credit.c

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

long exp10(int exponent);
long get_digit(long cc_num, int pos);
bool has_exact_digit(long cc_num, int digit);
string check_credit_card_number(long cc_num);

int main(void)
{
    long cc_num = get_long("Number: ");
    printf("%s\n", check_credit_card_number(cc_num));
}

string check_credit_card_number(long cc_num)
{
    // validate checksum with Luhn’s Algorithm
    long checksum = 0;
    int digit_count;

    // pos starts at 0, which corresponded to right-most digit
    // loop while 10^pos < cc_num
    for (int pos = 0; exp10(pos) < cc_num; pos++)
    {
        long digit = get_digit(cc_num, pos);

        // every other digit, starting with the number’s second-to-last digit
        // we need to x2 and sum all digit.
        // when pos starts with 0 from last. that means said condition are odd pos
        bool odd_pos = pos % 2 == 1;

        if (odd_pos)
        {
            long doubled_digit = digit * 2;
            if (doubled_digit >= 10)
            {
                long fst_digit = doubled_digit / 10;
                long snd_digit = doubled_digit % 10;
                checksum += fst_digit + snd_digit;
            }
            else
            {
                checksum += doubled_digit;
            }
        }
        else
        {
            checksum += digit;
        }

        // at last round of the loop, pos + 1 will determine digit count
        // since pos starts at 0 = 1st digit
        digit_count = pos + 1;
    }

    // valid only last checksum digit is 0
    if (checksum % 10 != 0)
    {
        return "INVALID";
    }

    long first_digit = cc_num / exp10(digit_count - 1);
    long first_two_digits = cc_num / exp10(digit_count - 2);

    // AMEX starts with 34, 37, 15-digit numbers
    if ((first_two_digits == 34 || first_two_digits == 37) && has_exact_digit(cc_num, 15))
    {
        return "AMEX";
    }

    // MASTERCARD starts with 51, 52, 53, 54, or 55, 16-digit numbers
    if (first_two_digits >= 51 && first_two_digits <= 55 && has_exact_digit(cc_num, 16))
    {
        return "MASTERCARD";
    }

    // VISA starts with 4, 13- and 16-digit numbers.
    if (first_digit == 4 && (has_exact_digit(cc_num, 13) || has_exact_digit(cc_num, 16)))
    {
        return "VISA";
    }

    // The rest are invalid
    return "INVALID";
}

long exp10(int exponent)
{
    return pow(10, (double)exponent);
}

// getting digit of a certain position
// for example: `1234`, digit at pos=1 -> 3
// 1234 % 10^(1+1) / 10^1
// => 1234 % 100 / 10
// => 34 / 10
// => 3 (dividing long yields long)
long get_digit(long cc_num, int pos)
{
    return (cc_num % exp10(pos + 1)) / exp10(pos);
}

bool has_exact_digit(long cc_num, int digit)
{
    return cc_num >= exp10(digit - 1) && cc_num < exp10(digit);
}