1#include <math.h>
2
3int is_prime(unsigned n)
4{
5  if (n <= 3)
6    return 1;
7  if (!(n & 1))
8    return 0;
9  if (n % 3 == 0)
10    return 0;
11  unsigned lim = unsigned(sqrt((double)n));
12  unsigned d = 5;
13  for (;;) {
14    if (d > lim)
15      break;
16    if (n % d == 0)
17      return 0;
18    d += 2;
19    if (d > lim)
20      break;
21    if (n % d == 0)
22      return 0;
23    d += 4;
24  }
25  return 1;
26}
27