From d275bf9643768cbf6472977f4262220e6c1c1bb5 Mon Sep 17 00:00:00 2001
From: Shawn Xu <xu107288696@gmail.com>
Date: Thu, 15 Aug 2024 16:24:56 -0600
Subject: [PATCH] Introduce Fail Low History Bonus

When a node fails low, give TT move a small bonus 1/4 of normal value.

Passed STC:
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 92384 W: 24094 L: 23691 D: 44599
Ptnml(0-2): 323, 10852, 23465, 11203, 349
https://tests.stockfishchess.org/tests/view/66be80794ff211be9d4eed68

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 114660 W: 29260 L: 28778 D: 56622
Ptnml(0-2): 97, 12506, 31653, 12966, 108
https://tests.stockfishchess.org/tests/view/66bf63ee4ff211be9d4eeef0

closes https://github.com/official-stockfish/Stockfish/pull/5539

bench 1463003
---
 src/search.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/search.cpp b/src/search.cpp
index b062aa46..c94d3c42 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1361,6 +1361,10 @@ moves_loop:  // When in check, search starts here
               << stat_bonus(depth) * bonus / 25;
     }
 
+    // Bonus when search fails low and there is a TT move
+    else if (moveCount > 1 && ttData.move && (cutNode || PvNode))
+        thisThread->mainHistory[us][ttData.move.from_to()] << stat_bonus(depth) / 4;
+
     if (PvNode)
         bestValue = std::min(bestValue, maxValue);