/*
 *  This file is part of the Jikes RVM project (http://jikesrvm.org).
 *
 *  This file is licensed to You under the Eclipse Public License (EPL);
 *  You may not use this file except in compliance with the License. You
 *  may obtain a copy of the License at
 *
 *      http://www.opensource.org/licenses/eclipse-1.0.php
 *
 *  See the COPYRIGHT.txt file distributed with this work for information
 *  regarding copyright ownership.
 */
package org.jikesrvm.compilers.opt.util;

import java.util.ArrayList;
import java.util.Enumeration;
import java.util.ListIterator;


/**
 *  This class provides enumeration of elements of a tree in a town-down manner
 *  It guarantees that all children of a node will only be visited after
 *  the parent.
 *  This is not necessarily the same as a top-down level walk.
 */
final class TreeTopDownEnumerator implements Enumeration<TreeNode> {

  /**
   * List of nodes in preorder
   */
  private final ArrayList<TreeNode> list;

  /**
   * an iterator of the above list
   */
  private final ListIterator<TreeNode> iterator;

  /**
   * constructor: it creates the list of nodes
   * @param   root Root of the tree to traverse
   */
  TreeTopDownEnumerator(TreeNode root) {
    list = new ArrayList<TreeNode>();

    if (root != null) {
      // Perform a DFS, saving nodes in preorder
      DFS(root);
    }

    iterator = list.listIterator();
  }

  /**
   * any elements left?
   * @return whether there are any elements left
   */
  @Override
  public boolean hasMoreElements() {
    return iterator.hasNext();
  }

  /**
   * returns the next element in the list iterator
   * @return the next element in the list iterator or null
   */
  @Override
  public TreeNode nextElement() {
    return iterator.next();
  }

  /**
   * A preorder depth first traversal, adding nodes to the list
   * @param node the node to start at
   */
  private void DFS(TreeNode node) {
    list.add(node);
    Enumeration<TreeNode> childEnum = node.getChildren();
    while (childEnum.hasMoreElements()) {
      TreeNode child = childEnum.nextElement();
      DFS(child);
    }
  }
}