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

void invalid(void);
void card_check(long user_input, int nDigits);
int start_digits(long);
bool luhn_algo(long user_input, int nDigits);

int main(void)
{
    //takes user input and check if the value is positive number
    long user_input = get_long("Enter credit card number:");
    if (user_input < 1)
    {
        invalid();
    }
    //calculates the lenght of input and if correct, continues with card_check
    int nDigits = floor(log10(user_input)) + 1;
    if (nDigits == 15 || nDigits == 13 || nDigits == 16)
    {
        card_check(user_input, nDigits);
    }
    else
    {
        invalid();
    }
}

//gives invalid output message to user
void invalid(void)
{
    printf("INVALID\n");
}

int start_digits(long user_input)
{
    while (user_input >= 100)
    {
        user_input /= 10;
    }
    return user_input;
}

//luhn algoritm to check validity of card number
bool luhn_algo(long user_input, int nDigits)
{
    //creates array and sets first value to last digit of user input number
    int n[nDigits];
    n[0] = user_input % 10;
    //loops through the remaining digits and stores them in the array as well (from back to the beginning)
    int i = 1;
    while (user_input >= 1)
    {
        user_input /= 10;
        n[i] = user_input % 10;
        i++;
    }
    //creates another array and fills it with every other digit multiplied by 2, starting at 2nd to last position
    int check_sum = 0;
    int check_digits[nDigits / 2];
    int j = 0;
    for (i = 1; i < nDigits; i += 2)
    {
        check_digits[j] = n[i] * 2;
        j++;
    }
    //creates sum of digits from array created above
    for (i = 0; i < nDigits / 2; i++)
    {
        //neccessary check for values with more than 1 digit, to create sum of their digits
        if (check_digits[i] > 9)
        {
            check_sum = check_sum + check_digits[i] % 10;
            while (check_digits[i] > 9)
            {
                check_digits[i] /= 10;
            }
            check_sum = check_sum + check_digits[i];

        }
        else
        {
            check_sum = check_digits[i] + check_sum;
        }
    }
    //adds sum of all digits that were not multiplied by 2 previously
    for (i = 0; i < nDigits; i += 2)
    {
        check_sum = n[i] + check_sum;
    }
    //checks if the last number of final check_sum is 0 and returns true or false
    if (check_sum % 10 == 0)
    {
        return true;
    }
    else
    {
        invalid();
        return false;
    }

}

//checks for the beginning numbers and branches into correct card provider
void card_check(long user_input, int nDigits)
{
    int beginning = start_digits(user_input);
    if (beginning == 34 || beginning == 37)
    {
        //check validity of card by using luhn alghoritm
        if (luhn_algo(user_input, nDigits))
        {
            printf("AMEX\n");
        }
    }
    else if (beginning > 39 && beginning < 50)
    {
        if (luhn_algo(user_input, nDigits))
        {
            printf("VISA\n");
        }
    }
    else if (beginning > 50 && beginning < 56)
    {
        if (luhn_algo(user_input, nDigits))
        {
            printf("MASTERCARD\n");
        }
    }
    else
    {
        invalid();
    }
}