Credit

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

int c2i(char c);
int add_digits(int n);
int LuhnsAlg(string CCnumber);

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

    do
    {
        CCnumber = get_string("Please enter a CC number: ");
        CCNumberLengh = strlen(CCnumber);
    }
    while (CCNumberLengh == 0 || CCnumber[0] > 57);

    if (CCNumberLengh < 15 || CCNumberLengh > 16)
    {
        printf("INVALID\n");
        return 0;
    }
    // Check starting numbers of credit cards & return VISA, AMEX or MASTERCARD
    // Verify CC is American Express. AA cards start with 34 or 37. Their card lengh can be 15 digits long
    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. MC's start with a 51, 52, 53, 54 or 55. Their card lengh can be 16 digits long
    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. VISA's start with a 4. Their card lengh can be 13 OR 16 digits long
    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)  // how this modulo works to add two digits together and why its needed for luhn's is in OneNote docco
{
    int d = 0;

    while (n > 0)
    {
        d = d + n % 10;
        n = n / 10;
    }
    return d;
}

int c2i(char c)  // to convert char to int you need to subtract a 48 based offset on the ascii table
{
    return c - (int)('0');
}

int LuhnsAlg(string CCnumber)  // detailed notes on how this function & forloops work are in your OneNote docco

{
    int i = 0;
    int s = 0;
    int t = 0;

    for (i = strlen(CCnumber) - 2; i >= 0; i -= 2)
    {
        t = c2i(CCnumber[i]);
        s = s + add_digits(t * 2);
    }
    for (i = strlen(CCnumber) - 1; i >= 0; i -= 2)
    {
        t = c2i(CCnumber[i]);
        s = s + t;
    }

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

    else
    {
        return 1;
    }
}