62 lines
1.6 KiB
C
62 lines
1.6 KiB
C
/* Finding the next prime >= a given small integer.
|
|
Copyright (C) 1995-2026 Free Software Foundation, Inc.
|
|
|
|
This file is free software: you can redistribute it and/or modify
|
|
it under the terms of the GNU Lesser General Public License as
|
|
published by the Free Software Foundation; either version 2.1 of the
|
|
License, or (at your option) any later version.
|
|
|
|
This file 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 Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public License
|
|
along with this program. If not, see <https://www.gnu.org/licenses/>. */
|
|
|
|
#include <config.h>
|
|
|
|
/* Specification. */
|
|
#include "next-prime.h"
|
|
|
|
#include <stdint.h> /* for SIZE_MAX */
|
|
|
|
/* Return true if CANDIDATE is a prime number or 1.
|
|
CANDIDATE should be an odd number >= 1. */
|
|
static bool _GL_ATTRIBUTE_CONST
|
|
is_prime (size_t candidate)
|
|
{
|
|
size_t divisor = 3;
|
|
size_t square = divisor * divisor;
|
|
|
|
for (;;)
|
|
{
|
|
if (square > candidate)
|
|
return true;
|
|
if ((candidate % divisor) == 0)
|
|
return false;
|
|
/* Increment divisor by 2. */
|
|
divisor++;
|
|
square += 4 * divisor;
|
|
divisor++;
|
|
}
|
|
}
|
|
|
|
size_t _GL_ATTRIBUTE_CONST
|
|
next_prime (size_t candidate)
|
|
{
|
|
#if !defined IN_LIBGETTEXTLIB
|
|
/* Skip small primes. */
|
|
if (candidate < 10)
|
|
candidate = 10;
|
|
#endif
|
|
|
|
/* Make it definitely odd. */
|
|
candidate |= 1;
|
|
|
|
while (SIZE_MAX != candidate && !is_prime (candidate))
|
|
candidate += 2;
|
|
|
|
return candidate;
|
|
}
|