Implement quiescence search for leaf nodes

src/search/mod.rs

@@ -1,2 +1,3 @@
 pub mod negamax;
 pub mod perft;
+pub mod quiescence;

src/search/negamax.rs

@@ -4,6 +4,8 @@ use crate::{
     movegen::r#move::Move,
 };
 
+use super::quiescence::quiescence;
+
 pub fn negamax(
     game: &mut Game,
     mut alpha: i32,
@@ -14,7 +16,7 @@ pub fn negamax(
     let color = game.current_player();
 
     if depth == 0 {
-        return (None, evaluate_position(&game.board));
+        return (None, quiescence(game, alpha, beta).1);
     }
 
     let (mut best_move, mut best_score, mate_score) = (None, -100000, -50000 + plies as i32);

src/search/quiescence.rs

@@ -0,0 +1,45 @@
+use crate::{
+    board::{game::Game, history::MoveParameters},
+    evaluation::evaluation::evaluate_position,
+    movegen::r#move::{Move, MoveType},
+};
+
+pub fn quiescence(game: &mut Game, mut alpha: i32, beta: i32) -> (Option<Move>, i32) {
+    let color = game.current_player();
+    let stand_pat = evaluate_position(&game.board);
+
+    if stand_pat >= beta {
+        return (None, beta);
+    }
+
+    if alpha < stand_pat {
+        alpha = stand_pat;
+    }
+
+    let captures = game
+        .board
+        .pseudo_moves_all(color)
+        .into_iter()
+        .filter(|m| !matches!(m.move_type, MoveType::Quiet));
+
+    for mv in captures {
+        let move_parameters = MoveParameters::build(&game.board, &mv);
+        game.board.make_move(&mv);
+
+        if game.board.king_under_check(color) {
+            game.board.unmake_move(move_parameters);
+            continue;
+        }
+
+        let move_score = -quiescence(game, -beta, -alpha).1;
+        game.board.unmake_move(move_parameters);
+
+        if move_score >= beta {
+            return (Some(mv), beta);
+        }
+
+        alpha = alpha.max(move_score);
+    }
+
+    (None, 0)
+}