From: Marco Costalba <mcostalba@gmail.com>
Date: Thu, 11 Oct 2012 19:11:11 +0000 (+0200)
Subject: Fix a minor bug in search
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=aba152ea3a0b97c1df7aab3a909d52349bdf6c76;hp=7f9ebf8e86e90ea1eb68388f1be12503a3f31e56

Fix a minor bug in search

As Joona says: "The problem is that when doing full
window search (-VALUE_INFINITE, VALUE_INFINITE), and
pruning all the moves will return fail low which is
mate score, because only clause touching alpha is
"mate distance pruning". So we are returning mate score
although we are just pruning all the moves. In reality
there probably is no mate in sight.

Bug spotted and fixed by Joona.
---

diff --git a/src/search.cpp b/src/search.cpp
index 4139ef96..cf98ca35 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -826,7 +826,8 @@ split_point_start: // At split points actual search starts from here
           && !inCheck
           && !dangerous
           &&  move != ttMove
-          && (bestValue > VALUE_MATED_IN_MAX_PLY || bestValue == -VALUE_INFINITE))
+          && (bestValue > VALUE_MATED_IN_MAX_PLY || (   bestValue == -VALUE_INFINITE
+                                                     && alpha > VALUE_MATED_IN_MAX_PLY)))
       {
           // Move count based pruning
           if (   depth < 16 * ONE_PLY