Author: Robert Hyatt
Date: 18:48:22 01/02/03
Go up one level in this thread
On January 02, 2003 at 15:51:30, Russell Reagan wrote: >[D]r3k2r/8/8/8/8/8/8/1R2K2R b Kkq - 0 1 > >In the above position, I calculate perft 3: > >> perft 3 >26 0 >583 0 >14251 0.032 >total time: 0.032 > >Other engines say that perft 3 is 14252, so I assume that I am wrong here and I >went looking for the position where the error is. I wrote a function to tell me >preft (n-1) for each of the legal moves so I can tell which branch the error is >in, and I continue to narrow it down until I find the offending position. This >has worked well so far, but now I'm getting this: > >> dperft 3 >h8h1 87 87 >a8a1 356 443 >a8a2 544 987 >a8a3 631 1618 >a8a4 631 2249 >a8a5 631 2880 >a8a6 631 3511 >a8a7 631 4142 >a8b8 524 4666 >a8c8 559 5225 >a8d8 488 5713 >h8h2 428 6141 >h8h3 547 6688 >h8h4 575 7263 >h8h5 602 7865 >h8h6 628 8493 >h8h7 650 9143 >h8f8 490 9633 >h8g8 560 10193 >e8d7 720 10913 >e8e7 742 11655 >e8f7 692 12347 >e8d8 510 12857 >e8f8 490 13347 >e8g8 474 13821 >e8c8 431 14252 > >This function reports perft n-1 for each move, and keeps a running total on the >right. The last number in the right column is, to my suprise, 14252, which is >the correct perft 3. > >So when I calculate perft 3, it is incorrect, but when I ask it to give me a >breakdown of perft 2 for each move, the sum (which should be the same as what I >am getting for perft 3) is suddenly correct. > >Any idea what this could mean? I'm stumped... Here are a couple of things to check. I've seen them all, and done most of them myself. :) (1) for castling, the king can not pass over a square that is under attack by the opponent. IE for kingside castling for white, the squares g1 and f1 must not be attacked. h1 can be attacked with no effect on castling. (2) for castling queenside, watch b1. It _can_ be attacked with no problem as the king doesn't pass over it. Only d1 and c1 must be free of attack and this is an easy mistake to make. I did it in an early crafty, and would not castle if b1 was attacked. (3) make certain that once you castle, you do not castle again deeper in the tree. :) This is also an easy mistake to make. (4) make certain that if a rook moves, castling to _that_ side is prevented, although castling the other way is still ok. (5) make certain that if the king moves, no castling can happen at all below that point. I'd bet one of those is your problem...
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