/*
1. Takes card number
2. Gets number of digits in card number. Invalid if not 13, 15 or 16.
3. Gets first two digits of card number to check later.
4. Uses Luhn's algorithm on card number to get "sum"
5. Checks if last digit of sum is 0. Invalid if not.
6. If it is, checks first digit(s) to match to correct card. Invalid if no match.
*/
#include <cs50.h>
#include <stdio.h>

int get_digit_length(long long num);

int main(void)
{
    printf("Card number: ");
    long long num = GetLongLong();

    // count digits in num
    int digit_length = get_digit_length(num);
    if (digit_length != 13 && digit_length != 15 && digit_length != 16 )
    {
        printf("INVALID\n");
        return 0;
    }

    // get first two digits of num to check amex/mastercard/visa
    long long first_two = num;
    while (first_two >= 100)
        first_two /= 10;

    // implement Luhn's algorithm
    int sum = 0;
    while (num)
    {
        // get last/second-last digits of num
        int last_digit = num % 10;
        int second_last_digit = (num / 10) % 10;

        // multiply by 2 and separate digits if product > 9
        int product = second_last_digit * 2;

        if (product > 9)
            product = 1 + (product - 10);

        sum += (product + last_digit);

        // divide to move to next pair of digits
        num /= 100;
    }

    // check that sum ends in zero and match to correct card if so.
    if (sum % 10 != 0)
        printf("INVALID\n");
    else if (first_two == 34 || first_two == 37)
        printf("AMEX\n");
    else if (first_two >= 51 && first_two <= 55)
        printf("MASTERCARD\n");
    else if (first_two / 10 == 4)
        printf("VISA\n");
    else
        printf("INVALID\n"); // in case Luhn's checks out, but not amex/mc/visa
}

// counts digits in num
int get_digit_length(long long num)
{
    int count = 0;
    while (num)
    {
        num = num / 10;
        count++;
    }
    return count;
}