🚀 go-pugleaf

From article <1989Nov27.204521.7311@Neon.Stanford.EDU>, by dalgic@bodega.stanford.edu (Ismail Dalgic):
> I have the GnuChess version 1.53, and several times I noticed a very
> disturbing bug.  The program "forgets" about the piece in front of a
> pawn in its original rank when it advances the pawn by 2.  Here is a
> game list which will indicate what I mean.  Black is GnuChess.  I
> played it with 60 moves/5 min. mode.
> 
>  1. e2e4             e7e5
>  2. g1f3             d7d6
>  3. d2d4             g8f6
>  4. c1g5             e5d4
>  5. d1d4             b8c6
>  6. f1b5             c8d7
>  7. b5c6             d7c6
>  8. b1c3             f8e7
>  9. c3d5             c7c5 !!!
> 
> 
> Any ideas, fixes etc???
> 
> --Ismail Dalgic

Yep, the problem is in GenMoves() in move.c

The proper GenMoves() function is below

GenMoves(ply,sq,side,xside)
     short ply,sq,side,xside;
 
/*
  Generate moves for a piece. The moves are taken from the
  precalulated array posdata. If the board is free, next move
  is choosen from nextpos else from nextdir.
*/
 
{
  register short u,piece;
  register struct sqdata *p;
	 
  piece = board[sq];
  p = posdata[side][piece][sq];
  if (piece == pawn) {
    u = p[sq].nextdir; /* follow captures thread */
    while (u != sq) {
      if (color[u] == xside) LinkMove(ply,sq,u,xside);
      u = p[u].nextdir;
    }
    u = p[sq].nextpos; /* and follow no captures thread */
    while (u != sq) {
      if (color[u] == neutral && (u != sq+16 || color[u-8] == neutral)
          && (u != sq-16 || color[u+8] == neutral)) {
        LinkMove(ply,sq,u,xside);
      }
      u = p[u].nextpos;
    }
  }  
  else {
    u = p[sq].nextpos;
    while (u != sq) {
      if (color[u] == neutral) {
        LinkMove(ply,sq,u,xside);
        u = p[u].nextpos;
      }
      else {
        if (color[u] == xside) LinkMove(ply,sq,u,xside);
	u = p[u].nextdir;
      }
    }
  }    
} 

-------
Ryan Moats
AT&T Bell Laboratories
Holmdel NJ
rdmi@homxb.att.com