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From Dennis Gearon <gear...@sbcglobal.net>
Subject Re: hash uniqueKey generation?
Date Wed, 17 Nov 2010 02:05:32 GMT
Good hash functions almost never have 'collisions' as they are called, 
duplicates, as long as you stay under a certain percentage of the bits for the 
number of entries. 


Read up on WikiPedia, but I believe that no Hash Function is much good above 50% 
of the address space it generates. Many are much worse. Some are exceptional. 
Just know what you are using.

Cosmic rays are not much of a problem at sea level . . . but that changes 
linearly with altitude. The astronauts regulary see micro flashes in there 
vision . . as cosmic rays go through their retina or optical nerves.
 Dennis Gearon 


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It is always a good idea to learn from your own mistakes. It is usually a better 
idea to learn from others’ mistakes, so you do not have to make them yourself. 
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otherwise we all die. 



----- Original Message ----
From: Yonik Seeley <yonik@lucidimagination.com>
To: solr-user@lucene.apache.org
Sent: Tue, November 16, 2010 1:46:43 PM
Subject: Re: hash uniqueKey generation?

On Tue, Nov 16, 2010 at 5:31 AM, Dennis Gearon <gearond@sbcglobal.net> wrote:
> hashing is not 100% guaranteed to produce unique values.

But if you go to enough bits with a good hash function, you can get
the odds lower than the odds of something else changing the value like
cosmic rays flipping a bit on you.

-Yonik
http://www.lucidimagination.com


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