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: 1649191 $
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);
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 positionof the character in the code, counting from right to left
068     * @return The weighted value of the character.
069     */
070    protected int weightedValue(int charValue, int leftPos, int rightPos) {
071        int weight = POSITION_WEIGHT[rightPos % 2];
072        int weightedValue = charValue * weight;
073        return weightedValue > 9 ? (weightedValue - 9) : weightedValue;
074    }
075}