c_check_credit_format

c_check_credit_format.c

#include <stdio.h>

unsigned long long countDigit(unsigned long long n);

int main(void)
{
    unsigned long long cardNumber, digitLength;
    char cardNumberText[255], cardType[255];
    do
    {
        printf("Number: ");
        scanf("%llu", &cardNumber);
        digitLength = countDigit(cardNumber);
        sprintf(cardNumberText, "%llu", cardNumber);

        if (cardNumberText[0] == '4' && (digitLength == 13 || digitLength == 16))
            sprintf(cardType, "%s", "VISA");
        else if (cardNumberText[0] == '5' && digitLength == 16)
        {
            if (cardNumberText[1] == '1' || cardNumberText[1] == '5')
                sprintf(cardType, "%s", "MASTERCARD");
            else
                sprintf(cardType, "%s", "INVALID");
        }
        else if (cardNumberText[0] == '3')
        {
            if ((cardNumberText[1] == '4' || cardNumberText[1] == '7') && digitLength == 15)
                sprintf(cardType, "%s", "AMEX");
            else
                sprintf(cardType, "%s", "INVALID");
        }

        else
            sprintf(cardType, "%s", "INVALID");
    } while (!cardNumber);

    unsigned long long sum = 0;
    for (int i = digitLength - 1; i >= 0; i--)
    {
        int digit = cardNumberText[i] - '0';
        if ((digitLength - i) % 2 == 0)
        {
            digit *= 2;
            sum += (digit > 9) ? (digit - 9) : digit;
        }
        else
            sum += digit;
    }
    char sumStringBuffer[20], sumDigitLength = countDigit(sum);
    sprintf(sumStringBuffer, "%llu", sum);
    if (sumStringBuffer[sumDigitLength - 1] - '0' != 0)
        sprintf(cardType, "%s", "INVALID");

    printf("%s\n", cardType);
    return 0;
}

unsigned long long countDigit(unsigned long long n)
{
    if (n < 0)
        return countDigit((n == INT_MIN) ? INT_MAX : -n);
    if (n < 10)
        return 1;
    return 1 + countDigit(n / 10);
}