/*
 * 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;
		}
	}
}
