Author: Alessandro Damiani
Date: 09:41:02 09/14/02
Go up one level in this thread
On September 14, 2002 at 07:27:17, Steffan Westcott wrote:
>On September 14, 2002 at 04:33:40, Sune Fischer wrote:
>
>>On September 14, 2002 at 04:10:46, Sune Fischer wrote:
>>
>>>I just thought of another application on the passed pawns.
>>>This way one doesn't need a 8x64 byte table or calls to firstone().
>>>I can't say if it's faster than the lookup though, I'll probably
>>>use it anyway.
>>>
>>>//===================================================================
>>>// find the passed and semi-passed pawns:
>>>
>>>BITBOARD WhitePassedPawns(BITBOARD w_pawns,BITBOARD b_pawns) {
>>> BITBOARD passed=0,tempA,tempB;
>>> int i=0;
>>>
>>> tempA=(w_pawns<<8)&~b_pawns;
>>> while (tempA) {
>>> tempB=tempA&rank8;
>>> i+=8;
>>> if (tempB) // a pawn has reached the 8th rank
>>> passed|=tempB>>i; // shift it back to where it originally came from
>>> tempA=(tempA<<8)&~b_pawns;
>>> }
>>> return passed;
>>>}
>>
>>Actually this may be faster, on 64 bit chips anyway:
>>
>> while (tempA) {
>> passed|=(tempA&rank8)>>(i+=8);
>> tempA=(tempA<<8)&~b_pawns;
>> }
>
>
>Sune,
>
>Somewhat simpler and faster still (!) is this approach :
>
>
>---- CUT HERE ----
>
>BitBoard FillDown(BitBoard b)
>{
> // Uses Kogge-Stone parallel prefix algorithm
> b |= b >> 8;
> b |= b >> 16;
> return b |= b >> 32;
>}
>
>
>BitBoard WhiteSemiPassedPawns(BitBoard w_pawns, BitBoard b_pawns)
>{
> return w_pawns & ~FillDown(b_pawns);
>}
>
>
>BitBoard WhitePassedPawns(BitBoard w_pawns, BitBoard b_pawns)
>{
> return w_pawns & ~FillDown( b_pawns
> | ShiftDownLeft (b_pawns)
> | ShiftDownRight(b_pawns));
>}
I like your approach very much. Until now I used table lookup to determine the
flooded squares for each file (as a function of the bit pattern of each file).
There is just a small improvement to your method "WhitePassedPawns()": factor
out the "shift down" to avoid shifting down twice.
BitBoard WhitePassedPawns(BitBoard w_pawns, BitBoard b_pawns)
{
return w_pawns & ~FillDown( b_pawns
| ShiftDown( ShiftLeft (b_pawns)
| ShiftRight(b_pawns));
}
Alessandro
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