71 lines
2.2 KiB
C
71 lines
2.2 KiB
C
/* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $ */
|
|
|
|
/*
|
|
* Copyright (c) 2008, Damien Miller <djm@openbsd.org>
|
|
*
|
|
* Permission to use, copy, modify, and distribute this software for any
|
|
* purpose with or without fee is hereby granted, provided that the above
|
|
* copyright notice and this permission notice appear in all copies.
|
|
*
|
|
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
|
|
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
|
|
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
|
|
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
|
|
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
|
|
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
|
|
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
|
|
*/
|
|
/*
|
|
* arngment - random music generator
|
|
*
|
|
* Written in 2020 by Lucas
|
|
*
|
|
* To the extent possible under law, the author(s) have dedicated all
|
|
* copyright and related and neighboring rights to this software to the
|
|
* public domain worldwide. This software is distributed without any
|
|
* warranty.
|
|
*
|
|
* You should have received a copy of the CC0 Public Domain Dedication
|
|
* along with this software. If not, see
|
|
* <http://creativecommons.org/publicdomain/zero/1.0/>.
|
|
*/
|
|
|
|
#include <stdint.h>
|
|
#include <stdlib.h>
|
|
|
|
/*
|
|
* Calculate a uniformly distributed random number less than upper_bound
|
|
* avoiding "modulo bias".
|
|
*
|
|
* Uniformity is achieved by generating new random numbers until the one
|
|
* returned is outside the range [0, 2**32 % upper_bound). This
|
|
* guarantees the selected random number will be inside
|
|
* [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
|
|
* after reduction modulo upper_bound.
|
|
*/
|
|
long
|
|
lrand48_uniform(uint32_t upper_bound)
|
|
{
|
|
uint32_t r, min;
|
|
|
|
if (upper_bound < 2)
|
|
return 0;
|
|
|
|
/* 2**32 % x == (2**32 - x) % x */
|
|
min = -upper_bound % upper_bound;
|
|
|
|
/*
|
|
* This could theoretically loop forever but each retry has
|
|
* p > 0.5 (worst case, usually far better) of selecting a
|
|
* number inside the range we need, so it should rarely need
|
|
* to re-roll.
|
|
*/
|
|
for (;;) {
|
|
r = lrand48();
|
|
if (r >= min)
|
|
break;
|
|
}
|
|
|
|
return r % upper_bound;
|
|
}
|