Author: Steffan Westcott
Date: 12:58:21 03/03/06
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Gerd,
I may have terribly misunderstood things, but there is direct algorithm for
mapping the 128 possible attack bitboards to 7 bit numbers. Additionally, the
algorithm does not need the location of the ray source square, nor any lookup
tables.
For a ray along a rank or diagonal, it is sufficient to collapse the files:
typedef unsigned __int64 BitBoard;
int collapsed_files_index(BitBoard b)
{
b |= b >> 32;
b |= b >> 16;
b |= b >> 8;
return b & 0xFF;
}
For a ray along a file, a little more trickery is needed:
int collapsed_ranks_index(BitBoard b)
{
b |= b >> 4; // No masking needed
b |= b >> 2; // " "
b |= b >> 1; // " "
return ((b & 0x0101010101010101) * 0x0102040810204080) >> 56;
}
Both of the above routines return 8 bit results. Depending on the left/right
orientation of the ray in relation to your chosen bitboard bit numbering scheme,
you may need to shift the result right by 1 bit.
In addition, if you know the co-ordinates of the source square (either at
runtime or compile time), simpler variants of the above routines are possible.
I don't use techniques like these as I avoid serialisation of moves or features
as much as possible. However, I offer you and Dann the above as it may be of
interest.
Cheers,
Steffan
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