Re: [Ocaml-lib-devel] BitSet

[Nicolas C. <war...@fr...>]

> > Thanks,
>
> You're welcome :)
>
> > The CVS have been updated with your changes. To answer more precisely
> > to your questions :
> > - we could actualy use ints as 16 bits values if we add the API to
> > extract them from strings, but we don't :)
>
> Why not to add this API ? It might also be useful in to improve other
> data structures. And why only 16 bits and not 32 ?

- We have to write C functions for that API.
- 16 bits are currently handled by a single unboxed OCaml int. Using all
32-bits is possible if we box integers -> that means that we will alloc/free
a lot of blocks around, that's not efficient.

> > - we can't use Pervasives.compare because bitsets might be of
> > different sizes but are actually the same integer
>
> I didn't want to say that Pervasives.compare should be used directly. I
> suggested something like that :
>
> (************************************************************)
> let extend t old_size new_size =
>    let b = bcreate new_size in
>    bblit !t 0 b 0 old_size;
>    bfill b old_size (new_size - old_size) 0;
>    b
>
> let compare t1 t2 =
>    let size1 , size2 = blen !t1 , blen !t2 in
>    if size1 < size2 then
>      Pervasives.compare (extend t1 size1 size2) !t2
>    else if size1 > size2 then
>      Pervasives.compare !t1 (extend t2 size2 size1)
>    else
>      Pervasives.compare !t1 !t2

In this case, I think custom compare function is better since it doesn't
require an allocation + a copy.
There is not so big speed difference between (correctly written) OCaml code
and C when compiled in native mode :)

Nicolas Cannasse