1276541Sdes/*      $OpenBSD: arc4random_uniform.c,v 1.1 2014/07/12 13:24:54 deraadt Exp $  */
2276541Sdes
3276541Sdes/*
4276541Sdes * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
5276541Sdes *
6276541Sdes * Permission to use, copy, modify, and distribute this software for any
7276541Sdes * purpose with or without fee is hereby granted, provided that the above
8276541Sdes * copyright notice and this permission notice appear in all copies.
9276541Sdes *
10276541Sdes * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
11276541Sdes * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
12276541Sdes * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
13276541Sdes * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
14276541Sdes * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
15276541Sdes * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
16276541Sdes * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
17276541Sdes */
18276541Sdes
19276541Sdes#include "config.h"
20276541Sdes#include <sys/types.h>
21276541Sdes#include <stdlib.h>
22276541Sdes
23276541Sdes/*
24276541Sdes * Calculate a uniformly distributed random number less than upper_bound
25276541Sdes * avoiding "modulo bias".
26276541Sdes *
27276541Sdes * Uniformity is achieved by generating new random numbers until the one
28276541Sdes * returned is outside the range [0, 2**32 % upper_bound).  This
29276541Sdes * guarantees the selected random number will be inside
30276541Sdes * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
31276541Sdes * after reduction modulo upper_bound.
32276541Sdes */
33276541Sdesuint32_t
34276541Sdesarc4random_uniform(uint32_t upper_bound)
35276541Sdes{
36276541Sdes	uint32_t r, min;
37276541Sdes
38276541Sdes	if (upper_bound < 2)
39276541Sdes		return 0;
40276541Sdes
41276541Sdes	/* 2**32 % x == (2**32 - x) % x */
42276541Sdes	min = -upper_bound % upper_bound;
43276541Sdes
44276541Sdes	/*
45276541Sdes	 * This could theoretically loop forever but each retry has
46276541Sdes	 * p > 0.5 (worst case, usually far better) of selecting a
47276541Sdes	 * number inside the range we need, so it should rarely need
48276541Sdes	 * to re-roll.
49276541Sdes	 */
50276541Sdes	for (;;) {
51276541Sdes		r = arc4random();
52276541Sdes		if (r >= min)
53276541Sdes			break;
54276541Sdes	}
55276541Sdes
56276541Sdes	return r % upper_bound;
57276541Sdes}
58