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From: =?UTF-8?B?Q2FuZWsgUGVsw6FleiBWYWxkw6lz?= <caneko@gmail.com>
Date: Thu, 5 Jun 2014 22:58:51 -0500
Message-ID: <CADPrc81Uz0rRcvscWSL-T57RBFHOru-oMrQ94oDaFToLyYrujA@mail.gmail.com>
Subject: Re: [gentoo-user] OT: Mapping random numbers (PRNG)
To: gentoo-user@lists.gentoo.org
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On Thu, Jun 5, 2014 at 9:56 PM, <meino.cramer@gmx.de> wrote:
> Hi,
>
> I am experimenting with the C code of the ISAAC pseudo random number gene=
rator
> (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[] =3D { '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 =3D 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 =3D 0; i < 20; i++) // --std=3Dc99
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.
--=20
Canek Pel=C3=A1ez Vald=C3=A9s
Profesor de asignatura, Facultad de Ciencias
Universidad Nacional Aut=C3=B3noma de M=C3=A9xico