001/*
002 * Copyright (C) 2012 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020
021import com.google.common.annotations.Beta;
022import com.google.common.annotations.GwtCompatible;
023import com.google.common.base.Function;
024import java.util.ArrayDeque;
025import java.util.Deque;
026import java.util.Iterator;
027import java.util.Queue;
028import javax.annotation.CheckForNull;
029
030/**
031 * Views elements of a type {@code T} as nodes in a tree, and provides methods to traverse the trees
032 * induced by this traverser.
033 *
034 * <p>For example, the tree
035 *
036 * <pre>{@code
037 *        h
038 *      / | \
039 *     /  e  \
040 *    d       g
041 *   /|\      |
042 *  / | \     f
043 * a  b  c
044 * }</pre>
045 *
046 * <p>can be iterated over in preorder (hdabcegf), postorder (abcdefgh), or breadth-first order
047 * (hdegabcf).
048 *
049 * <p>Null nodes are strictly forbidden.
050 *
051 * <p>Because this is an abstract class, not an interface, you can't use a lambda expression to
052 * implement it:
053 *
054 * <pre>{@code
055 * // won't work
056 * TreeTraverser<NodeType> traverser = node -> node.getChildNodes();
057 * }</pre>
058 *
059 * Instead, you can pass a lambda expression to the {@code using} factory method:
060 *
061 * <pre>{@code
062 * TreeTraverser<NodeType> traverser = TreeTraverser.using(node -> node.getChildNodes());
063 * }</pre>
064 *
065 * @author Louis Wasserman
066 * @since 15.0
067 * @deprecated Use {@link com.google.common.graph.Traverser} instead. All instance methods have
068 *     their equivalent on the result of {@code Traverser.forTree(tree)} where {@code tree}
069 *     implements {@code SuccessorsFunction}, which has a similar API as {@link #children} or can be
070 *     the same lambda function as passed into {@link #using(Function)}.
071 *     <p>This class is scheduled to be removed in October 2019.
072 */
073// TODO(b/68134636): Remove by 2019-10
074@Deprecated
075@Beta
076@GwtCompatible
077@ElementTypesAreNonnullByDefault
078public abstract class TreeTraverser<T> {
079
080  /**
081   * Returns a tree traverser that uses the given function to navigate from a node to its children.
082   * This is useful if the function instance already exists, or so that you can supply a lambda
083   * expressions. If those circumstances don't apply, you probably don't need to use this; subclass
084   * {@code TreeTraverser} and implement its {@link #children} method directly.
085   *
086   * @since 20.0
087   * @deprecated Use {@link com.google.common.graph.Traverser#forTree} instead. If you are using a
088   *     lambda, these methods have exactly the same signature.
089   */
090  @Deprecated
091  public static <T> TreeTraverser<T> using(
092      final Function<T, ? extends Iterable<T>> nodeToChildrenFunction) {
093    checkNotNull(nodeToChildrenFunction);
094    return new TreeTraverser<T>() {
095      @Override
096      public Iterable<T> children(T root) {
097        return nodeToChildrenFunction.apply(root);
098      }
099    };
100  }
101
102  /** Returns the children of the specified node. Must not contain null. */
103  public abstract Iterable<T> children(T root);
104
105  /**
106   * Returns an unmodifiable iterable over the nodes in a tree structure, using pre-order traversal.
107   * That is, each node's subtrees are traversed after the node itself is returned.
108   *
109   * <p>No guarantees are made about the behavior of the traversal when nodes change while iteration
110   * is in progress or when the iterators generated by {@link #children} are advanced.
111   *
112   * @deprecated Use {@link com.google.common.graph.Traverser#depthFirstPreOrder} instead, which has
113   *     the same behavior.
114   */
115  @Deprecated
116  public final FluentIterable<T> preOrderTraversal(final T root) {
117    checkNotNull(root);
118    return new FluentIterable<T>() {
119      @Override
120      public UnmodifiableIterator<T> iterator() {
121        return preOrderIterator(root);
122      }
123    };
124  }
125
126  UnmodifiableIterator<T> preOrderIterator(T root) {
127    return new PreOrderIterator(root);
128  }
129
130  private final class PreOrderIterator extends UnmodifiableIterator<T> {
131    private final Deque<Iterator<T>> stack;
132
133    PreOrderIterator(T root) {
134      this.stack = new ArrayDeque<>();
135      stack.addLast(Iterators.singletonIterator(checkNotNull(root)));
136    }
137
138    @Override
139    public boolean hasNext() {
140      return !stack.isEmpty();
141    }
142
143    @Override
144    public T next() {
145      Iterator<T> itr = stack.getLast(); // throws NSEE if empty
146      T result = checkNotNull(itr.next());
147      if (!itr.hasNext()) {
148        stack.removeLast();
149      }
150      Iterator<T> childItr = children(result).iterator();
151      if (childItr.hasNext()) {
152        stack.addLast(childItr);
153      }
154      return result;
155    }
156  }
157
158  /**
159   * Returns an unmodifiable iterable over the nodes in a tree structure, using post-order
160   * traversal. That is, each node's subtrees are traversed before the node itself is returned.
161   *
162   * <p>No guarantees are made about the behavior of the traversal when nodes change while iteration
163   * is in progress or when the iterators generated by {@link #children} are advanced.
164   *
165   * @deprecated Use {@link com.google.common.graph.Traverser#depthFirstPostOrder} instead, which
166   *     has the same behavior.
167   */
168  @Deprecated
169  public final FluentIterable<T> postOrderTraversal(final T root) {
170    checkNotNull(root);
171    return new FluentIterable<T>() {
172      @Override
173      public UnmodifiableIterator<T> iterator() {
174        return postOrderIterator(root);
175      }
176    };
177  }
178
179  UnmodifiableIterator<T> postOrderIterator(T root) {
180    return new PostOrderIterator(root);
181  }
182
183  private static final class PostOrderNode<T> {
184    final T root;
185    final Iterator<T> childIterator;
186
187    PostOrderNode(T root, Iterator<T> childIterator) {
188      this.root = checkNotNull(root);
189      this.childIterator = checkNotNull(childIterator);
190    }
191  }
192
193  private final class PostOrderIterator extends AbstractIterator<T> {
194    private final ArrayDeque<PostOrderNode<T>> stack;
195
196    PostOrderIterator(T root) {
197      this.stack = new ArrayDeque<>();
198      stack.addLast(expand(root));
199    }
200
201    @Override
202    @CheckForNull
203    protected T computeNext() {
204      while (!stack.isEmpty()) {
205        PostOrderNode<T> top = stack.getLast();
206        if (top.childIterator.hasNext()) {
207          T child = top.childIterator.next();
208          stack.addLast(expand(child));
209        } else {
210          stack.removeLast();
211          return top.root;
212        }
213      }
214      return endOfData();
215    }
216
217    private PostOrderNode<T> expand(T t) {
218      return new PostOrderNode<>(t, children(t).iterator());
219    }
220  }
221
222  /**
223   * Returns an unmodifiable iterable over the nodes in a tree structure, using breadth-first
224   * traversal. That is, all the nodes of depth 0 are returned, then depth 1, then 2, and so on.
225   *
226   * <p>No guarantees are made about the behavior of the traversal when nodes change while iteration
227   * is in progress or when the iterators generated by {@link #children} are advanced.
228   *
229   * @deprecated Use {@link com.google.common.graph.Traverser#breadthFirst} instead, which has the
230   *     same behavior.
231   */
232  @Deprecated
233  public final FluentIterable<T> breadthFirstTraversal(final T root) {
234    checkNotNull(root);
235    return new FluentIterable<T>() {
236      @Override
237      public UnmodifiableIterator<T> iterator() {
238        return new BreadthFirstIterator(root);
239      }
240    };
241  }
242
243  private final class BreadthFirstIterator extends UnmodifiableIterator<T>
244      implements PeekingIterator<T> {
245    private final Queue<T> queue;
246
247    BreadthFirstIterator(T root) {
248      this.queue = new ArrayDeque<>();
249      queue.add(root);
250    }
251
252    @Override
253    public boolean hasNext() {
254      return !queue.isEmpty();
255    }
256
257    @Override
258    public T peek() {
259      return queue.element();
260    }
261
262    @Override
263    public T next() {
264      T result = queue.remove();
265      Iterables.addAll(queue, children(result));
266      return result;
267    }
268  }
269}