package net.sourceforge.fenixedu.domain.util;

/**
 * @see <a href="http://en.wikipedia.org/wiki/Verhoeff_algorithm/">More Info</a>
 * @see <a href="http://en.wikipedia.org/wiki/Dihedral_group">Dihedral Group</a>
 * @see <a href="http://mathworld.wolfram.com/DihedralGroupD5.html">Dihedral
 *      Group Order 10</a>
 * @author Colm Rice
 */
public class Verhoeff {

    // The multiplication table
    static int[][] d = new int[][] { { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }, { 1, 2, 3, 4, 0, 6, 7, 8, 9, 5 },
	{ 2, 3, 4, 0, 1, 7, 8, 9, 5, 6 }, { 3, 4, 0, 1, 2, 8, 9, 5, 6, 7 }, { 4, 0, 1, 2, 3, 9, 5, 6, 7, 8 },
	{ 5, 9, 8, 7, 6, 0, 4, 3, 2, 1 }, { 6, 5, 9, 8, 7, 1, 0, 4, 3, 2 }, { 7, 6, 5, 9, 8, 2, 1, 0, 4, 3 },
	{ 8, 7, 6, 5, 9, 3, 2, 1, 0, 4 }, { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 } };

    // The permutation table
    static int[][] p = new int[][] { { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }, { 1, 5, 7, 6, 2, 8, 3, 0, 9, 4 },
	{ 5, 8, 0, 3, 7, 9, 6, 1, 4, 2 }, { 8, 9, 1, 6, 0, 4, 3, 5, 2, 7 }, { 9, 4, 5, 3, 1, 2, 6, 8, 7, 0 },
	{ 4, 2, 8, 6, 5, 7, 3, 9, 0, 1 }, { 2, 7, 9, 3, 8, 0, 6, 4, 1, 5 }, { 7, 0, 4, 6, 9, 1, 3, 2, 5, 8 } };

    // The inverse table
    static int[] inv = { 0, 4, 3, 2, 1, 5, 6, 7, 8, 9 };

    /*
     * For a given number generates a Verhoeff digit
     */
    public static String generateVerhoeff(String num) {

	int c = 0;
	int[] myArray = StringToReversedIntArray(num);

	for (int i = 0; i < myArray.length; i++) {
	    c = d[c][p[((i + 1) % 8)][myArray[i]]];
	}

	return Integer.toString(inv[c]);
    }

    /*
     * Validates that an entered number is Verhoeff compliant. NB: Make sure the
     * check digit is the last one.
     */
    public static boolean validateVerhoeff(String num) {

	int c = 0;
	int[] myArray = StringToReversedIntArray(num);

	for (int i = 0; i < myArray.length; i++) {
	    c = d[c][p[(i % 8)][myArray[i]]];
	}

	return (c == 0);
    }

    /*
     * Converts a string to a reversed integer array.
     */
    private static int[] StringToReversedIntArray(String num) {

	int[] myArray = new int[num.length()];

	for (int i = 0; i < num.length(); i++) {
	    myArray[i] = Integer.parseInt(num.substring(i, i + 1));
	}

	myArray = Reverse(myArray);

	return myArray;

    }

    /*
     * Reverses an int array
     */
    private static int[] Reverse(int[] myArray) {
	int[] reversed = new int[myArray.length];

	for (int i = 0; i < myArray.length; i++) {
	    reversed[i] = myArray[myArray.length - (i + 1)];
	}

	return reversed;
    }

}