C program from course work that validates a credit card number

credit.c

#include <stdio.h>

#include <cs50.h>

#include <string.h>



int main(void)

{

    // promt user for credit card

    // validate that we have a positive integer between 13 and 16 digits

    long long ccNum;

    // if user gives a negative number, prompt again

    do

    {

        ccNum = get_long_long("Enter Credit Card Number: ");

    }

    while (ccNum < 0);

    // reset value of ccNum

    long long copyCCNum = ccNum;

    int count = 0;

    // count the number of digits in the integer provided by user

    if (ccNum > 0)

    {

        while (ccNum)

        {

            ccNum = ccNum / 10;

            count++;

        }

    }

    // if user provides a number less than thirteen or more than 16, print "INVALID"

    if (count < 13 || count > 16)

    {

        printf("INVALID\n");

        return 0;

    }

    // reset value of ccNum

    ccNum = copyCCNum;

    // access company identifier (2 digits) at the beginning of card number

    long divNum;

    switch (count)

    {

        case 13:

            divNum = 100000000000;

            break;

        case 14:

            divNum = 1000000000000;

            break;

        case 15:

            divNum = 10000000000000;

            break;

        case 16:

            divNum = 100000000000000;

            break;

    }

    int identifier = ccNum / divNum;

    // validate the identifier and assign company to the number if valid

    string company = " ";

    switch (identifier)

    {

        case 34:

        case 37:

            company = "AMEX";

            break;

        case 40:

        case 41:

        case 42:

        case 43:

        case 44:

        case 45:

        case 46:

        case 47:

        case 48:

        case 49:

            company = "VISA";

            break;

        case 51:

        case 52:

        case 53:

        case 54:

        case 55:

            company = "MASTERCARD";

            break;

    }

    // If no valid identifier is found, print "INVALID"

    int result;

    result = strncmp(company, " ", 3);

    if (result == 0)

    {

        printf("INVALID\n");

        return 0;

    }

    // check the string value of the company variable

    int amex = strncmp(company, "AMEX", 3);

    int visa = strncmp(company, "VISA", 3);

    int mastercard = strncmp(company, "MASTERCARD", 3);

    // if company is not AMEX, VISA, or MASTERCARD, print "INVALID"

    if (amex != 0 && visa != 0 && mastercard != 0)

    {

        printf("INVALID\n");

        return 0;

    }

    // if company is AMEX, VISA or MASTERCARD, but number doesn't have required number of digits, print "INVALID"

    if (amex == 0 && count != 15)

    {

        printf("INVALID\n");

        return 0;

    }

    if (visa == 0 && count < 13)

    {

        printf("INVALID\n");

        return 0;

    }

    if (visa == 0 && count > 16)

    {

        printf("INVALID\n");

        return 0;

    }

    if (mastercard == 0 && count != 16)

    {

        printf("INVALID\n");

        return 0;

    }

    // for valid company and number, start with second-to-last digit & multiply every other digit by 2 & then sum those digits

    int product = 0;

    int splitA = 0;

    int splitB = 0;

    int sum = 0;

    int secondLastNum = 0;

    while (ccNum)

    {

        ccNum /= 10;

        secondLastNum = ccNum % 10;

        product = secondLastNum * 2;

        splitA = product % 10;

        splitB = product / 10;

        sum += (splitA + splitB);

        ccNum /= 10;

    }

    ccNum = copyCCNum;

    // add sum to remaining digits

    int lastNum = 0;

    while (ccNum)

    {

        lastNum = ccNum % 10;

        ccNum /= 100;

        sum += lastNum;

    }

    // validate checksum

    int lastNumDigit = sum % 10;

    if (amex == 0 && lastNumDigit == 0)

    {

        printf("AMEX\n");

        return 0;

    }

    else if (visa == 0 && lastNumDigit == 0)

    {

        printf("VISA\n");

        return 0;

    }

    else if (mastercard == 0 && lastNumDigit == 0)

    {

        printf("MASTERCARD\n");

        return 0;

    }

    else

    {

        printf("INVALID\n");

        return 0;

    }

}