Author: Gerd Isenberg
Date: 15:49:39 03/03/06
Go up one level in this thread
On March 03, 2006 at 18:29:46, Dann Corbit wrote:
>On March 03, 2006 at 17:14:54, Tony Werten wrote:
>
>>On March 03, 2006 at 15:58:21, Steffan Westcott wrote:
>>
>>>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.
>>
>>Hi Steffan,
>>
>>although the algoritm looks simpler (allthough I'm not sure I understand) , (BB
>>& mask)*magic_number will give a better performance.
>
>Why not just: switch (BB) {/*128 cases go here */}
>?
>What is the value of the transform into a shorter integer type?
>Has anyone measured that it makes the switch faster? For me it is only
>important if it makes it faster for 64 bit hardware anyway.
Branchless stuff and memory access versus a 7 folded cmp-jxx chain and a lot of
btb-entries on some good chance of a miss-prediction here and there.
Of course if you index a function pointer table a "save" missprediction" occurs
as well. So the folding is intented to avoid branches at all and to index
precaluclated attack bitboards the same way as in a rotated framework.
While your switch approach seems to cover much more items. All precalculated
raywise properties of a square one may think about, "indexed" somehow by all up
to seven other man on that ray - not only occupied states.
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