Add aspiration windows

src/search/iterative_deepening.rs

@@ -15,6 +15,7 @@ pub fn iterative_deepening(
     time: &TimeInfo,
 ) -> anyhow::Result<Option<Move>> {
     let mut best_move = None;
+    let mut best_score = MIN_SCORE;
 
     for depth in 1..=max_depth {
         if time.exceed_soft_limit() {
@@ -26,7 +27,45 @@ pub fn iterative_deepening(
         }
 
         let mut nodes = 0;
-        let score = negamax::negamax(game, MIN_SCORE, MAX_SCORE, depth, 0, time, &mut nodes, true);
+
+        let mut score;
+
+        if depth < 4 {
+            score = negamax::negamax(game, MIN_SCORE, MAX_SCORE, depth, 0, time, &mut nodes, true);
+        } else {
+            let mut window_size = 20;
+            let mut alpha = (best_score - window_size).max(MIN_SCORE);
+            let mut beta = (best_score + window_size).min(MAX_SCORE);
+            let mut break_from_aw = true;
+
+            loop {
+                score = negamax::negamax(game, alpha, beta, depth, 0, time, &mut nodes, true);
+
+                let score = match score {
+                    Ok(score) => score,
+                    Err(ref e) => {
+                        write_response(&mut io::stdout(), &Response::Info(format!("{e}")))?;
+                        break_from_aw = false;
+                        break;
+                    }
+                };
+
+                if score >= beta {
+                    beta = MAX_SCORE.min(beta.saturating_add(window_size));
+                    window_size = window_size.saturating_mul(2);
+                } else if score <= alpha {
+                    alpha = MIN_SCORE.max(alpha.saturating_sub(window_size));
+                    window_size = window_size.saturating_mul(2);
+                } else {
+                    best_score = score;
+                    break;
+                }
+            }
+
+            if !break_from_aw {
+                break;
+            }
+        }
 
         if let Err(e) = score {
             write_response(&mut io::stdout(), &Response::Info(format!("{e}")))?;