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Chess - Programming - BitBoards

A BitBoard is comprised of a 64-bit word, which is used to store the positions of different Chess Pieces.

Every square on a Chess Board is represented by one bit in the 64-bit word.

Since there are 12 different pieces in chess, 12 BitBoards are needed to store a chess position:

whiteKing - bitboard with the position bit of the white king set to 1 (and other bits set to 0).
whiteQueens - bitboard with the position bits of all white queens set to 1 (and other bits set to 0), etc
whiteRooks
whiteBishops
whiteKnights
whitePawns
blackKing
blackQueens
blackRooks
blackBishops
blackKnights
blackPawns

The bit-numbering convention for the bitboards:

bit #0 will be the rightmost bit in the 64-bit word (least significant bit, or LSB), and
bit #63 will be the leftmost bit in the 64-bit word (most significant bit, or MSB).
Furthermore, square a1 = bit #0, h1 = bit #7, a8 = bit #56 and h8 = bit #63.

 RANKS:
   8 |       56   57   58   59   60   61   62   63  (MSB,
   7 |       48   49   50   51   52   53   54   55  left)  
   6 |       40   41   42   43   44   45   46   47  
   5 |       32   33   34   35   36   37   38   39 
   4 |       24   25   26   27   28   29   30   31  
   3 |       16   17   18   19   20   21   22   23  
   2 |        8    9   10   11   12   13   14   15
   1 | (LSB,  0    1    2    3    4    5    6    7
       right)
           -------------------------------------------
 FILES:      a     b    c    d    e    f    g    h

NOTE: If FILE and RANK are numbered from 1..8, then SQUARE (0..63) can be calculated as follows: SQUARE = 8 * RANK + FILE - 9;

There are some fast bit-wise operations that we can perform on bitboards.

For instance, if we want to have a bitboard of all white pieces, then we can get this by using the bit-wise OR operator:

whitePieces = whiteKing | whiteQueens | whiteRooks | whiteBishops | whiteKnights | whitePawns;

Or all occupied squares:

occupiedSquares = whitePieces | blackPieces;

There are six bitwise operators in C++, that are used in combination with bitboards:

&	The AND operator, compares two values, and returns a value that has its bits set if, and only if, the two values being compared both have their bits set.
\|The OR operator, compares two values, and returns a value that has its bits set if one or the other value, or both, have their bits set.
The XOR operator, compares two values, and returns a value that has its bits set if one or the other value has its corresponding bits set, but not both.
~	The Ones Complement, or Inversion, operator acts only on one value and inverts it, turning all ones into zeros, and all zeros into ones.
»	The Right Shift operator, shifts the bits from the high bit (MSB) to the low bit (LSB), the number of bit positions specified.
«	The Left Shift operator, shifts the bits from the low bit (LSB) to the high bit (MSB), the number of bit positions specified.