You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
80 lines
1.3 KiB
80 lines
1.3 KiB
3 months ago
|
/*
|
||
|
|
* 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
|