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 */
017 package org.apache.commons.validator.routines.checkdigit;
018
019 import java.io.Serializable;
020
021
022 /**
023 * Modulus 10 <b>Luhn</b> Check Digit calculation/validation.
024 * <p>
025 * Luhn check digits are used, for example, by:
026 * <ul>
027 * <li><a href="http://en.wikipedia.org/wiki/Credit_card">Credit Card Numbers</a></li>
028 * <li><a href="http://en.wikipedia.org/wiki/IMEI">IMEI Numbers</a> - International
029 * Mobile Equipment Identity Numbers</li>
030 * </ul>
031 * Check digit calculation is based on <i>modulus 10</i> with digits in
032 * an <i>odd</i> position (from right to left) being weighted 1 and <i>even</i>
033 * position digits being weighted 2 (weighted values greater than 9 have 9 subtracted).
034 * <p>
035 * See <a href="http://en.wikipedia.org/wiki/Luhn_algorithm">Wikipedia</a>
036 * for more details.
037 *
038 * @version $Revision: 493905 $ $Date: 2007-01-08 03:11:38 +0100 (Mo, 08. Jan 2007) $
039 * @since Validator 1.4
040 */
041 public final class LuhnCheckDigit extends ModulusCheckDigit implements Serializable {
042
043 /** Singleton Luhn Check Digit instance */
044 public static final CheckDigit INSTANCE = new LuhnCheckDigit();
045
046 /** weighting given to digits depending on their right position */
047 private static final int[] POSITION_WEIGHT = new int[] {2, 1};
048
049 /**
050 * Construct a modulus 10 Luhn Check Digit routine.
051 */
052 public LuhnCheckDigit() {
053 super(10);
054 }
055
056 /**
057 * <p>Calculates the <i>weighted</i> value of a charcter in the
058 * code at a specified position.</p>
059 *
060 * <p>For Luhn (from right to left) <b>odd</b> digits are weighted
061 * with a factor of <b>one</b> and <b>even</b> digits with a factor
062 * of <b>two</b>. Weighted values > 9, have 9 subtracted</p>
063 *
064 * @param charValue The numeric value of the character.
065 * @param leftPos The position of the character in the code, counting from left to right
066 * @param rightPos The positionof the character in the code, counting from right to left
067 * @return The weighted value of the character.
068 */
069 protected int weightedValue(int charValue, int leftPos, int rightPos) {
070 int weight = POSITION_WEIGHT[rightPos % 2];
071 int weightedValue = (charValue * weight);
072 return (weightedValue > 9 ? (weightedValue - 9) : weightedValue);
073 }
074 }