Re: [Caml-list] Size of Bigarray elements, or matching with 'kind'?

[Florent Monnier <monnier.florent@gmail.com>]

Le jeudi 05 avril 2012 07:57:45, Anthony Tavener a écrit :
> I'm trying to determine the size of a Bigarray, in bytes.
> 
> I've tried several things... ending up with this as the only solution:
> 
> --------------------------
> let ba_char = Hashtbl.hash Bigarray.char;;
> let ba_int8_signed = Hashtbl.hash int8_signed;;
> let ba_int8_unsigned = Hashtbl.hash int8_unsigned;;
> let ba_int16_signed = Hashtbl.hash int16_signed;;
> let ba_int16_unsigned = Hashtbl.hash int16_unsigned;;
> let ba_int32 = Hashtbl.hash Bigarray.int32;;
> let ba_float32 = Hashtbl.hash float32;;
> let ba_int64 = Hashtbl.hash Bigarray.int64;;
> let ba_float64 = Hashtbl.hash float64;;
> let ba_complex32 = Hashtbl.hash complex32;;
> let ba_complex64 = Hashtbl.hash complex64;;
> let ba_int = Hashtbl.hash Bigarray.int;;
> let ba_nativeint = Hashtbl.hash Bigarray.nativeint;;
> 
> let bigarray_bytes ba =
>   let elt_bytes = match Hashtbl.hash (Array1.kind ba) with
> 
>     | x when x=ba_char || x=ba_int8_signed || x=ba_int8_unsigned -> 1
>     | x when x=ba_int16_signed || x=ba_int16_unsigned            -> 2
>     | x when x=ba_int32 || x=ba_float32                          -> 4
>     | x when x=ba_int64 || x=ba_float64 || x=ba_complex32        -> 8
>     | x when x=ba_complex64                                      -> 16
>     | x when x=ba_int || x=ba_nativeint                          ->
> 
> Nativeint.size lsr 8
> 
>     | _ -> failwith "Unknown Bigarray kind."
> 
>   in (Array1.dim ba) * elt_bytes;;
> --------------------------
> 
> Which is a rather ugly hack! It feels dirty like Obj.magic, while being
> completely inelegant, and even has a fail case! Can't get much worse. :)
> Of course, internally, bigarray will know it's own size... sometimes the
> price of abstraction... *sigh*
> 
> I found a related Mantis entry from 6 years back:
> http://caml.inria.fr/mantis/view.php?id=3962
> 
> Well... does anyone have some better suggestions? Please? :)
> Oh, I can make it look a little prettier by plugging in the constants eked
> out by Hashtbl.hash. ;D
> 
> Why do I want this? Interfacing with OpenGL (glcaml).
> 
> Thanks for looking, and more again if you have a good tip!

Hi Anthony,

Had the same problem with glMLite.
I use the function below, but if there is a better solution I would be 
interested too:

val ba_sizeof: ba:('a, 'b, Bigarray.c_layout) Bigarray.Array1.t -> int

external elem_size: ba:('a, 'b, Bigarray.c_layout) Bigarray.Array1.t -> int
  = "ml_ba_elem_size" "noalloc"

let ba_sizeof ~ba =
  ((Bigarray.Array1.dim ba) * (elem_size ba)) ;;

CAMLprim value ml_ba_elem_size( value _ba )
{
    struct caml_bigarray *ba = Bigarray_val(_ba);
    int size;
    switch (ba->flags & BIGARRAY_KIND_MASK)
    {
        case BIGARRAY_SINT8:
        case BIGARRAY_UINT8:
            size = 1; break;

        case BIGARRAY_SINT16:
        case BIGARRAY_UINT16:
            size = 2; break;

        case BIGARRAY_INT32:
        case BIGARRAY_FLOAT32:
        case BIGARRAY_COMPLEX32:
            size = 4; break;

        case BIGARRAY_INT64:
        case BIGARRAY_FLOAT64:
        case BIGARRAY_COMPLEX64:
            size = 8; break;

        case BIGARRAY_CAML_INT:
        case BIGARRAY_NATIVE_INT:
            size = sizeof(value); break;
    }
    return Val_int(size);
}