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/* Generate random integers.
Copyright (C) 2006-2023 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Paul Eggert. */
#include <config.h>
#include "randint.h"
#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include <string.h>
#if TEST
# include <inttypes.h>
# include <stdio.h>
int
main (int argc, char **argv)
{
randint i;
randint n = strtoumax (argv[1], nullptr, 10);
randint choices = strtoumax (argv[2], nullptr, 10);
char const *name = argv[3];
struct randint_source *ints = randint_all_new (name, SIZE_MAX);
for (i = 0; i < n; i++)
printf ("%"PRIuMAX"\n", randint_choose (ints, choices));
return (randint_all_free (ints) == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
}
#endif
#include "xalloc.h"
/* A source of random data for generating random integers. */
struct randint_source
{
/* The source of random bytes. */
struct randread_source *source;
/* RANDNUM is a buffered random integer, whose information has not
yet been delivered to the caller. It is uniformly distributed in
the range 0 <= RANDNUM <= RANDMAX. If RANDMAX is zero, then
RANDNUM must be zero (and in some sense it is not really
"random"). */
randint randnum;
randint randmax;
};
/* Create a new randint_source from SOURCE. */
struct randint_source *
randint_new (struct randread_source *source)
{
struct randint_source *s = xmalloc (sizeof *s);
s->source = source;
s->randnum = s->randmax = 0;
return s;
}
/* Create a new randint_source by creating a randread_source from
NAME and ESTIMATED_BYTES. Return nullptr (setting errno) if
unsuccessful. */
struct randint_source *
randint_all_new (char const *name, size_t bytes_bound)
{
struct randread_source *source = randread_new (name, bytes_bound);
return (source ? randint_new (source) : nullptr);
}
/* Return the random data source of *S. */
struct randread_source *
randint_get_source (struct randint_source const *s)
{
return s->source;
}
/* HUGE_BYTES is true on hosts hosts where randint and unsigned char
have the same width and where shifting by the word size therefore
has undefined behavior. */
enum { HUGE_BYTES = RANDINT_MAX == UCHAR_MAX };
/* Return X shifted left by CHAR_BIT bits. */
static inline randint shift_left (randint x)
{
return HUGE_BYTES ? 0 : x << CHAR_BIT;
}
/* Consume random data from *S to generate a random number in the range
0 .. GENMAX. */
randint
randint_genmax (struct randint_source *s, randint genmax)
{
struct randread_source *source = s->source;
randint randnum = s->randnum;
randint randmax = s->randmax;
randint choices = genmax + 1;
while (1)
{
if (randmax < genmax)
{
/* Calculate how many input bytes will be needed, and read
the bytes. */
size_t i = 0;
randint rmax = randmax;
unsigned char buf[sizeof randnum];
do
{
rmax = shift_left (rmax) + UCHAR_MAX;
i++;
}
while (rmax < genmax);
randread (source, buf, i);
/* Increase RANDMAX by appending random bytes to RANDNUM and
UCHAR_MAX to RANDMAX until RANDMAX is no less than
GENMAX. This may lose up to CHAR_BIT bits of information
if (HUGE_BYTES ? 0 : RANDINT_MAX >> CHAR_BIT) < GENMAX,
but it is not worth the programming hassle of saving
these bits since GENMAX is rarely that large in practice. */
i = 0;
do
{
randnum = shift_left (randnum) + buf[i];
randmax = shift_left (randmax) + UCHAR_MAX;
i++;
}
while (randmax < genmax);
}
if (randmax == genmax)
{
s->randnum = s->randmax = 0;
return randnum;
}
else
{
/* GENMAX < RANDMAX, so attempt to generate a random number
by taking RANDNUM modulo GENMAX+1. This will choose
fairly so long as RANDNUM falls within an integral
multiple of GENMAX+1; otherwise, LAST_USABLE_CHOICE < RANDNUM,
so discard this attempt and try again.
Since GENMAX cannot be RANDINT_MAX, CHOICES cannot be
zero and there is no need to worry about dividing by
zero. */
randint excess_choices = randmax - genmax;
randint unusable_choices = excess_choices % choices;
randint last_usable_choice = randmax - unusable_choices;
randint reduced_randnum = randnum % choices;
if (randnum <= last_usable_choice)
{
s->randnum = randnum / choices;
s->randmax = excess_choices / choices;
return reduced_randnum;
}
/* Retry, but retain the randomness from the fact that RANDNUM fell
into the range LAST_USABLE_CHOICE+1 .. RANDMAX. */
randnum = reduced_randnum;
randmax = unusable_choices - 1;
}
}
}
/* Clear *S so that it no longer contains undelivered random data. */
void
randint_free (struct randint_source *s)
{
explicit_bzero (s, sizeof *s);
free (s);
}
/* Likewise, but also clear the underlying randread object. Return
0 if successful, -1 (setting errno) otherwise. */
int
randint_all_free (struct randint_source *s)
{
int r = randread_free (s->source);
int e = errno;
randint_free (s);
errno = e;
return r;
}
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