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 > 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}