telodendria/src/Rand.c

86 lines
2.8 KiB
C

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