credit.c

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>
#include <math.h>
#include <string.h>

int c2i(char c);  //prototypes for my functions
int add_digits(int n);
int LuhnsAlg(string CCnumber);

int main(void)
{
    string CCnumber;  // declare a string called CCnumber
    int CCNumberLengh;  // declare an int called CCNumberLengh

    do
    {
        CCnumber = get_string("Please enter a CC number: ");  // Assign user input with get_string function to CCnumber
        CCNumberLengh = strlen(CCnumber);  // Declare CCNumberLengh == the strlengh of the inputted CCnumber
    }
    while (CCNumberLengh == 0 || c2i(CCnumber[0]) >= 9);  // do above while CCNumberLengh is == 0 OR CCnumber at index [0] is greater or equal to asci value of 57 (int)(9)

    if (CCNumberLengh < 15 || CCNumberLengh > 16)  // if CCNumberLengh is less than 15 characters or greater than 16 characters
    {
        printf("INVALID\n");
        return 0;  // kill program
    }

    // Check starting numbers of the credit card & return VISA, AMEX or MASTERCARD
    // Verify CC is American Express. All AA cards start with 34 or 37
    if ((CCNumberLengh == 15) && c2i(CCnumber[0]) == 3 && (c2i(CCnumber[1]) == 4 || c2i(CCnumber[1]) == 7))
    {
        if (LuhnsAlg(CCnumber) == 0)
        {
            printf("AMEX\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    // Verify CC is Mastercard. All MC's start with a 51, 52, 53, 54 or 55
    else if ((CCNumberLengh == 16) && (c2i(CCnumber[0]) == 5 || c2i(CCnumber[0]) == 2) && c2i(CCnumber[1]) >= 1 && c2i(CCnumber[1]) <= 5)
    {
        if (LuhnsAlg(CCnumber) == 0)
        {
            printf("MASTERCARD\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    // Verify CC is visa. All Visa's are 13 or 16 digits long and start with the string value of '4'
    else if ((CCNumberLengh == 13 || CCNumberLengh == 16) && CCnumber[0] == '4')
    {
        if (LuhnsAlg(CCnumber) == 0)
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}

int add_digits(int n) // my function to split a number like 15 and add it as literally 1 + 5
{
    int d = 0;

    while (n > 0)  // how this modulus works and why its needed for luhn's in OneNote docco
    {
        d = d + n % 10;
        n = n / 10;
    }
    return d;
}

int c2i(char c)  // my function to convert a character to an integer value with the ascii table
{
    // https://www.rapidtables.com/code/text/ascii-table.html
    // To convert char to int you need to subtract a 48 based offset on the ascii table
    return c - (int)('0');  // return this new value for char c into the vairable of wherther this copy is being called (in main for this program)
}

int LuhnsAlg(string CCnumber)  // my function to execute Luhn's algorith on the credit card number to check it
{
    int i = 0;  // detailed notes on how this forloop works are in OneNote docco
    int s = 0;
    int t = 0;

    for (i = strlen(CCnumber) - 2; i >= 0; i -= 2)  // start counting though even indexes of digits in CC array while i > 0, then move over and check every second CC index
    {
        t = c2i(CCnumber[i]);
        s = s + add_digits(t * 2);  // if t * 2 makes a number larger than "10" add each digit in that number together. eg 13 would become 1 + 3 = 4, thenr eturn "4" and add it to "s"
    }
    for (i = strlen(CCnumber) - 1; i >= 0; i -= 2)  // start counting though odd indexes of digits in CC array while i > 0, then move over and check every second CC index
    {
        t = c2i(CCnumber[i]);
        s = s + t;
    }

    if (s % 10 == 0)  // If the total of the last digit of this card is 0, card is valid
    {
        return 0;
    }

    else
    {
        return 1;
    }
}