combinations with repitition iterative

comb_with_rep_iter.c

#include <stdio.h>

const char *donuts[] = {"iced", "jam", "plain",
                        "something completely different"};
int pos[] = {0, 0, 0, 0};

void printDonuts(int k) {
  for (size_t i = 1; i < k + 1; i += 1) // offset: i:1..N, N=k+1
    printf("%s\t", donuts[pos[i]]);     // str:0..N-1
  printf("\n");
}

// idea: custom number system with 2s complement like 0b10...0==MIN stop case
int step_comb_with_rep(int n, int k) {
  pos[k] += 1; // xxxxN -> xxxxN+1
  for (int i = k; i > 0; i -= 1) {
    if (pos[i] > n - 1) // if number spilled over: xx0(n-1)xx
    {
      pos[i - 1] += 1; // set xx1(n-1)xx
      for (int j = i; j <= k; j += 1)
        pos[j] = pos[j - 1]; // set xx11..1
    }
  }
  if (pos[0] > 0) // stop condition: 1xxxx
    return 1; // true
  return 0; // false
}

int main() {
  int n = 3;
  int k = 2;
  printDonuts(k);
  while(step_comb_with_rep(n, k) == 0)
    printDonuts(k);
  return 0;
}