forked from lda/telodendria
84 lines
2.7 KiB
C
84 lines
2.7 KiB
C
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/*
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* Copyright (C) 2022-2023 Jordan Bancino <@jordan:bancino.net>
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*
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* Permission is hereby granted, free of charge, to any person
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* obtaining a copy of this software and associated documentation files
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* (the "Software"), to deal in the Software without restriction,
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* including without limitation the rights to use, copy, modify, merge,
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* publish, distribute, sublicense, and/or sell copies of the Software,
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* and to permit persons to whom the Software is furnished to do so,
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* subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be
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* included in all copies or portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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* SOFTWARE.
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*/
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#include <Rand.h>
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#include <Util.h>
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#include <stdlib.h>
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#include <pthread.h>
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#include <unistd.h>
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/* Generate random numbers using rejection sampling.
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* The basic idea is to "reroll" if a number happens to be
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* outside the range. However this could be extremely inefficient.
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*
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* Another idea would just be to "reroll" if the generated number
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* ends up in the previously "biased" range, and THEN do a modulo.
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*
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* This would be far more efficient for small values of max,
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* and fixes the bias issue. */
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/* This algorithm therefore computes N random numbers generally in
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* O(N) time, while being less biased. */
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void
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RandIntN(int * buf, size_t size, unsigned int max)
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{
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static pthread_mutex_t seedLock = PTHREAD_MUTEX_INITIALIZER;
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static unsigned int seed = 0;
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int tmp;
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/* Limit the range to banish all previously biased results */
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const int allowed = RAND_MAX - RAND_MAX % max;
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size_t i;
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pthread_mutex_lock(&seedLock);
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if (!seed)
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{
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/* Generate a seed from the system time, PID, and TID */
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seed = UtilServerTs() ^ getpid() ^ (unsigned long) pthread_self();
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}
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/* Generate {size} random numbers. */
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for (i = 0; i < size; i++)
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{
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/* Most of the time, this will take about 1 loop */
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do
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{
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tmp = rand_r(&seed);
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} while (tmp >= allowed);
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/* Since a generated number here is never in the biased range,
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* we can now safely use modulo. */
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buf[i] = tmp % max;
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}
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pthread_mutex_unlock(&seedLock);
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}
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/* Generate just 1 random number */
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int
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RandInt(unsigned int max)
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{
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int val = 0;
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RandIntN(&val, 1, max);
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return val;
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}
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