[Mono-dev] String.GetHashCode Discussion.

[Andreas Nahr]

Imho we should revert the change, especially as MS seems to also have an
implementation with low collisions (and in fact this will be what people are
expecting from a hashcode without any further explanation) we should do the
same. Otherwise this might drive some implementations into huge perf
problems.

Happy Hacking
Andreas

> -----Ursprüngliche Nachricht-----
> Von: mono-devel-list-bounces at lists.ximian.com [mailto:mono-devel-list-
> bounces at lists.ximian.com] Im Auftrag von Marek Safar
> Gesendet: Donnerstag, 17. Juli 2008 01:13
> An: Bill Holmes
> Cc: mono-devel-list
> Betreff: Re: [Mono-dev] String.GetHashCode Discussion.
> 
> Hello,
> 
> Here are some data which hopefully bring some light to this topic. I
> didn't measure uniqueness of hashcodes as I consider it less important
> as range distribution. Enclosed you can find the result for you data
> with old and new string.GetHashCode implementation.
> 
> Some numbers to look at:
> 
> ** Old implementation
> 
> Highest contentions: 2985, 1155, 1137
> 
> ** Current implementation
> 
> Highest contentions: 236, 217, 210
> 
> Marek
> 
> 
> > I have been asked to move this discussion to the e-mail list from
> IRC.
> >  Basically we (my company and I) have new unit tests errors in 2.0
> > that did not occur at 1.9.  The errors were traced to the
> > String.GetHashCode change.  I had one of our interns (Mike) research
> > the change and I wanted to share his findings with you.  His final
> > document s attached.  I can also send the test case Mike used to
> > generate his data.  I believe it is too large to send to all of your
> > inboxes.
> >
> > http://docs.google.com/Doc?id=dhnnms3p_3fpmfq8dm
> >
> > (1:28:15 PM) holmes: Have there been any discussions about r106887
> > which was a change to String.GetHashCode?
> > (1:31:54 PM) holmes: This change is causing some of our tests to
> fail.
> >  I will admit that our tests are relying on the sting hash code a bit
> > too much.  However after some investigation I am wondering if the new
> > algorithm is generating a unique enough result.  (I say that as
> > respectfully as I can.)
> > (1:34:33 PM) holmes: We conducted a short test where 58000 English
> > string were tested and the new code only results in 60% unique
> results
> > compared to the previous code's performance of 100%
> > (1:38:36 PM) holmes: We have found the current algorithm only
> > considers the last ~6 characters of a string, which may be the
> > problem.  That is the last thing I will say here I promise.
> > (1:42:21 PM) kumpera: marek: how have you tested the distribution of
> > your new string hashing algorithm?
> > (1:42:54 PM) marek: kumpera: I used english dictionary
> > (1:43:57 PM) marek: holmes: how big is your dictionary?
> > (1:45:05 PM) holmes: marek: 58110
> > (1:46:12 PM) holmes: I think the real problem is not that a word
> > hashes unique enough as a sentence producing a unique result.
> > (1:46:56 PM) holmes: the last 6 characters define the whole
> algorithm.
> >  (or at least I think so)
> > (1:56:50 PM) lupus: it's likely better to revert the change, string
> > hashing is also supposed to be synced with the implementation in C in
> > the runtime anyway
> >
> > -bill
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> >