Re: g_str_hash

[Tim Janik <timj gtk org>]

On 11 Feb 2000, Owen Taylor wrote:

> Tim Janik <timj@gtk.org> writes:

> > guint
> > g_str_hash (gconstpointer string)
> > {
> >   register const gchar *p = string;
> >   register guint h = *p;
> > 
> >   for (p = string + 1; *p; p++)
> >     h += (h << 4) + *p;
> > 
> >   return h;
> > }
> > 
> > and for high-bit feedback:

> I'm not sure this is a good idea: one point of glib is that people can
> use it without having to understand things to the level of detail
> of having to understand tradeoffs between different hash functions.

that's merely a matter of documenting the intended use of the hash functions,
e.g. we recommend using g_str_hash_long() if the average string of your
hash table is longer than >=10 characters and tends to have a higher
distribution in the string head than the tail.

> The perl version effectively uses '<< 5' instead of '<< 4',
> and also differs by having 

thanks for catching that, g_str_hash() should have read h += (h << 5) + *p;

>  return (h >> 5) + h;
> 
> instead of 'return h'; something I don't completely understand.

hm, odd indeed, unless they use powers of 2 instead of prime numbers
for their hash table sizes ;) i don't really see how that would make
a difference.

> It would take a bit of experimentation to see how the Tcl/Perl
> versions perform on long strings that differ only in their tail, but I
> suspect that
> 
>  a) Inserting such strings into a hash table is rare (and
>     slow anyways)

well that depends on what you mean with "such" (long) strings,
the ASU version starts feedback after the 7th character.

>  b) They'll probably do OK anyways
> 
> Regards,
>                                         Owen
> 

---
ciaoTJ