Update #0 - First Release

sources/main/java/com/google/common/collect/TreeTraverser.java

@@ -0,0 +1,229 @@
+/*
+ * Copyright (C) 2012 The Guava Authors
+ *
+ * Licensed 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 com.google.common.collect;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+import java.util.Iterator;
+import java.util.Queue;
+
+import com.google.common.annotations.Beta;
+import com.google.common.annotations.GwtCompatible;
+
+/**
+ * Views elements of a type {@code T} as nodes in a tree, and provides methods
+ * to traverse the trees induced by this traverser.
+ *
+ * <p>
+ * For example, the tree
+ *
+ * <pre>
+ *           {@code
+ *          h
+ *        / | \
+ *       /  e  \
+ *      d       g
+ *     /|\      |
+ *    / | \     f
+ *   a  b  c       }
+ * </pre>
+ *
+ * <p>
+ * can be iterated over in preorder (hdabcegf), postorder (abcdefgh), or
+ * breadth-first order (hdegabcf).
+ *
+ * <p>
+ * Null nodes are strictly forbidden.
+ *
+ * @author Louis Wasserman
+ * @since 15.0
+ */
+@Beta
+@GwtCompatible(emulated = true)
+public abstract class TreeTraverser<T> {
+	// TODO(user): make this GWT-compatible when we've checked in ArrayDeque
+	// emulation
+
+	/**
+	 * Returns the children of the specified node. Must not contain null.
+	 */
+	public abstract Iterable<T> children(T root);
+
+	/**
+	 * Returns an unmodifiable iterable over the nodes in a tree structure, using
+	 * pre-order traversal. That is, each node's subtrees are traversed after the
+	 * node itself is returned.
+	 *
+	 * <p>
+	 * No guarantees are made about the behavior of the traversal when nodes change
+	 * while iteration is in progress or when the iterators generated by
+	 * {@link #children} are advanced.
+	 */
+	public final FluentIterable<T> preOrderTraversal(final T root) {
+		checkNotNull(root);
+		return new FluentIterable<T>() {
+			@Override
+			public UnmodifiableIterator<T> iterator() {
+				return preOrderIterator(root);
+			}
+		};
+	}
+
+	// overridden in BinaryTreeTraverser
+	UnmodifiableIterator<T> preOrderIterator(T root) {
+		return new PreOrderIterator(root);
+	}
+
+	private final class PreOrderIterator extends UnmodifiableIterator<T> {
+		private final Deque<Iterator<T>> stack;
+
+		PreOrderIterator(T root) {
+			this.stack = new ArrayDeque<Iterator<T>>();
+			stack.addLast(Iterators.singletonIterator(checkNotNull(root)));
+		}
+
+		@Override
+		public boolean hasNext() {
+			return !stack.isEmpty();
+		}
+
+		@Override
+		public T next() {
+			Iterator<T> itr = stack.getLast(); // throws NSEE if empty
+			T result = checkNotNull(itr.next());
+			if (!itr.hasNext()) {
+				stack.removeLast();
+			}
+			Iterator<T> childItr = children(result).iterator();
+			if (childItr.hasNext()) {
+				stack.addLast(childItr);
+			}
+			return result;
+		}
+	}
+
+	/**
+	 * Returns an unmodifiable iterable over the nodes in a tree structure, using
+	 * post-order traversal. That is, each node's subtrees are traversed before the
+	 * node itself is returned.
+	 *
+	 * <p>
+	 * No guarantees are made about the behavior of the traversal when nodes change
+	 * while iteration is in progress or when the iterators generated by
+	 * {@link #children} are advanced.
+	 */
+	public final FluentIterable<T> postOrderTraversal(final T root) {
+		checkNotNull(root);
+		return new FluentIterable<T>() {
+			@Override
+			public UnmodifiableIterator<T> iterator() {
+				return postOrderIterator(root);
+			}
+		};
+	}
+
+	// overridden in BinaryTreeTraverser
+	UnmodifiableIterator<T> postOrderIterator(T root) {
+		return new PostOrderIterator(root);
+	}
+
+	private static final class PostOrderNode<T> {
+		final T root;
+		final Iterator<T> childIterator;
+
+		PostOrderNode(T root, Iterator<T> childIterator) {
+			this.root = checkNotNull(root);
+			this.childIterator = checkNotNull(childIterator);
+		}
+	}
+
+	private final class PostOrderIterator extends AbstractIterator<T> {
+		private final ArrayDeque<PostOrderNode<T>> stack;
+
+		PostOrderIterator(T root) {
+			this.stack = new ArrayDeque<PostOrderNode<T>>();
+			stack.addLast(expand(root));
+		}
+
+		@Override
+		protected T computeNext() {
+			while (!stack.isEmpty()) {
+				PostOrderNode<T> top = stack.getLast();
+				if (top.childIterator.hasNext()) {
+					T child = top.childIterator.next();
+					stack.addLast(expand(child));
+				} else {
+					stack.removeLast();
+					return top.root;
+				}
+			}
+			return endOfData();
+		}
+
+		private PostOrderNode<T> expand(T t) {
+			return new PostOrderNode<T>(t, children(t).iterator());
+		}
+	}
+
+	/**
+	 * Returns an unmodifiable iterable over the nodes in a tree structure, using
+	 * breadth-first traversal. That is, all the nodes of depth 0 are returned, then
+	 * depth 1, then 2, and so on.
+	 *
+	 * <p>
+	 * No guarantees are made about the behavior of the traversal when nodes change
+	 * while iteration is in progress or when the iterators generated by
+	 * {@link #children} are advanced.
+	 */
+	public final FluentIterable<T> breadthFirstTraversal(final T root) {
+		checkNotNull(root);
+		return new FluentIterable<T>() {
+			@Override
+			public UnmodifiableIterator<T> iterator() {
+				return new BreadthFirstIterator(root);
+			}
+		};
+	}
+
+	private final class BreadthFirstIterator extends UnmodifiableIterator<T> implements PeekingIterator<T> {
+		private final Queue<T> queue;
+
+		BreadthFirstIterator(T root) {
+			this.queue = new ArrayDeque<T>();
+			queue.add(root);
+		}
+
+		@Override
+		public boolean hasNext() {
+			return !queue.isEmpty();
+		}
+
+		@Override
+		public T peek() {
+			return queue.element();
+		}
+
+		@Override
+		public T next() {
+			T result = queue.remove();
+			Iterables.addAll(queue, children(result));
+			return result;
+		}
+	}
+}