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Apache XML Security for Java supports XML-Signature Syntax and Processing, W3C Recommendation 12 February 2002, and XML Encryption Syntax and Processing, W3C Recommendation 10 December 2002. As of version 1.4, the library supports the standard Java API JSR-105: XML Digital Signature APIs.

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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.xml.security.c14n.helper;

import org.apache.xml.security.utils.Constants;
import org.w3c.dom.Attr;
import java.io.Serializable;
import java.util.Comparator;

/**
 * Compares two attributes based on the C14n specification.
 *
 * 
 * Namespace nodes have a lesser document order position than attribute 
 *   nodes.
 * 
 An element's namespace nodes are sorted lexicographically by
 *   local name (the default namespace node, if one exists, has no
 *   local name and is therefore lexicographically least).
 * 
 An element's attribute nodes are sorted lexicographically with
 *   namespace URI as the primary key and local name as the secondary
 *   key (an empty namespace URI is lexicographically least).
 * 
 *
 * @author Christian Geuer-Pollmann
 */
public class AttrCompare implements Comparator, Serializable {

    private static final long serialVersionUID = -7113259629930576230L;
    private static final int ATTR0_BEFORE_ATTR1 = -1;
    private static final int ATTR1_BEFORE_ATTR0 = 1;
    private static final String XMLNS = Constants.NamespaceSpecNS;

    /**
     * Compares two attributes based on the C14n specification.
     *
     * 
     * Namespace nodes have a lesser document order position than 
     *   attribute nodes.
     * 
 An element's namespace nodes are sorted lexicographically by
     *   local name (the default namespace node, if one exists, has no
     *   local name and is therefore lexicographically least).
     * 
 An element's attribute nodes are sorted lexicographically with
     *   namespace URI as the primary key and local name as the secondary
     *   key (an empty namespace URI is lexicographically least).
     * 
     *
     * @param attr0
     * @param attr1
     * @return returns a negative integer, zero, or a positive integer as 
     *   obj0 is less than, equal to, or greater than obj1
     *
     */
    public int compare(Attr attr0, Attr attr1) {
        String namespaceURI0 = attr0.getNamespaceURI();      
        String namespaceURI1 = attr1.getNamespaceURI();
      
        boolean isNamespaceAttr0 = XMLNS.equals(namespaceURI0);
        boolean isNamespaceAttr1 = XMLNS.equals(namespaceURI1);

        if (isNamespaceAttr0) {
            if (isNamespaceAttr1) {
                // both are namespaces
                String localname0 = attr0.getLocalName();
                String localname1 = attr1.getLocalName();

                if ("xmlns".equals(localname0)) {
                    localname0 = "";
                }

                if ("xmlns".equals(localname1)) {
                    localname1 = "";
                }

                return localname0.compareTo(localname1);
            }
            // attr0 is a namespace, attr1 is not
            return ATTR0_BEFORE_ATTR1;
        } else if (isNamespaceAttr1) {
            // attr1 is a namespace, attr0 is not
            return ATTR1_BEFORE_ATTR0;
        } 

        // none is a namespace
        if (namespaceURI0 == null) {
            if (namespaceURI1 == null) {
                String name0 = attr0.getName();
                String name1 = attr1.getName();
                return name0.compareTo(name1);
            }
            return ATTR0_BEFORE_ATTR1;
        } else if (namespaceURI1 == null) {
            return ATTR1_BEFORE_ATTR0;
        } 

        int a = namespaceURI0.compareTo(namespaceURI1);
        if (a != 0) {
            return a;
        }
      
        return attr0.getLocalName().compareTo(attr1.getLocalName());
    }
}