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 }