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/*-
* SPDX-License-Identifier: 0BSD
*
* Copyright (c) Robert Clausecker <fuz@FreeBSD.org>
* Based on a publication by Daniel Lemire.
* Public domain where applicable.
*
* Daniel Lemire, "Fast Random Integer Generation in an Interval",
* Association for Computing Machinery, ACM Trans. Model. Comput. Simul.,
* no. 1, vol. 29, pp. 1--12, New York, NY, USA, January 2019.
*/
#include <stdint.h>
#include <stdlib.h>
uint32_t
arc4random_uniform(uint32_t upper_bound)
{
uint64_t product;
/*
* The paper uses these variable names:
*
* L -- log2(UINT32_MAX+1)
* s -- upper_bound
* x -- arc4random() return value
* m -- product
* l -- (uint32_t)product
* t -- threshold
*/
if (upper_bound <= 1)
return (0);
product = upper_bound * (uint64_t)arc4random();
if ((uint32_t)product < upper_bound) {
uint32_t threshold;
/* threshold = (2**32 - upper_bound) % upper_bound */
threshold = -upper_bound % upper_bound;
while ((uint32_t)product < threshold)
product = upper_bound * (uint64_t)arc4random();
}
return (product >> 32);
}
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