From 2678606e8dbeac8332909f0b3e43638936570835 Mon Sep 17 00:00:00 2001
From: xoto10 <23479932+xoto10@users.noreply.github.com>
Date: Fri, 14 Jun 2024 17:27:09 +0100
Subject: [PATCH] Consider wider range of moves near leaves.

try to avoid missing good moves for opponent or engine, by updating bestMove
also when value == bestValue (i.e. value == alpha) under certain conditions.
In particular require this is at higher depth in the tree, leaving the logic
near the root unchanged, and only apply randomly. Avoid doing this near mate
scores, leaving mate PVs intact.

Passed SMP STC 6+0.06 th7 :
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 42040 W: 10930 L: 10624 D: 20486
Ptnml(0-2): 28, 4682, 11289, 4998, 23
https://tests.stockfishchess.org/tests/view/66608b00c340c8eed7757d1d

Passed SMP LTC 24+0.24 th7 :
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 73692 W: 18978 L: 18600 D: 36114
Ptnml(0-2): 9, 7421, 21614, 7787, 15
https://tests.stockfishchess.org/tests/view/666095e8c340c8eed7757d49

closes https://github.com/official-stockfish/Stockfish/pull/5367

Bench 1205168
---
 src/search.cpp | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index 8fb65fe7..75eea2fd 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1283,11 +1283,17 @@ moves_loop:  // When in check, search starts here
                 rm.score = -VALUE_INFINITE;
         }
 
-        if (value > bestValue)
+        // In case we have an alternative move equal in eval to the current bestmove,
+        // promote it to bestmove by pretending it just exceeds alpha (but not beta).
+        int inc = (value == bestValue && (int(nodes) & 15) == 0
+                   && ss->ply + 2 + ss->ply / 32 >= thisThread->rootDepth
+                   && std::abs(value) + 1 < VALUE_TB_WIN_IN_MAX_PLY);
+
+        if (value + inc > bestValue)
         {
             bestValue = value;
 
-            if (value > alpha)
+            if (value + inc > alpha)
             {
                 bestMove = move;