// UPC code checker.cpp : This file contains the 'main' function. Program execution begins and ends there.
//

#include <iostream>

int getCheckDigit(long long int upc, int firstDigit);
int getDigit(int numOfPlaces, int placeIndex, long long int number);
int main()
{
    while (true) {
        std::cout << "Do you have a code to test [y/n]? ";
        std::string test = "";
        std::cin >> test;
        if (test=="y") {
            long long int upcCode;
            int firstDigit;

            //get first digit
            std::cout << "Enter the first digit: ";
            std::cin >> firstDigit;

            //get upc code
            std::cout << "Please enter a upc code: ";
            std::cin >> upcCode;



            //remove checkdigit
            long long int codeWithoutCheckDigit = upcCode / 10;


            //get what check digit is supposed to be
            int checkDigit = getCheckDigit(codeWithoutCheckDigit, firstDigit);
            //compare calculated check digit to actual check digit and determain if the code is valid or not
            if (checkDigit == getDigit(12, 12, upcCode)) {
                std::cout << "The code is valid\n\n";
            }
            else {
                std::cout << "The code is invalid\n\n";
            }
        }
        else if (test=="n") {
            std::cout << ("Exiting program");

            //quit
            return 0;
        }
        else {
            std::cout << "That is not valid please try again \n\n";
        }
    }
}

//gets the check digit from a upc code
int getCheckDigit(long long int upc,int firstDigit) {
    //odd and even digit sums
    int oddDigits=0, evenDigits=0;

    //determains how many digits are in the code based off the first digit
    int digitCount = firstDigit == 0 ? digitCount = 10 : digitCount = 11;

    //sums all the odd and even digits
    for (int i = 1; i <= digitCount; i++) {

        //gets digit from code
        int digit = getDigit(digitCount, i, upc);

        //if the first digit is zero then make the odds even and evens odd because there is one less digit at the begining
        if (firstDigit == 0) {
            if (i % 2 == 0) {
                oddDigits += digit;
            }
            else {
                evenDigits += digit;
            }
        }

        //if the first digit is not zero don't do anything and parse evens and odds normally
        else {
            if (i % 2 == 0) {
                evenDigits += digit;
            }
            else {
                oddDigits += digit;
            }
        }
    }
    //math to get check digit
    oddDigits *= 3;
    evenDigits += oddDigits;
    evenDigits %= 10;
    int digit = 10 - evenDigits;
    digit = digit == 10 ? 0 : digit;
    //return check digit
    return digit;
}

//parses int
int getDigit(int numOfPlaces, int placeIndex, long long int number) {
    //make a temp var for number as it is being parsed
    long long int temp = number;

    //make a counter for the loop to keep track of which place the loop is at
    int counter = 0;

    /*
    * For loop that loops via powers of 10 downwards starting at 10^(numOfPlaces-1)
    * Ex: 100->10->1
    * if the counter is equal to the requested digit stop the loop and return
    * loop will eventually stop when i == 1
    */
    for (long long int i = pow(10, numOfPlaces - 1); i != 1; i /= 10) {
        //get digit
        long long int digit = temp / i;

        //removes last digit
        temp %= i;

        //check if this is the required digit
        if (counter == (placeIndex - 1)) {
            return digit;
        }

        //if the loop did not return, increment counter and move onto next digit
        counter++;
    }
    return temp;
}