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Fix an issue with only capture moves

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When the only legal moves are captures, then
probe_wdl_table() returns a wrong score.
This commit is contained in:
Marco Costalba 2016-05-24 09:42:45 +02:00
parent f852b0ed8f
commit 09139d87cb
1 changed files with 22 additions and 20 deletions
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38
src/syzygy/tbprobe.cpp
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@ -1235,18 +1235,21 @@ int probe_dtz_table(const Position& pos, WDLScore wdl, ProbeState* result)
template<bool CheckZeroingMoves = false>
WDLScore search(Position& pos, WDLScore alpha, WDLScore beta, ProbeState* result)
{
WDLScore value, epValue = WDLScoreNone;
WDLScore value;
StateInfo st;
CheckInfo ci(pos);
auto moveList = MoveList<LEGAL>(pos);
size_t moveCount = moveList.size();
size_t totalCount = moveList.size();
size_t moveCount = 0;
for (const Move& move : moveList)
{
if ( !pos.capture(move)
&& (!CheckZeroingMoves || type_of(pos.moved_piece(move)) != PAWN))
continue;
moveCount++;
pos.do_move(move, st, pos.gives_check(move, ci));
value = -search(pos, -beta, -alpha, result);
pos.undo_move(move);
@ -1254,13 +1257,6 @@ WDLScore search(Position& pos, WDLScore alpha, WDLScore beta, ProbeState* result
if (*result == FAIL)
return WDLDraw;
if (type_of(move) == ENPASSANT)
{
moveCount--; // We can have up to 2 ep
if (value > epValue)
epValue = value;
}
if (value >= beta)
{
*result = ZEROING_BEST_MOVE; // Winning DTZ-zeroing move
@ -1271,22 +1267,28 @@ WDLScore search(Position& pos, WDLScore alpha, WDLScore beta, ProbeState* result
alpha = value;
}
// In case we have already searched all the legal moves we don't have to probe
// the TB because the stored score could be wrong. For instance TB tables
// do not contain information on position with ep rights, so in this case
// the result of probe_wdl_table is wrong. Also in case of only capture
// moves, for instance here 4K3/4q3/6p1/2k5/6p1/8/8/8 w - - 0 7, we have to
// return with ZEROING_BEST_MOVE set.
bool noMoreMoves = (moveCount && moveCount == totalCount);
if (noMoreMoves)
value = alpha;
else
{
value = probe_wdl_table(pos, result);
if (*result == FAIL)
return WDLDraw;
// TB tables do not contain information on position with ep rights, so in
// this case probe_wdl_table could be wrong. In particular could be higher
// then epValue and if epValue is the only move then we are forced to play
// the losing ep capture.
if (epValue != WDLScoreNone && !moveCount)
value = epValue;
}
// Here alpha stores the best value of the ply-1 search, note that in case
// we only have a losing ep move alpha == value.
// we have already searched all the possible moves alpha == value.
if (alpha >= value)
return *result = (alpha > WDLDraw || alpha == epValue ? ZEROING_BEST_MOVE : OK), alpha;
return *result = (alpha > WDLDraw || noMoreMoves ? ZEROING_BEST_MOVE : OK), alpha;
return *result = OK, value;
}