Re: [Chicken-users] u8vector to numbers bignum

[Peter Bex]

On Wed, May 27, 2015 at 03:53:39PM -0700, chi wrote:
> How would I convert a u8vector to a bignum?
> 
> I'd like a good large random number, and there's srfi 27 for decent random
> sources, and there's 'numbers' for bignum support, but srfi-27 only produces
> fixnums or u8vectors.

I don't know anything about the srfi-27 egg, but after a quick look,
there seems to be an undocumented "srfi-27-numbers" module.  Since srfi-27
depends on the "numbers" egg, this should probably do the trick.

> Logically I can't imagine a bignum isn't represented under
> the hood by a block of bytes somehow, so it should be easy to turn a random 
> byte
> vector into a random bignum?
> 
> numbers itself only provides (random ...) which uses rand(3), and that's not
> what I would call a good random number. Random sources don't really seem like
> something a bignum library should worry about, anyway.

Yeah, the numbers random implementation is shitty, which is why I
decided to omit it from my port to CHICKEN core (CHICKEN 5's random is
still fixnum only).  Personally, I think it would be better if we simply
got rid of all "random" support in core, because there's no way this is ever
going to satisfy everyone.  Besides, the standard C library functions are
deeply flawed, especially on some platforms like OS X.

> I could always do (number->string (u8vec_to_hex (random-u8vector #x20)) #x10)
> but having a double sized hexadecimal intermediate isn't terribly appealing,
> especially for a random number I might have to generate for hundreds of peers 
> a
> second in a high volume scenario. (Okay now I really /am/ being optimistic.)
> 
> Looking at the numbers source, it seems to be analagous to a vector whose 
> first
> element is a magic number indicating it's a bignum, and the rest of the vector
> is the digits.

That's correct: it's a record object which has 'bignum as its type tag and
is followed by a string which contains the binary data.  The first C_word of
that string is actually 1 or 0 indicating its sign (1 if negative).

Cheers,
Peter

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