From: Joona Kiiski <joona.kiiski@gmail.com>
Date: Fri, 22 Mar 2013 07:54:03 +0000 (+0100)
Subject: Fix bogus mate scores in some positions
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=09f1fdf52fe67b500f1a0159da9e53e9e3456679

Fix bogus mate scores in some positions

Always before pruning the move, it's important to check that:

bestValue > VALUE_MATED_IN_MAX_PLY

See example position:

8/2p1p3/P1NpP3/3k4/1P1BN3/2P1P3/2Q5/6K1 w - - 0 1
http://support.stockfishchess.org/discussions/problems/268-wrong-declaring-a-forced-mate-in-3-moves

This problem was present in 2.3.1, then it was fixed by my patch.

After 24000 games at 15+0.05
ELO: 2.40 +-4.4 (95%) LOS: 95.7%
Total: 24000 W: 4774 L: 4608 D: 14618

bench: 4465997
---

diff --git a/src/search.cpp b/src/search.cpp
index 7a06dad9..6dcd9f1d 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -856,7 +856,8 @@ split_point_start: // At split points actual search starts from here
           && !captureOrPromotion
           && !inCheck
           && !dangerous
-          &&  move != ttMove)
+          &&  move != ttMove
+          &&  bestValue > VALUE_MATED_IN_MAX_PLY)
       {
           // Move count based pruning
           if (   depth < 16 * ONE_PLY