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Detect stalemate in KXK endgames

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Also, handle cases where there are 2 bishops of the same color.
This commit is contained in:
Gary Linscott 2012-02-05 10:24:53 -05:00
parent 40e939421f
commit 0347339970
2 changed files with 29 additions and 3 deletions
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23
src/endgame.cpp
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@ -82,6 +82,21 @@ namespace {
template<typename M>
void delete_endgame(const typename M::value_type& p) { delete p.second; }
// Fast stalemate detection with lone king
bool is_kxk_stalemate(const Position &pos, const Color c) {
if ( pos.side_to_move() == c &&
!pos.in_check()) {
const Square from = pos.king_square(c);
Bitboard b = pos.attacks_from<KING>(from);
while (b) {
// Assume there are no pinned pieces, as it is a lone king
if (pos.pl_move_is_legal(make_move(from, pop_1st_bit(&b)), 0))
return false;
}
return true;
}
return false;
}
} // namespace
@ -132,6 +147,10 @@ Value Endgame<KXK>::operator()(const Position& pos) const {
assert(pos.non_pawn_material(weakerSide) == VALUE_ZERO);
assert(pos.piece_count(weakerSide, PAWN) == VALUE_ZERO);
if (is_kxk_stalemate(pos, weakerSide)) {
return VALUE_DRAW;
}
Square winnerKSq = pos.king_square(strongerSide);
Square loserKSq = pos.king_square(weakerSide);
@ -142,9 +161,9 @@ Value Endgame<KXK>::operator()(const Position& pos) const {
if ( pos.piece_count(strongerSide, QUEEN)
|| pos.piece_count(strongerSide, ROOK)
|| pos.piece_count(strongerSide, BISHOP) > 1)
// TODO: check for two equal-colored bishops!
|| pos.both_color_bishops(strongerSide)) {
result += VALUE_KNOWN_WIN;
}
return strongerSide == pos.side_to_move() ? result : -result;
}

7
src/position.h
View file

@ -189,6 +189,7 @@ public:
template<bool SkipRepetition> bool is_draw() const;
int startpos_ply_counter() const;
bool opposite_colored_bishops() const;
bool both_color_bishops(Color c) const;
bool has_pawn_on_7th(Color c) const;
bool is_chess960() const;
@ -432,6 +433,12 @@ inline bool Position::opposite_colored_bishops() const {
&& opposite_colors(pieceList[WHITE][BISHOP][0], pieceList[BLACK][BISHOP][0]);
}
inline bool Position::both_color_bishops(Color c) const {
// Assumes that there are only two bishops
return pieceCount[c][BISHOP] >= 2 &&
opposite_colors(pieceList[c][BISHOP][0], pieceList[c][BISHOP][1]);
}
inline bool Position::has_pawn_on_7th(Color c) const {
return pieces(PAWN, c) & rank_bb(relative_rank(c, RANK_7));
}