Author: Sune Fischer
Date: 03:42:52 12/05/01
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On December 05, 2001 at 06:02:45, David Rasmussen wrote: >>Whether or not 32 bit is sufficient depends on which "resolution" you require. >>If you need to distinguish between millions of different positions, then 32 bit >>will create too many collisions, probably somewhere near the numbers you've been >>talking. But if you "only" have a few thousand different positions (which is >>typical if you hash pawns _only_) then on average you should have about 1% >>chance of a collision in an entire game. >> >>Try and count how many different positions you get, this is really the key >>factor. >> > >First of all, we're both talking about hashing pawns only. And that is what I >have been talking about all along. Now: > >I disagree. They key factor is how many actual collisions returning wrong scores >(and other wrong data from the pawn hash table) you have, regardless of how many >positions etc. But I showed you the math, the odds can be calculated and that is what I base my opinion on. I think either you have way more than a few thousand positions (why won't you count them?), or you have overlooked something else. >I know that there aren't that many pawn positions during the >cause of a game, but I really don't care. The important thing is to measure how >often the hashtable adds incorrectness that _would_ not have been there, had it >not been for the pawn hashtable, i.e. a defect or, if you will, a bug of the >pawn hashtable. This process is way more complicated, I suspect you get "collisions" due some other parts of the code. If you insist on the empirical data (which is great actaully), then perhaps try a simpler approach, like someone suggested use 64 bit and see how often the reduced 32 bit keys will collide. >And whether this rate is "high" or "low", I don't know. Until I >am sure it is not "too high", I will stick with a rate of zero. Your argument >that there is about 1% chance of collision per game is all well and good. It >just doesn't matter, since Crafty have more than 1 collision a second with appx. >150 kn/s from the initial position. I am asking several questions: > >Is this expected or a bug? >Is it too high? >If not, why? > >/David -S.
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