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Fix small bug in move_attacks_square()

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We test if the piece moved in 'to' attacks the square 's' with:

bit_is_set(attacks_from(piece, to), s))

But we should instead consider the new occupancy, changed after
the piece is moved, and so test with:

bit_is_set(attacks_from(piece, to, occ), s))

Otherwise we can miss some cases, for instance a queen in b1 that
moves in c1 is not detected to attack a1 while instead she does.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2011-10-30 18:59:12 +01:00
parent 29be28e1a2
commit c6f497f09d
1 changed files with 15 additions and 12 deletions
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27
src/position.cpp
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@ -377,7 +377,7 @@ template Bitboard Position::hidden_checkers<true>() const;
template Bitboard Position::hidden_checkers<false>() const;
/// Position::attackers_to() computes a bitboard of all pieces which attacks a
/// Position::attackers_to() computes a bitboard of all pieces which attack a
/// given square. Slider attacks use occ bitboard as occupancy.
Bitboard Position::attackers_to(Square s, Bitboard occ) const {
@ -417,22 +417,25 @@ bool Position::move_attacks_square(Move m, Square s) const {
assert(square_is_ok(s));
Bitboard occ, xray;
Square f = move_from(m), t = move_to(m);
Square from = move_from(m);
Square to = move_to(m);
Piece piece = piece_on(from);
assert(!square_is_empty(f));
assert(!square_is_empty(from));
if (bit_is_set(attacks_from(piece_on(f), t), s))
// Update occupancy as if the piece is moving
occ = occupied_squares();
do_move_bb(&occ, make_move_bb(from, to));
// The piece moved in 'to' attacks the square 's' ?
if (bit_is_set(attacks_from(piece, to, occ), s))
return true;
// Move the piece and scan for X-ray attacks behind it
occ = occupied_squares();
do_move_bb(&occ, make_move_bb(f, t));
xray = ( (rook_attacks_bb(s, occ) & pieces(ROOK, QUEEN))
|(bishop_attacks_bb(s, occ) & pieces(BISHOP, QUEEN)))
& pieces(color_of(piece_on(f)));
// Scan for possible X-ray attackers behind the moved piece
xray = (rook_attacks_bb(s, occ) & pieces(ROOK, QUEEN, color_of(piece)))
|(bishop_attacks_bb(s, occ) & pieces(BISHOP, QUEEN, color_of(piece)));
// If we have attacks we need to verify that are caused by our move
// and are not already existent ones.
// Verify attackers are triggered by our move and not already existing
return xray && (xray ^ (xray & attacks_from<QUEEN>(s)));
}