credit.c

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

int getLength(long n);
long getTens(int distanceFromRight);
int getDigit(long n, int s);
bool isValid(long n);


int main(void)
{
    double cardNumber = 0;
    int length = 0;
    cardNumber = get_double("Number: ");

    length = getLength(cardNumber);
    if (isValid(cardNumber) == true)
    {
        // AMEX: Starts with 34 or 37 && is 15 digits long
        if (length == 15 && getDigit(cardNumber, 15) == 3 && (getDigit(cardNumber, 14) == 4 || getDigit(cardNumber, 14) == 7))
        {
            printf("AMEX\n");
        }

        // MASTERCARD: Starts with 51, 52, 53, 54, or 55 && is 16 digits long
        else if (length == 16 && getDigit(cardNumber, 16) == 5 && getDigit(cardNumber, 15) > 0 && getDigit(cardNumber, 15) < 6)
        {
            printf("MASTERCARD\n");
        }

        // VISA: Starts with 4 && is 13 or 16 digits long
        else if ((length == 13 && getDigit(cardNumber, 13) == 4) || (length == 16 && getDigit(cardNumber, 16) == 4))
        {
            printf("VISA\n");
        }

        // Otherwise, it's invalid
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}

long getTens(int distanceFromRight)
{
    long number = 1;

    for (int i = 0; i < distanceFromRight; i++)
    {
        number *= 10;

    }
    return number;
}

int getDigit(long n, int s)
{
    return (n % getTens(s) - n % getTens(s - 1)) / getTens(s - 1);
}

bool isValid(long n)
{
    int evenDigitsSum = 0;
    int oddDigitsSum = 0;
    int length = getLength(n);
    int lastDigit = 1;
    bool valid = false;

    // Add all the even numbers
    for (int i = 2; i <= length; i += 2)
    {
        int newDigit = getDigit(n, i) * 2;
        // printf("New digit (to parse): %d\n", getDigit(n, i));

        if (newDigit > 9)
        {
            // printf("New digit (to add): %d\n", getDigit(newDigit, i) * 2);
            newDigit = getDigit(newDigit, 1) + getDigit(newDigit, 2);
        }
        // printf("New digit (added): %d\n", newDigit);
        evenDigitsSum += newDigit;
    }

    // Add all the odd numbers
    for (int i = 1; i <= length; i += 2)
    {
        int newDigit = getDigit(n, i);

        oddDigitsSum += newDigit;
    }

    lastDigit = getDigit(oddDigitsSum + evenDigitsSum, 1);

    if (getDigit(oddDigitsSum + evenDigitsSum, 1) == 0)
    {
        valid = true;
    }

    return valid;
}

int getLength(long n)
{
    int length = 0;

    // Must be 13, 15, or 16 digits long!
    if (n >= 1000000000000 && n <= 9999999999999)
    {
        length = 13;
    }
    else if (n >= 100000000000000 && n <= 999999999999999)
    {
        length = 15;
    }
    else if (n >= 1000000000000000 && n <= 9999999999999999)
    {
        length = 16;
    }

    return length;
}


// Test method
// int main(void)
// {
//     long n = 1234567890123456;
//     long s = 10000000000000000;

//     printf("%ld", getDigit(n, 15));
//     printf("%ld", getDigit(n, 14));
// }