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see.c
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#include "chess.h"
#include "data.h"
/* last modified 05/16/14 */
/*
*******************************************************************************
* *
* SEE() is used to analyze capture moves to see whether or not they appear *
* to be profitable. The basic algorithm is extremely fast since it uses *
* the bitmaps to determine which squares are attacking the [target] square. *
* *
* The algorithm is quite simple. Using the attack bitmaps, we enumerate *
* all the pieces that are attacking [target] for either side. Then we *
* simply use the lowest piece (value) for the correct side to capture on *
* [target]. we continually "flip" sides taking the lowest piece each time. *
* *
* As a piece is used, if it is a sliding piece (pawn, bishop, rook or *
* queen) we remove the piece, then generate moves of bishop/queen or *
* rook/queen and then add those in to the attackers, removing any attacks *
* that have already been used. *
* *
*******************************************************************************
*/
int SEE(TREE * RESTRICT tree, int wtm, int move) {
uint64_t attacks, temp = 0, toccupied = OccupiedSquares;
uint64_t bsliders =
Bishops(white) | Bishops(black) | Queens(white) | Queens(black);
uint64_t rsliders =
Rooks(white) | Rooks(black) | Queens(white) | Queens(black);
int attacked_piece, piece, nc = 1, see_list[32];
int source = From(move), target = To(move);
/*
************************************************************
* *
* Determine which squares attack <target> for each side. *
* initialize by placing the piece on <target> first in *
* the list as it is being captured to start things off. *
* *
************************************************************
*/
attacks = AttacksTo(tree, target);
attacked_piece = pcval[Captured(move)];
/*
************************************************************
* *
* The first piece to capture on <target> is the piece *
* standing on <source>. *
* *
************************************************************
*/
wtm = Flip(wtm);
see_list[0] = attacked_piece;
piece = Piece(move);
attacked_piece = pcval[piece];
Clear(source, toccupied);
if (piece & 1)
attacks |= BishopAttacks(target, toccupied) & bsliders;
if (piece != king && (piece == pawn || piece & 4))
attacks |= RookAttacks(target, toccupied) & rsliders;
/*
************************************************************
* *
* Now pick out the least valuable piece for the correct *
* side that is bearing on <target>. As we find one, we *
* update the attacks (if this is a sliding piece) to get *
* the attacks for any sliding piece that is lined up *
* behind the attacker we are removing. *
* *
* Once we know there is a piece attacking the last *
* capturing piece, add it to the see list and repeat *
* until one side has no more captures. *
* *
************************************************************
*/
for (attacks &= toccupied; attacks; attacks &= toccupied) {
for (piece = pawn; piece <= king; piece++)
if ((temp = Pieces(wtm, piece) & attacks))
break;
if (piece > king)
break;
toccupied ^= (temp & -temp);
if (piece & 1)
attacks |= BishopAttacks(target, toccupied) & bsliders;
if (piece != king && piece & 4)
attacks |= RookAttacks(target, toccupied) & rsliders;
see_list[nc] = -see_list[nc - 1] + attacked_piece;
attacked_piece = pcval[piece];
if (see_list[nc++] - attacked_piece > 0)
break;
wtm = Flip(wtm);
}
/*
************************************************************
* *
* Starting at the end of the sequence of values, use a *
* "minimax" like procedure to decide where the captures *
* will stop. *
* *
************************************************************
*/
while (--nc)
see_list[nc - 1] = -Max(-see_list[nc - 1], see_list[nc]);
return see_list[0];
}
/* last modified 05/16/14 */
/*
*******************************************************************************
* *
* SEEO() is used to analyze a move already made to see if it appears to be *
* safe. It is similar to SEE() except that the move has already been made *
* and we are checking to see whether the opponent can gain material by *
* capturing the piece just moved. *
* *
*******************************************************************************
*/
int SEEO(TREE * RESTRICT tree, int wtm, int move) {
uint64_t attacks, temp = 0, toccupied = OccupiedSquares;
uint64_t bsliders =
Bishops(white) | Bishops(black) | Queens(white) | Queens(black);
uint64_t rsliders =
Rooks(white) | Rooks(black) | Queens(white) | Queens(black);
int attacked_piece, piece, nc = 1, see_list[32], target = To(move);
/*
************************************************************
* *
* Determine which squares attack <target> for each side. *
* initialize by placing the piece on <target> first in *
* the list as it is being captured to start things off. *
* *
************************************************************
*/
attacks = AttacksTo(tree, target);
attacked_piece = pcval[Piece(move)];
/*
************************************************************
* *
* The first piece to capture on <target> is the piece *
* standing on that square. We have to find out the least *
* valuable attacker for that square first. *
* *
************************************************************
*/
wtm = Flip(wtm);
see_list[0] = attacked_piece;
for (piece = pawn; piece <= king; piece++)
if ((temp = Pieces(wtm, piece) & attacks))
break;
if (piece > king)
return 0;
toccupied ^= (temp & -temp);
if (piece & 1)
attacks |= BishopAttacks(target, toccupied) & bsliders;
if (piece != king && piece & 4)
attacks |= RookAttacks(target, toccupied) & rsliders;
attacked_piece = pcval[piece];
wtm = Flip(wtm);
/*
************************************************************
* *
* Now pick out the least valuable piece for the correct *
* side that is bearing on <target>. As we find one, we *
* update the attacks (if this is a sliding piece) to get *
* the attacks for any sliding piece that is lined up *
* behind the attacker we are removing. *
* *
* Once we know there is a piece attacking the last *
* capturing piece, add it to the see list and repeat *
* until one side has no more captures. *
* *
************************************************************
*/
for (attacks &= toccupied; attacks; attacks &= toccupied) {
for (piece = pawn; piece <= king; piece++)
if ((temp = Pieces(wtm, piece) & attacks))
break;
if (piece > king)
break;
toccupied ^= (temp & -temp);
if (piece & 1)
attacks |= BishopAttacks(target, toccupied) & bsliders;
if (piece != king && piece & 4)
attacks |= RookAttacks(target, toccupied) & rsliders;
see_list[nc] = -see_list[nc - 1] + attacked_piece;
attacked_piece = pcval[piece];
if (see_list[nc++] - attacked_piece > 0)
break;
wtm = Flip(wtm);
}
/*
************************************************************
* *
* Starting at the end of the sequence of values, use a *
* "minimax" like procedure to decide where the captures *
* will stop. *
* *
************************************************************
*/
while (--nc)
see_list[nc - 1] = -Max(-see_list[nc - 1], see_list[nc]);
return see_list[0];
}