zeal/src/evaluation/evaluation.rs

use u64 as Bitboard;

use crate::{
    board::{
        bitboard::{self, lsb},
        board::{Board, Color, PieceType},
    },
    evaluation::psqt::{mirror_index, piece_square_score},
};

use super::psqt::piece_square_score_endgame;

pub const fn material_score(piece_type: PieceType) -> i32 {
    match piece_type {
        PieceType::Pawn => 100,
        PieceType::Knight => 320,
        PieceType::Bishop => 330,
        PieceType::Rook => 500,
        PieceType::Queen => 900,
        PieceType::King => 0,
    }
}

fn is_end_game(board: &Board) -> bool {
    let white_pieces = board.white_pieces.iter().fold(0, |acc, p| {
        acc + bitboard::bit_count(p.bitboard) * material_score(p.piece_type) as usize
    });

    let black_pieces = board.black_pieces.iter().fold(0, |acc, p| {
        acc + bitboard::bit_count(p.bitboard) * material_score(p.piece_type) as usize
    });

    (white_pieces + black_pieces) < 2000
}

fn evaluate_side_for(board: &Board, color: Color) -> i32 {
    let mut total_score = 0;
    let pieces = match color {
        Color::White => &board.white_pieces,
        Color::Black => &board.black_pieces,
    };

    let psqt = if is_end_game(board) {
        piece_square_score_endgame
    } else {
        piece_square_score
    };

    for piece in pieces {
        let (piece_type, mut bitboard): (PieceType, Bitboard) = (piece.piece_type, piece.bitboard);
        let mut score = 0;
        while bitboard != 0 {
            let psqt_index = match color {
                Color::White => lsb(bitboard),
                Color::Black => mirror_index(lsb(bitboard)),
            };

            score += material_score(piece_type);
            score += psqt(piece_type, psqt_index);
            bitboard &= bitboard - 1;
        }
        total_score += score;
    }
    total_score
}

pub fn evaluate_position(board: &Board) -> i32 {
    let total_white_score = evaluate_side_for(board, Color::White);
    let total_black_score = evaluate_side_for(board, Color::Black);

    match board.state.current_player() {
        Color::White => total_white_score - total_black_score,
        Color::Black => total_black_score - total_white_score,
    }
}

#[cfg(test)]
mod tests {
    use crate::{
        board::{
            board::{Color, PieceType},
            fen::from_fen,
        },
        evaluation::evaluation::{evaluate_position, evaluate_side_for, material_score},
    };

    const FEN_QUIET: [&str; 2] = [
        "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1",
        "rnbqkbnr/ppp2ppp/4p3/3pN3/3P4/8/PPP1PPPP/RNBQKB1R w KQkq - 0 1",
    ];

    #[test]
    fn test_material_score() -> Result<(), String> {
        assert_eq!(100, material_score(PieceType::Pawn));
        assert_eq!(320, material_score(PieceType::Knight));
        assert_eq!(330, material_score(PieceType::Bishop));
        assert_eq!(500, material_score(PieceType::Rook));
        assert_eq!(900, material_score(PieceType::Queen));
        assert_eq!(0, material_score(PieceType::King));

        Ok(())
    }

    #[test]
    fn test_evaluate_side_for() -> Result<(), String> {
        let game = from_fen(FEN_QUIET[0])?;

        assert_eq!(
            evaluate_side_for(&game.board, Color::White),
            evaluate_side_for(&game.board, Color::Black)
        );

        let game_2 = from_fen(FEN_QUIET[1])?;
        let evaluate_white = evaluate_side_for(&game_2.board, Color::White);
        let evaluate_black = evaluate_side_for(&game_2.board, Color::Black);

        assert_eq!(4005, evaluate_white);
        assert_eq!(3965, evaluate_black);

        Ok(())
    }

    #[test]
    fn test_evaluate_position() -> Result<(), String> {
        let game = from_fen(FEN_QUIET[1])?;
        assert_eq!(40, evaluate_position(&game.board));

        Ok(())
    }
}