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how to measure frequency of hash collisions.
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Ronald de Man



Joined: 28 Feb 2012
Posts: 841
PostPost subject: Re: how to measure frequency of hash collisions.    Posted: Sun Jun 17, 2012 9:14 pm Reply to topic Reply with quote
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.
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Subject Author Date/Time
how to measure frequency of hash collisions. Daniel Shawul Sat Jun 16, 2012 4:45 pm
      Re: how to measure frequency of hash collisions. Marcel van Kervinck Sat Jun 16, 2012 5:25 pm
            Some tests: Attn Don Daniel Shawul Sat Jun 16, 2012 6:25 pm
                  Re: Some tests: Attn Don Daniel Shawul Sat Jun 16, 2012 7:15 pm
      Re: how to measure frequency of hash collisions. Don Dailey Sat Jun 16, 2012 9:17 pm
            Re: how to measure frequency of hash collisions. Daniel Shawul Sat Jun 16, 2012 10:05 pm
                  Re: how to measure frequency of hash collisions. Don Dailey Sat Jun 16, 2012 10:23 pm
                        Re: how to measure frequency of hash collisions. Don Dailey Sat Jun 16, 2012 10:29 pm
                        Re: how to measure frequency of hash collisions. Daniel Shawul Sat Jun 16, 2012 11:06 pm
                              Re: how to measure frequency of hash collisions. Don Dailey Sat Jun 16, 2012 11:39 pm
                                    Re: how to measure frequency of hash collisions. Kevin Hearn Sat Jun 16, 2012 11:50 pm
                                          Re: how to measure frequency of hash collisions. Don Dailey Sun Jun 17, 2012 12:08 am
                                          Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 12:40 am
                                    Re: how to measure frequency of hash collisions. Daniel Shawul Sat Jun 16, 2012 11:54 pm
                                          Re: how to measure frequency of hash collisions. Don Dailey Sun Jun 17, 2012 12:23 am
                                    Re: how to measure frequency of hash collisions. Marcel van Kervinck Sun Jun 17, 2012 7:55 am
                  Re: how to measure frequency of hash collisions. Don Dailey Sun Jun 17, 2012 1:26 am
      Re: how to measure frequency of hash collisions. Ed Schroder Sat Jun 16, 2012 9:36 pm
            Re: how to measure frequency of hash collisions. Daniel Shawul Sat Jun 16, 2012 10:01 pm
      Re: how to measure frequency of hash collisions. H.G.Muller Sat Jun 16, 2012 10:12 pm
            Re: how to measure frequency of hash collisions. Daniel Shawul Sat Jun 16, 2012 10:31 pm
            Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 2:00 am
                  Re: how to measure frequency of hash collisions. Rémi Coulom Sun Jun 17, 2012 8:30 am
                        Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 10:49 am
                        Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 2:58 pm
                              Re: how to measure frequency of hash collisions. Ronald de Man Sun Jun 17, 2012 3:41 pm
                                    Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 5:58 pm
                                          Re: how to measure frequency of hash collisions. Ronald de Man Sun Jun 17, 2012 9:06 pm
                                                Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 9:57 pm
      Re: how to measure frequency of hash collisions. Lucas Braesch Sun Jun 17, 2012 3:02 am
            Re: how to measure frequency of hash collisions. Ronald de Man Sun Jun 17, 2012 3:08 pm
            Re: how to measure frequency of hash collisions. Daniel Shawul Sun Jun 17, 2012 6:12 pm
                  Re: how to measure frequency of hash collisions. 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
      Re: how to measure frequency of hash collisions. Joshua Shriver Wed Jun 20, 2012 8:45 pm
            Re: how to measure frequency of hash collisions. Ronald de Man Wed Jun 20, 2012 10:44 pm
                  Re: how to measure frequency of hash collisions. Joshua Shriver Thu Jun 21, 2012 1:51 am
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