Author: Christophe Theron
Date: 08:15:01 07/26/98
Go up one level in this thread
On July 26, 1998 at 03:54:36, Pat King wrote:
>
>On July 24, 1998 at 06:14:04, Robert Hyatt wrote:
>
>
>>
>>
>>There are two issues:
>>
>>(1) *all* hash table scores are technically wrong almost all the time. Because
>>*none* include path information in the key, yet most include path information
>>in the score. Examples include repetition and 50-move draws.
>
>I have a solution to this problem, which may only work for my implementation,
>but here goes... I implement undo by storing a stack of positions for the entire
>game. It wouldn't be hard for me to modify this to include the current variation
>being considered. Then, when the terminal position is reached, you...
> a) Examine the game history for draws by repetition or 50 moves. If found,
> return 0, else...
> b) Consult hash. If found, return result, else...
> c) Call static evaluator (or qsearch) store result in hash, and return result.
The whole discussion seemed cryptic to me, as I never experienced the problem.
But maybe it's because I do in Chess Tiger what you are suggesting: I check for
draw by repetition or 50 moves rules just after doing the move, and before
calling the search recursively. If I detect a draw, the position is given a nul
score, then I undo the move and go to the next one without storing anything in
the hash table.
I guess this is not a perfect protection anyway.
Note that a similar problem exists with draw detection: all modern programs
(except DB?) are using a threat detection scheme to forward prune useless lines
(it can be null move or just static threats detector, depending on your taste).
It works by pruning lines where a side has a score greater than beta and no
threats against him.
The problem is that it simply ignores the fact that doing a move here can draw
the position by 50 moves rules. So the score would not be greater than beta
anymore. It happened to me several times in won endgames.
Christophe
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