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.configuration2.tree;
018
019import java.util.LinkedList;
020import java.util.List;
021
022/**
023 * <p>
024 * A class providing different algorithms for traversing a hierarchy of
025 * configuration nodes.
026 * </p>
027 * <p>
028 * The methods provided by this class accept a {@link ConfigurationNodeVisitor}
029 * and visit all nodes in a hierarchy starting from a given root node. Because a
030 * {@link NodeHandler} has to be passed in, too, arbitrary types of nodes can be
031 * processed. The {@code walk()} methods differ in the order in which nodes are
032 * visited. Details can be found in the method documentation.
033 * </p>
034 * <p>
035 * An instance of this class does not define any state; therefore, it can be
036 * shared and used concurrently. The {@code INSTANCE} member field can be used
037 * for accessing a default instance. If desired (e.g. for testing purposes), new
038 * instances can be created.
039 * </p>
040 *
041 * @since 2.0
042 */
043public class NodeTreeWalker
044{
045    /** The default instance of this class. */
046    public static final NodeTreeWalker INSTANCE = new NodeTreeWalker();
047
048    /**
049     * Visits all nodes in the hierarchy represented by the given root node in
050     * <em>depth first search</em> manner. This means that first
051     * {@link ConfigurationNodeVisitor#visitBeforeChildren(Object, NodeHandler)}
052     * is called on a node, then recursively all of its children are processed,
053     * and eventually
054     * {@link ConfigurationNodeVisitor#visitAfterChildren(Object, NodeHandler)}
055     * gets invoked.
056     *
057     * @param root the root node of the hierarchy to be processed (may be
058     *        <b>null</b>, then this call has no effect)
059     * @param visitor the {@code ConfigurationNodeVisitor} (must not be
060     *        <b>null</b>)
061     * @param handler the {@code NodeHandler} (must not be <b>null</b>)
062     * @param <T> the type of the nodes involved
063     * @throws IllegalArgumentException if a required parameter is <b>null</b>
064     */
065    public <T> void walkDFS(final T root, final ConfigurationNodeVisitor<T> visitor,
066            final NodeHandler<T> handler)
067    {
068        if (checkParameters(root, visitor, handler))
069        {
070            dfs(root, visitor, handler);
071        }
072    }
073
074    /**
075     * Visits all nodes in the hierarchy represented by the given root node in
076     * <em>breadth first search</em> manner. This means that the nodes are
077     * visited in an order corresponding to the distance from the root node:
078     * first the root node is visited, then all direct children of the root
079     * node, then all direct children of the first child of the root node, etc.
080     * In this mode of traversal, there is no direct connection between the
081     * encounter of a node and its children. <strong>Therefore, on the visitor
082     * object only the {@code visitBeforeChildren()} method gets
083     * called!</strong>.
084     *
085     * @param root the root node of the hierarchy to be processed (may be
086     *        <b>null</b>, then this call has no effect)
087     * @param visitor the {@code ConfigurationNodeVisitor} (must not be
088     *        <b>null</b>)
089     * @param handler the {@code NodeHandler} (must not be <b>null</b>)
090     * @param <T> the type of the nodes involved
091     * @throws IllegalArgumentException if a required parameter is <b>null</b>
092     */
093    public <T> void walkBFS(final T root, final ConfigurationNodeVisitor<T> visitor,
094            final NodeHandler<T> handler)
095    {
096        if (checkParameters(root, visitor, handler))
097        {
098            bfs(root, visitor, handler);
099        }
100    }
101
102    /**
103     * Recursive helper method for performing a DFS traversal.
104     *
105     * @param node the current node
106     * @param visitor the visitor
107     * @param handler the handler
108     * @param <T> the type of the nodes involved
109     */
110    private static <T> void dfs(final T node, final ConfigurationNodeVisitor<T> visitor,
111            final NodeHandler<T> handler)
112    {
113        if (!visitor.terminate())
114        {
115            visitor.visitBeforeChildren(node, handler);
116            for (final T c : handler.getChildren(node))
117            {
118                dfs(c, visitor, handler);
119            }
120            if (!visitor.terminate())
121            {
122                visitor.visitAfterChildren(node, handler);
123            }
124        }
125    }
126
127    /**
128     * Helper method for performing a BFS traversal. Implementation node: This
129     * method organizes the nodes to be visited in structures on the heap.
130     * Therefore, it can deal with larger structures than would be the case in a
131     * recursive approach (where the stack size limits the size of the
132     * structures which can be traversed).
133     *
134     * @param root the root node to be navigated
135     * @param visitor the visitor
136     * @param handler the handler
137     * @param <T> the type of the nodes involved
138     */
139    private static <T> void bfs(final T root, final ConfigurationNodeVisitor<T> visitor,
140            final NodeHandler<T> handler)
141    {
142        final List<T> pendingNodes = new LinkedList<>();
143        pendingNodes.add(root);
144        boolean cancel = false;
145
146        while (!pendingNodes.isEmpty() && !cancel)
147        {
148            final T node = pendingNodes.remove(0);
149            visitor.visitBeforeChildren(node, handler);
150            cancel = visitor.terminate();
151            for (final T c : handler.getChildren(node))
152            {
153                pendingNodes.add(c);
154            }
155        }
156    }
157
158    /**
159     * Helper method for checking the parameters for the walk() methods. If
160     * mandatory parameters are missing, an exception is thrown. The return
161     * value indicates whether an operation can be performed.
162     *
163     * @param root the root node
164     * @param visitor the visitor
165     * @param handler the handler
166     * @param <T> the type of the nodes involved
167     * @return <b>true</b> if a walk operation can be performed, <b>false</b>
168     *         otherwise
169     * @throws IllegalArgumentException if a required parameter is missing
170     */
171    private static <T> boolean checkParameters(final T root,
172            final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler)
173    {
174        if (visitor == null)
175        {
176            throw new IllegalArgumentException("Visitor must not be null!");
177        }
178        if (handler == null)
179        {
180            throw new IllegalArgumentException("NodeHandler must not be null!");
181        }
182        return root != null;
183    }
184}