CS50 Credit

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

int main(void)
{
    long number = get_long("Number: ");

    long card_number = number; // create a copy of number to use for card type validation

    int digit_count = 0;
    int sum_of_products = 0;
    int sum_of_digits = 0;

    while (number != 0)
    {
        int digit = number % 10;
        digit_count++;

        // multiply every other digit by 2, starting with second-to-last
        if (digit_count % 2 == 0)
        {
            int product = digit * 2;
            sum_of_products += product / 10 + product % 10;
        }
        else
        {
            sum_of_digits += digit;
        }

        number /= 10;
    }

    // add the two sums together
    int total_sum = sum_of_products + sum_of_digits;

    // validate card type using card_number variable
    if (total_sum % 10 == 0)
    {
        if (digit_count == 15 && (card_number / (long)pow(10, 13) == 34 || card_number / (long)pow(10, 13) == 37))
        {
            printf("AMEX\n");
        }
        else if (digit_count == 16 && (card_number / (long)pow(10, 14) >= 51 && card_number / (long)pow(10, 14) <= 55))
        {
            printf("MASTERCARD\n");
        }
        else if ((digit_count == 13 || digit_count == 16) && (card_number / (long)pow(10, digit_count - 1) == 4))
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }

    return 0;
}