/*
 *	UCW Library -- Prime Number Tests
 *
 *	(c) 1997 Martin Mares <mj@ucw.cz>
 *
 *	This software may be freely distributed and used according to the terms
 *	of the GNU Lesser General Public License.
 */

#include <ucw/lib.h>
#include <ucw/prime.h>

static int				/* Sequential search */
__isprime(uint x)			/* We know x != 2 && x != 3 */
{
  uint test = 5;

  if (x == 5)
    return 1;
  for(;;)
    {
      if (!(x % test))
	return 0;
      if (x / test <= test)
	return 1;
      test += 2;			/* 6k+1 */
      if (!(x % test))
	return 0;
      if (x / test <= test)
	return 1;
      test += 4;			/* 6k-1 */
    }
}

int
isprime(uint x)
{
  if (x < 5)
    return (x == 2 || x == 3);
  switch (x % 6)
    {
    case 1:
    case 5:
      return __isprime(x);
    default:
      return 0;
    }
}

uint
nextprime(uint x)			/* Returns some prime greater than x */
{
  x += 5 - (x % 6);			/* x is 6k-1 */
  for(;;)
    {
      x += 2;				/* 6k+1 */
      if (__isprime(x))
	return x;
      x += 4;				/* 6k-1 */
      if (__isprime(x))
	return x;
    }
}

#ifdef TEST

#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char **argv)
{
  uint k = atol(argv[1]);
  printf("%d is%s prime\n", k, isprime(k) ? "" : "n't");
  printf("Next prime is %d\n", nextprime(k));
  return 0;
}

#endif