From f852b0ed8f469940e9cd83e45292b147bfa628b2 Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Tue, 24 May 2016 08:52:53 +0200
Subject: [PATCH] Hanlde mate position in dtz probe

Stick to the original code behaviour.
---
 src/syzygy/tbprobe.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/syzygy/tbprobe.cpp b/src/syzygy/tbprobe.cpp
index 78cc89ca..efca279c 100644
--- a/src/syzygy/tbprobe.cpp
+++ b/src/syzygy/tbprobe.cpp
@@ -1520,7 +1520,10 @@ int Tablebases::probe_dtz(Position& pos, ProbeState* result)
             minDTZ = dtz;
     }
 
-    return minDTZ + sign_of(minDTZ); // Convert result from 1-ply search
+    // Convert result from 1-ply search. Special handle a mate position, when
+    // there are no legal moves. Return value is somewhat arbitrary, so stick
+    // to the original TB code that returns -1 in this case.
+    return minDTZ == 0xFFFF ? - 1 : minDTZ + sign_of(minDTZ);
 }
 
 // Check whether there has been at least one repetition of positions