feat: aspiration window

Author: UndefinedCpp

src/search.cpp

@@ -137,6 +137,7 @@ class Searcher {
 
         Value alpha = Value::matedIn(0);
         Value beta = Value::mateIn(0);
+        Value window = 50;
         Value bestEvalRoot = Value::none();
         Move bestMoveRoot = Move::NO_MOVE;
 
@@ -168,6 +169,24 @@ class Searcher {
                 break;
             }
 
+            // Aspiration window
+            if (stats.depth >= 3) {
+                if (score <= alpha) {
+                    beta = (alpha + beta) / 2;
+                    alpha = alpha - window;
+                    window = window * 2;
+                    continue;
+                }
+                if (score >= beta) {
+                    beta += window;
+                    window = window * 2;
+                    continue;
+                }
+                window = 50;
+                alpha = score - window;
+                beta = score + window;
+            }
+
             stats.depth++;
         }