Compare with original alpha to decide node type

src/search/negamax.rs

@@ -106,6 +106,9 @@ pub fn negamax(
         )
     });
 
+    // Keep alpha before the loop raises it, so the TT node type uses
+    // the window we actually searched for this node
+    let alpha_original = alpha;
     let mut legal_moves = 0;
     let mut best_move = entry.and_then(|entry| entry.mv);
     let mut best_score = MIN_SCORE;
@@ -187,7 +190,7 @@ pub fn negamax(
 
     let node_type = match best_score {
         s if s >= beta => NodeType::LowerBound,
-        s if s <= alpha => NodeType::UpperBound,
+        s if s <= alpha_original => NodeType::UpperBound,
         _ => NodeType::Exact,
     };