Luhn's Algorithm in C

luhns.c

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

string cardType;

// Prompt for a valid card number
long getCardNumber(void)
{
    long cardNumber;
    cardNumber = get_long("Please enter the card number: ");
    return cardNumber;
}

int getDigitCount(long n)
{
    int digits = 0;
    while (n > 0)
    {
        n /= 10;
        digits++;
    }

    return digits;
}

int getFirstDigit(long n)
{
    while (n > 10)
    {
        n /= 10;
    }

    return n;
}

int getFirstTwoDigits(long n)
{
    while (n > 100)
    {
        n /= 10;
    }

    return n;
}

// Basic validation against known card types using digit count & card prefixes
bool digitCheck(long cardNumber)
{
    int cardDigits = getDigitCount(cardNumber);
    int firstTwoDigits = getFirstTwoDigits(cardNumber);
    int firstDigit = getFirstDigit(cardNumber);

    switch (cardDigits)
    {
        case 13: // Visa

            if (firstDigit == 4)
            {
                cardType = "VISA";
                return true;
            }

        case 16: // Visa & Mastercard

            // Visa Cards begin with a 4
            if (firstDigit == 4)
            {
                cardType = "VISA";
                return true;
            }

           	// Master cards are 51, 52, 53, 54, or 55
            if (firstTwoDigits >= 51 && firstTwoDigits <= 55)
            {
                cardType = "MASTERCARD";
                return true;
            }

        case 15: // Amex

           	// Amex cards only begin with 34 &37
            if (firstTwoDigits == 34 || firstTwoDigits == 37)
            {
                cardType = "AMEX";
                return true;
            }

    }


    return false;
}

bool cardRangeCheck(long cardNumber)
{
    int cardDigits = getDigitCount(cardNumber);
    if (cardDigits < 13) return false;
    if (cardDigits > 16) return false;
    if (cardDigits == 14) return false;
    return true;
}

bool luhnsCheck(long cardNumber)
{
    // Multiply every other digit by 2, starting with the number’s second-to-last digit then add those products’ digits together.
    int count = 0;
    int result = 0;
    long odd_check = cardNumber;
    while (odd_check > 0)
    {
        if (count % 2 == 1)
        {
            int value = (odd_check % 10) * 2;
            int sum = 0;
            while (value != 0)
            {
                sum += value % 10;
                value /= 10;
            }

            result += sum;
        }

        odd_check /= 10;
        count++;
    }

   	// Sum the result with each of the digits that weren’t multiplied by 2.
    count = 0;
    long even_check = cardNumber;
    while (even_check > 0)
    {
        if (count % 2 == 0)
        {
            int value = (even_check % 10);
            int sum = 0;
            while (value != 0)
            {
                sum += value % 10;
                value /= 10;
            }

            result += sum;
        }

        even_check /= 10;
        count++;
    }

   	// If the total modulo 10 is congruent to 0, the number is valid!
    return result % 10 == 0;
}

bool validateCard(long cardNumber)
{
   	// Basic range check for the number of digits
    if (cardRangeCheck(cardNumber) == false) return false;

   	// Check the digit length for known cards
    if (digitCheck(cardNumber) == false) return false;

    // Finally, run Luhn's Algorithm against the num check approved card number
    if (luhnsCheck(cardNumber) == false) return false;

    return true;
}

int main(void)
{
   	// Prompt the user for a valid card number
    long cardNumber = getCardNumber();

   	// Validate the card
    bool isValidCard = validateCard(cardNumber);
    if (isValidCard)
    {
        printf("%s\n", cardType);
    }
    else
    {
        printf("INVALID\n");
    }
}