    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.validator.routines.checkdigit;
    018
    019/**
    020 * Modulus 10 <b>Luhn</b> Check Digit calculation/validation.
    021 *
    022 * Luhn check digits are used, for example, by:
    023 * <ul>
    024 *    <li><a href="http://en.wikipedia.org/wiki/Credit_card">Credit Card Numbers</a></li>
    025 *    <li><a href="http://en.wikipedia.org/wiki/IMEI">IMEI Numbers</a> - International
    026 *        Mobile Equipment Identity Numbers</li>
    027 * </ul>
    028 * Check digit calculation is based on <i>modulus 10</i> with digits in
    029 * an <i>odd</i> position (from right to left) being weighted 1 and <i>even</i>
    030 * position digits being weighted 2 (weighted values greater than 9 have 9 subtracted).
    031 *
    032 * <p>
    033 * See <a href="http://en.wikipedia.org/wiki/Luhn_algorithm">Wikipedia</a>
    034 * for more details.
    035 * </p>
    036 *
    037 * @version $Revision$
    038 * @since Validator 1.4
    039 */
    040public final class LuhnCheckDigit extends ModulusCheckDigit {
    041
    042    private static final long serialVersionUID = -2976900113942875999L;
    043
    044    /** Singleton Luhn Check Digit instance */
    045    public static final CheckDigit LUHN_CHECK_DIGIT = new LuhnCheckDigit();
    046
    047    /** weighting given to digits depending on their right position */
    048    private static final int[] POSITION_WEIGHT = new int[] {2, 1};
    049
    050    /**
    051     * Construct a modulus 10 Luhn Check Digit routine.
    052     */
    053    public LuhnCheckDigit() {
    054        super(10); // CHECKSTYLE IGNORE MagicNumber
    055    }
    056
    057    /**
    058     * <p>Calculates the <i>weighted</i> value of a charcter in the
    059     * code at a specified position.</p>
    060     *
    061     * <p>For Luhn (from right to left) <b>odd</b> digits are weighted
    062     * with a factor of <b>one</b> and <b>even</b> digits with a factor
    063     * of <b>two</b>. Weighted values &gt; 9, have 9 subtracted</p>
    064     *
    065     * @param charValue The numeric value of the character.
    066     * @param leftPos The position of the character in the code, counting from left to right
    067     * @param rightPos The position of the character in the code, counting from right to left
    068     * @return The weighted value of the character.
    069     */
    070    @Override
    071    protected int weightedValue(int charValue, int leftPos, int rightPos) {
    072        int weight = POSITION_WEIGHT[rightPos % 2]; // CHECKSTYLE IGNORE MagicNumber
    073        int weightedValue = charValue * weight;
    074        return weightedValue > 9 ? (weightedValue - 9) : weightedValue; // CHECKSTYLE IGNORE MagicNumber
    075    }
    076}