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Update Algorithm explanation.txt
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Coding with Tom
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@@ -202,11 +202,12 @@ will be stored.
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}
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For pins it's a bit more complicated. We first need INBETWEEN_BITBOARDS.
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This is a multi dimensional array that simple has all of this bits inbetween every square combination.
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This is a multi dimensional array that simply has all of this bits inbetween every square combination.
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It also includes the last square.
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Example:
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INBETWEEN_BITBOARD[E1][E8] =
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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@@ -215,15 +216,17 @@ Example:
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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INBETWEEN_BITBOARD[E1][E8] =
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INBETWEEN_BITBOARD[B7][G2] =
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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0 0 1 0 0 0 0 0
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0 0 0 1 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 1 0 0
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0 0 0 0 0 0 1 0
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0 0 0 0 0 0 0 0
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We use these for pins and check for rooks, queens and bishops.
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int pinArray[8][2] =
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{
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@@ -247,31 +250,223 @@ We create a pin array. The first index is the square being pinned. The second in
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while (temp_bitboard != 0) //if there is a black bishop there
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{
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const int piece_square = (DEBRUIJN64[MAGIC * (temp_bitboard ^ (temp_bitboard - 1)) >> 58]); //find the square
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temp_pin_bitboard = INBETWEEN_BITBOARDS[whiteKingPosition][piece_square] & WHITE_OCCUPANCIES_LOCAL;
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temp_pin_bitboard = INBETWEEN_BITBOARDS[whiteKingPosition][piece_square] & WHITE_OCCUPANCIES_LOCAL;
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//for the squares inbetween, is there a white piece there
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if (temp_pin_bitboard == 0)
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if (temp_pin_bitboard == 0) //if there is no white piece, the bishop is attacking the king
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{
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if (check_bitboard == 0)
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{
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check_bitboard = INBETWEEN_BITBOARDS[whiteKingPosition][piece_square];
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check_bitboard = INBETWEEN_BITBOARDS[whiteKingPosition][piece_square]; //add the check
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}
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whiteKingCheckCount++;
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}
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else
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else //if there is a white piece inbetween, there is a potential pin
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{
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const int pinned_square = (DEBRUIJN64[MAGIC * (temp_pin_bitboard ^ (temp_pin_bitboard - 1)) >> 58]);
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temp_pin_bitboard &= temp_pin_bitboard - 1;
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const int pinned_square = (DEBRUIJN64[MAGIC * (temp_pin_bitboard ^ (temp_pin_bitboard - 1)) >> 58]); //get the square
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temp_pin_bitboard &= temp_pin_bitboard - 1; //remove one bit from bitboard
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if (temp_pin_bitboard == 0)
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if (temp_pin_bitboard == 0) //if the bitboard is not empty then that piece was pinned.
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{
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pinArray[pinNumber][PINNED_SQUARE_INDEX] = pinned_square;
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pinArray[pinNumber][PINNING_PIECE_INDEX] = piece_square;
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pinNumber++;
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pinArray[pinNumber][PINNED_SQUARE_INDEX] = pinned_square; //add the pinned square
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pinArray[pinNumber][PINNING_PIECE_INDEX] = piece_square; //add the pinning piece
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pinNumber++; //increase the pin number
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}
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}
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temp_bitboard &= temp_bitboard - 1; //remove the bit to stop infinitive loop
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}
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We do the same for queen and rook.
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We then check if a piece is pinned like so:
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temp_bitboard = bitboard_array_global[WN];
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while (temp_bitboard != 0)
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{
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starting_square = (DEBRUIJN64[MAGIC * (temp_bitboard ^ (temp_bitboard - 1)) >> 58]);
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temp_bitboard &= temp_bitboard - 1; //removes the knight from that square to not infinitely loop
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temp_pin_bitboard = MAX_ULONG; //set it max, so allows all moves first
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if (pinNumber != 0) //if there is a pin somewhere
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{
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for (int i = 0; i < pinNumber; i++) //loop through them
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{
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if (pinArray[i][PINNED_SQUARE_INDEX] == starting_square) //if this piece is pinned
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{
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temp_pin_bitboard = INBETWEEN_BITBOARDS[whiteKingPosition][pinArray[i][PINNING_PIECE_INDEX]];
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//set the temp_pin_bitboard to the bits in between the king and pinning piece
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//This means the knight can only move
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}
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}
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}
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temp_attack = ((KNIGHT_ATTACKS[starting_square] & BLACK_OCCUPANCIES_LOCAL) & check_bitboard) & temp_pin_bitboard;
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//Then you AND the attacks with the check and pins
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//add moves if temp_attack isn't zero etc...
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while (temp_attack != 0) ...
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Let's say in this position:
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__ __ __ __ BK __ __ __
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__ __ __ __ BR __ __ __
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__ __ __ __ __ __ __ __
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__ __ __ __ __ __ __ __
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__ __ __ __ __ __ __ __
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__ __ __ __ __ __ __ __
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__ __ __ __ WR __ __ __
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__ __ __ __ WK __ __ __
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Here both rooks are pinning each other.
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The white rook bitboard looks like this:
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tempBitboard = bitboard_array[WR];
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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The rook moves will look like this:
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rook_attacks = GetRookAttackFast(rook_square, COMBINED_OCCUPANCIES);
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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1 1 1 1 0 1 1 1
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0 0 0 0 1 0 0 0
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We AND this with empty squares to get non capture moves:
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EMPTY_OCCUPANCIES:
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1 1 1 1 0 1 1 1
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1 1 1 1 0 1 1 1
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1 1 1 1 1 1 1 1
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1 1 1 1 1 1 1 1
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1 1 1 1 1 1 1 1
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1 1 1 1 1 1 1 1
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1 1 1 1 0 1 1 1
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1 1 1 1 0 1 1 1
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unsigned long long non_capture_moves = EMPTY_OCCUPANCIES & rook_attacks;
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non_capture_moves:
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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1 1 1 1 0 1 1 1
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0 0 0 0 0 0 0 0
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We get the capture moves by ANDing the attacks with black occupancies:
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BLACK_OCCUPANCIES:
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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unsigned long long capture_moves = BLACK_OCCUPANCIES & rook_attacks;
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capture_moves:
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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The pin bitboard will look like this:
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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We then AND that with captures and non captures. So if there is a pin, only these squares are valid.
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Similar with check:
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__ __ __ __ BK __ __ __
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__ __ __ __ BR __ __ __
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__ __ __ __ __ __ __ __
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__ __ __ __ __ __ __ __
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__ __ __ __ __ __ __ __
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__ __ __ __ __ __ __ __
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WR __ __ __ __ __ __ __
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__ __ __ __ WK __ __ __
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The check bitboard here:
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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The white rook attacks:
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1 0 0 0 0 0 0 0
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1 0 0 0 0 0 0 0
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1 0 0 0 0 0 0 0
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1 0 0 0 0 0 0 0
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1 0 0 0 0 0 0 0
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1 0 0 0 0 0 0 0
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0 1 1 1 1 1 1 1
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1 0 0 0 0 0 0 0
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AND then together:
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unsigned long long valid_rook_moves = checkBitboard & rook_attacks;
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 0 0 0 0
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0 0 0 0 1 0 0 0
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0 0 0 0 0 0 0 0
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Only these moves are valid.
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The lazy and slow way to do pins and check:
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for (int i = 0; i < move_count; ++i) {
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MakeMove(move_list[i]);
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if (isWhiteToMove)
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{
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if (is_attacked_by_black(white_king_position, COMBINED_OCCUPANCIES) == true) {
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UnmakeMove(move_list[i]);
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continue;
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}
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} else {
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if (is_attacked_by_white(black_king_position, COMBINED_OCCUPANCIES) == true) {
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UnmakeMove(move_list[i]);
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continue;
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}
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}
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}
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__Finding Pieces__
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I didn't write BitscanForward. It uses the DEBRUJN method to get "the least significant bit".
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