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Subject: Re: Pawn hashing without Zobrist keys

Author: Omid David Tabibi

Date: 14:06:15 09/12/03

Go up one level in this thread


On September 12, 2003 at 17:02:04, Gerd Isenberg wrote:

>On September 12, 2003 at 16:53:44, Omid David Tabibi wrote:
>
>>On September 12, 2003 at 16:49:09, Gerd Isenberg wrote:
>>
>>>On September 12, 2003 at 16:39:33, Omid David Tabibi wrote:
>>>
>>>>On September 12, 2003 at 16:34:48, Gian-Carlo Pascutto wrote:
>>>>
>>>>>On September 12, 2003 at 16:33:13, Omid David Tabibi wrote:
>>>>>
>>>>>>I still don't understand why modulus is needed, and
>>>>>>I have a pawn hash of 2^17 entries. What is so special
>>>>>>about pawn hash that you need the modulus operation
>>>>>>there while you don't need it in the normal hash table?
>>>>>
>>>>>You don't?
>>>>>
>>>>>I assume you have 32 or 64 bit hash keys.
>>>>
>>>>64 bit.
>>>>
>>>>>
>>>>>How do you map 2^64 -> 2^17 ?
>>>>
>>>>Please, don't tell me that you guys are mapping the hash key as following:
>>>>
>>>>address = key % table_size;
>>>
>>>
>>>Yes, unless you use zobrist keys and power of two table sizes for pawn hashing,
>>>which is probably smarter. Do you have a smarter way of getting a hashkey from
>>>white/black pawn bitboards?
>>>
>>>>
>>>>???
>>>>
>>>>Do you know how many cycles does the modolus operation take???
>>>>
>>>
>>>Yes, that's the reason for my initial posting.
>>>
>>
>>assuming 'table_size' is a power of 2, the following two expressions are
>>equivalent:
>>
>>address = key % table_size;
>>address = key & (table_size - 1);
>>
>
>
>Yes, but if "key" is whitePawns-blackPawns, bitboards as 64-bit ints, you may
>loose some pawns. That's the advantage with zobrist keys. I guess a 32-bit key,
>incremental updated if pawn structure changed, is good enough to address the
>pawn hash table.
>

Well, well... In this case I must quote Vincent: don't waste time with bitboards
:)

But seriously, I have always used zobrist keys for pawn hash tables.
Incrementally updating the pawn key, the cost is negligible.



>Gerd
>
>
>>
>>
>>>Gerd
>>>
>>>>
>>>>>
>>>>>--
>>>>>GCP



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