credit.c

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

long get_ccn();

int main(void)
{
    //Get card number, copy to temp value so it can be manipulated and reset
    long ccn = get_ccn();
    long temp_ccn = ccn;

    //Reject if card number is not 13, 15, or 16 digits
    int length = (log10(ccn) + 1);
    if (length != 13 && length != 15 && length != 16)
    {
        printf("INVALID\n");
        return (0);
    }

    //Iterate through every other digit starting from the right
    temp_ccn = ccn;
    int sum = 0;

    for (int i = 1; i <= length; i++)
    {
        int digit = temp_ccn % 10L;

        if (i % 2 == 0)
        {
            //Multiply every other digit by two
            digit *= 2;
            //Subtract 9 to turn 2 digit product into a single int, because math
            if (digit > 9)
            {
                digit -= 9;
            }
        }
        //Add digit or digit*2 to running total, divide by 10 to get to next number
        sum += digit;
        temp_ccn /= 10L;
    }

    //Check if last digit of sum is 0
    if (sum % 10 != 0)
    {
        printf("INVALID\n");
        return (0);
    }

    //Determine issuer based on first two digits
    temp_ccn = ccn;
    while (temp_ccn > 100L)
    {
        temp_ccn = temp_ccn / 10L;
    }

    if ((temp_ccn / 10 == 4) && (length == 13 || length == 16))
    {
        printf("VISA\n");
    }
    else if ((temp_ccn == 34 || temp_ccn == 37) && length == 15)
    {
        printf("AMEX\n");
    }
    else if ((temp_ccn > 50 && temp_ccn < 56) && length == 16)
    {
        printf("MASTERCARD\n");
    }
    else
    {
        printf("INVALID\n");
    }
}

//FUNCTIONS

//Get Card Number
long get_ccn()
{
    long i = 0L;
    while (i < 1L || i > 9999999999999999L)
    {
        i = get_long("Enter card number:\n");
    }
    return i;
}