#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>

// Returns -1 if a string is not a number (e.g. 'two')
// Otherwise, returns the number.
// Only works for single digits.
// Only accepts lower case.
int8_t parse_num(char *str)
{
    const char *num_strings[10] =  { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

    // The lengths of the strings above, 
    const uint8_t num_string_lens[10] =  { 4, 3, 3, 5, 4, 4, 3, 5, 5, 4 };

    bool possible_nums[10] = { [0 ... 9] = true };
    uint8_t num_idx = 0;
    uint8_t char_incriment = 0;
    char current;
    bool result_still_possible = false;

    for (char_incriment = 0; char_incriment < 6; char_incriment++)
    {
        // Grab out next character in the string
        current = *(str + char_incriment);

        for (num_idx = 0; num_idx < 10; num_idx++)
        {
            if (!possible_nums[num_idx]) continue;

            // If we have passed the end of our number string and every character has matched,
            // then we return our valid character.
            if (char_incriment == num_string_lens[num_idx] && possible_nums[num_idx]) return num_idx;

            // If the character is not the letter it should be, disqualify this number.
            // e.g. if num_idx == 1 and char_incriment == 2, current should equal 'e'
            // because 'e' is the 3rd number in the word 'one.'
            if (current != num_strings[num_idx][char_incriment]) possible_nums[num_idx] = false;
            else result_still_possible = true;
        }
        if (!result_still_possible) return -1;
    }

    return -1;
}

int main(int argc, char **argv) {

    const char *num_strings[10] =  { "zerorr", "oneqw", "twoqw", "threere", "fourer", "fiveer", "sixer", "sevener", "eighter", "nineer" };

    uint8_t i = 0;

    for (i = 0; i < 10; i++)
    {
        printf("%d\n", parse_num(num_strings[i]));
    }

    return 0;
}