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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="http://en.wikipedia.org/wiki/Credit_card">Credit Card Numbers</a></li>
25   *    <li><a href="http://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="http://en.wikipedia.org/wiki/Luhn_algorithm">Wikipedia</a>
34   * for more details.
35   * </p>
36   *
37   * @version $Revision: 1649191 $
38   * @since Validator 1.4
39   */
40  public final class LuhnCheckDigit extends ModulusCheckDigit {
41  
42      private static final long serialVersionUID = -2976900113942875999L;
43  
44      /** Singleton Luhn Check Digit instance */
45      public static final CheckDigit LUHN_CHECK_DIGIT = new LuhnCheckDigit();
46  
47      /** weighting given to digits depending on their right position */
48      private static final int[] POSITION_WEIGHT = new int[] {2, 1};
49  
50      /**
51       * Construct a modulus 10 Luhn Check Digit routine.
52       */
53      public LuhnCheckDigit() {
54          super(10);
55      }
56  
57      /**
58       * <p>Calculates the <i>weighted</i> value of a charcter in the
59       * code at a specified position.</p>
60       *
61       * <p>For Luhn (from right to left) <b>odd</b> digits are weighted
62       * with a factor of <b>one</b> and <b>even</b> digits with a factor
63       * of <b>two</b>. Weighted values &gt; 9, have 9 subtracted</p>
64       *
65       * @param charValue The numeric value of the character.
66       * @param leftPos The position of the character in the code, counting from left to right
67       * @param rightPos The positionof the character in the code, counting from right to left
68       * @return The weighted value of the character.
69       */
70      protected int weightedValue(int charValue, int leftPos, int rightPos) {
71          int weight = POSITION_WEIGHT[rightPos % 2];
72          int weightedValue = charValue * weight;
73          return weightedValue > 9 ? (weightedValue - 9) : weightedValue;
74      }
75  }