#include <stdio.h>
#include <cs50.h>

int main(void)
{
printf("This program is designed to check whether your credit card number is valid or not.\n");
long long card_number = get_long_long("Please enter your credit card number: ");

int valid = 0; // Initialize the condition under which the loop will run.

while(valid == 0) // Initialize the validation loop. Loop will cease once a valid credit card number is entered.
    if(card_number > 0) // Validate the number as a positive integer.
    {
        // Now validate the checksum.
        int digits = 0;
        int checksum = 0;
        long long x = card_number;
        while(x > 0)      // If our working number is greater than 0, we can continue calculating the checksum.
        {
        digits++;         // Adds the last digit to the checksum. (This will target every other digit).
        checksum = checksum + (x % 10);
        x = x / 10;       // Removes the last digit to continue processing.
            if(x > 0)
            {
            digits++;     // Adds the last digit multiplied by 2 (this targets every other digit) to the checksum.
            checksum = checksum + (2 *(x % 10));
            x = x / 10;
            }
        }
        if(checksum % 10 == 0)     // If the checksum is evenly divisible by 10, it is valid.
            {
                printf("Checksum is valid. Now checking which type of card you have.\n");
            // Now, validate the type. If 15 digits and starts with 34 or 37, it's American Express.
            if(digits == 15 && (card_number / 10000000000000 == 34 || card_number / 10000000000000 == 37))
            {
                printf("Your card is a valid AmEx card.\n");
                valid = 1;
            }
            // If it's 16 digits and begins with 51-55, it's Master Card.
            else if(digits == 16 && card_number / 100000000000000 >= 51 && card_number / 100000000000000 <= 55)
            {
                printf("Your card is a valid Master Card card.\n");
                valid = 1;
            }
            // If it's 13 or 16 digits and starts with 4, it's Visa.
            else if(digits == 16 && card_number / 1000000000000000 == 4)
            {
                printf("Your card is a valid Visa card.\n");
                valid = 1;
            }
            else
            {
                // If the number passes the checksum test but isn't a recognized AmEx, Visa, or Master Card, this will handle it.
                card_number = get_long_long("Valid checksum, but not a recognized company. Please retry: ");
            }
            }
        else
        {
            card_number = get_long_long("Invalid checksum. Please retry: ");
        }
    }
}