Author: Sune Fischer
Date: 09:52:48 12/07/02
Go up one level in this thread
On December 07, 2002 at 11:42:54, David Rasmussen wrote: >On December 07, 2002 at 11:25:44, Robert Hyatt wrote: > >>> >>>I hadn't either until now. But I am pretty sure that Bob sais something about >>>this happening once in 100 games or so. >> >>That was an estimate. Think about the math. You are collapsing some 2^168 >>positions into 2^64 hash space. That is a _huge_ aliasing problem. So the >>probability of getting duplicates is not just a probability, it is an absolute >>certainty... > >I know that. I am not saying that it will not happen. I know it will. But I am >asking about the frequency. When doing math on these beasts people tend to just >assume that everything is random. It isn't. The subspace visited by a 12-ply >search with a 64 MB hashtable is not the same as the whole game tree. As I said: >we don't care if there are signature collisions between two positions that are >"far" from eachother. The probablility for it happening within the history of a >seach with a hash table, is far less (apparently). And it is not simple to >estimate how much. So I was asking about people's experiences of this frequency, >because I didn't believe such naive reasoning anyway. If the theory isn't >corresponding with experience, fix the theory. You could try making keys with less bits. I did that experiment once, IIRC I started to get collisions at around 45-48 bits. So 32 bit is useless but 50 bits is okay. I would think that you about double the collision rate everytime you lose a bit, so if you don't get any collisions during a full game with 50 bits then you have something like 2^14 games before you can expect a collision using full 64 bit keys. In other words it doesn't happen, if it does maybe you need a better PRNG. -S. P.S. Thanks for the game, I see I need to work on those rook-on-7th and rooks-in-openfiles evaluations :) All I can is: avoid cxd5! LOL [D]2r3k1/qprbbppp/p7/P2np3/2Pp4/1P1P4/2NBBPPP/RQ3RK1 w - - 0 19 >/David
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