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127709d30a
Daniel Lemire has published a more efficient range reduction algorithm
for finding a random number in a given range without bias, reducing the
number of divisions to none in the common case and 1 in case the initial
sample is rejected.
This speeds up performance by 22% on amd64, 15% on i386, and 70% on armv7.
os: FreeBSD
arch: amd64
cpu: Intel(R) Core(TM) i7-4910MQ CPU @ 2.90GHz
│ benchmark.out │
│ sec/op │
Arc4random_uniform 56.53n ± 0%
Fast_uniform 44.00n ± 0%
geomean 49.87n
Reviewed by: cem
Approved by: emaste
Differential Revision: https://reviews.freebsd.org/D47659
47 lines
1 KiB
C
47 lines
1 KiB
C
/*-
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* SPDX-License-Identifier: 0BSD
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*
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* Copyright (c) Robert Clausecker <fuz@FreeBSD.org>
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* Based on a publication by Daniel Lemire.
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* Public domain where applicable.
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*
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* Daniel Lemire, "Fast Random Integer Generation in an Interval",
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* Association for Computing Machinery, ACM Trans. Model. Comput. Simul.,
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* no. 1, vol. 29, pp. 1--12, New York, NY, USA, January 2019.
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*/
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#include <stdint.h>
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#include <stdlib.h>
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uint32_t
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arc4random_uniform(uint32_t upper_bound)
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{
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uint64_t product;
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/*
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* The paper uses these variable names:
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*
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* L -- log2(UINT32_MAX+1)
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* s -- upper_bound
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* x -- arc4random() return value
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* m -- product
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* l -- (uint32_t)product
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* t -- threshold
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*/
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if (upper_bound <= 1)
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return (0);
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product = upper_bound * (uint64_t)arc4random();
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if ((uint32_t)product < upper_bound) {
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uint32_t threshold;
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/* threshold = (2**32 - upper_bound) % upper_bound */
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threshold = -upper_bound % upper_bound;
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while ((uint32_t)product < threshold)
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product = upper_bound * (uint64_t)arc4random();
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}
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return (product >> 32);
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}
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