Author: Dieter Buerssner
Date: 16:35:02 05/02/02
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On May 02, 2002 at 14:52:22, Uri Blass wrote: >On May 02, 2002 at 11:48:59, Robert Hyatt wrote: > > ><snipped> >>It doesn't. Here is a better explanation... >> >> >> >>root----------YYY----------------------XXX--------------TIP >> >>root-------------------XXX-----------------YYY----------------XXX-------TIP >> >>You search the first path first. And when you finish searching position >>YYY, you store a score of +1.2... Which represents what happens between YYY >>and the TIP position. >> >>You search the second path next. When you reach position YYY you retrieve >>that +1.2 score and use it. It is _wrong_. Because position XXX will be >>reached before the tip position is reached, producing a correct score of >>draw. But you are claiming +1.2 which is wrong... > >The only way to use hash tables without these problems seems to be to store >a score for yyy in the hash tables only in cases that the last move before yyy >is a conversion and to look at hash tables in order to cut the tree only after >conversions(you can only use hash tables not after conversions but in this case >using hash tables should be done only for better order of moves and not to cut >the tree). You can also use hash tables in the other cases for move ordering. I have no numbers, but I think in the middle game move ordering is a significant part of the strength increase of hash tables. Also, hashing the path since the last irreversible move, would be enough, but would probably in most cases make hash tables almost useless. One could have two hash keys, one including the path since the last irreversible move, that can be safely used for cutoffs from the HT. The other, without this path information, that would only be used for move ordering. I would guess, that it is not worth the trouble. >I do not say that it is a good idea but if someone wants to avoid the errors and >still use hash tables then it seems to be the only way. These errors are "self healing" (no idea, if this is a correct English phrase). Sooner or later you will not have enough draft in the hash table to allow a cutoff, and must search it. A good reading about this is also Dennis Breuker's thesis, which contains a chapter about this. http://members.ams.chello.nl/dbreuker/thesis/index.html Regards, Dieter
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