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Daniel Shawul

Joined: 14 Mar 2006
Posts: 2285
Location: Ethiopia

Post subject: Re: how to measure frequency of hash collisions.    Posted: Sun Jun 17, 2012 10:01 pm

syzygy wrote:
 Daniel Shawul wrote: I tested both and it seems I get 240 collisions/million on average for 28-bit key length with any decent PRNG. So p = 0.24*10^-3 collision rate for 28 bit key and a hash table of 1<<16 entries with bucket size of 1. I am quoting this detail for anyone that wants to do some calculations to arrive at an approximate figure. Once the mean rate of collision is known poisson equation can be used for prediction.

If the total key has 28 bits, and you use 16 for the index into the hash table, there are 12 bits left to distinguish between nodes mapped to the same entry. 2^12 = 4096, so the expected number of collisions per million of nodes is 10^6 / 4096 = 244.

Your slightly lower number might be explained by actual hash hits among the 10^6 probes, which by definition cannot lead to a collision.

Great! I skipped over the assumption of a full hash table but that will probably not have a significant effect since it that is much smaller than number of trials.
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Daniel Shawul Sat Jun 16, 2012 4:45 pm
Marcel van Kervinck Sat Jun 16, 2012 5:25 pm
Daniel Shawul Sat Jun 16, 2012 6:25 pm
Daniel Shawul Sat Jun 16, 2012 7:15 pm
Don Dailey Sat Jun 16, 2012 9:17 pm
Daniel Shawul Sat Jun 16, 2012 10:05 pm
Don Dailey Sat Jun 16, 2012 10:23 pm
Don Dailey Sat Jun 16, 2012 10:29 pm
Daniel Shawul Sat Jun 16, 2012 11:06 pm
Don Dailey Sat Jun 16, 2012 11:39 pm
Kevin Hearn Sat Jun 16, 2012 11:50 pm
Don Dailey Sun Jun 17, 2012 12:08 am
Daniel Shawul Sun Jun 17, 2012 12:40 am
Daniel Shawul Sat Jun 16, 2012 11:54 pm
Don Dailey Sun Jun 17, 2012 12:23 am
Marcel van Kervinck Sun Jun 17, 2012 7:55 am
Don Dailey Sun Jun 17, 2012 1:26 am
Ed Schroder Sat Jun 16, 2012 9:36 pm
Daniel Shawul Sat Jun 16, 2012 10:01 pm
H.G.Muller Sat Jun 16, 2012 10:12 pm
Daniel Shawul Sat Jun 16, 2012 10:31 pm
Daniel Shawul Sun Jun 17, 2012 2:00 am
Rémi Coulom Sun Jun 17, 2012 8:30 am
Daniel Shawul Sun Jun 17, 2012 10:49 am
Daniel Shawul Sun Jun 17, 2012 2:58 pm
Ronald de Man Sun Jun 17, 2012 3:41 pm
Daniel Shawul Sun Jun 17, 2012 5:58 pm
Ronald de Man Sun Jun 17, 2012 9:06 pm
Daniel Shawul Sun Jun 17, 2012 9:57 pm
Lucas Braesch Sun Jun 17, 2012 3:02 am
Ronald de Man Sun Jun 17, 2012 3:08 pm
Daniel Shawul Sun Jun 17, 2012 6:12 pm
Ronald de Man Sun Jun 17, 2012 9:14 pm
Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 10:01 pm
Joshua Shriver Wed Jun 20, 2012 8:45 pm
Ronald de Man Wed Jun 20, 2012 10:44 pm
Joshua Shriver Thu Jun 21, 2012 1:51 am

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