cs50/credit.c

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

// Prototypes
long long get_card_number(void);
void find_issuer(long long card_number);
int validate(long long card_number);
int get_id(long long card_number);
int get_length(long long card_number);

int main(void)
{
    long long card_number = get_card_number();

    if (validate(card_number))
    {
        find_issuer(card_number);
    }
    return 0;
}

long long get_card_number()
{
    long long number;
    do
    {
        number = get_long("Insert card number: ");
    }
    while (number < 0);

    return number;
}

void find_issuer(long long card_number)
{
    int id = get_id(card_number), count = get_length(card_number);

    // Number beginning with 34 or 37 and length is 15
    if ((id / 100 == 34 || id / 100 == 37) && count == 15)
    {
        printf("AMEX\n");
        return;
    }

    // Number beginning with 4 and length is 13 or 16
    if ((id / 1000 == 4) && (count == 13 || count == 16))
    {
        printf("VISA\n");
        return;
    }

    // Number beginning with 51-55 or 2221-2720 and length is 16
    if (((id / 100 >= 51 && id / 100 <= 55) || (id >= 2221 && id <= 2720)) && count == 16)
    {
        printf("MASTERCARD\n");
        return;
    }
    else
    {
        printf("INVALID\n");
        return;
    }
}

int validate(long long card_number)
{
    // Add together individual digits while multiplying every other digit from one after least significant before addition
    // If multiplication product is geater than 9 then add individual digits together. 3456 => 6 + (1 + 0) + 4 + 6
    int sum = 0, multiply = 0;

    while (card_number > 0)
    {
        if (multiply)
        {
            if (card_number % 10 * 2 > 9) // Is multiplication product greater than 9?
            {
                int n = card_number % 10 * 2;
                while (n > 0) // If so, add individual digits to the sum
                {
                    sum += n % 10;
                    n /= 10;
                }
            }
            else
            {
                sum += card_number % 10 * 2;
            }
            card_number /= 10;
            multiply = 0;
        }
        else
        {
            sum += card_number % 10;
            card_number /= 10;
            multiply = 1;
        }
    }

    if (sum % 10 != 0)
    {
        printf("INVALID\n");
        return 0; // Invalid number
    }
    else
    {
        return 1; // Valid number
    }
}

int get_id(long long card_number)
{
    int count = get_length(card_number), id;
    long long n, trunc = 1;

    if (count < 13 || count > 16)
    {
        return 0;
    }

    for (int i = 0; i < count - 4; i++) // Get four most significant digits
    {
        trunc *= 10;
    }
    id = card_number / trunc;
    return id;
}

int get_length(long long card_number)
{
    int count = 0;

    while (card_number > 0)
    {
        card_number /= 10;
        count++;
    }
    return count;
}