/*
* 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.configuration.tree;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
/**
* <p>
* A specialized implementation of the <code>NodeCombiner</code> interface
* that constructs a union from two passed in node hierarchies.
* </p>
* <p>
* The given source hierarchies are traversed and their nodes are added to the
* resulting structure. Under some circumstances two nodes can be combined
* rather than adding both. This is the case if both nodes are single children
* (no lists) of their parents and do not have values. The corresponding check
* is implemented in the <code>findCombineNode()</code> method.
* </p>
* <p>
* Sometimes it is not possible for this combiner to detect whether two nodes
* can be combined or not. Consider the following two node hierarchies:
* </p>
* <p>
*
* <pre>
* Hierarchy 1:
*
* Database
* +--Tables
* +--Table
* +--name [users]
* +--fields
* +--field
* | +--name [uid]
* +--field
* | +--name [usrname]
* ...
* </pre>
*
* </p>
* <p>
*
* <pre>
* Hierarchy 2:
*
* Database
* +--Tables
* +--Table
* +--name [documents]
* +--fields
* +--field
* | +--name [docid]
* +--field
* | +--name [docname]
* ...
* </pre>
*
* </p>
* <p>
* Both hierarchies contain data about database tables. Each describes a single
* table. If these hierarchies are to be combined, the result should probably
* look like the following:
* <p>
*
* <pre>
* Database
* +--Tables
* +--Table
* | +--name [users]
* | +--fields
* | +--field
* | | +--name [uid]
* | ...
* +--Table
* +--name [documents]
* +--fields
* +--field
* | +--name [docid]
* ...
* </pre>
*
* </p>
* <p>
* i.e. the <code>Tables</code> nodes should be combined, while the
* <code>Table</code> nodes should both be added to the resulting tree. From
* the combiner's point of view there is no difference between the
* <code>Tables</code> and the <code>Table</code> nodes in the source trees,
* so the developer has to help out and give a hint that the <code>Table</code>
* nodes belong to a list structure. This can be done using the
* <code>addListNode()</code> method; this method expects the name of a node,
* which should be treated as a list node. So if
* <code>addListNode("Table");</code> was called, the combiner knows that it
* must not combine the <code>Table</code> nodes, but add it both to the
* resulting tree.
* </p>
*
* @author <a
* href="http://commons.apache.org/configuration/team-list.html">Commons
* Configuration team</a>
* @version $Id: UnionCombiner.java 561230 2007-07-31 04:17:09Z rahul $
* @since 1.3
*/
public class UnionCombiner extends NodeCombiner
{
/**
* Combines the given nodes to a new union node.
*
* @param node1 the first source node
* @param node2 the second source node
* @return the union node
*/
public ConfigurationNode combine(ConfigurationNode node1,
ConfigurationNode node2)
{
ViewNode result = createViewNode();
result.setName(node1.getName());
result.appendAttributes(node1);
result.appendAttributes(node2);
// Check if nodes can be combined
List children2 = new LinkedList(node2.getChildren());
for (Iterator it = node1.getChildren().iterator(); it.hasNext();)
{
ConfigurationNode child1 = (ConfigurationNode) it.next();
ConfigurationNode child2 = findCombineNode(node1, node2, child1,
children2);
if (child2 != null)
{
result.addChild(combine(child1, child2));
children2.remove(child2);
}
else
{
result.addChild(child1);
}
}
// Add remaining children of node 2
for (Iterator it = children2.iterator(); it.hasNext();)
{
result.addChild((ConfigurationNode) it.next());
}
return result;
}
/**
* <p>
* Tries to find a child node of the second source node, with whitch a child
* of the first source node can be combined. During combining of the source
* nodes an iteration over the first source node's children is performed.
* For each child node it is checked whether a corresponding child node in
* the second source node exists. If this is the case, these corresponsing
* child nodes are recursively combined and the result is added to the
* combined node. This method implements the checks whether such a recursive
* combination is possible. The actual implementation tests the following
* conditions:
* </p>
* <p>
* <ul>
* <li>In both the first and the second source node there is only one child
* node with the given name (no list structures).</li>
* <li>The given name is not in the list of known list nodes, i.e. it was
* not passed to the <code>addListNode()</code> method.</li>
* <li>None of these matching child nodes has a value.</li>
* </ul>
* </p>
* <p>
* If all of these tests are successfull, the matching child node of the
* second source node is returned. Otherwise the result is <b>null</b>.
* </p>
*
* @param node1 the first source node
* @param node2 the second source node
* @param child the child node of the first source node to be checked
* @param children a list with all children of the second source node
* @return the matching child node of the second source node or <b>null</b>
* if there is none
*/
protected ConfigurationNode findCombineNode(ConfigurationNode node1,
ConfigurationNode node2, ConfigurationNode child, List children)
{
if (child.getValue() == null && !isListNode(child)
&& node1.getChildrenCount(child.getName()) == 1
&& node2.getChildrenCount(child.getName()) == 1)
{
ConfigurationNode child2 = (ConfigurationNode) node2.getChildren(
child.getName()).iterator().next();
if (child2.getValue() == null)
{
return child2;
}
}
return null;
}
}