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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 11 ISBN-10 Check Digit calculation/validation.
 * 
 * ISBN-10 Numbers are a numeric code except for the last (check) digit
 * which can have a value of "X".
 * 

 * Check digit calculation is based on modulus 11 with digits being weighted
 * based by their position, from right to left  with the first digit being weighted
 * 1, the second 2 and so on. If the check digit is calculated as "10" it is converted
 * to "X".
 * 

 * N.B. From 1st January 2007 the book industry will start to use a new 13 digit
 * ISBN number (rather than this 10 digit ISBN number) which uses the EAN-13 / UPC
 * (see {@link EAN13CheckDigit}) standard.
 * 

 * For further information see:
 * 

 *   Wikipedia - International
 *       Standard Book Number (ISBN).
 *   ISBN-13
 *       Transition details.
 * 
 *
 * @version $Revision: 1739356 $
 * @since Validator 1.4
 */
public final class ISBN10CheckDigit extends ModulusCheckDigit {

    private static final long serialVersionUID = 8000855044504864964L;

    /** Singleton ISBN-10 Check Digit instance */
    public static final CheckDigit ISBN10_CHECK_DIGIT = new ISBN10CheckDigit();

    /**
     * Construct a modulus 11 Check Digit routine for ISBN-10.
     */
    public ISBN10CheckDigit() {
        super(11);  // CHECKSTYLE IGNORE MagicNumber
    }

    /**
     * Calculates the weighted value of a charcter in the
     * code at a specified position.
     *
     * For ISBN-10 (from right to left) digits are weighted
     * by their position.
     *
     * @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) {
        return charValue * rightPos;
    }

    /**
     * Convert a character at a specified position to an
     * integer value.
     *
     * Character 'X' check digit converted to 10.
     *
     * @param character The character to convert.
     * @param leftPos The position of the character in the code, counting from left to right
     * @param rightPos The position of the character in the code, counting from right to left
     * @return The integer value of the character.
     * @throws CheckDigitException if an error occurs.
     */
    @Override
    protected int toInt(char character, int leftPos, int rightPos)
            throws CheckDigitException {
        if (rightPos == 1 && character == 'X') {
            return 10;  // CHECKSTYLE IGNORE MagicNumber
        }
        return super.toInt(character, leftPos, rightPos);
    }

    /**
     * Convert an integer value to a character at a specified position.
     *
     * Value '10' for position 1 (check digit) converted to 'X'.
     *
     * @param charValue The integer value of the character.
     * @return The converted character.
     * @throws CheckDigitException if an error occurs.
     */
    @Override
    protected String toCheckDigit(int charValue)
            throws CheckDigitException {
        if (charValue == 10) {  // CHECKSTYLE IGNORE MagicNumber
            return "X";
        }
        return super.toCheckDigit(charValue);
    }

}