/*  $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.

 */

#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.

 */

uint32_t

arc4random_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 = arc4random();

    if (r >= min)

      break;

  }

  return r % upper_bound;

}