chessfriend/board/src/move_generator/king.rs

// Eryn Wells <eryn@erynwells.me>

//! Declares the KingMoveGenerator type. This struct is responsible for
//! generating the possible moves for the king in the given position.

use super::{move_generator_declaration, MoveGeneratorInternal, MoveSet};
use crate::{r#move::Castle, Move, MoveBuilder, Position};
use chessfriend_bitboard::BitBoard;
use chessfriend_core::{Color, Piece, PlacedPiece};

move_generator_declaration!(KingMoveGenerator, struct);
move_generator_declaration!(KingMoveGenerator, new);
move_generator_declaration!(KingMoveGenerator, getters);

impl KingMoveGenerator {
    #[allow(unused_variables)]
    fn king_side_castle(position: &Position, color: Color) -> Option<Move> {
        if !position.player_has_right_to_castle(color, Castle::KingSide) {
            return None;
        }

        // TODO: Implement king side castle.
        None
    }

    #[allow(unused_variables)]
    fn queen_side_castle(position: &Position, color: Color) -> Option<Move> {
        if !position.player_has_right_to_castle(color, Castle::QueenSide) {
            return None;
        }

        // TODO: Implement queen side castle.
        None
    }
}

impl MoveGeneratorInternal for KingMoveGenerator {
    fn piece(color: Color) -> Piece {
        Piece::king(color)
    }

    fn move_set_for_piece(position: &Position, placed_piece: PlacedPiece) -> MoveSet {
        let piece = placed_piece.piece();
        let color = piece.color();
        let square = placed_piece.square();

        let empty_squares = position.empty_squares();
        let opposing_pieces = position.bitboard_for_color(color.other());

        let all_moves = BitBoard::king_moves(square);
        let quiet_moves_bb = all_moves & empty_squares;
        let capture_moves_bb = all_moves & opposing_pieces;

        // TODO: Handle checks. Prevent moving a king to a square attacked by a
        // piece of the opposite color.

        let map_to_move = |sq| MoveBuilder::new(*piece, square, sq).build();

        let king_side_castle = Self::king_side_castle(position, color);
        let queen_side_castle = Self::queen_side_castle(position, color);
        let quiet_moves = quiet_moves_bb
            .occupied_squares()
            .map(map_to_move)
            .chain(king_side_castle.iter().cloned())
            .chain(queen_side_castle.iter().cloned());
        let capture_moves = capture_moves_bb.occupied_squares().map(map_to_move);

        MoveSet::new(placed_piece)
            .quiet_moves(quiet_moves_bb, quiet_moves)
            .capture_moves(capture_moves_bb, capture_moves)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{position, r#move::AlgebraicMoveFormatter, PositionBuilder};
    use chessfriend_bitboard::bitboard;
    use chessfriend_core::{piece, Square};
    use std::collections::HashSet;

    #[test]
    fn one_king() {
        let pos = position![White King on E4];

        let generator = KingMoveGenerator::new(&pos, Color::White);

        assert_eq!(
            generator.bitboard(),
            bitboard![E5, F5, F4, F3, E3, D3, D4, D5]
        );

        let expected_moves = [
            MoveBuilder::new(piece!(White King), Square::E4, Square::D5).build(),
            MoveBuilder::new(piece!(White King), Square::E4, Square::E5).build(),
            MoveBuilder::new(piece!(White King), Square::E4, Square::F5).build(),
            MoveBuilder::new(piece!(White King), Square::E4, Square::F4).build(),
            MoveBuilder::new(piece!(White King), Square::E4, Square::F3).build(),
            MoveBuilder::new(piece!(White King), Square::E4, Square::E3).build(),
            MoveBuilder::new(piece!(White King), Square::E4, Square::D3).build(),
            MoveBuilder::new(piece!(White King), Square::E4, Square::D4).build(),
        ];

        let mut generated_moves: HashSet<Move> = generator.iter().cloned().collect();

        for ex_move in expected_moves {
            assert!(
                generated_moves.remove(&ex_move),
                "{:#?} was not generated",
                &ex_move
            );
        }

        assert!(
            generated_moves.is_empty(),
            "Moves unexpectedly present: {:#?}",
            generated_moves
        );
    }

    #[test]
    fn one_king_corner() {
        let pos = position![White King on A1];

        let generator = KingMoveGenerator::new(&pos, Color::White);

        let generated_bitboard = generator.bitboard();
        let expected_bitboard = bitboard![A2, B2, B1];
        assert_eq!(
            generator.bitboard(),
            bitboard![A2, B2, B1],
            "Generated:\n{generated_bitboard}\nExpected:\n{expected_bitboard}"
        );

        let expected_moves = [
            MoveBuilder::new(piece!(White King), Square::A1, Square::A2).build(),
            MoveBuilder::new(piece!(White King), Square::A1, Square::B1).build(),
            MoveBuilder::new(piece!(White King), Square::A1, Square::B2).build(),
        ];

        let mut generated_moves: HashSet<Move> = generator.iter().cloned().collect();

        for ex_move in expected_moves {
            assert!(
                generated_moves.remove(&ex_move),
                "{:#?} was not generated",
                &ex_move
            );
        }

        assert!(
            generated_moves.is_empty(),
            "Moves unexpectedly present: {:#?}",
            generated_moves
        );
    }

    #[test]
    fn black_king_in_check_by_rook() {
        let pos = PositionBuilder::new()
            .place_piece(piece!(White King on E1))
            .place_piece(piece!(White Rook on E4))
            .place_piece(piece!(Black King on E7))
            .to_move(Color::Black)
            .build();

        assert!(pos.is_king_in_check());

        let generator = KingMoveGenerator::new(&pos, Color::Black);
        let generated_moves: HashSet<Move> = generator.iter().cloned().collect();

        let king = piece!(Black King);
        let from_square = Square::E7;
        let expected_moves = HashSet::from_iter([
            MoveBuilder::new(king, from_square, Square::D6).build(),
            MoveBuilder::new(king, from_square, Square::D7).build(),
            MoveBuilder::new(king, from_square, Square::D8).build(),
            MoveBuilder::new(king, from_square, Square::F6).build(),
            MoveBuilder::new(king, from_square, Square::F7).build(),
            MoveBuilder::new(king, from_square, Square::F8).build(),
        ]);

        assert_eq!(
            generated_moves,
            expected_moves,
            "Difference: {:?}",
            generated_moves
                .symmetric_difference(&expected_moves)
                .map(|mv| format!("{}", AlgebraicMoveFormatter::new(mv, &pos)))
                .collect::<Vec<_>>()
        );
    }
}