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Subject: Re: Hamming distance and lower hash table indexing

Author: Dieter Buerssner

Date: 10:57:07 09/04/03

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On September 03, 2003 at 22:59:17, Tom Likens wrote:

>On September 03, 2003 at 16:28:14, Dieter Buerssner wrote:
>
>>On September 03, 2003 at 13:43:13, Tom Likens wrote:
>>
>>>Getting the hamming distance in pseudo-code for N random numbers:
>>>
>>>1. Generate a random number (index=M)
>>>
>>>2. Compare it to the 0 ... M-1 valid random numbers alread saved
>>>
>>>   if (popcnt64(new_rand64 ^ array[0..M-1]) >= MIN_HAM) then OK
>>>
>>>3. If valid, save it into slot M
>>>   If not valid (hamming distance is too small) goto 1

BTW. This can give an infinite loop.

>>>4. Repeat until you have N random numbers
>>

>>They aren't random anymore, after you filtered them like that.
>You're correct of course, the set of numbers generated with the above
>algorithm is no longer pseudo-random and I should have been more precise
>in my wording.

I hope, I didn't sound too pedantic. It was just meant as a side note. I
assumed, you were already aware of it. The main point of my posting was, to give
a reference to that earlier discussion.

I tried different PRNGs (including Mersenne twister) of different quality, and
could not see any change (besides noise) in some test suites.

Regards,
Dieter




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