chessfriend/board/src/position/builders/move_builder.rs

// Eryn Wells <eryn@erynwells.me>

use crate::{
    position::flags::Flags, r#move::Castle, sight::SightExt, MakeMoveError, Move, Position,
};
use chessfriend_bitboard::BitBoard;
use chessfriend_core::{Color, Direction, Piece, PlacedPiece, Shape, Square};

/// A position builder that builds a new position by making a move.
#[derive(Clone)]
pub struct Builder<'p, M: MoveToMake> {
    position: &'p Position,
    move_to_make: M,
}

pub trait MoveToMake {}

pub struct NoMove;

pub enum ValidatedMove {
    RegularMove {
        from_square: Square,
        to_square: Square,
        moving_piece: PlacedPiece,
        captured_piece: Option<PlacedPiece>,
        promotion: Option<Shape>,
        flags: Flags,
        en_passant_square: Option<Square>,
        increment_ply: bool,
    },
    Castle {
        castle: Castle,
        king: PlacedPiece,
        rook: PlacedPiece,
        flags: Flags,
    },
}

impl MoveToMake for NoMove {}
impl MoveToMake for ValidatedMove {}

impl<'p> Builder<'p, NoMove> {
    pub fn new(position: &'p Position) -> Builder<'p, NoMove> {
        Builder {
            position,
            move_to_make: NoMove,
        }
    }
}

impl<'p, M> Builder<'p, M>
where
    M: MoveToMake,
{
    pub fn make(self, mv: &Move) -> Result<Builder<'p, ValidatedMove>, MakeMoveError> {
        let from_square = mv.from_square();

        let piece = self
            .position
            .piece_on_square(from_square)
            .ok_or(MakeMoveError::NoPiece)?;

        let to_square = mv.to_square();

        let sight = self.position.sight_of_piece(&piece);
        if !sight.is_set(to_square) {
            return Err(MakeMoveError::IllegalSquare(to_square));
        }

        let player = self.position.player_to_move();

        let captured_piece: Option<PlacedPiece> = if mv.is_en_passant() {
            // En passant captures the pawn directly ahead (in the player's direction) of the en passant square.
            let capture_square = match player {
                Color::White => to_square.neighbor(Direction::South),
                Color::Black => to_square.neighbor(Direction::North),
            }
            .ok_or(MakeMoveError::NoCapturedPiece)?;

            Some(
                self.position
                    .piece_on_square(capture_square)
                    .ok_or(MakeMoveError::NoCapturedPiece)?,
            )
        } else if mv.is_capture() {
            Some(
                self.position
                    .piece_on_square(to_square)
                    .ok_or(MakeMoveError::NoCapturedPiece)?,
            )
        } else {
            None
        };

        // TODO: Check whether the move is legal.

        let piece_is_king = piece.is_king();
        let mut flags = self.position.flags().clone();

        if piece_is_king {
            flags.clear_player_has_right_to_castle_flag(player, Castle::KingSide);
            flags.clear_player_has_right_to_castle_flag(player, Castle::QueenSide);
        } else if piece.is_kingside_rook() {
            flags.clear_player_has_right_to_castle_flag(player, Castle::KingSide);
        } else if piece.is_queenside_rook() {
            flags.clear_player_has_right_to_castle_flag(player, Castle::QueenSide);
        }

        if let Some(castle) = mv.castle() {
            println!("piece is king: {}", piece_is_king);
            if !piece_is_king || !self.position.player_can_castle(player, castle) {
                return Err(MakeMoveError::IllegalCastle);
            }

            let rook = self
                .position
                .rook_for_castle(player, castle)
                .ok_or(MakeMoveError::NoPiece)?;

            Ok(Builder {
                position: self.position,
                move_to_make: ValidatedMove::Castle {
                    castle,
                    king: piece,
                    rook,
                    flags,
                },
            })
        } else {
            let en_passant_square: Option<Square> = if mv.is_double_push() {
                match piece.color() {
                    Color::White => to_square.neighbor(Direction::South),
                    Color::Black => to_square.neighbor(Direction::North),
                }
            } else {
                None
            };

            Ok(Builder {
                position: self.position,
                move_to_make: ValidatedMove::RegularMove {
                    from_square,
                    to_square,
                    moving_piece: piece,
                    captured_piece,
                    promotion: mv.promotion(),
                    flags,
                    en_passant_square,
                    increment_ply: !(mv.is_capture() || piece.is_pawn()),
                },
            })
        }
    }
}

impl<'p> Builder<'p, ValidatedMove> {
    pub fn build(&self) -> Position {
        let player = self.position.player_to_move();

        let updated_move_number =
            self.position.move_number() + if player == Color::Black { 1 } else { 0 };

        match self.move_to_make {
            ValidatedMove::RegularMove {
                from_square,
                to_square,
                moving_piece,
                captured_piece,
                promotion,
                flags,
                en_passant_square,
                increment_ply,
            } => {
                let mut pieces = self.position.piece_bitboards().clone();

                if let Some(captured_piece) = captured_piece {
                    pieces.remove_piece(&captured_piece);
                }

                if let Some(promotion) = promotion {
                    pieces.remove_piece(&moving_piece);
                    let _ = pieces
                        .place_piece(&PlacedPiece::new(Piece::new(player, promotion), to_square));
                } else {
                    pieces.move_piece(moving_piece.piece(), from_square, to_square);
                }

                let ply = if increment_ply {
                    self.position.ply_counter() + 1
                } else {
                    0
                };

                Position::new(
                    self.position.player_to_move().other(),
                    flags,
                    pieces,
                    en_passant_square,
                    ply,
                    updated_move_number,
                )
            }
            ValidatedMove::Castle {
                castle,
                king,
                rook,
                flags,
            } => {
                let mut pieces = self.position.piece_bitboards().clone();

                let target_squares = castle.target_squares(player);

                let king_from: BitBoard = king.square().into();
                let king_to: BitBoard = target_squares.king.into();
                *pieces.bitboard_for_piece_mut(king.piece()) ^= king_from | king_to;

                let rook_from: BitBoard = rook.square().into();
                let rook_to: BitBoard = target_squares.rook.into();
                *pieces.bitboard_for_piece_mut(rook.piece()) ^= rook_from | rook_to;

                *pieces.bitboard_for_color_mut(player) &=
                    !(king_from | rook_from) | (king_to | rook_to);

                Position::new(
                    player.other(),
                    flags,
                    pieces,
                    None,
                    self.position.ply_counter() + 1,
                    updated_move_number,
                )
            }
        }
    }
}

impl<'p> From<&'p Position> for Builder<'p, NoMove> {
    fn from(position: &'p Position) -> Self {
        Self {
            position,
            move_to_make: NoMove,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{position, r#move::Castle, MoveBuilder, PositionBuilder};
    use chessfriend_core::piece;

    #[test]
    fn move_white_pawn_one_square() -> Result<(), MakeMoveError> {
        let pos = position![White Pawn on E2];
        let mv = MoveBuilder::new(piece!(White Pawn), Square::E2, Square::E3).build();

        let new_position = Builder::<NoMove>::new(&pos).make(&mv)?.build();
        println!("{}", &new_position);

        assert_eq!(
            new_position.piece_on_square(Square::E3),
            Some(piece!(White Pawn on E3))
        );

        Ok(())
    }

    #[test]
    fn move_white_pawn_two_squares() -> Result<(), MakeMoveError> {
        let pos = position![White Pawn on E2];
        let mv = MoveBuilder::new(piece!(White Pawn), Square::E2, Square::E4).build();

        let new_position = Builder::<NoMove>::new(&pos).make(&mv)?.build();
        println!("{}", &new_position);

        assert_eq!(
            new_position.piece_on_square(Square::E4),
            Some(piece!(White Pawn on E4))
        );
        assert_eq!(new_position.en_passant_square(), Some(Square::E3));

        Ok(())
    }

    #[test]
    fn white_kingside_castle() -> Result<(), MakeMoveError> {
        let pos = position![
            White King on E1,
            White Rook on H1,
            White Pawn on E2,
            White Pawn on F2,
            White Pawn on G2,
            White Pawn on H2
        ];
        println!("{}", &pos);

        let mv = MoveBuilder::new(piece!(White King), Square::E1, Square::G1)
            .castle(Castle::KingSide)
            .build();

        let new_position = Builder::<NoMove>::new(&pos).make(&mv)?.build();
        println!("{}", &new_position);

        assert_eq!(
            new_position.piece_on_square(Square::G1),
            Some(piece!(White King on G1))
        );
        assert_eq!(
            new_position.piece_on_square(Square::F1),
            Some(piece!(White Rook on F1))
        );

        Ok(())
    }

    #[test]
    fn en_passant_capture() -> Result<(), MakeMoveError> {
        let pos = PositionBuilder::new()
            .place_piece(piece!(White Pawn on B5))
            .place_piece(piece!(Black Pawn on A7))
            .to_move(Color::Black)
            .build();
        println!("{pos}");

        let black_pawn_move = MoveBuilder::new(piece!(Black Pawn), Square::A7, Square::A5).build();
        assert!(black_pawn_move.is_double_push());
        assert!(!black_pawn_move.is_en_passant());

        let en_passant_position = Builder::<NoMove>::new(&pos).make(&black_pawn_move)?.build();
        println!("{en_passant_position}");

        assert_eq!(
            en_passant_position.piece_on_square(Square::A5),
            Some(piece!(Black Pawn on A5))
        );
        assert_eq!(
            en_passant_position.piece_on_square(Square::B5),
            Some(piece!(White Pawn on B5))
        );

        let white_pawn_capture = MoveBuilder::new(piece!(White Pawn), Square::B5, Square::A6)
            .capturing_en_passant(piece!(Black Pawn on A5))
            .build();
        let en_passant_capture = Builder::<NoMove>::new(&en_passant_position)
            .make(&white_pawn_capture)?
            .build();
        println!("{en_passant_capture}");

        assert_eq!(en_passant_capture.piece_on_square(Square::A5), None);
        assert_eq!(en_passant_capture.piece_on_square(Square::B5), None);
        assert_eq!(
            en_passant_capture.piece_on_square(Square::A6),
            Some(piece!(White Pawn on A6))
        );

        Ok(())
    }
}