Re: [Caml-list] Hashtbl.hash and Hashtbl.hash_param

[Florian Douetteau <douet@clipper.ens.fr>]

On Fri, 23 Aug 2002, sebastien FURIC wrote:

After a quick inspection in the source byterun/hash.c,
it appears that hash_param C implementation
decrements a C static variable 'counter' whenever an object
is considered and  returns 0 whenever this counter reach 0
(that's a way to handle cycles).

As a consequence (..details omitted..), in the case of the list type,
if the length of your list
is greater that the initial value of the counter,
hash will always
return 0.

A quick fix is to use:
hash_param <number_bigger_than_the_length_of_any_list> 100
instead of
hash (== hash_param 10 100)

By the way, beware that hash_param is not thread-safe,
since it uses a C global variable.


--
Florian Douetteau



>
>
> sebastien FURIC a écrit :
> >
> >  What kind of algorithm is used to compute the hash code of objects in
> > O'Caml ?
> >
> >  Hashtbl.hash (List.map (fun x -> Random.int 100)
> > [1;2;3;4;5;6;7;8;9;10]);;
> >  always returns 0 (Hashtbl.hash_param has the same properties) which is
> > a poor result !
> >
> >  What can I do if I don't know the size of the list in advance ?
>
>  Of course, I know I could use functors to specify my own hash function.
>  While prototyping I just would prefer an acceptable Hashtbl.hash
> function to avoid writing "boring" code.
>
>  Sebastien.
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