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      1 /*
      2  * Copyright (c) 2008, Damien Miller <djm (at) openbsd.org>
      3  *
      4  * Permission to use, copy, modify, and distribute this software for any
      5  * purpose with or without fee is hereby granted, provided that the above
      6  * copyright notice and this permission notice appear in all copies.
      7  *
      8  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
      9  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
     10  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
     11  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
     12  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
     13  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
     14  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
     15  */
     16 
     17 #include <stdint.h>
     18 #include <stdlib.h>
     19 
     20 uint32_t uniform_random(uint32_t);
     21 unsigned long prng_uint32(void);
     22 
     23 /*
     24  * Calculate a uniformly distributed random number less than upper_bound
     25  * avoiding "modulo bias".
     26  *
     27  * Uniformity is achieved by generating new random numbers until the one
     28  * returned is outside the range [0, 2**32 % upper_bound).  This
     29  * guarantees the selected random number will be inside
     30  * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
     31  * after reduction modulo upper_bound.
     32  */
     33 uint32_t
     34 uniform_random(uint32_t upper_bound)
     35 {
     36 	uint32_t r, min;
     37 
     38 	if (upper_bound < 2)
     39 		return 0;
     40 
     41 	/* 2**32 % x == (2**32 - x) % x */
     42 	min = -upper_bound % upper_bound;
     43 
     44 	/*
     45 	 * This could theoretically loop forever but each retry has
     46 	 * p > 0.5 (worst case, usually far better) of selecting a
     47 	 * number inside the range we need, so it should rarely need
     48 	 * to re-roll.
     49 	 */
     50 	for (;;) {
     51 		r = (uint32_t)prng_uint32();
     52 		if (r >= min)
     53 			break;
     54 	}
     55 
     56 	return r % upper_bound;
     57 }
     58