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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
pull/3552/head
J. Oster 2021-06-14 17:28:30 +02:00
parent 900f249f59
commit 4c4e104cad
1 changed files with 13 additions and 1 deletions
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14
src/syzygy/tbprobe.cpp
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@ -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]);