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Diffstat (limited to 'lib/randint.c')
| -rw-r--r-- | lib/randint.c | 216 |
1 files changed, 216 insertions, 0 deletions
diff --git a/lib/randint.c b/lib/randint.c new file mode 100644 index 0000000..02c04ed --- /dev/null +++ b/lib/randint.c @@ -0,0 +1,216 @@ +/* Generate random integers. + + Copyright (C) 2006-2022 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], NULL, 10); + randint choices = strtoumax (argv[2], NULL, 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 NULL (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) : NULL); +} + +/* 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; +} |