From: J. Oster <osterj165@googlemail.com>
Date: Mon, 14 Jun 2021 15:28:30 +0000 (+0200)
Subject: Fix a rare case of wrong TB ranking
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=4c4e104cadceae678668f1738b5ca6296b2b4ef8;hp=900f249f596417b35129f1dc357cd5392018e575

Fix a rare case of wrong TB ranking
of a root move leading to a 3-fold repetition.
With this small fix a draw ranking and thus a draw score is being applied.
This works for both, ranking by dtz or wdl tables.

Fixes https://github.com/official-stockfish/Stockfish/issues/3542

(No functional change without TBs.)
Bench: 4877339
---

diff --git a/src/syzygy/tbprobe.cpp b/src/syzygy/tbprobe.cpp
index a0ac727f..96b2970f 100644
--- a/src/syzygy/tbprobe.cpp
+++ b/src/syzygy/tbprobe.cpp
@@ -1536,6 +1536,14 @@ bool Tablebases::root_probe(Position& pos, Search::RootMoves& rootMoves) {
             WDLScore wdl = -probe_wdl(pos, &result);
             dtz = dtz_before_zeroing(wdl);
         }
+        else if (pos.is_draw(1))
+        {
+            // In case a root move leads to a draw by repetition or
+            // 50-move rule, we set dtz to zero. Note: since we are
+            // only 1 ply from the root, this must be a true 3-fold
+            // repetition inside the game history.
+            dtz = 0;
+        }
         else
         {
             // Otherwise, take dtz for the new position and correct by 1 ply
@@ -1586,6 +1594,7 @@ bool Tablebases::root_probe_wdl(Position& pos, Search::RootMoves& rootMoves) {
 
     ProbeState result;
     StateInfo st;
+    WDLScore wdl;
 
     bool rule50 = Options["Syzygy50MoveRule"];
 
@@ -1594,7 +1603,10 @@ bool Tablebases::root_probe_wdl(Position& pos, Search::RootMoves& rootMoves) {
     {
         pos.do_move(m.pv[0], st);
 
-        WDLScore wdl = -probe_wdl(pos, &result);
+        if (pos.is_draw(1))
+            wdl = WDLDraw;
+        else
+            wdl = -probe_wdl(pos, &result);
 
         pos.undo_move(m.pv[0]);