From 248e679a3b744a21c4eecb05a66734b18acdd322 Mon Sep 17 00:00:00 2001
From: Thomas Marsh <thomas.marsh2@gmail.com>
Date: Tue, 27 Feb 2024 12:14:44 -0500
Subject: [PATCH] Use non-zero initial Zobrist hash

---
 src/games/ttt.rs |  2 +-
 src/zobrist.rs   | 11 ++++++++++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/games/ttt.rs b/src/games/ttt.rs
index b64fd36..943f914 100644
--- a/src/games/ttt.rs
+++ b/src/games/ttt.rs
@@ -167,7 +167,7 @@ impl HashedPosition {
     pub fn new() -> Self {
         Self {
             position: Position::new(),
-            hashes: [0; 8],
+            hashes: [HASHES.initial(); 8],
         }
     }
 }
diff --git a/src/zobrist.rs b/src/zobrist.rs
index 3e7e618..4845030 100644
--- a/src/zobrist.rs
+++ b/src/zobrist.rs
@@ -66,6 +66,7 @@ impl<T> ZobristHashMap<T> {
 
 pub struct ZobristTable<const N: usize> {
     hashes: [u64; N],
+    initial: u64,
     // We have a unique path via node_id in mcts, but other approaches might
     // benefit from having a path hash.  See: Kishimoto, A., Müller, M., A
     // General Solution to the Graph History Interaction Problem.
@@ -82,7 +83,10 @@ impl<const N: usize> ZobristTable<N> {
             *h = rng.gen::<u64>();
         }
 
-        ZobristTable { hashes }
+        ZobristTable {
+            hashes,
+            initial: rng.gen::<u64>(),
+        }
     }
 
     fn hash(&self, index: usize) -> u64 {
@@ -114,4 +118,9 @@ impl<const N: usize> LazyZobristTable<N> {
     pub fn hash(&self, index: usize) -> u64 {
         self.get_or_init().hash(index)
     }
+
+    #[inline(always)]
+    pub fn initial(&self) -> u64 {
+        self.get_or_init().initial
+    }
 }