C - Alphabet permutations without arrays or recursion

gistfile1.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <math.h>

int main() {
	int string_len = 4;
	char alphabet[] = "abc";
	int alphabet_len = ((sizeof(alphabet) / sizeof(alphabet[0])) - 1);
	
	// null-initialize a char-array to hold a max-length of `string_len` characters
	char *current_key = (char *)calloc((string_len + 1), sizeof(char));
	
	// iterate over each keysize
	for (int keysize = 1; keysize <= string_len; keysize++) {
		// set `keysize` indexes to the first letter in our alphabet
		for (int i = 0; i < keysize; i++) {
			current_key[i] = alphabet[0];
		}
		
		// determine the max number of character combinations for the current keysize
		double num_combinations = pow(alphabet_len, keysize);
		int index = 0;
		while (index++ < num_combinations) {
			printf("%s\n", current_key);
			
			// update the first character in our key
			int alphabet_index = (index % alphabet_len);
			current_key[0] = alphabet[alphabet_index];
			
			if ((alphabet_index == 0) && (keysize != 1)) {
				// our first character has wrapped back to the beginning; begin a waterfall updating the next
				// character in the key (and repeat until the *next* character doesn't wrap)
				int key_index = 1;
				alphabet_index = ((int)(strchr(alphabet, current_key[key_index]) - alphabet) + 1) % alphabet_len;
				current_key[key_index] = alphabet[alphabet_index];
				printf("current_key[%d] = alphabet[%d], %d = %c\n", key_index, alphabet_index, key_index, alphabet[alphabet_index]);

				if (alphabet_index == 0) {
					key_index = (key_index + 1) % keysize;
					while (key_index > 0) {
						alphabet_index = ((int)(strchr(alphabet, current_key[key_index]) - alphabet) + 1) % alphabet_len;
						current_key[key_index] = alphabet[alphabet_index];
						printf("current_key[%d] = alphabet[%d], %d = %c\n", key_index, alphabet_index, key_index, alphabet[alphabet_index]);
						key_index = (key_index + 1) % keysize;
						
						if (alphabet_index != 0) break;
					}
				}
			}
		}
	}
	
	return 0;
}