95 lines
2.5 KiB
Rust
95 lines
2.5 KiB
Rust
use crate::{
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board::{game::Game, history::MoveParameters},
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evaluation::evaluation::evaluate_position,
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movegen::r#move::Move,
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};
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pub fn negamax(
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game: &mut Game,
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mut alpha: i32,
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beta: i32,
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depth: u8,
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plies: u8,
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) -> (Option<Move>, i32) {
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let color = game.current_player();
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if depth == 0 {
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return (None, evaluate_position(&game.board));
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}
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let (mut best_move, mut best_score, mate_score) = (None, -100000, -50000 + plies as i32);
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let mut legal_moves = 0;
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for mv in game.board.pseudo_moves_all(color) {
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let move_parameters = MoveParameters::build(&game.board, &mv);
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game.board.make_move(&mv);
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if !game.board.king_under_check(color) {
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legal_moves += 1;
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let move_score = -negamax(game, -beta, -alpha, depth - 1, plies + 1).1;
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if move_score > best_score {
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best_score = move_score;
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best_move = Some(mv)
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}
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if move_score >= beta {
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game.board.unmake_move(move_parameters);
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return (Some(mv), beta); // fail hard beta-cutoff
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}
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if move_score > alpha {
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alpha = move_score; // alpha acts like max in minimax
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}
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}
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game.board.unmake_move(move_parameters);
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}
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if legal_moves == 0 {
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if game.board.king_under_check(color) {
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return (None, mate_score);
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} else {
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return (None, 0);
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}
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}
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(best_move, best_score)
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}
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#[cfg(test)]
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mod tests {
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use crate::board::{fen::from_fen, square::Square};
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use crate::evaluation::evaluation::{MAX_EVAL, MIN_EVAL};
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use crate::movegen::attack::init_attacks;
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use crate::movegen::r#move::Move;
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use crate::search::negamax::negamax;
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const FEN_MATE_IN_1: &str = "8/8/8/8/8/4q1k1/8/5K2 b - - 0 1";
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#[test]
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fn test_negamax() -> Result<(), String> {
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init_attacks();
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let mut game = from_fen(FEN_MATE_IN_1)?;
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let e3f2 = Move::new(Square::E3, Square::F2);
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assert_eq!(
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e3f2,
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negamax(&mut game, MIN_EVAL, MAX_EVAL, 2, 0).0.unwrap()
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);
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assert_eq!(
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e3f2,
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negamax(&mut game, MIN_EVAL, MAX_EVAL, 3, 0).0.unwrap()
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);
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assert_eq!(
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e3f2,
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negamax(&mut game, MIN_EVAL, MAX_EVAL, 4, 0).0.unwrap()
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);
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assert_eq!(
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e3f2,
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negamax(&mut game, MIN_EVAL, MAX_EVAL, 5, 0).0.unwrap()
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);
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Ok(())
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}
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}
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