Author: Edward Screven
Date: 11:44:19 06/17/99
Go up one level in this thread
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
This page took 0 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.