Author: Steffan Westcott
Date: 04:27:17 09/14/02
Go up one level in this thread
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));
}
---- CUT HERE ----
FillDown is an example of a simple flood fill, which 'smears' set bits in just
one direction. Instead of using a normal fill loop, I've borrowed an idea
normally found in fast adder hardware. This sort of fill is great for things
like attack or move generator bitboards.
Cheers,
Steffan
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