chessfriend/position/src/evaluation.rs

// Eryn Wells <eryn@erynwells.me>

use chessfriend_board::Board;
use chessfriend_core::{Color, Piece, Shape, score::Score};

struct Evaluator;

impl Evaluator {
    pub fn evaluate_symmetric_unwrapped(board: &Board, color: Color) -> Score {
        let material_balance = Self::material_balance(board, color);

        let to_move_factor = color.score_factor();

        to_move_factor * material_balance
    }

    /// Evaluate a board using the symmetric evaluation algorithm defined by
    /// Claude Shannon.
    fn material_balance(board: &Board, color: Color) -> Score {
        let other_color = color.other();

        Shape::into_iter().fold(Score::zero(), |acc, shape| {
            let (active_pieces, other_pieces) = (
                board.count_piece(&Piece::new(color, shape)) as i32,
                board.count_piece(&Piece::new(other_color, shape)) as i32,
            );

            let factor = shape.score() * (active_pieces - other_pieces);

            acc + factor
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use chessfriend_board::fen;

    #[test]
    fn pawn_material_balance() -> Result<(), Box<dyn std::error::Error>> {
        let board = fen!("8/8/8/8/8/3P4/8/8 w - - 0 1")?;
        assert_eq!(
            Evaluator::material_balance(&board, Color::White),
            100i32.into()
        );

        let board = fen!("8/8/3p4/8/8/3P4/8/8 w - - 0 1")?;
        assert_eq!(Evaluator::material_balance(&board, Color::White), 0.into());

        Ok(())
    }

    #[test]
    fn starting_position_is_even() {
        let board = Board::starting(None);
        assert_eq!(
            Evaluator::evaluate_symmetric_unwrapped(&board, Color::White),
            Evaluator::evaluate_symmetric_unwrapped(&board, Color::Black)
        );
    }
}