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Apache Commons Validator provides the building blocks for both client side validation and server side data validation. It may be used standalone or with a framework like Struts.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.validator.routines.checkdigit;

/**
 * Modulus 10 Luhn Check Digit calculation/validation.
 *
 * Luhn check digits are used, for example, by:
 * 
 *    Credit Card Numbers
 *    IMEI Numbers - International
 *        Mobile Equipment Identity Numbers
 * 
 * Check digit calculation is based on modulus 10 with digits in
 * an odd position (from right to left) being weighted 1 and even
 * position digits being weighted 2 (weighted values greater than 9 have 9 subtracted).
 *
 * 
 * See Wikipedia
 * for more details.
 * 
 *
 * @version $Revision: 1739356 $
 * @since Validator 1.4
 */
public final class LuhnCheckDigit extends ModulusCheckDigit {

    private static final long serialVersionUID = -2976900113942875999L;

    /** Singleton Luhn Check Digit instance */
    public static final CheckDigit LUHN_CHECK_DIGIT = new LuhnCheckDigit();

    /** weighting given to digits depending on their right position */
    private static final int[] POSITION_WEIGHT = new int[] {2, 1};

    /**
     * Construct a modulus 10 Luhn Check Digit routine.
     */
    public LuhnCheckDigit() {
        super(10); // CHECKSTYLE IGNORE MagicNumber
    }

    /**
     * Calculates the weighted value of a charcter in the
     * code at a specified position.
     *
     * For Luhn (from right to left) odd digits are weighted
     * with a factor of one and even digits with a factor
     * of two. Weighted values > 9, have 9 subtracted
     *
     * @param charValue The numeric value of the character.
     * @param leftPos The position of the character in the code, counting from left to right
     * @param rightPos The positionof the character in the code, counting from right to left
     * @return The weighted value of the character.
     */
    @Override
    protected int weightedValue(int charValue, int leftPos, int rightPos) {
        int weight = POSITION_WEIGHT[rightPos % 2]; // CHECKSTYLE IGNORE MagicNumber
        int weightedValue = charValue * weight;
        return weightedValue > 9 ? (weightedValue - 9) : weightedValue; // CHECKSTYLE IGNORE MagicNumber
    }
}