From: Marco Costalba <mcostalba@gmail.com>
Date: Wed, 14 Apr 2010 05:13:24 +0000 (+0100)
Subject: Don't overwrite exsisting TT with null search value
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=ec0a650dff9ad78eff89e0c7fb04491f79fa7945

Don't overwrite exsisting TT with null search value

Real search is considered of higher quality then null
search one.

This allows to fix the zugzwang issue with a minimal
impact on ELO.

Zugzwang verified on position:

8/7P/8/8/K2b4/p7/1k6/1B6 b - -

After 999 games at 1+0 on my QUAD
Mod vs Orig(94bb196) +168 =657 -174  -2 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---

diff --git a/src/search.cpp b/src/search.cpp
index 043c433a..4c7e1e46 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1395,7 +1395,9 @@ namespace {
             {
                 assert(value_to_tt(nullValue, ply) == nullValue);
 
-                TT.store(posKey, nullValue, VALUE_TYPE_NS_LO, depth, MOVE_NONE);
+                if (!tte)
+                    TT.store(posKey, nullValue, VALUE_TYPE_NS_LO, depth, MOVE_NONE);
+
                 return nullValue;
             }
         } else {