Author: Will Singleton
Date: 22:54:29 05/01/02
Go up one level in this thread
On May 02, 2002 at 00:02:53, Robert Hyatt wrote: >On May 01, 2002 at 21:53:01, Will Singleton wrote: > >>On May 01, 2002 at 11:41:54, Arwin Smit wrote: >> >>>Hi, >>> >>>I programmed a hash table for my chess engine. It seems to be faster >>>and stronger now, but as far as I know the disadvantage of using a >>>hashtable is that the engine will not always find the best solution >>>anymore. And this is caused by not being able to see a 3-fold repetition >>>draw anymore in some cases. A position may be in the hash table and >>>returning a non-draw score, but it was reached in a different part of the >>>search tree by repetition of positions. >>> >>>This worries me a bit since this is the first time I made a change to my >>>engine causing it to be "non-perfect". >>>Is the advantage bigger than the disadvantage? >>>What is the best way to test which version is better anyway? Just let it >>>play a lot of games against eachother? >>> >>>Arwin >> >>It's possible you can fix this by 1) not storing draw scores in the hash table, >>and 2) testing for repetition prior to probing the hash (if you don't use the >>main hash for your rep detection). > > > >There is really no way to close all the holes. It is possible that the >path from the current position to the tip, which is found by a hash hit, >repeats a position between the current position and the root. But it is >impossible to know this without storing the complete path in a hash entry. > >Not storing draw scores is (IMHO) a bad idea. A draw score is as legitimate >as any other score. Not storing them to avoid one problem simply creates >another on the other side... And it still doesn't solve the first problem >I gave, which means errors are going to occur, period... > >I simply ignore them... > > Hmmm... I don't know about that. You say that a position occurring between the current position to the tip may contain a repetition of a position that occurred between the current position and the root, and that the score of the current position wouldn't reflect the draw without storing draws in the hash. My point was that, if you don't hash draw scores, the current position would not appear in the hash, and the draw would have to be found by the normal search. Which seems to work fine in my prog. Let's take another example. Suppose you find a draw in a search, at ply 7 (draft 0), which gets stored as exact in the hash. Now you and the opponent move, and in the next search, you find that draw score at ply 5 (draft 0). But it turns out that the original draw score was from a two-fold repetition in the search, and that two-fold rep doesn't exist in the current search. Hence, the draw score in the hash is incorrect. That was part of my thinking in not storing the draw scores. In any case, the search will find any draws without the need for storing them. Will
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