Branchless FizzBuz

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>

// 0 or 1
unsigned is_divisible(unsigned x, unsigned d) {
  unsigned rem = x % d;
  rem--;
  return ((rem & (1 << sizeof(rem) * CHAR_BIT - 1)) >>
          (sizeof(rem) * CHAR_BIT - 1));
}

typedef void (*do_num_f)(unsigned i);

void do_last(unsigned i) {
  (void)(i);
  printf("Good bye!\n");
  exit(0);
}
void do_3_and_5(unsigned i) {
  (void)(i);
  printf("FizzBuzz!\n");
}
void do_3(unsigned i) {
  (void)(i);
  printf("Fizz!\n");
}
void do_5(unsigned i) {
  (void)(i);
  printf("Buzz!\n");
}
void do_other(unsigned i) { printf("%u\n", i); }

do_num_f do_num[5] = {do_other, do_3, do_5, do_3_and_5, do_last};

// 0 - others
// 1 - 3
// 2 - 5
// 3 - 15
// 4 - last
unsigned get_do_index(unsigned i, unsigned last) {
  unsigned is_divisible_last = is_divisible(i, last);
  unsigned is_divisible_5 = is_divisible(i, 5);
  unsigned is_divisible_3 = is_divisible(i, 3);
  return is_divisible_last * 4 +
         (is_divisible_5 * 2 + is_divisible_3 * 1) * (is_divisible_last ^ 1);
}

int main(int argc, char *argv[]) {
  printf("Branchless FizzBuzz!\n");
  for (unsigned i = 1;; i++)
    do_num[get_do_index(i, 101)](i);

  return -1;
}