Author: James Robertson
Date: 14:20:38 06/17/99
Go up one level in this thread
On June 17, 1999 at 14:44:19, Edward Screven wrote:
Thanks! I will save this code and go through it carefully.
James
>On June 17, 1999 at 00:10:11, James Robertson wrote:
>
>>I am trying to find the most efficient way to detect backward pawns, either with
>>bitboards or the more conventional piece array. Any help is greatly
>>appreciated!!
>>
>>James
>
>if bitboards are useful anywhere, it has to be calculating
>pawn structure information. here's how i calculate backward
>pawns. my definition is a little different than yours, but
>it could easily be made the same.
>
>here's how i calculate the set of squares attacked by white
>pawns. "Squares" is a 64-bit unsigned integer; "wpawns"
>are the squares occupied by white pawns.
>
> inline static Squares WhiteAttacks(Squares wpawns)
> {
> return (((wpawns & ~FILE_A) << 9) |
> ((wpawns & ~FILE_H) << 7));
> }
>
>
>WhiteWeak() returns white's weak pawns. "apawns" are the
>squares occupied by white or black pawns. "wattacks" are
>squares attacked by white pawns before any pawn moves. i
>consider a pawn weak if it isn't defended by a pawn, or
>can't be defended after one pawn move.
>
> inline static Squares WhiteWeak(Squares wpawns,
> Squares apawns,
> Squares wattacks)
> {
> /*
> ** a pawn is weak if
> **
> ** 1) it is not currently defended
> ** 2) it cannot be pushed onto a defended square
> ** 3) it cannot be double-pushed onto a defended square
> */
>
> Squares weak = wpawns;
>
> weak &= ~wattacks;
> weak &= ~((wattacks & ~apawns) >> 8);
> weak &= ~(((wattacks & ~apawns & ~(apawns << 8)) >> 16)
> & RANK_2);
>
> /*
> ** and if
> **
> ** 4) a pawn cannot be pushed to defend it
> ** 5) a pawn cannot be double-pushed to defend it.
> */
>
> Squares wstep1 = (wpawns << 8) & ~apawns;
> Squares wstep2 = (wstep1 << 8) & ~apawns & RANK_4;
>
> weak &= ~WhiteAttacks(wstep1 | wstep2);
>
> return weak;
> }
>
>
>finally we get to backward pawns. i consider a pawn backward
>if it is weak and the square in front of it is attacked by
>an enemy pawn, and the square is occupied by another pawn.
>
> inline static Squares WhiteBackward(Squares wweak,
> Squares apawns,
> Squares battacks)
> {
> return wweak & ((~apawns & battacks) >> 8);
> }
>
>
>i use similar kinds of code to calculate isolated and passed pawns.
>
> - edward
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