path: root/ai/src/lib.rs

pub use model::{CheckersBitBoard, Move, PieceColor, PossibleMoves};

const KING_WORTH: u32 = 2;

fn eval_position(board: CheckersBitBoard) -> f32 {
	let light_pieces = board.pieces_bits() & !board.color_bits();
	let dark_pieces = board.pieces_bits() & board.color_bits();

	let light_peasants = light_pieces & !board.king_bits();
	let dark_peasants = dark_pieces & !board.king_bits();

	let light_kings = light_pieces & board.king_bits();
	let dark_kings = dark_pieces & board.king_bits();

	// if we assume the black player doesn't exist, how good is this for white?
	let light_eval =
		(light_peasants.count_ones() as f32) + ((light_kings.count_ones() * KING_WORTH) as f32);
	let dark_eval =
		(dark_peasants.count_ones() as f32) + ((dark_kings.count_ones() * KING_WORTH) as f32);

	// avoiding a divide by zero error
	if dark_eval + light_eval != 0.0 {
		light_eval / (dark_eval + light_eval)
	} else {
		0.0
	}
}

pub fn eval(depth: usize, mut alpha: f32, beta: f32, board: CheckersBitBoard) -> f32 {
	if depth == 0 {
		eval_position(board)
	} else {
		let turn = board.turn();
		let mut best_eval = f32::NEG_INFINITY;
		for current_move in PossibleMoves::moves(board) {
			let board = unsafe { current_move.apply_to(board) };
			let current_eval = if board.turn() != turn {
				1.0 - eval(depth - 1, 1.0 - beta, 1.0 - alpha, board)
			} else {
				eval(depth - 1, alpha, beta, board)
			};

			if current_eval >= beta {
				return beta;
			}

			if best_eval < current_eval {
				best_eval = current_eval;
			}
			if alpha < best_eval {
				alpha = best_eval;
			}
		}

		best_eval
	}
}