/tags/release-0.0.0-rc0/hive/external/ql/src/java/org/apache/hadoop/hive/ql/lib/PreOrderWalker.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.hadoop.hive.ql.lib;

import org.apache.hadoop.hive.ql.parse.SemanticException;

/**
 * base class for operator graph walker this class takes list of starting ops
 * and walks them one by one.
 */
public class PreOrderWalker extends DefaultGraphWalker {

  /*
   * Since the operator tree is a DAG, nodes with mutliple parents will be
   * visited more than once. This can be made configurable.
   */

  /**
   * Constructor.
   * 
   * @param disp
   *          dispatcher to call for each op encountered
   */
  public PreOrderWalker(Dispatcher disp) {
    super(disp);
  }

  /**
   * Walk the current operator and its descendants.
   * 
   * @param nd
   *          current operator in the graph
   * @throws SemanticException
   */
  @Override
  public void walk(Node nd) throws SemanticException {
    opStack.push(nd);
    dispatch(nd, opStack);

    // move all the children to the front of queue
    if (nd.getChildren() != null) {
      for (Node n : nd.getChildren()) {
        walk(n);
      }
    }

    opStack.pop();
  }
}