1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.validator.routines.checkdigit;
18
19 /**
20 * Modulus 10 <b>Luhn</b> Check Digit calculation/validation.
21 *
22 * Luhn check digits are used, for example, by:
23 * <ul>
24 * <li><a href="https://en.wikipedia.org/wiki/Credit_card">Credit Card Numbers</a></li>
25 * <li><a href="https://en.wikipedia.org/wiki/IMEI">IMEI Numbers</a> - International
26 * Mobile Equipment Identity Numbers</li>
27 * </ul>
28 * Check digit calculation is based on <i>modulus 10</i> with digits in
29 * an <i>odd</i> position (from right to left) being weighted 1 and <i>even</i>
30 * position digits being weighted 2 (weighted values greater than 9 have 9 subtracted).
31 *
32 * <p>
33 * See <a href="https://en.wikipedia.org/wiki/Luhn_algorithm">Wikipedia</a>
34 * for more details.
35 * </p>
36 *
37 * @since 1.4
38 */
39 public final class LuhnCheckDigit extends ModulusCheckDigit {
40
41 private static final long serialVersionUID = -2976900113942875999L;
42
43 /** Singleton Luhn Check Digit instance */
44 public static final CheckDigit LUHN_CHECK_DIGIT = new LuhnCheckDigit();
45
46 /** Weighting given to digits depending on their right position */
47 private static final int[] POSITION_WEIGHT = {2, 1};
48
49 /**
50 * Constructs a modulus 10 Luhn Check Digit routine.
51 */
52 public LuhnCheckDigit() {
53 }
54
55 /**
56 * <p>Calculates the <i>weighted</i> value of a character in the
57 * code at a specified position.</p>
58 *
59 * <p>For Luhn (from right to left) <b>odd</b> digits are weighted
60 * with a factor of <b>one</b> and <b>even</b> digits with a factor
61 * of <b>two</b>. Weighted values > 9, have 9 subtracted</p>
62 *
63 * @param charValue The numeric value of the character.
64 * @param leftPos The position of the character in the code, counting from left to right
65 * @param rightPos The position of the character in the code, counting from right to left
66 * @return The weighted value of the character.
67 */
68 @Override
69 protected int weightedValue(final int charValue, final int leftPos, final int rightPos) {
70 final int weight = POSITION_WEIGHT[rightPos % 2]; // CHECKSTYLE IGNORE MagicNumber
71 final int weightedValue = charValue * weight;
72 return weightedValue > 9 ? weightedValue - 9 : weightedValue; // CHECKSTYLE IGNORE MagicNumber
73 }
74 }