//algorytm Luhn'a
//www.algorytm.org
//(c)2007 by Tomasz Lubinski

public class Luhn {
	
	private static final byte DoubleArray[] = {0, 2, 4, 6, 8, 1, 3, 5, 7, 9};

	/**
	 * Check number
	 * @param number
	 * @return check digit
	 */
	public static int luhnCheckDigit(String number) {
		int sum = 0;
		for (int i=2; i<=number.length(); i++) {
			if (i%2 != 0) {
				sum += Integer.parseInt(number.charAt(number.length() - i) + "");
			} else {
				sum += DoubleArray[Integer.parseInt(number.charAt(number.length() - i) + "")];
			}
		}
		sum %= 10;
		sum = 10 - sum;
		sum %= 10;

		return sum;
	}
	
	/**
	 * Check number - Luhn algorithm
	 * @param number
	 * @return true if check digit in number is correct, false otherwise
	 */
	public static boolean luhnCheck(String number) {
		int sum = 0;
		for (int i=1; i<=number.length(); i++) {
			if (i%2 != 0) {
				sum += Integer.parseInt(number.charAt(number.length() - i) + "");
			} else {
				sum += DoubleArray[Integer.parseInt(number.charAt(number.length() - i) + "")];
			}
		}
		
		if (sum % 10 == 0) {
			return true;
		}
		return false;
	}	

	/**
	 * Get number and check it
	 * 
	 * @param args
	 */
	public static void main(String[] args) {
		String number;
		
		System.out.println("Podaj numer do sprawdzenia");
		number = Console.readString();
		
		if (luhnCheck(number)) {
			System.out.println("Numer jest prawidłowy");
		} else {
			System.out.println("Numer jest nieprawidłowy");
		}
	}

}