Credit

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

int main(void)
{
    long long   card_number, orig_number;
    string      card_type;
    int         digits = 0, temp = 0, current = 0,
                even_digits = 0, odd_digits = 0, total_sum = 0;
    bool        is_even = false;

// Get Card number

    do
    {
        printf("Number: ");
        card_number = get_long_long();
    }
    while (card_number < 0);

// Save card number for later check;

    orig_number = card_number;

// Process Card Number

    while (card_number != 0)
    {

// Get last digit of number

        current = (card_number % 10);

// Increase number of digits in card number

        digits += 1;

// Check if digit is even or odd and increase appropriate sum

        if (is_even == true)
        {

// Multiply digit by 2

            temp = current * 2;

// Add each digit of product

            while (temp != 0)
            {
                even_digits += temp % 10;
                temp /= 10;
            }

// Tell program next digit is odd

            is_even = false;
        }
        else
        {
            odd_digits += current;

// Tell program next digit is even

            is_even = true;
        }

// Remove last digit and continue

        card_number /= 10;
    }

// Get total sum

    total_sum = even_digits + odd_digits;

// Check sum and card type by first digit

    if (total_sum % 10 == 0)
    {

// Check AMEX

        if (digits == 15)
        {
            if ((orig_number / (long long) 10E12) == 34 ||
                (orig_number / (long long) 10E12) == 37)
            {
                card_type = "AMEX";
            }
            else
            {
                card_type = "INVALID";
            }
        }
        else if (digits == 16)
        {

// Check Mastercard

            if ((orig_number / (long long) 10E13) >= 51 &&
                (orig_number / (long long) 10E13) <= 55)
            {
                card_type = "MASTERCARD";
            }

// Check some Visas

            else if ((orig_number / (long long) 10E14) == 4)
            {
                card_type = "VISA";
            }
            else
            {
                card_type = "INVALID";
            }
        }

// Check Visa

        else if (digits == 13)
        {
            if ((orig_number / (long long) 10E11) == 4)
            {
                card_type = "VISA";
            }
            else
            {
                card_type = "INVALID";
            }
        }
        else
        {
            card_type = "INVALID";
        }
    }
    else
    {
        card_type = "INVALID";
    }

// Print result

    printf("%s\n", card_type);

    return 0;
}