credit_version_1.c

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

int main(void)
{
    // valid cc numbers for testing
    // Amex - 378282246310005
    // MC - 5404000000000001
    // Visa - 4242424242424242
    
    long long user_input, cc_num_test_1, cc_num_test_2;
    
    // prompt for user input
    do
    {
        printf("Please enter your credit card number: ");
        user_input = get_long_long();
    }
    while (user_input < 0);
    
    
    int n1, n2, sum_of_n1, sum_of_n2_products;
    sum_of_n1 = 0;
    sum_of_n2_products = 0;
    
    cc_num_test_1 = user_input;
    cc_num_test_2 = user_input;
    
    // validate checksum
    while (cc_num_test_1 > 0)
    {
        n1 = cc_num_test_1 % 10;
        sum_of_n1 += n1;
        cc_num_test_1 -= n1;
        cc_num_test_1 /= 10;
        
        n2 = cc_num_test_1 % 10;
        
        if ( (n2 * 2) > 9)
        {
            int n2_t2 = n2 * 2;
            sum_of_n2_products += n2_t2 % 10;
            sum_of_n2_products += (n2_t2 - (n2_t2 % 10)) / 10;
        }
        else{
            sum_of_n2_products += (n2 * 2);
        }
            
        cc_num_test_1 -= n2;
        cc_num_test_1 /= 10;
    }
    
    
    // validate company's identifier
    // validate number's length
    if ( (sum_of_n1 + sum_of_n2_products) % 10 == 0)
    {
        bool amex, mc, visa;
        
        amex = 
            (cc_num_test_2 >= 340000000000000 && cc_num_test_2 < 350000000000000) || 
            (cc_num_test_2 >= 370000000000000 && cc_num_test_2 < 380000000000000);
            
        mc = 
            (cc_num_test_2 >= 5100000000000000 && cc_num_test_2 < 5600000000000000);
            
        visa = 
            (cc_num_test_2 >= 4000000000000 && cc_num_test_2 < 5000000000000) || 
            (cc_num_test_2 >= 4000000000000000 && cc_num_test_2 < 5000000000000000);
        
        if (amex)
            printf("Success! Valid American Express!\n");
        else if (mc)
            printf("Success! Valid MasterCard\n");
        else if (visa)
            printf("Success! Valid Visa\n");
        else
            printf("ERROR: checksum passed, however card type not found!\n");

    }
    else
    {
        printf("ERROR: checksum did not pass!\n");
    }
    
}