In the last KGS tournament, a triple ko arose and Orego tried to play
an illegal move (superko violation). We looked into it this morning
and (we think) fixed the problem.
It had to do with the way we stored hash codes, used both for superko
detection and the transposition table. Our hash codes are longs,
incorporating:
- The stones on the board (Zobrist style)
- The current color to play (all bits are flipped if it's white to play)
- The current simple ko point (also Zobrist style)
The last item was the catch. In a transposition table, you want to
include the simple ko point, because you don't want to conflate two
states, in only one of which some point (possibly a key point) is
illegal. In the superko table, on the other hand, you do NOT want to
include this information, because a repetition of a previous position
(with the same color to play) is a repetition, regardless of the
simple ko point.
It's fixed now, and will be so in the next release. Obviously, these
situations are exceedingly rare, which is why it took us so long to
discover the error in our thinking.
Peter Drake
http://www.lclark.edu/~drake/
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