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

int check(long long c);

int main(void)
{
    long long card = 0;  /* card is to get card input, i is to run while loops until every digit
                            of card is checked */

    while ((1000000000000 > card) || (card > 9999999999999999))
    {
        printf("Number: ");
        card = get_long_long();
        if ((1000000000000 > card) || (card > 9999999999999999))
        {
            printf("INVALID\n");
            break;
        }

    }


    if (check(card) == 1)
    {
        if ((card / 10000000000000) == 34 || (card / 10000000000000) == 37)
        {
            printf("AMEX\n");
        }
        else if (50 < (card / 100000000000000) && (card / 100000000000000) < 56)
        {
            printf("MASTERCARD\n");
        }
        else if ((card / 1000000000000) == 4 || (card / 1000000000000000))
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}

int check(long long card)
{
    int store1, sum1 = 0;   /* variables for gathering every other digit starting with the second
                            multiplying by 2 and summing */
    int sum2 = 0;   /* variables for gethering every other digit starting with the first
                            one and summing */
    long long i;  /* i is to run while loops until every digit
                            of card is checked */

    i = 10;
    while (i < card)
    {
        store1 = ((card / i) % 10) * 2;
        if (store1 > 9)
        {
            store1 = (store1 / 10) + (store1 % 10);
        }
        sum1 += store1;
        i *= 100;
    }

    i = 1;
    while (i < card)
    {
        sum2 += ((card / i) % 10);
        i *= 100;
    }

    if ((sum1 + sum2) % 10 == 0)
    {
        return 1;
    }
    else
    {
        return 0;
    }
}