// Checks for a valid credit card number

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

int get_length(long x);
long first_two(long x);
long first_digit(long x);

int main(void)
{

    long test = get_long("Number: ");
    long test2 = test;
    long digit = test/10;

    int len = get_length(test);

    int i = 0;
    int num;
    int luhn[10];
    while (digit != 0)
    {
        num = digit % 10;
        luhn[i] = 2*num;
        i++;
        digit /= 100;
    }
    int count = 9;
    int total = 0;
    int t, sum = 0, remainder;
    for (i = 0; i < 9; i++)
    {
        t = luhn[i];
        while (t != 0)
        {
            remainder = t % 10;
            sum  = sum + remainder;
            t = t / 10;
        }
    }

    int num2;
    int remain[10];
    int i2 = 0;
    while (test != 0)
    {
        num2 = test % 10;
        remain[i2] = num2;
        i2++;
        test /= 100;
    }
    int count2 = 9;
    int total2 = 0;
    int t2, sum2 = 0, remainder2,i3 = 0;
    for (i3 = 0; i3 < 9; i3++)
    {
        t2 = remain[i3];
        while (t2 != 0)
            {
                remainder2 = t2 % 10;
                sum2  = sum2 + remainder2;
                t2 = t2 / 10;
            }
    }

    int total_sum = sum + sum2;
    int final_sum = total_sum % 10;
    long pair = first_two(test2);
    long single = first_digit(test2);

    if (final_sum ==0 && single == 4)
    {
        printf("VISA\n");
    }
    else if (final_sum == 0 && (pair == 34 || pair == 37))
    {
        printf("AMEX\n");
    }
    else if (final_sum == 0 && (pair > 50 && pair < 56))
    {
        printf("MASTERCARD\n");
    }
    else
    {
        printf("INVALID\n");
    }
}


int get_length(long x)
{
    if (x >= 1000000000000000) return 16;
    if (x >= 100000000000000) return 15;
    if (x >= 10000000000000) return 14;
    if (x >= 1000000000000) return 13;
    if (x >= 100000000000) return 12;
    if (x >= 10000000000) return 11;
    if (x >= 1000000000) return 10;
    if (x >= 100000000)  return 9;
    if (x >= 10000000)   return 8;
    if (x >= 1000000)    return 7;
    if (x >= 100000)     return 6;
    if (x >= 10000)      return 5;
    if (x >= 1000)       return 4;
    if (x >= 100)        return 3;
    if (x >= 10)         return 2;
    return 1;
}

// Returns first pair of digits
long first_two(long x)
{
    long first;
    first = x;

    while (first >= 100)
    {
        first = first / 10;
    }
    return first;
}

// Returns first digit of input
long first_digit(long x)
{
    long first;
    first = x;

    while (first >= 10)
    {
        first = first / 10;
    }
    return first;
}