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d20ea4a703
It used to require two 32-bit integer divisions to get a random number less than some limit. Now we use Daniel Lemire's "nearly-divisionless" algorithm for unbiased bounded random numbers, which requires one 64-bit integer multiply in the usual case, and one 32-bit integer division in rare slow cases. Even the slow cases are faster than before; there are also fewer branches. I think this algorithm is exceptionally beautiful. It also has more clever tricks than lines of code, so I have done my best to explain how it works.
71 lines
1.7 KiB
C
71 lines
1.7 KiB
C
/*
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* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
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*
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* SPDX-License-Identifier: MPL-2.0
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*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, you can obtain one at https://mozilla.org/MPL/2.0/.
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*
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* See the COPYRIGHT file distributed with this work for additional
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* information regarding copyright ownership.
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*/
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#pragma once
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#include <inttypes.h>
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#include <stdlib.h>
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#include <isc/lang.h>
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#include <isc/types.h>
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/*! \file isc/random.h
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* \brief Implements wrapper around a non-cryptographically secure
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* pseudo-random number generator.
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*
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*/
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ISC_LANG_BEGINDECLS
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uint8_t
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isc_random8(void);
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/*!<
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* \brief Returns a single 8-bit random value.
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*/
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uint16_t
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isc_random16(void);
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/*!<
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* \brief Returns a single 16-bit random value.
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*/
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uint32_t
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isc_random32(void);
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/*!<
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* \brief Returns a single 32-bit random value.
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*/
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void
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isc_random_buf(void *buf, size_t buflen);
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/*!<
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* \brief Fills the region buf of length buflen with random data.
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*/
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uint32_t
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isc_random_uniform(uint32_t upper_bound);
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/*!<
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* \brief Returns a single 32-bit uniformly distributed random value
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* less than upper_bound.
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*
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* This is better than ``isc_random() % upper_bound'' as it avoids
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* "modulo bias" when the upper bound is not a power of two. This
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* function is also faster, because it usually avoids doing any
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* divisions (which are typically very slow).
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*
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* It uses rejection sampling to ensure uniformity, so it may require
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* multiple iterations to get a result; the probability of needing to
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* resample is very small when the upper_bound is small, rising to 0.5
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* when upper_bound is UINT32_MAX/2.
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*/
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ISC_LANG_ENDDECLS
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