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Subject: Re: hamming distance...

Author: Robert Hyatt

Date: 08:08:02 04/07/04

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On April 07, 2004 at 09:14:40, martin fierz wrote:

>On April 07, 2004 at 08:56:26, James Swafford wrote:
>
>>On April 07, 2004 at 06:55:31, Andrew Williams wrote:
>>
>>>On April 07, 2004 at 06:49:59, Renze Steenhuisen wrote:
>>>
>>>>
>>>>Hi all,
>>>>
>>>>could someone give me some numbers that are common with hashkey collisions?
>>>>Because I guess my % is little too high...
>>>>
>>>>I'm getting like 0.03% [which is 1 every 3000, if I'm not mistaken]
>>>>
>>>>This is when using TT=32MB (haven't got the exact number of entries)
>>>>
>>>>If you think it is an error, any suggestions on where to start looking?
>>>>
>>>>Thanks!
>>>>
>>>>   Renze
>>>
>>>One in 3000 seems very high. How many bits are there in your hashkey?
>>>
>>>Andrew
>>
>>
>>Even though you said you're using Crafty's random num gen,
>>I would start by doing some hamming-distance checks.
>>
>>For reference, my program gets:
>>Checking minimum hamming distance between random keys: 14 bits
>>Checking average hamming distance between random keys: 31 bits
>>
>>If your hamming distances are comparable, you can conclude
>>your zobrist keys are ok, and go from there.
>>
>>--
>>James
>
>i never understood why people think hamming distance is a good measure for the
>quality of random numbers. e.g. for 8-bit numbers i can produce a collision with
>the numbers
>
>a = 11111000
>b = 11100011
>c = 00011011
>
>because b^c = a. the mutual hamming distances all come out to 3-5 :-)
>
>cheers
>  martin

It is about the chess tree.  Burton Wendroff and Tony Warnock wrote a paper
published in the JICCA years ago, which addressed this topic...  They explained
why this is important.  Ideal hamming distance is 64, but there are only two
64-bit numbers with this property across the entire set...





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