Re: [gentoo-user] OT: Mapping random numbers (PRNG)

[Canek Peláez Valdés]

On Thu, Jun 5, 2014 at 9:56 PM,  <meino.cramer@×××.de> wrote:
> Hi,
>
> I am experimenting with the C code of the ISAAC pseudo random number generator
> (http://burtleburtle.net/bob/rand/isaacafa.html).
>
> Currently the implementation creates (on my embedded linux) 32 bit
> hexadecimal output.

So it's a 32 bit integer.

> From this I want to create random numbers in the range of [a-Za-z0-9]
> *without violating randomness* and (if possible) without throwing
> away bits of the output.

You mean *characters* int the range [A-Za-z0-9]?

> How can I do this mathemtically (in concern of the quality of output)
> correct?

The easiest thing to do would be:

-------------------------------------------------------------------------------
#include <time.h>
#include <stdio.h>
#include <stdlib.h>

#define N (26+26+10)

static char S[] = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
'K', 'L', 'M',
                    'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W',
'X', 'Y', 'Z',
                    'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
'k', 'l', 'm',
                    'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w',
'x', 'y', 'z',
                    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' };

int
next_character()
{
        // Use the correct call for ISAAC instead of rand()
        unsigned int idx = rand() % N;
        return S[idx];
}

int
main(int argc, char* argv[])
{
        // Use the correct call for initializing the ISAAC seed
        srand((unsigned int)time(NULL));
        for (int i = 0; i < 20; i++) // --std=c99
                printf("%c\n", next_character());
        return 0;
}
-------------------------------------------------------------------------------

If the ISAAC RNG has a good distribution, then the next_character()
function will give a good distribution among the set [A-Za-z0-9].

Unless I missunderstood what you meant with "create random numbers in
the range of [a-Za-z0-9]".

Regards.
-- 
Canek Peláez Valdés
Profesor de asignatura, Facultad de Ciencias
Universidad Nacional Autónoma de México