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Subject: Re: regular hash key & pawn hash key together--good idea?

Author: Robert Hyatt

Date: 13:08:23 09/16/99

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On September 16, 1999 at 12:10:29, Bas Hamstra wrote:

>I don't agree. I store 32 bits, but use the Index as to store 48 or more bits
>*effectively*. I store a hashmove for extra checking. If the hashmove is not
>legal it prints a message. I never see such messages.
>
>Regards,
>Bas Hamstra.

I am not sure what you "don't agree" with.  Storing 32 bits is fine (main
hash) but it will _not_ work if that is all you compute for the hash signature.
If you store the left-most 32 bits, and use the rightmost log2(size_of_table)
bits for a probe address, you are effectively using far more than 32 bits,
which was the point.. you _must_ use more than 32 or you will get collisions
like nobody's business...



>
>On September 16, 1999 at 09:44:56, Robert Hyatt wrote:
>
>>On September 16, 1999 at 04:00:43, Tom Kerrigan wrote:
>>
>>>If you have a program with 64 bit hash keys, is it a good idea to devote n bits
>>>to pawns and the rest to pieces and side-to-move?
>>>
>>>I was thinking a good balance may be 24 bits for pawns, but I have no data to
>>>back this up. Does anybody else?
>>>
>>>-Tom
>>
>>If you are going to use them independently to probe tables (ie using the 24
>>bits to probe the pawn hash table) then this won't work well.  24 bits is only
>>16 million possibilities...  that will get more than a few collisions.  And
>>the same for the upper 40 bits and pieces.  I know that 32 bits is not enough
>>for normal hashing.  Stanback/I ran a bunch of tests years ago in a discussion
>>on r.g.c.c and we got collisions by the bucketload with 32 bit keys.  64 was
>>totally safe back then, although speeds are way up now compared to then.  I
>>am using a 32 bit pawn hash signature, but a full 64 bit total hash signature.



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