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 *      https://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
018package org.apache.commons.validator.routines.checkdigit;
019
020/**
021 * Modulus 10 <strong>Luhn</strong> Check Digit calculation/validation.
022 *
023 * <p>
024 * Luhn check digits are used, for example, by:
025 * </p>
026 * <ul>
027 * <li><a href="https://en.wikipedia.org/wiki/Credit_card">Credit Card Numbers</a></li>
028 * <li><a href="https://en.wikipedia.org/wiki/IMEI">IMEI Numbers</a> - International Mobile Equipment Identity Numbers</li>
029 * </ul>
030 * <p>
031 * Check digit calculation is based on <em>modulus 10</em> with digits in an <em>odd</em> position (from right to left) being weighted 1 and <em>even</em>
032 * position digits being weighted 2 (weighted values greater than 9 have 9 subtracted).
033 * </p>
034 * <p>
035 * See <a href="https://en.wikipedia.org/wiki/Luhn_algorithm">Wikipedia</a> for more details.
036 * </p>
037 *
038 * @since 1.4
039 */
040public final class LuhnCheckDigit extends ModulusCheckDigit {
041
042    private static final long serialVersionUID = -2976900113942875999L;
043
044    /**
045     * Singleton Luhn Check Digit instance.
046     */
047    public static final CheckDigit LUHN_CHECK_DIGIT = new LuhnCheckDigit();
048
049    /** Weighting given to digits depending on their right position */
050    private static final int[] POSITION_WEIGHT = { 2, 1 };
051
052    /**
053     * Constructs a modulus 10 Luhn Check Digit routine.
054     */
055    public LuhnCheckDigit() {
056    }
057
058    /**
059     * Calculates the <em>weighted</em> value of a character in the code at a specified position.
060     * <p>
061     * For Luhn (from right to left) <strong>odd</strong> digits are weighted with a factor of <strong>one</strong> and <strong>even</strong> digits with a
062     * factor of <strong>two</strong>. Weighted values &gt; 9, have 9 subtracted
063     * </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(final int charValue, final int leftPos, final int rightPos) {
072        final int weight = POSITION_WEIGHT[rightPos % 2]; // CHECKSTYLE IGNORE MagicNumber
073        final int weightedValue = charValue * weight;
074        return weightedValue > 9 ? weightedValue - 9 : weightedValue; // CHECKSTYLE IGNORE MagicNumber
075    }
076}