div 7

div7.c

#include <stdio.h>

int div7(int n)
{
  int res = 0;
  int cnt = 0;

  while (n > 0) {
    int nn = (8 - (n & 7));
    res = res + (nn  << cnt);
    n -= ((nn << 3) - nn);
    n >>= 3;
    cnt += 3;
  }

  return res;
}

main(){
  for(int i = 0; i < 1000; i++) {
    printf("%d %d \n", i, div7(i));
  }
}