001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration.tree;
018
019import java.util.LinkedList;
020import java.util.List;
021
022/**
023 * <p>
024 * A specialized implementation of the {@code NodeCombiner} interface
025 * that constructs a union from two passed in node hierarchies.
026 * </p>
027 * <p>
028 * The given source hierarchies are traversed and their nodes are added to the
029 * resulting structure. Under some circumstances two nodes can be combined
030 * rather than adding both. This is the case if both nodes are single children
031 * (no lists) of their parents and do not have values. The corresponding check
032 * is implemented in the {@code findCombineNode()} method.
033 * </p>
034 * <p>
035 * Sometimes it is not possible for this combiner to detect whether two nodes
036 * can be combined or not. Consider the following two node hierarchies:
037 * </p>
038 * <p>
039 *
040 * <pre>
041 * Hierarchy 1:
042 *
043 * Database
044 *   +--Tables
045 *        +--Table
046 *             +--name [users]
047 *             +--fields
048 *                   +--field
049 *                   |    +--name [uid]
050 *                   +--field
051 *                   |    +--name [usrname]
052 *                     ...
053 * </pre>
054 *
055 * </p>
056 * <p>
057 *
058 * <pre>
059 * Hierarchy 2:
060 *
061 * Database
062 *   +--Tables
063 *        +--Table
064 *             +--name [documents]
065 *             +--fields
066 *                   +--field
067 *                   |    +--name [docid]
068 *                   +--field
069 *                   |    +--name [docname]
070 *                     ...
071 * </pre>
072 *
073 * </p>
074 * <p>
075 * Both hierarchies contain data about database tables. Each describes a single
076 * table. If these hierarchies are to be combined, the result should probably
077 * look like the following:
078 * <p>
079 *
080 * <pre>
081 * Database
082 *   +--Tables
083 *        +--Table
084 *        |    +--name [users]
085 *        |    +--fields
086 *        |          +--field
087 *        |          |    +--name [uid]
088 *        |            ...
089 *        +--Table
090 *             +--name [documents]
091 *             +--fields
092 *                   +--field
093 *                   |    +--name [docid]
094 *                     ...
095 * </pre>
096 *
097 * </p>
098 * <p>
099 * i.e. the {@code Tables} nodes should be combined, while the
100 * {@code Table} nodes should both be added to the resulting tree. From
101 * the combiner's point of view there is no difference between the
102 * {@code Tables} and the {@code Table} nodes in the source trees,
103 * so the developer has to help out and give a hint that the {@code Table}
104 * nodes belong to a list structure. This can be done using the
105 * {@code addListNode()} method; this method expects the name of a node,
106 * which should be treated as a list node. So if
107 * {@code addListNode("Table");} was called, the combiner knows that it
108 * must not combine the {@code Table} nodes, but add it both to the
109 * resulting tree.
110 * </p>
111 *
112 * @author <a
113 * href="http://commons.apache.org/configuration/team-list.html">Commons
114 * Configuration team</a>
115 * @version $Id: UnionCombiner.java 1206486 2011-11-26 16:41:12Z oheger $
116 * @since 1.3
117 */
118public class UnionCombiner extends NodeCombiner
119{
120    /**
121     * Combines the given nodes to a new union node.
122     *
123     * @param node1 the first source node
124     * @param node2 the second source node
125     * @return the union node
126     */
127    @Override
128    public ConfigurationNode combine(ConfigurationNode node1,
129            ConfigurationNode node2)
130    {
131        ViewNode result = createViewNode();
132        result.setName(node1.getName());
133        result.appendAttributes(node1);
134        result.appendAttributes(node2);
135
136        // Check if nodes can be combined
137        List<ConfigurationNode> children2 = new LinkedList<ConfigurationNode>(node2.getChildren());
138        for (ConfigurationNode child1 : node1.getChildren())
139        {
140            ConfigurationNode child2 = findCombineNode(node1, node2, child1,
141                    children2);
142            if (child2 != null)
143            {
144                result.addChild(combine(child1, child2));
145                children2.remove(child2);
146            }
147            else
148            {
149                result.addChild(child1);
150            }
151        }
152
153        // Add remaining children of node 2
154        for (ConfigurationNode c : children2)
155        {
156            result.addChild(c);
157        }
158
159        return result;
160    }
161
162    /**
163     * <p>
164     * Tries to find a child node of the second source node, with which a child
165     * of the first source node can be combined. During combining of the source
166     * nodes an iteration over the first source node's children is performed.
167     * For each child node it is checked whether a corresponding child node in
168     * the second source node exists. If this is the case, these corresponding
169     * child nodes are recursively combined and the result is added to the
170     * combined node. This method implements the checks whether such a recursive
171     * combination is possible. The actual implementation tests the following
172     * conditions:
173     * </p>
174     * <p>
175     * <ul>
176     * <li>In both the first and the second source node there is only one child
177     * node with the given name (no list structures).</li>
178     * <li>The given name is not in the list of known list nodes, i.e. it was
179     * not passed to the {@code addListNode()} method.</li>
180     * <li>None of these matching child nodes has a value.</li>
181     * </ul>
182     * </p>
183     * <p>
184     * If all of these tests are successful, the matching child node of the
185     * second source node is returned. Otherwise the result is <b>null</b>.
186     * </p>
187     *
188     * @param node1 the first source node
189     * @param node2 the second source node
190     * @param child the child node of the first source node to be checked
191     * @param children a list with all children of the second source node
192     * @return the matching child node of the second source node or <b>null</b>
193     * if there is none
194     */
195    protected ConfigurationNode findCombineNode(ConfigurationNode node1,
196            ConfigurationNode node2, ConfigurationNode child, List<ConfigurationNode> children)
197    {
198        if (child.getValue() == null && !isListNode(child)
199                && node1.getChildrenCount(child.getName()) == 1
200                && node2.getChildrenCount(child.getName()) == 1)
201        {
202            ConfigurationNode child2 = node2.getChildren(
203                    child.getName()).iterator().next();
204            if (child2.getValue() == null)
205            {
206                return child2;
207            }
208        }
209        return null;
210    }
211}