Absolutely horrible default string hashing

[Steve Teale]

dsimcha Wrote:

> == Quote from Steve Teale (steve.teale at britseyeview.com)'s article
> > Kristian Kilpi Wrote:
> > > On Sun, 03 May 2009 15:33:12 +0300, Jérôme M. Berger <jeberger at free.fr>
> > > wrote:
> > > > |
> > > > | foreach(c; str)
> > > > | {
> > > > |     ret = (ret << 4) + c;
> > > > | }
> > > > |
> > > > 	That one is very bad because it only takes into account the last
> > > > few characters of the string (how many depends on the size of the
> > > > hash). However, several hashing algorithms use 2^n+1 as their
> > > > multiplier, which is very fast even on old/small/embedded hardware
> > > > because it can be implemented as a shift and an addition.
> > >
> > > You are of course right; thanks for the correction. Lets scrap that
> > > algorithm! :)
> > Would something like
> > foreach(i, c; str)
> > {
> >    ret ^= cast(uint) c;
> >    if (i & 1)
> >       ret <<= 1;
> > }
> > do a better job. That could cover the first 64 chars reasonably well, and I
> suspect that could cover a lot of use cases.
> 
> I guess a lot of people missed my follow-up post on this.  This is largely a
> typeinfo bug, not a string hashing bug.  typeid(string) returns a TypeInfo
> reference to generic array TypeInfo, whereas typeid(char[]) returns a TypeInfo
> reference for which the hashing works well.  What is needed is for
> typeid(typemodifier(char)[]) to return the same thing as typeid(char[]).
> 
> Then again, I guess that, given how bad the generic array hashing is, it wouldn't
> hurt to improve that, too.  If someone implements a decent meta-hashing function
> (i.e. take a bunch of hashes of elements of some object, like an array, a tuple,
> or a struct, and produce a single good hash out of these), there would be no
> shortage of use cases for it.

Sorry, I missed that. The topic listing does get a bit fragmented at times.

Thanks Steve