credit.c

Harvard CS50 - Problem Set 1

// get length of creditcard number
long l = card_number;
int length = 0;
while (l > 0)
{
    l /= 10;
    length++;
}

// check length
if (length != 13 && length != 15 && length != 16)
{
    printf("INVALID\n");
    return 0;
}

// calculate checksum
int last_digit, second_last_digit;
int Sum1 = 0;
int Sum2 = 0;
int Total = 0;
long x = card_number;
do
{
    // get last digit
    last_digit = x % 10;
    x /= 10;
    Sum1 += last_digit;

    // get second last digit
    second_last_digit = x % 10;
    x /= 10;

    // multiply * 2
    second_last_digit *= 2;

    // add up d1 + d2
    int d1 = second_last_digit % 10;
    int d2 = second_last_digit / 10;
    Sum2 += d1 + d2;
}

while (x > 0);
Total = Sum1 + Sum2;

// check algorithm result
if (Total % 10 != 0)
{
    printf("INVALID\n");
    return 0;
}

// get the first 2 digits
long num;
while (card_number >= 100)
{
    card_number /= 10;
    num = card_number % 100;
}

// check which card
if ((length == 15) && (num == 34 || num == 37))
{
    printf("AMEX\n");
}
else if ((length == 16) && (num >= 51 && num <= 55))
{
    printf("MASTERCARD\n");
}
else if ((length == 13 || length == 16) && (num >= 40 && num <= 49))
{
    printf("VISA\n");
}
else
{
    printf("INVALID\n");
}