#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

int main(void)
{
    // Acquire credit card number as string for invalid character search iteration
    string credit_card_1 = get_string("CC#: ");

    // Convert credit card string to long long integer.
    long long credit_card_2 = atoll(cc_1);

    // Tally for the sum of every second digit x 2
    int sum1 = 0;

    // Establish divisors increased my a factor of 100 every iteration to isolate the current number place affected
    long long upper_divisor = 100;
    long long lower_divisor = 10;

    //For the string length of the credit card number
    for (int i = 0; i < strlen(credit_card_1); i++)
    {
        // Islolate and remove numbers prior to the affected number place
        int upper_isolate = credit_card_2 % upper_divisor;

        // Isolate and remove the numbers after the affected number place
        int lower_isolate = upper_isolate % lower_divisor;
        int lower_removal = upper_isolate - lower_isolate;

        // Divide by number place to reduce number to a single digit
        int isolated_number_place_1 = lower_removal / lower_divisor;

        // Double isolated number
        int current_digit = isolated_number_place_1 * 2;

        // Add doubled isolated number to the sum1 tally
        sum1 = sum1 + current_digit;

        // Multiply divisors to isolated the next highest number place
        upper_divisor = upper_divisor * 100;
        lower_divisor = lower_divisor * 100;

        // Diagnostic: print each new sum tally to ensure the math is correct.
        printf("%i\n", sum1);
    }

    printf("\n");
}