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