/*
This program takes 11 digits of a UPC(Universal Product Code) number and generates the 12th.
A UPC number is the number under the barcode on products in some countries,
not all countries use this number.
*/

public class UPC {
    public static void main(String[] args){
        System.out.println(findValidUPC("1234567"));
    }
    private static int[] turnIntoValidUPC(String UPCString){
        int expectedUPCLength = 11;
        int[] UPCArray = new int[expectedUPCLength];
        int UPCLength = UPCString.length();

        //If the UPC is shorter than the expected 11 digits, put zeroes in front of the UPC until
        //it is 11 digits long
        int counter = 0;
        while(UPCLength < expectedUPCLength){
            UPCArray[counter] = 0;
            UPCLength++;
            counter++;
        }

        //After putting the zeroes in front, copy the rest of the UPC into the array
        int stringCounter = 0;
        while(counter != expectedUPCLength){
            int currentNumber = Integer.parseInt(UPCString.charAt(stringCounter) + "");
            UPCArray[counter] = currentNumber;
            stringCounter++;
            counter++;
        }
        return UPCArray;
    }
    private static int findValidUPC(String UPCString){
        int[] UPCArray = turnIntoValidUPC(UPCString);
        int sumOfOddPlaces = 0;
        int sumOfEvenPlaces = 0;

        //Loop through the UPC array and add the even and odd spots together
        for(int counter = 0; counter < 11; counter++){
            if(counter % 2 == 0){
                sumOfOddPlaces += UPCArray[counter];
            }
            else{
                sumOfEvenPlaces += UPCArray[counter];
            }
        }
        //Math to calculate the last digit
        int lastUPCDigit = sumOfOddPlaces * 3 + sumOfEvenPlaces;
        lastUPCDigit = lastUPCDigit % 10;
        if(lastUPCDigit == 0){
            return 0;
        }
        return 10 - lastUPCDigit;
    }
}