inkwell/src/eval.rs

use rand::prelude::*;
use std::f64::{INFINITY, NEG_INFINITY};

use chess::{Board, BoardStatus, ChessMove, Color, MoveGen, Piece};

fn static_board_eval(
    mov: ChessMove,
    board: Board,
    playing_as: Color,
    move_history: Vec<(ChessMove, Board)>,
) -> f64 {
    if move_history.len() > 6 {
        if move_history[move_history.len() - 1].0 == mov
            && move_history[move_history.len() - 2].0 == mov
        {
            return -666.0;
        }
    }

    let mut current_eval = 0.0;

    // basic material counting
    board.color_combined(playing_as).for_each(|square| {
        match board.piece_on(square) {
            Some(Piece::Pawn) => current_eval += 1.0,
            Some(Piece::Knight) => current_eval += 3.0,
            Some(Piece::Bishop) => current_eval += 5.0,
            Some(Piece::Rook) => current_eval += 5.0,
            Some(Piece::Queen) => current_eval += 8.0,
            _ => { /* do nothing */ }
        }
    });

    board.color_combined(!playing_as).for_each(|square| {
        match board.piece_on(square) {
            Some(Piece::Pawn) => current_eval -= 1.0,
            Some(Piece::Knight) => current_eval -= 3.0,
            Some(Piece::Bishop) => current_eval -= 5.0,
            Some(Piece::Rook) => current_eval -= 5.0,
            Some(Piece::Queen) => current_eval -= 8.0,
            _ => { /* do nothing */ }
        }
    });

    // checking heuristic
    let mut are_we_in_check = false;
    let mut is_opponent_in_check = false;
    board
        .checkers()
        .filter(|sq| board.color_on(*sq).unwrap() == playing_as)
        .for_each(|_| {
            if !are_we_in_check {
                current_eval -= 66.6;
                are_we_in_check = true;
            } else {
                current_eval -= 12.5;
            }
        });
    board
        .checkers()
        .filter(|sq| board.color_on(*sq).unwrap() == !playing_as)
        .for_each(|_| {
            if !is_opponent_in_check
                && move_history.len()
                    > *vec![10, 11, 12, 13, 14, 15]
                        .choose(&mut rand::thread_rng())
                        .unwrap() as usize
            {
                current_eval += 17.5;
                is_opponent_in_check = true;
            } else {
                current_eval += 3.5;
            }
        });

    // deprioritize king movement
    if board.piece_on(mov.get_source()).unwrap_or(Piece::Pawn) == Piece::King && !are_we_in_check {
        current_eval -= 12.5;
    }

    current_eval
}

pub fn eval_board(
    board: Board,
    is_maximizing_player: bool,
    depth: u8,
    move_history: Vec<(ChessMove, Board)>,
    last_move: Option<ChessMove>,
    original_playing_as: Color,
    mut alpha: f64,
    mut beta: f64,
) -> f64 {
    let moves_iter = MoveGen::new_legal(&board).into_iter();

    if depth == 0 || board.status() == BoardStatus::Checkmate {
        return static_board_eval(
            last_move.unwrap_or(move_history.last().unwrap().0),
            board,
            original_playing_as,
            move_history,
        );
    } else {
        if is_maximizing_player {
            let mut max_eval = NEG_INFINITY;
            for mov in moves_iter {
                let mut new_move_history = move_history.clone();

                let new_board = board.make_move_new(mov);
                new_move_history.push((mov, new_board));

                let eval = eval_board(
                    new_board,
                    false,
                    depth - 1,
                    new_move_history,
                    Some(mov),
                    original_playing_as,
                    alpha,
                    beta,
                );
                max_eval = max_eval.max(eval);
                alpha = alpha.max(eval);
                if beta <= alpha {
                    break;
                }
            }
            return max_eval;
        } else {
            let mut min_eval = INFINITY;
            for mov in moves_iter {
                let mut new_move_history = move_history.clone();

                let new_board = board.make_move_new(mov);
                new_move_history.push((mov, new_board));

                let eval = eval_board(
                    new_board,
                    true,
                    depth - 1,
                    new_move_history,
                    Some(mov),
                    original_playing_as,
                    alpha,
                    beta,
                );
                min_eval = min_eval.min(eval);
                beta = beta.min(eval);
                if beta <= alpha {
                    break;
                }
            }
            return min_eval;
        }
    }
}