credit.c

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

// Prototypes
long get_card_no(void);
bool checksum_pass(long card_no);
string get_card_type(long card_no);

// Main
int main(void)
{
    // Prompt the user for the card number
    long card_no = get_card_no();
    // a card number with invalid digits will return -1 and this be invalid
    if (card_no < 0)
    {
        printf("INVALID\n");
    }
    // If it passes the checksum
    else if (checksum_pass(card_no) == true)
    {
        // Get the card type and print it
        printf("%s", get_card_type(card_no));
    }
    else
    {
        // Fails the checksum
        printf("INVALID\n");
    }
}

string get_card_type(long card_no)
{
    // Setting up variables
    int first_digit = 0;
    int second_digit = 0;
    int third_digit = 0;
    long working_card_no = card_no;
    long multiplier = 1;
    int working_digit;
    // Loop 
    do
    {
        // Calculate the latest digit
        working_digit = (card_no / multiplier) % 10;
        working_card_no = working_card_no / 10;
        multiplier = multiplier * 10;
        // Bump the storage of the last 3 over by one
        first_digit = second_digit;
        second_digit = third_digit;
        third_digit = working_digit;
    }
    // Until the numbers are empty
    while (working_card_no > 0);
    // Swap third and first
    int leading_digit = third_digit;
    // If AMEX card
    if (leading_digit == 3 && (second_digit == 4 || second_digit == 7))
    {
        return "AMEX\n";
    }
    // If MASTERCARD card
    else if (leading_digit == 5 && (second_digit <= 5 && second_digit > 0))
    {
        return "MASTERCARD\n";
    }
    // If VISA card
    else if (leading_digit == 4)
    {
        return "VISA\n";
    }
    // Else it is invalid
    else
    {
        return "INVALID\n";
    }
}

// Get and check the card number
long get_card_no(void)
{
    long card_no;
    card_no = get_long("What is the card number?\n> ");
    // if it is   over 16 digits                 under 13 digits                14 digits, reject it
    if (card_no > 10000000000000000 || card_no < 999999999999 || (99999999999999 < card_no && card_no < 100000000000000))
    {
        // It is invalid
        return -1;
    }
    return card_no;
}

// Returns true if checksum is passed
bool checksum_pass(long card_no)
{
    // Setup variables
    long working_card_no = card_no;
    bool multiply_by_two = false;
    long multiplier = 1;
    int working_digit;
    int twos_sum = 0;
    int ones_sum = 0;
    int totals_sum = 0;
    int check_digit;
    do
    {
        // set working digit to the final digit in working number
        working_digit = (card_no / multiplier) % 10;
        // update working number
        working_card_no = working_card_no / 10;
        // every other digit
        if (multiply_by_two)
        {
            // multiply every other digit by two
            int step_result = working_digit * 2;
            // is this result two digits?
            if (step_result > 9)
            {
                // take the first digit
                int part_sum = step_result % 10;
                // add the second digit
                part_sum = part_sum + ((step_result / 10) % 10);
                // add to the sum of the digits multiplied
                twos_sum = twos_sum + part_sum;
            }
            // if not, just add
            else
            {
                twos_sum = twos_sum + step_result;
            }
        }
        // digits not multiplied
        else
        {
            // just add to the total
            ones_sum = ones_sum + working_digit;
        }    
        // flipflop for multiplying by 1 or 2
        multiply_by_two = !multiply_by_two;
        // update multiplier
        multiplier = multiplier * 10;
    }
    // stop when there are no more card digits
    while (working_card_no > 0);
    // get the total sum
    totals_sum = twos_sum + ones_sum;
    // get the check digit
    check_digit = totals_sum % 10;
    // printf("check digit: %i\n", check_digit);
    if (check_digit == 0)
    {
        return true;
    }
    else
    {
        return false;
    }
}