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);
     }

src/search/quiescence.rs

@@ -15,7 +15,7 @@ pub fn quiescence(game: &mut Game, mut alpha: i32, beta: i32, time_info: &TimeIn
     let stand_pat = pesto().eval(game);
 
     if stand_pat >= beta {
-        return Ok(beta);
+        return Ok(stand_pat);
     }
 
     if stand_pat > alpha {
@@ -26,6 +26,7 @@ pub fn quiescence(game: &mut Game, mut alpha: i32, beta: i32, time_info: &TimeIn
     let all_moves = game.board.pseudo_moves_all_captures();
     let tt_move = game.tt.lookup(game.hash).and_then(|entry| entry.mv);
     let moves = move_ordering::sort_moves(all_moves, &game.mailbox, tt_move);
+    let mut best_score = stand_pat;
 
     for mv in moves {
         game.make_move(&mv);
@@ -38,14 +39,19 @@ pub fn quiescence(game: &mut Game, mut alpha: i32, beta: i32, time_info: &TimeIn
         let score = -quiescence(game, -beta, -alpha, time_info)?;
         game.unmake_move();
 
-        if score >= beta {
-            return Ok(beta);
+
+        if score > best_score {
+            best_score = score;
         }
 
         if score > alpha {
-            alpha = score
+            alpha = score;
+        }
+
+        if score >= beta {
+            break;
         }
     }
 
-    Ok(alpha)
+    Ok(best_score)
 }