Re: [gnugo-devel] You stepped on a bug

[Gunnar Farneback]

Stephen123 wrote:
> ***assertion failure:
> board.c:909 - board[pos] == EMPTY near C3***
> 
> 
>    A B C D E F G H J K L M N
> 13 . . . X O O . . . . . O X 13
> 12 . . . X X O . . O O . O X 12
> 11 . . . . X O . . O X O O X 11
> 10 . . . X O O + . X X X X X 10
>  9 . . . . X O O X X . X . . 9
>  8 . . . . X O O O O X O X X 8
>  7 . . . X . X O X X + O O . 7
>  6 . . . X . X O O X O O . . 6
>  5 . . . . X O O . O O X . . 5     WHITE has captured 2 stones
>  4 . X O X O O X O O + O . . 4     BLACK has captured 1 stones
>  3 . O X X X X X X O O . . . 3
>  2 . X O X . . X X O . . . . 2
>  1 . . . . . . X O O . . . . 1
>    A B C D E F G H J K L M N
> 
> gnugo 3.2 (seed 1037253629): You stepped on a bug.
> Please save this game as an sgf file and mail it to address@hidden
> If you can, please also include the debug output above this message.
> 
> ----
> 
> Save didn't work.  There were 2 each of Apple, File and Edit menus. I tried 
> to save in both file menus.  The first didn't do anything.  The second save a 
> copy of the message above as a ".out" file.

Thanks for the report. We have received a couple of very similar bug
reports which, as far as I can tell, all come from the MacOS 9 port of
GNU Go 3.2. The particular assertion failure can only happen under two
circumstances:
1. When undoing a move, provided some horrible bug in the undo
   implementation.
2. The interface enters an invalid move in the engine.

Of these two number 2 is far more likely since we haven't seen the
slightest trace of 1 on any platform. My guess is that the Mac UI for
some reason told the engine that the human played at C3 (which is
occupied) and caused the crash. Whether this was a failure to filter
out an unreasonable move or some other bug I can't say. Do you
remember what you did (or tried to do) when it crashed?

> Attached is a game which it decided I won. I didn't excually win
> until it started filling in it's own territory and then in the final
> analysis is geve me credit for the territory it felt the need to
> fill in. I've seen this three times now (all 13x13 games, different
> skill levels) all MacOS 9.2.2.

I think this definitely confirms that the MacOS 9 port is flawed.
There is absolutely no way that the original 3.2 engine would play the
sequence of moves from 76 to 120. The C4 and B5 moves may be plausible
at a very low level, but moves like G13, H12, J12, and in particular
H13 are totally out of the question.

/Gunnar