    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="https://en.wikipedia.org/wiki/Credit_card">Credit Card Numbers</a></li>
    025 *    <li><a href="https://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="https://en.wikipedia.org/wiki/Luhn_algorithm">Wikipedia</a>
    034 * for more details.
    035 * </p>
    036 *
    037 * @since 1.4
    038 */
    039public final class LuhnCheckDigit extends ModulusCheckDigit {
    040
    041    private static final long serialVersionUID = -2976900113942875999L;
    042
    043    /** Singleton Luhn Check Digit instance */
    044    public static final CheckDigit LUHN_CHECK_DIGIT = new LuhnCheckDigit();
    045
    046    /** Weighting given to digits depending on their right position */
    047    private static final int[] POSITION_WEIGHT = {2, 1};
    048
    049    /**
    050     * Constructs a modulus 10 Luhn Check Digit routine.
    051     */
    052    public LuhnCheckDigit() {
    053    }
    054
    055    /**
    056     * <p>Calculates the <i>weighted</i> value of a character in the
    057     * code at a specified position.</p>
    058     *
    059     * <p>For Luhn (from right to left) <b>odd</b> digits are weighted
    060     * with a factor of <b>one</b> and <b>even</b> digits with a factor
    061     * of <b>two</b>. Weighted values &gt; 9, have 9 subtracted</p>
    062     *
    063     * @param charValue The numeric value of the character.
    064     * @param leftPos The position of the character in the code, counting from left to right
    065     * @param rightPos The position of the character in the code, counting from right to left
    066     * @return The weighted value of the character.
    067     */
    068    @Override
    069    protected int weightedValue(final int charValue, final int leftPos, final int rightPos) {
    070        final int weight = POSITION_WEIGHT[rightPos % 2]; // CHECKSTYLE IGNORE MagicNumber
    071        final int weightedValue = charValue * weight;
    072        return weightedValue > 9 ? weightedValue - 9 : weightedValue; // CHECKSTYLE IGNORE MagicNumber
    073    }
    074}