Convert minimax to negamax and implement alpha-beta pruning

src/search/negamax.rs

@@ -0,0 +1,94 @@
+use crate::{
+    board::{game::Game, history::MoveParameters},
+    evaluation::evaluation::evaluate_position,
+    movegen::r#move::Move,
+};
+
+pub fn negamax(
+    game: &mut Game,
+    mut alpha: i32,
+    beta: i32,
+    depth: u8,
+    plies: u8,
+) -> (Option<Move>, i32) {
+    let color = game.current_player();
+
+    if depth == 0 {
+        return (None, evaluate_position(&game.board));
+    }
+
+    let (mut best_move, mut best_score, mate_score) = (None, -100000, -50000 + plies as i32);
+    let mut legal_moves = 0;
+
+    for mv in game.board.pseudo_moves_all(color) {
+        let move_parameters = MoveParameters::build(&game.board, &mv);
+        game.board.make_move(&mv);
+
+        if !game.board.king_under_check(color) {
+            legal_moves += 1;
+            let move_score = -negamax(game, -beta, -alpha, depth - 1, plies + 1).1;
+
+            if move_score > best_score {
+                best_score = move_score;
+                best_move = Some(mv)
+            }
+
+            if move_score >= beta {
+                game.board.unmake_move(move_parameters);
+                return (Some(mv), beta); // fail hard beta-cutoff
+            }
+
+            if move_score > alpha {
+                alpha = move_score; // alpha acts like max in minimax
+            }
+        }
+        game.board.unmake_move(move_parameters);
+    }
+
+    if legal_moves == 0 {
+        if game.board.king_under_check(color) {
+            return (None, mate_score);
+        } else {
+            return (None, 0);
+        }
+    }
+    (best_move, best_score)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::board::{fen::from_fen, square::Square};
+    use crate::evaluation::evaluation::{MAX_EVAL, MIN_EVAL};
+    use crate::movegen::attack::init_attacks;
+    use crate::movegen::r#move::Move;
+    use crate::search::negamax::negamax;
+
+    const FEN_MATE_IN_1: &str = "8/8/8/8/8/4q1k1/8/5K2 b - - 0 1";
+
+    #[test]
+    fn test_negamax() -> Result<(), String> {
+        init_attacks();
+        let mut game = from_fen(FEN_MATE_IN_1)?;
+
+        let e3f2 = Move::new(Square::E3, Square::F2);
+
+        assert_eq!(
+            e3f2,
+            negamax(&mut game, MIN_EVAL, MAX_EVAL, 2, 0).0.unwrap()
+        );
+        assert_eq!(
+            e3f2,
+            negamax(&mut game, MIN_EVAL, MAX_EVAL, 3, 0).0.unwrap()
+        );
+        assert_eq!(
+            e3f2,
+            negamax(&mut game, MIN_EVAL, MAX_EVAL, 4, 0).0.unwrap()
+        );
+        assert_eq!(
+            e3f2,
+            negamax(&mut game, MIN_EVAL, MAX_EVAL, 5, 0).0.unwrap()
+        );
+
+        Ok(())
+    }
+}