My Credit Card Validation Program

credit.c

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

int initialvalidate(long cardnumber); // Checks if cardnumber is 13, 15, or 16 digits long.
int validate(long cardnumber); // Luhn's algorithm.
string whichbrand(long cardnumber); // Tells which brand i.e. VISA, MASTERCARD, AMEX.
int countdigits(long num); // Used for initial validation.

int main(void)
{
    int valid, initialvalidation;
    string brand;
    long cardnumber = get_long("Number: ");
    initialvalidation = initialvalidate(cardnumber);
    if (initialvalidation == 1)
    {
        valid = validate(cardnumber); // should be 0 or 1
        if (valid == 1)
        {
            brand = whichbrand(cardnumber);
            printf("%s", brand);
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}

int countdigits(long num)
{
    int count = 0;
    do
    {
        count++;
        num = num / 10;
    }
    while (num != 0);

    return count;
}

int initialvalidate(long cardnumber) // check if cardnumber is less than 13 or more than 16
{
    if (countdigits(cardnumber) < 13)
    {
        return 0;
    }
    else if (countdigits(cardnumber) == 13)
    {
        return 1;
    }
    else if (countdigits(cardnumber) == 14)
    {
        return 0;
    }
    else if (countdigits(cardnumber) == 15)
    {
        return 1;
    }
    else if (countdigits(cardnumber) == 16)
    {
        return 1;
    }
    else
    {
        return 0;
    }
}

int validate(long cardnumber) // Luhn’s Algorithm
{
    int sum = 0, othersum = 0, counter = 1, total = 0, currentnum, currentnumtimes2;
    long restofcardno;
    restofcardno = cardnumber;

    do
    {
        currentnum = restofcardno % 10;
        if (counter % 2 == 0) // if it is even, do this
        {
            currentnumtimes2 = currentnum * 2;
            if (currentnumtimes2 > 9)
            {
                sum = sum + currentnumtimes2 / 10 + currentnumtimes2 % 10;
            }
            else
            {
                sum = sum + currentnumtimes2;
            }
        }
        else // if it is odd, do this.
        {
            othersum = othersum + currentnum;
        }
        counter++;
        restofcardno = restofcardno / 10;
    }
    while (restofcardno != 0);

    total = sum + othersum;
    if (total % 10 == 0)
    {
        return 1;
    }
    else
    {
        return 0;
    }
}

int getFirst2Digits(long input)
{
    while (input >= 100)
    {
        input /= 10;
    }

    return input;
}

string whichbrand(long cardnumber)
{
    int First2 = getFirst2Digits(cardnumber);
    if (First2 == 34 || First2 == 37)
    {
        return "AMEX\n";
    }
    else if (First2 == 51 || First2 == 52 || First2 == 53 || First2 == 54 || First2 == 55)
    {
        return "MASTERCARD\n";
    }
    else if (First2 / 10 == 4)
    {
        return "VISA\n";
    }
    else
    {
        return "INVALID\n";
    }

}