chessfriend/board/src/position/position.rs

// Eryn Wells <eryn@erynwells.me>

use super::{flags::Flags, Pieces};
use crate::{
    move_generator::Moves,
    piece::{Color, Piece, PiecePlacementError, PlacedPiece, Shape},
    sight::Sight,
    BitBoard, Square,
};
use std::cell::OnceCell;

/// A lateral side of the board relative to the player. Kingside is the side the
/// player's king starts on. Queenside is the side of the board the player's
/// queen starts on.
pub(crate) enum BoardSide {
    King,
    Queen,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Position {
    color_to_move: Color,
    flags: Flags,

    /// Composite bitboards for all the pieces of a particular color.
    pieces_per_color: [BitBoard; 2],

    /// Bitboards representing positions of particular piece types per color.
    pieces_per_type: [[BitBoard; 6]; 2],

    en_passant_square: Option<Square>,

    sight: [OnceCell<BitBoard>; 2],
}

impl Position {
    pub fn empty() -> Position {
        Position {
            color_to_move: Color::White,
            flags: Default::default(),
            pieces_per_color: [BitBoard::empty(); 2],
            pieces_per_type: [
                [
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                ],
                [
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                    BitBoard::empty(),
                ],
            ],
            en_passant_square: None,
            sight: [OnceCell::new(), OnceCell::new()],
        }
    }

    /// Return a starting position.
    pub fn starting() -> Position {
        let white_pieces = [
            BitBoard::new(0x00FF000000000000),
            BitBoard::new(0x4200000000000000),
            BitBoard::new(0x2400000000000000),
            BitBoard::new(0x8100000000000000),
            BitBoard::new(0x1000000000000000),
            BitBoard::new(0x0800000000000000),
        ];

        let black_pieces = [
            BitBoard::new(0xFF00),
            BitBoard::new(0x0042),
            BitBoard::new(0x0024),
            BitBoard::new(0x0081),
            BitBoard::new(0x0010),
            BitBoard::new(0x0008),
        ];

        Position {
            color_to_move: Color::White,
            flags: Default::default(),
            pieces_per_color: [
                white_pieces.iter().fold(BitBoard::empty(), |a, b| a | *b),
                black_pieces.iter().fold(BitBoard::empty(), |a, b| a | *b),
            ],
            pieces_per_type: [white_pieces, black_pieces],
            en_passant_square: None,
            sight: [OnceCell::new(), OnceCell::new()],
        }
    }

    /// Returns true if the player has the right to castle on the given side of
    /// the board.
    ///
    /// The right to castle on a particular side of the board is retained as
    /// long as the player has not moved their king, or the rook on that side of
    /// the board.
    ///
    /// The following requirements must also be met:
    ///
    /// 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(crate) fn player_has_right_to_castle(&self, color: Color, side: BoardSide) -> bool {
        self.flags.player_has_right_to_castle(color, side)
    }

    pub fn place_piece(&mut self, piece: Piece, square: Square) -> Result<(), PiecePlacementError> {
        let type_bb = self.bitboard_for_piece_mut(piece);

        if type_bb.is_set(square) {
            return Err(PiecePlacementError::ExistsOnSquare);
        }

        type_bb.set_square(square);

        self.bitboard_for_color_mut(piece.color())
            .set_square(square);

        Ok(())
    }

    pub fn moves(&self) -> Moves {
        Moves::new(self, self.color_to_move)
    }

    /// Return a BitBoard representing the set of squares containing a piece.
    #[inline]
    pub(crate) fn occupied_squares(&self) -> BitBoard {
        self.pieces_per_color[Color::White as usize] | self.pieces_per_color[Color::Black as usize]
    }

    #[inline]
    pub(crate) fn friendly_pieces(&self) -> BitBoard {
        self.bitboard_for_color(self.color_to_move)
    }

    #[inline]
    pub(crate) fn opposing_pieces(&self) -> BitBoard {
        self.bitboard_for_color(self.color_to_move.other())
    }

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

    pub(crate) fn bitboard_for_piece(&self, piece: Piece) -> BitBoard {
        self.pieces_per_type[piece.color() as usize][piece.shape() as usize]
    }

    fn bitboard_for_piece_mut(&mut self, piece: Piece) -> &mut BitBoard {
        &mut self.pieces_per_type[piece.color() as usize][piece.shape() as usize]
    }

    pub(crate) fn bitboard_for_color(&self, color: Color) -> BitBoard {
        self.pieces_per_color[color as usize]
    }

    fn bitboard_for_color_mut(&mut self, color: Color) -> &mut BitBoard {
        &mut self.pieces_per_color[color as usize]
    }

    pub fn piece_on_square(&self, sq: Square) -> Option<PlacedPiece> {
        for color in Color::iter() {
            for shape in Shape::iter() {
                let piece = Piece::new(color, *shape);
                let bb = self.bitboard_for_piece(piece);
                if bb.is_set(sq) {
                    return Some(PlacedPiece::new(piece, sq));
                }
            }
        }

        None
    }

    pub fn pieces(&self, color: Color) -> Pieces {
        Pieces::new(&self, color)
    }

    pub fn en_passant_square(&self) -> Option<Square> {
        self.en_passant_square
    }

    pub(crate) fn sight_of_player(&self, color: Color) -> BitBoard {
        *self.sight[color as usize].get_or_init(|| {
            self.pieces(color).fold(BitBoard::empty(), |acc, pp| {
                acc | pp.sight_in_position(&self)
            })
        })
    }

    pub(crate) fn is_king_in_check(&self) -> bool {
        let sight_of_opposing_player = self.sight_of_player(self.color_to_move.other());
        sight_of_opposing_player.is_set(self.king_square())
    }

    fn king_square(&self) -> Square {
        self.bitboard_for_piece(Piece::king(self.color_to_move))
            .occupied_squares()
            .next()
            .unwrap()
    }
}

#[cfg(test)]
impl Position {
    pub(crate) fn test_set_en_passant_square(&mut self, square: Square) {
        self.en_passant_square = Some(square);
    }
}

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

impl FromIterator<PlacedPiece> for Position {
    fn from_iter<T: IntoIterator<Item = PlacedPiece>>(iter: T) -> Self {
        let mut position = Position::empty();
        for placed_piece in iter {
            _ = position.place_piece(placed_piece.piece(), placed_piece.square());
        }

        position
    }
}

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

    #[test]
    fn place_piece() {
        let mut position = Position::empty();

        let piece = Piece::new(Color::White, Shape::Queen);
        let square = Square::E4;

        position
            .place_piece(piece, square)
            .expect("Unable to place white queen on e4");

        assert_eq!(
            position.bitboard_for_color(piece.color()).clone(),
            BitBoard::new(1 << 28)
        );
        assert_eq!(
            position.bitboard_for_piece(piece).clone(),
            BitBoard::new(1 << 28)
        );

        position
            .place_piece(piece, square)
            .expect_err("Placed white queen on e4 a second time?!");
    }

    #[test]
    fn king_is_in_check() {
        let pos = position![
            White King on E1,
            Black Rook on E8,
        ];
        assert!(pos.is_king_in_check());
    }

    #[test]
    fn king_is_not_in_check() {
        let pos = position![
            White King on F1,
            Black Rook on E8,
        ];
        assert!(!pos.is_king_in_check());
    }
}