gnu/dist/groff/src/libs/libgroff/prime.cpp

   1 /*      $NetBSD$        */
   2
   3 #include <math.h>
   4
   5 int is_prime(unsigned n)
   6 {
   7   if (n <= 3)
   8     return 1;
   9   if (!(n & 1))
  10     return 0;
  11   if (n % 3 == 0)
  12     return 0;
  13   unsigned lim = unsigned(sqrt((double)n));
  14   unsigned d = 5;
  15   for (;;) {
  16     if (d > lim)
  17       break;
  18     if (n % d == 0)
  19       return 0;
  20     d += 2;
  21     if (d > lim)
  22       break;
  23     if (n % d == 0)
  24       return 0;
  25     d += 4;
  26   }
  27   return 1;
  28 }