter::new(self)
    }

    #[must_use]
    pub fn bitboard_for_color(&self, color: Color) -> BitBoard {
        self.pieces.bitboard_for_color(color)
    }

    #[must_use]
    pub fn bitboard_for_piece(&self, piece: Piece) -> BitBoard {
        self.pieces.bitboard_for_piece(piece)
    }
}

impl Default for Board {
    fn default() -> Self {
        Self {
            castling_rights: castle::Rights::default(),
            pieces: PieceSet::default(),
            en_passant: None,
            move_counter: MoveCounter::default(),
        }
    }
}

impl PartialEq for Board {
    fn eq(&self, other: &Self) -> bool {
        self.pieces == other.pieces
            && self.castling_rights == other.castling_rights
            && self.en_passant == other.en_passant
            && self.move_counter == other.move_counter
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::test_board;
    use chessfriend_core::piece;

    #[test]
    fn piece_on_square() {
        let pos = test_board![
            Black Bishop on F7,
        ];

        let piece = pos.piece_on_square(Square::F7);
        assert_eq!(piece, Some(piece!(Black Bishop on F7)));
    }

    #[test]
    fn piece_in_starting_position() {
        let board = test_board!(starting);

        assert_eq!(
            board.piece_on_square(Square::H1),
            Some(piece!(White Rook on H1))
        );
        assert_eq!(
            board.piece_on_square(Square::A8),
            Some(piece!(Black Rook on A8))
        );
    }

    #[test]
    fn king_not_on_starting_square_cannot_castle() {
        let board = test_board!(White King on E4);
        assert!(!board
            .castling_rights
            .player_has_right_to_castle(Color::White, Castle::KingSide));
        assert!(!board
            .castling_rights
            .player_has_right_to_castle(Color::White, Castle::QueenSide));
    }

    #[test]
    fn king_on_starting_square_can_castle() {
        let board = test_board!(
            White King on E1,
            White Rook on A1,
            White Rook on H1
        );

        assert!(board
            .castling_rights
            .player_has_right_to_castle(Color::White, Castle::KingSide));
        assert!(board
            .castling_rights
            .player_has_right_to_castle(Color::White, Castle::QueenSide));
    }

    #[test]
    fn rook_for_castle() {
        let board = test_board![
            White King on E1,
            White Rook on H1,
            White Rook on A1,
        ];

        assert_eq!(
            board.rook_for_castle(Color::White, Castle::KingSide),
            Some(piece!(White Rook on H1))
        );
        assert_eq!(
            board.rook_for_castle(Color::White, Castle::QueenSide),
            Some(piece!(White Rook on A1))
        );
    }
}