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Teach SEE about pinned pieces

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Remove pinned pieces from attacks when calculating
SEE value.

Algorithm is not perfect, there should be no false
positives, but can happen that we miss to remove a
pinned piece. Currently we don't cach 100% of cases,
but is a tradeoff between speed and accuracy. In any
case we stay on the safe side, so we remove an attacker
when we are sure it is pinned.

About only 0,5% of cases are affected by this patch, not
a lot given the hard work: this is a difficult patch!

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2009-02-23 15:19:51 +01:00
parent 243fa483d7
commit 962216440c
1 changed files with 24 additions and 2 deletions
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26
src/position.cpp
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@ -1491,7 +1491,7 @@ void Position::undo_null_move() {
/// Position::see() is a static exchange evaluator: It tries to estimate the /// Position::see() is a static exchange evaluator: It tries to estimate the
/// material gain or loss resulting from a move. There are three versions of /// material gain or loss resulting from a move. There are three versions of
/// this function: One which takes a destination square as input, one takes a /// this function: One which takes a destination square as input, one takes a
/// move, and one which takes a 'from' and a 'to' square. The function does /// move, and one which takes a 'from' and a 'to' square. The function does
/// not yet understand promotions captures. /// not yet understand promotions captures.
@ -1528,6 +1528,11 @@ int Position::see(Square from, Square to) const {
Color us = (from != SQ_NONE ? color_of_piece_on(from) : opposite_color(color_of_piece_on(to))); Color us = (from != SQ_NONE ? color_of_piece_on(from) : opposite_color(color_of_piece_on(to)));
Color them = opposite_color(us); Color them = opposite_color(us);
// Initialize pinned and pinners bitboards
Bitboard pinned[2], pinners[2];
pinned[us] = pinned_pieces(us, pinners[us]);
pinned[them] = pinned_pieces(them, pinners[them]);
// Initialize pieces // Initialize pieces
Piece piece = piece_on(from); Piece piece = piece_on(from);
Piece capture = piece_on(to); Piece capture = piece_on(to);
@ -1560,6 +1565,17 @@ int Position::see(Square from, Square to) const {
| (pawn_attacks(WHITE, to) & pawns(BLACK)) | (pawn_attacks(WHITE, to) & pawns(BLACK))
| (pawn_attacks(BLACK, to) & pawns(WHITE)); | (pawn_attacks(BLACK, to) & pawns(WHITE));
// Remove our pinned pieces from attacks if the captured piece is not
// a pinner, otherwise we could remove a valid "capture the pinner" attack.
if (pinned[us] != EmptyBoardBB && !bit_is_set(pinners[us], to))
attackers &= ~pinned[us];
// Remove opponent pinned pieces from attacks if the moving piece is not
// a pinner, otherwise we could remove a piece that is no more pinned
// due to our pinner piece is moving away.
if (pinned[them] != EmptyBoardBB && !bit_is_set(pinners[them], from))
attackers &= ~pinned[them];
if (from != SQ_NONE) if (from != SQ_NONE)
break; break;
@ -1597,7 +1613,7 @@ int Position::see(Square from, Square to) const {
swapList[0] = seeValues[capture]; swapList[0] = seeValues[capture];
do { do {
// Locate the least valuable attacker for the side to move. The loop // Locate the least valuable attacker for the side to move. The loop
// below looks like it is potentially infinite, but it isn't. We know // below looks like it is potentially infinite, but it isn't. We know
// that the side to move still has at least one attacker left. // that the side to move still has at least one attacker left.
for (pt = PAWN; !(attackers & pieces_of_color_and_type(c, pt)); pt++) for (pt = PAWN; !(attackers & pieces_of_color_and_type(c, pt)); pt++)
@ -1622,6 +1638,12 @@ int Position::see(Square from, Square to) const {
lastCapturingPieceValue = seeValues[pt]; lastCapturingPieceValue = seeValues[pt];
c = opposite_color(c); c = opposite_color(c);
// Remove pinned pieces from attackers
if ( pinned[c] != EmptyBoardBB
&& !bit_is_set(pinners[c], to)
&& !(pinners[c] & attackers))
attackers &= ~pinned[c];
// Stop after a king capture // Stop after a king capture
if (pt == KING && (attackers & pieces_of_color(c))) if (pt == KING && (attackers & pieces_of_color(c)))
{ {