From a924641aafb03eeb56feb121af5b245f741ef8ec Mon Sep 17 00:00:00 2001
From: kuel27 <35112637+kuel27@users.noreply.github.com>
Date: Sat, 29 Aug 2026 18:49:43 +0300
Subject: [PATCH] Fix false TT hits for zero verification keys

When a TT slot is cleared, it has a verification key zero, but zero is also a valid 21-bit verification key. So a probe with this key can match an unused slot and read zeroed data as a valid TT entry.

To fix this, reserve verification key zero for unused slots. Skip TT reads and writes for positions with this key.

Bench: 2957947
---
 src/transposition.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/src/transposition.rs b/src/transposition.rs
index cd37eef2c..618ecfe43 100644
--- a/src/transposition.rs
+++ b/src/transposition.rs
@@ -193,6 +193,11 @@ impl TranspositionTable {
         };
 
         let key = verification_key(hash);
+
+        if key.0 == 0 {
+            return None;
+        }
+
         let index = cluster.lookup_key(key);
 
         if index < cluster.entries.len() {
@@ -227,6 +232,11 @@ impl TranspositionTable {
         };
 
         let key = verification_key(hash);
+
+        if key.0 == 0 {
+            return;
+        }
+
         let tt_age = self.age();
 
         let replacement_index = {