Change a/b to fail-soft

src/search/negamax.rs

@@ -36,7 +36,6 @@ pub fn negamax(
     let color = game.current_player();
     let mut best_move = None;
     let mut best_score = MIN_SCORE;
-    let mate_score = -MATE_SCORE + plies as i32;
     let mut legal_moves = 0;
     let all_moves = game.board.pseudo_moves_all();
 
@@ -57,23 +56,21 @@ pub fn negamax(
 
         if score > best_score {
             best_score = score;
-            best_move = Some(mv);
-        }
-
-        if score >= beta {
-            best_score = beta;
-            best_move = Some(mv);
-            break;
         }
 
         if score > alpha {
             alpha = score;
+            best_move = Some(mv);
+        }
+
+        if score >= beta {
+            break;
         }
     }
 
     if legal_moves == 0 {
         if game.board.king_under_check(color) {
-            return Ok(mate_score);
+            return Ok(-MATE_SCORE + plies as i32);
         }
         return Ok(0);
     }