org.springframework.statemachine.support.tree.TreeTraverser.java Source code

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Here is the source code for org.springframework.statemachine.support.tree.TreeTraverser.java

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/*
 * Copyright 2015-2018 the original author or 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
 *
 * https://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 org.springframework.statemachine.support.tree;

import java.util.ArrayDeque;
import java.util.Iterator;

import org.springframework.util.Assert;

public abstract class TreeTraverser<T> {

    /**
     * Returns the children of the specified node. Must not contain null.
     *
     * @param root the node
     * @return child iterables
     */
    public abstract Iterable<T> children(T root);

    public final Iterable<T> postOrderTraversal(final T root) {
        Assert.notNull(root, "root cannot be null");
        return new Iterable<T>() {
            @Override
            public Iterator<T> iterator() {
                return postOrderIterator(root);
            }
        };
    }

    Iterator<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) {
            Assert.notNull(root, "root cannot be null");
            Assert.notNull(childIterator, "childIterator cannot be null");
            this.root = root;
            this.childIterator = 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());
        }
    }

}