arc4random_uniform.c

/*
 * Copyright (c) 1996, David Mazieres <dm@uun.org>
 * 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.
 */

#include <clens.h>

#ifdef NEED_ARC4RANDOM_UNIFORM

#include <stdlib.h>
#include <stdint.h>
#include <sys/types.h>

#if defined(__linux__) || defined(__CYGWIN__)
#include <bsd/stdlib.h>
#endif

/*
 * 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.
 */
uint32_t
arc4random_uniform(u_int32_t upper_bound)
{
	uint32_t r, min;

	if (upper_bound < 2)
		return 0;

#if (ULONG_MAX > 0xffffffffUL)
	min = 0x100000000UL % upper_bound;
#else
	/* Calculate (2**32 % upper_bound) avoiding 64-bit math */
	if (upper_bound > 0x80000000)
		min = 1 + ~upper_bound;		/* 2**32 - upper_bound */
	else {
		/* (2**32 - (x * 2)) % x == 2**32 % x when x <= 2**31 */
		min = ((0xffffffff - (upper_bound * 2)) + 1) % upper_bound;
	}
#endif

	/*
	 * 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 = arc4random();
		if (r >= min)
			break;
	}

	return r % upper_bound;
}
#endif /* NEED_ARC4RANDOM_UNIFORM */