Author: Robert Hyatt
Date: 20:37:39 04/14/03
Go up one level in this thread
On April 14, 2003 at 18:09:37, Gerd Isenberg wrote: >On April 14, 2003 at 17:40:03, Robert Hyatt wrote: > >>On April 14, 2003 at 15:48:48, Gerd Isenberg wrote: >> >>>Hi all, >>> >>>a minor pawn pattern idea, i like to share ... >>> >>>With (pawn) bitboards it's pretty easy to get all passers and even to get all >>>outside passers. The second was new to me, and i had the idea recently (thinking >>>bitboards ;-), but i guess the "trick" is already common or well known. >>> >>>Instead of pattern branching after a distance compare in an inner loop over >>>passers of one color, i use a separate bitscan and reset loop over this >>>dedicated set of outside passers now, eg. in pawn endings. >>> >>>For outside passers one need a precalculated array of 2*64*2 bitboards. >>>This array is indexed by color of passed pawns we are looking for, square of >>>opposite king and color of side to move. It contains set bits for all squares >>>with distance to promotion < king distance to promote squares, considering the >>>tempo. >>> >>>Next step may be to avoid such loops at all, to process some single properties >>>without further piece interactions. Most often one uses some piece-square >>>tables, indexed by the squares of the traversed bitboard. If only the >>>rank-position is significant, a simultanious (mmx) popcount of six consecutive >>>bytes may be interesting, where the bytewise population counts are multipied >>>afterwards with some weighting factors. >> >>I do what you are close to hitting on. I have a bitmask of 8 bits, indicating >>whether white has a passer on a file (one bitmap) or whether black has a passer >>on a file (the other bitmap). I can now use a simple table lookup >>is_outside[256][256] >>that works just fine. The first subscript is the white outside passers (files >>with passers) >>and the second is a bitmap of _all_ files that have black pawns (or the inverse >>for looking >>at black outside passers). >> >>A single probe says "white has an outside passer or not" and another probe says >>"black has >>an outside passer or not." The value returned is 0, 1 or 2, which indicate how >>many outside >>passers that side has. And when I say outside passers I mean on each wing. 0 = >>no outside >>passers, 1 = outside passer on one wing. 2 = two outside passers on opposite >>wings, which >>is almost impossible to defend against. >> > >Nice idea. > >Not sure whether my definition of _outside_ passer is correct. I mean pawns, the >enemy king can't catch anymore. Where the opposite king is outside the pawns >square. How do call them? That's not "outside". That is "unstoppable" and the idea is commonly called "the square of the king". It is a simple idea about whether the king can reach the promoting square quick enough to win the pawn. An "outside passer" is a passed pawn that is closest to one side of the board, where the other side of the board has pawns by both sides that may or may not be passed. The idea is that the king must stop the outside passer and after it does, it is too far away from the other side and the pawns there go lost...
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