#include
#define CACHE 256
enum { h_unknown = 0, h_yes, h_no };
unsigned char buf[CACHE] = {0, h_yes, 0};
int happy(int n)
{
int sum = 0, x, nn;
if (n < CACHE) {
if (buf[n]) return 2 - buf[n];
buf[n] = h_no;
}
for (nn = n; nn; nn /= 10) x = nn % 10, sum += x * x;
x = happy(sum);
if (n < CACHE) buf[n] = 2 - x;
return x;
}
int main()
{
int i, cnt = 8;
for (i = 1; cnt || !printf("\n"); i++)
if (happy(i)) --cnt, printf("%d ", i);
printf("The %dth happy number: ", cnt = 1000000);
for (i = 1; cnt; i++)
if (happy(i)) --cnt || printf("%d\n", i);
return 0;
}
from http://rosettacode.org/wiki/Happy_numbers#C