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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;

import java.io.Serializable;

import org.apache.commons.validator.routines.checkdigit.CheckDigitException;
import org.apache.commons.validator.routines.checkdigit.EAN13CheckDigit;
import org.apache.commons.validator.routines.checkdigit.ISSNCheckDigit;

/**
 * International Standard Serial Number (ISSN)
 * is an eight-digit serial number used to
 * uniquely identify a serial publication.
 *  * The format is:
 * 
 * ISSN dddd-dddC
 * where:
 * d = decimal digit (0-9)
 * C = checksum (0-9 or X)
 * 
 * The checksum is formed by adding the first 7 digits multiplied by
 * the position in the entire number (counting from the right).
 * 
 * For example, abcd-efg would be 8a + 7b + 6c + 5d + 4e +3f +2g.
 * The check digit is modulus 11, where the value 10 is represented by 'X'
 * For example:
 * ISSN 0317-8471
 * ISSN 1050-124X
 *
 * This class strips off the 'ISSN ' prefix if it is present before passing
 * the remainder to the checksum routine.
 * 
 * 
 * 
 * Note: the {@link #isValid(String)} and {@link #validate(String)} methods strip off any leading
 * or trailing spaces before doing the validation.
 * To ensure that only a valid code (without 'ISSN ' prefix) is passed to a method,
 * use the following code:
 * 
 * Object valid = validator.validate(input); 
 * if (valid != null) {
 *    some_method(valid.toString());
 * }
 * 
 * @since 1.5.0
 */
public class ISSNValidator implements Serializable {

    private static final long serialVersionUID = 4319515687976420405L;

    private static final String ISSN_REGEX = "(?:ISSN )?(\\d{4})-(\\d{3}[0-9X])$"; // We don't include the '-' in the code, so it is 8 chars

    private static final CodeValidator VALIDATOR = new CodeValidator(ISSN_REGEX, 8, ISSNCheckDigit.ISSN_CHECK_DIGIT);

    /** ISSN Code Validator */
    private static final ISSNValidator ISSN_VALIDATOR = new ISSNValidator();

    /**
     * Return a singleton instance of the ISSN validator
     *
     * @return A singleton instance of the ISSN validator.
     */
    public static ISSNValidator getInstance() {
        return ISSN_VALIDATOR;
    }

    /**
     * Check the code is a valid ISSN code after any transformation
     * by the validate routine.
     * @param code The code to validate.
     * @return true if a valid ISSN
     * code, otherwise false.
     */
    public boolean isValid(String code) {
        return VALIDATOR.isValid(code);
    }

    /**
     * Check the code is valid ISSN code.
     * 
     * If valid, this method returns the ISSN code with
     * the 'ISSN ' prefix removed (if it was present)
     *
     * @param code The code to validate.
     * @return A valid ISSN code if valid, otherwise null.
     */
    public Object validate(String code) {
        return VALIDATOR.validate(code);
    }

    /**
     * Convert an ISSN code to an EAN-13 code.
     * 
     * This method requires a valid ISSN code.
     * It may contain a leading 'ISSN ' prefix,
     * as the input is passed through the {@link #validate(String)}
     * method.
     *
     * @param issn The ISSN code to convert
     * @param suffix the two digit suffix, e.g. "00" 
     * @return A converted EAN-13 code or null
     * if the input ISSN code is not valid
     */
    public String convertToEAN13(String issn, String suffix) {

        if (suffix == null || !suffix.matches("\\d\\d")) {
            throw new IllegalArgumentException("Suffix must be two digits: '" + suffix + "'");            
        }

        Object result = validate(issn);
        if (result == null) {
            return null;
        }

        // Calculate the new EAN-13 code
        final String input = result.toString();
        String ean13 = "977" + input.substring(0, input.length() -1) + suffix;
        try {
            String checkDigit = EAN13CheckDigit.EAN13_CHECK_DIGIT.calculate(ean13);
            ean13 += checkDigit;
            return ean13;
        } catch (CheckDigitException e) { // Should not happen
            throw new IllegalArgumentException("Check digit error for '" + ean13 + "' - " + e.getMessage());
        }

    }
}