C中的Luhn算法

如何解决C中的Luhn算法

我对编码非常陌生，其中一项任务是编写 Luhn 算法。在互联网上搜索后，每个人的解决方案看起来如此不同和陌生:(所以我不知道我的解决方案的问题在哪里。感谢任何帮助！

int main(void)
{
    // get card number from user
    long number;
    do
    {
        number = get_long("Number: ");
    } while (number < 0);

    // isolate digits of card number
    int digit;
    int product;
    int sum;
    int totalSum;
    int counter;
    for (counter = 1; number > 9; counter++) {
        for (int i = 1; number > 9; i = i * -1) {
            digit = number % 10;

            // isolate digits that need to be multiplied by 2
            if (i == 1) {
                product = digit * 2;

                // add products' digits
                if (product > 9) {
                    sum = (product % 10) + 1;
                }
            }
            // add sum of digits that weren't multiplied by 2
            totalSum = product + sum + digit;

            // update "new" number
            number = (number - digit) / 10;
        }
    }

    // checksum
    int check = totalSum % 10;
    if (check != 0) {
        printf("INVALID\n");
    } else {
        printf("VALID\n");
    }
}