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Properly handle checkmate occuring exactly when reaching the 50 move rule #754

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merged 1 commit into from
Dec 21, 2024
Merged

Properly handle checkmate occuring exactly when reaching the 50 move rule #754

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7 changes: 7 additions & 0 deletions src/movegen.c
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Original file line number Diff line number Diff line change
Expand Up @@ -187,3 +187,10 @@ void GenLegalMoves(Position *pos, MoveList *list) {
}
pos->nodes = 0;
}

int LegalMoveCount(Position *pos) {
MoveList list;
list.count = list.next = 0;
GenLegalMoves(pos, &list);
return list.count;
}
1 change: 1 addition & 0 deletions src/movegen.h
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Expand Up @@ -38,3 +38,4 @@ void GenNoisyMoves(const Position *pos, MoveList *list);
void GenQuietMoves(const Position *pos, MoveList *list);
void GenAllMoves(const Position *pos, MoveList *list);
void GenLegalMoves(Position *pos, MoveList *list);
int LegalMoveCount(Position *pos);
21 changes: 14 additions & 7 deletions src/search.c
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Expand Up @@ -105,8 +105,12 @@ static int Quiescence(Thread *thread, Stack *ss, int alpha, int beta) {
return alpha;
}

// Position is drawn
if (IsRepetition(pos) || pos->rule50 >= 100)
// Position is drawn by repetition
if (IsRepetition(pos))
return DrawScore(pos);

// Position is drawn by 50 move rule
if (pos->rule50 >= 100 && (!inCheck || LegalMoveCount(pos) > 0))
return DrawScore(pos);

// If we are at max depth, return static eval
Expand Down Expand Up @@ -244,7 +248,8 @@ static int AlphaBeta(Thread *thread, Stack *ss, int alpha, int beta, Depth depth
ss->doubleExtensions = (ss-1)->doubleExtensions;

const bool pvNode = alpha != beta - 1;
const bool root = ss->ply == 0;
const bool root = ss->ply == 0;
const bool inCheck = pos->checkers;

// Check time situation
if (OutOfTime(thread) || loadRelaxed(ABORT_SIGNAL))
Expand All @@ -260,8 +265,12 @@ static int AlphaBeta(Thread *thread, Stack *ss, int alpha, int beta, Depth depth
return alpha;
}

// Position is drawn
if (IsRepetition(pos) || pos->rule50 >= 100)
// Position is drawn by repetition
if (IsRepetition(pos))
return DrawScore(pos);

// Position is drawn by 50 move rule
if (pos->rule50 >= 100 && (!inCheck || LegalMoveCount(pos) > 0))
return DrawScore(pos);

// Max depth reached
Expand Down Expand Up @@ -325,8 +334,6 @@ static int AlphaBeta(Thread *thread, Stack *ss, int alpha, int beta, Depth depth
}
}

const bool inCheck = pos->checkers;

// Do a static evaluation for pruning considerations
int eval = ss->staticEval = inCheck ? NOSCORE
: lastMoveNullMove ? -(ss-1)->staticEval + 2 * Tempo
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