Or modify the interface to Random. Or, even easier, use another PRNG that's
easy to pull bits from. E.g. here's a simple implementation of RC4 (written
more for cleanliness than performance, you can tweak this, or use Cryptokit
to use a faster C based version if you need speed).

external get_byte : string -> int -> int = "%string_unsafe_get"
external set_byte : string -> int -> int -> unit = "%string_unsafe_set"

class rc4 =
object (self)
val state = Array.init 256 (fun i -> i)
val mutable x = 0
val mutable y = 0

method private swap i j =
let t = state.(i) in state.(i) <- state.(j); state.(j) <- t

method private mask =
x <- (x+1) land 0xff;
y <- ((y + state.(x)) land 0xff);
self#swap x y;
state.((state.(x) + state.(y)) land 0xff)

method init key =
let state_index = ref 0
and len = String.length key in
for i = 0 to 255 do
state_index := ((!state_index + state.(i) + (get_byte key (i mod
len)))) land 0xff;
self#swap i !state_index
done

method crypt src =
let dest = String.copy src in
for i = 0 to String.length dest - 1 do
set_byte dest i ((get_byte dest i) lxor self#mask)
done;
dest

method gen_bytes num_bytes =
let r = String.create num_bytes in
for i = 0 to num_bytes - 1 do set_byte r i self#mask done;
r
end

-------------------

# let prng = new rc4;;
val prng : rc4 = <obj>
# prng#init "this is a simple seed";;
- : unit = ()
# let random_bits = prng#gen_bytes 100;;
val random_bits : string =
"÷­N´öjæ-’á§\007iÔãb)öø’úÓÕw¨5Y}d8«\1579RE\007ŽHƒ6Ý\143Ã[\153T)«µ\136¢)!
ÿ܇”\152àI—âÓ/“eò©á\025´ØXÃÿ²î–‰Îtr\018H–Á‡ãê_\019MøÞ+\008\028ê\007\143"

On Fri, 7 Nov 2008 14:37:07 +0000, Jon Harrop wrote
> On Friday 07 November 2008 13:23:49 Hugo Ferreira wrote:
> > Hello,
> >
> > Dose anyone know how I may generate a arbitrary precision random number.
> > The 64 bits is not enough. I looked at the Big_Int module but this
> > has no function for random number generation.
>
> Call the 64-bit generator as many times as necessary and concatenate
> the bits in the results together to get a big random number.
>
> --
> Dr Jon Harrop, Flying Frog Consultancy Ltd.
> http://www.ffconsultancy.com/?e
>
> ------------------------------------
>
> Archives up to December 31, 2007 are also downloadable at
http://www.connettivo.net/cntprojects/ocaml_beginners/
> The archives of the very official ocaml list (the seniors' one) can
> be found at http://caml.inria.fr Attachments are banned and you're
> asked to be polite, avoid flames etc.Yahoo! Groups Links
>
>
>

--

William D. Neumann

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> On a Linux platform, you can read bytes from /dev/urandom
> (and get true random numbers instead of pseudo-random ones).
^^^^

And by "true", you mean "also pseudo though generally better",
of course...

Cheers,
Dario Teixeira

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-[ Fri, Nov 07, 2008 at 10:59:20AM -0500, Eric Cooper ]----
> On a Linux platform, you can read bytes from /dev/urandom
> (and get true random numbers instead of pseudo-random ones).

Last time I checked you coul not read many of these. It's
good for an initial seed, not a long sequence.

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