chessfriend/board/src/position/position.rs

// Eryn Wells <eryn@erynwells.me>

use super::{flags::Flags, Pieces};
use crate::{
    bitboard::BitBoard,
    moves::Moves,
    piece::{Color, Piece, PiecePlacementError, PlacedPiece, Shape},
    Square,
};
use std::fmt;
use std::fmt::Write;

/// 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,
}

#[macro_export]
macro_rules! position {
    [$($color:ident $shape:ident on $square:ident,)*] => {
        Position::from_iter([
            $(piece!($color $shape on $square)),*
        ].into_iter())
    };
}

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

    /// Position flags indicating various bits of game state.  The flags are as
    /// follows:
    ///
    /// 0. white can castle king-side
    /// 1. white can castle queen-side
    /// 2. black can castle king-side
    /// 3. black can castle queen-side
    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],
}

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(),
                ],
            ],
        }
    }

    /// 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],
        }
    }

    /// 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)
    }

    fn set_player_has_right_to_castle_flag(&mut self, color: Color, side: BoardSide) {
        self.flags.set_player_has_right_to_castle_flag(color, side);
    }

    fn clear_player_has_right_to_castle_flag(&mut self, color: Color, side: BoardSide) {
        self.flags
            .clear_player_has_right_to_castle_flag(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.has_piece_at(square) {
            return Err(PiecePlacementError::ExistsOnSquare);
        }

        type_bb.place_piece_at(square);

        let color_bb = &mut self.bitboard_for_color_mut(piece.color());
        color_bb.place_piece_at(square);

        Ok(())
    }

    pub fn move_generator(&self, color: Color) -> Moves {
        Moves::new(self, color)
    }

    /// 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]
    }

    /// 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.has_piece_at(sq) {
                    return Some(PlacedPiece::new(piece, sq));
                }
            }
        }

        None
    }

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

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
    }
}

impl fmt::Debug for Position {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut output = String::new();

        output.push_str("Position {\n");
        write!(output, "    color_to_move: {:?},\n", self.color_to_move)?;

        output.push_str("    pieces_per_color: [\n");
        for bb in self.pieces_per_color {
            write!(output, "        {bb:?},\n")?;
        }
        output.push_str("    ],\n");

        output.push_str("    pieces_per_type: [\n");
        for color_bbs in self.pieces_per_type {
            output.push_str("        [\n");
            for bb in color_bbs {
                write!(output, "            {bb:?},\n")?;
            }
            output.push_str("        ],\n");
        }
        output.push_str("    ],\n}");

        write!(f, "{}", output)
    }
}

#[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?!");
    }
}