Re: [Chicken-users] u8vector to numbers bignum

[Kon Lovett]

> On May 27, 2015, at 3:53 PM, chi <address@hidden> 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.

The integer result range extends to that of the limit parameter. Ex:

#;1> (use numbers srfi-27)
; loading /usr/local/lib/chicken/7/numbers.import.so ...
; loading /usr/local/lib/chicken/7/srfi-27.import.so ...
; loading /usr/local/lib/chicken/7/srfi-4.import.so …
        ...
; loading /usr/local/lib/chicken/7/srfi-4-checks.so ...
; loading /usr/local/lib/chicken/7/srfi-4-errors.so ..
#;2> (random-integer (expt 2 64))
15816285179193592418
#;3> (bignum? 15816285179193592418)
#t

> 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.
> 
> 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. I'm not qualified to say how hard that would be to turn a 
> vector
> into that, but it really doesn't seem like it would be too much trouble for
> someone who was. Of course it would be seen as native endian, but since it's a
> random number it doesn't matter too much.
> 
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