001package org.xbib.standardnumber.check;
002
003/**
004 * Verhoeff's algorithm for createChecksum computation
005 *
006 * J. Verhoeff, Error Detecting Decimal Codes, Mathematical Centre Tract 29, The Mathematical Centre, Amsterdam, 1969
007 */
008public class DihedralGroup implements Digit {
009
010    /**
011     * dihedral addition matrix A + B = a[A][B]
012     */
013    private static final int a[][] = { { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 },
014            { 1, 2, 3, 4, 0, 6, 7, 8, 9, 5 }, { 2, 3, 4, 0, 1, 7, 8, 9, 5, 6 },
015            { 3, 4, 0, 1, 2, 8, 9, 5, 6, 7 }, { 4, 0, 1, 2, 3, 9, 5, 6, 7, 8 },
016            { 5, 9, 8, 7, 6, 0, 4, 3, 2, 1 }, { 6, 5, 9, 8, 7, 1, 0, 4, 3, 2 },
017            { 7, 6, 5, 9, 8, 2, 1, 0, 4, 3 }, { 8, 7, 6, 5, 9, 3, 2, 1, 0, 4 },
018            { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 } };
019
020    /**
021     *  dihedral inverse map, A + inverse[A] = 0
022     */
023    private static final int inverse[] = { 0, 4, 3, 2, 1, 5, 6, 7, 8, 9 };
024
025    /**
026     * permutation weighting matrix P[position][value]
027     */
028    private static final int p[][] = { { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 },
029            { 1, 5, 7, 6, 2, 8, 3, 0, 9, 4 }, { 5, 8, 0, 3, 7, 9, 6, 1, 4, 2 },
030            { 8, 9, 1, 6, 0, 4, 3, 5, 2, 7 }, { 9, 4, 5, 3, 1, 2, 6, 8, 7, 0 },
031            { 4, 2, 8, 6, 5, 7, 3, 9, 0, 1 }, { 2, 7, 9, 3, 8, 0, 6, 4, 1, 5 },
032            { 7, 0, 4, 6, 9, 1, 3, 2, 5, 8 } };
033
034
035    @Override
036    public String encode(String digits) {
037        return Integer.toString(compute(digits)) + digits;
038    }
039
040    @Override
041    public boolean verify(String digits){
042        int check = 0;
043        for (int i = 0; i < digits.length(); ++i) {
044            check = a[check][p[i % 8][digits.charAt(i) - '0']];
045        }
046        return check == 0;
047    }
048
049    @Override
050    public int compute(String digits) {
051        int check = 0;
052        for (int i = 0; i < digits.length(); ++i) {
053            int c = digits.charAt(i) - '0';
054            check = a[check][p[(i + 1) % 8][c]];
055        }
056        return inverse[check];
057    }
058
059    @Override
060    public int getDigit(String digits) {
061        return Integer.parseInt(digits.substring(0, 1));
062    }
063
064    @Override
065    public String getNumber(String digits) {
066        return digits.substring(1);
067    }
068}