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Another see() shortcut

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Because we only generate legal moves we can assume
a king cannot be recaptured, so we can safely return
immediately with the captured piece score. If the move
turns out to be illegal it will be pruned anyhow,
independently from SEE value. This gives a good speed up
especially now that we SEE-test all the evasions that
are always legal and very often are king moves.

Another optimization catches almost 15% of cases, unfortunatly
we have already calculated the very expensive attacks, so
benefits are not so big anyway.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2009-10-31 11:16:40 +01:00
parent 941d923bf8
commit dddaeff7d8
1 changed files with 20 additions and 6 deletions
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26
src/position.cpp
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@ -1337,6 +1337,7 @@ int Position::see(Square from, Square to, bool shortcut) const {
};
Bitboard attackers, stmAttackers, b;
int pieceDiff = 0;
assert(!shortcut || from != SQ_NONE);
assert(square_is_ok(from) || from == SQ_NONE);
@ -1351,20 +1352,24 @@ int Position::see(Square from, Square to, bool shortcut) const {
Piece capture = piece_on(to);
Bitboard occ = occupied_squares();
// King cannot be recaptured
if (type_of_piece(piece) == KING)
return seeValues[capture];
// If captured piece is defended by enemy pawns or knights then SEE is negative
// when captured piece value is not enough to compensate the lost of capturing one.
// when captured piece value does not compensate the lost of capturing one.
if (shortcut)
{
int diff = seeValues[piece] - seeValues[capture];
pieceDiff = seeValues[piece] - seeValues[capture];
if ( diff > seeValues[PAWN]
if ( pieceDiff > seeValues[PAWN]
&&(attacks_from<PAWN>(to, us) & pieces(PAWN, them)))
return -(diff - seeValues[PAWN] / 2);
return -(pieceDiff - seeValues[PAWN] / 2);
if ( diff > seeValues[KNIGHT]
if ( pieceDiff > seeValues[KNIGHT]
&& pieces(KNIGHT, them)
&&(pieces(KNIGHT, them) & attacks_from<KNIGHT>(to)))
return -(diff - seeValues[KNIGHT] / 2);
return -(pieceDiff - seeValues[KNIGHT] / 2);
}
// Handle en passant moves
@ -1437,6 +1442,15 @@ int Position::see(Square from, Square to, bool shortcut) const {
for (pt = PAWN; !(stmAttackers & pieces(pt)); pt++)
assert(pt < KING);
// If captured piece is defended by an enemy piece then SEE is negative
// if captured piece value does not compensate the lost of capturing one.
if (pieceDiff > seeValues[pt])
{
assert(shortcut);
return -(pieceDiff - seeValues[pt] / 2);
} else
pieceDiff = 0; // Only first cycle
// Remove the attacker we just found from the 'attackers' bitboard,
// and scan for new X-ray attacks behind the attacker.
b = stmAttackers & pieces(pt);