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#include "chess.h"
#include "data.h"
/* last modified 08/11/10 */
/*
*******************************************************************************
* *
* Swap() 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 Swap(TREE * RESTRICT tree, int move, int wtm) {
BITBOARD attacks, temp = 0, toccupied = OccupiedSquares;
BITBOARD bsliders =
Bishops(white) | Bishops(black) | Queens(white) | Queens(black);
BITBOARD rsliders =
Rooks(white) | Rooks(black) | Queens(white) | Queens(black);
int attacked_piece, piece, color, nc = 1, swap_list[32];
int source = From(move);
int 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 = pc_values[Captured(move)];
/*
************************************************************
* *
* The first piece to capture on <target> is the piece *
* standing on <source>. *
* *
************************************************************
*/
color = Flip(wtm);
swap_list[0] = attacked_piece;
piece = Piece(move);
attacked_piece = pc_values[piece];
Clear(source, toccupied);
if (piece != knight && piece != king) {
if (piece & 1)
attacks |= AttacksBishop(target, toccupied) & bsliders;
if (piece == pawn || piece & 4)
attacks |= AttacksRook(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 swap 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(color, piece) & attacks))
break;
if (piece > king)
break;
toccupied ^= (temp & -temp);
if (piece != knight && piece != king) {
if (piece & 1)
attacks |= AttacksBishop(target, toccupied) & bsliders;
if (piece & 4)
attacks |= AttacksRook(target, toccupied) & rsliders;
}
swap_list[nc] = -swap_list[nc - 1] + attacked_piece;
attacked_piece = pc_values[piece];
if (swap_list[nc++] - attacked_piece > 0)
break;
color = Flip(color);
}
/*
************************************************************
* *
* Starting at the end of the sequence of values, use a *
* "minimax" like procedure to decide where the captures *
* will stop. *
* *
************************************************************
*/
while (--nc)
swap_list[nc - 1] = -Max(-swap_list[nc - 1], swap_list[nc]);
return (swap_list[0]);
}
/* last modified 08/11/10 */
/*
*******************************************************************************
* *
* SwapO() is used to analyze a move already made to see if it appears to be *
* safe. It is similar to Swap() 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 SwapO(TREE * RESTRICT tree, int move, int wtm) {
BITBOARD attacks, temp = 0, toccupied = OccupiedSquares;
BITBOARD bsliders =
Bishops(white) | Bishops(black) | Queens(white) | Queens(black);
BITBOARD rsliders =
Rooks(white) | Rooks(black) | Queens(white) | Queens(black);
int attacked_piece, piece, color, nc = 1, swap_list[32];
int 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 = pc_values[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. *
* *
************************************************************
*/
color = Flip(wtm);
swap_list[0] = attacked_piece;
for (piece = pawn; piece <= king; piece++)
if ((temp = Pieces(color, piece) & attacks))
break;
if (piece > king)
return (0);
toccupied ^= (temp & -temp);
if (piece != knight && piece != king) {
if (piece & 1)
attacks |= AttacksBishop(target, toccupied) & bsliders;
if (piece & 4)
attacks |= AttacksRook(target, toccupied) & rsliders;
}
attacked_piece = pc_values[piece];
color = Flip(color);
/*
************************************************************
* *
* 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 swap 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(color, piece) & attacks))
break;
if (piece > king)
break;
toccupied ^= (temp & -temp);
if (piece != knight && piece != king) {
if (piece & 1)
attacks |= AttacksBishop(target, toccupied) & bsliders;
if (piece & 4)
attacks |= AttacksRook(target, toccupied) & rsliders;
}
swap_list[nc] = -swap_list[nc - 1] + attacked_piece;
attacked_piece = pc_values[piece];
if (swap_list[nc++] - attacked_piece > 0)
break;
color = Flip(color);
}
/*
************************************************************
* *
* Starting at the end of the sequence of values, use a *
* "minimax" like procedure to decide where the captures *
* will stop. *
* *
************************************************************
*/
while (--nc)
swap_list[nc - 1] = -Max(-swap_list[nc - 1], swap_list[nc]);
return (swap_list[0]);
}
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