lib/next_prime.c

/* Determine prime number.
   Copyright (C) 2000, 2002 Free Software Foundation, Inc.
   Written by Ulrich Drepper <drepper@redhat.com>, 2000.

   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, version 2.

   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, write to the Free Software Foundation,
   Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */

#include <stddef.h>


/* Test whether CANDIDATE is a prime.  */
static int
is_prime (size_t candidate)
{
  /* No even number and none less than 10 will be passed here.  */
  size_t divn = 3;
  size_t sq = divn * divn;

  while (sq < candidate && candidate % divn != 0)
    {
      size_t old_sq = sq;
      ++divn;
      sq += 4 * divn;
      if (sq < old_sq)
	return 1;
      ++divn;
    }

  return candidate % divn != 0;
}


/* We need primes for the table size.  */
size_t
next_prime (size_t seed)
{
  /* Make it definitely odd.  */
  seed |= 1;

  while (!is_prime (seed))
    seed += 2;

  return seed;
}