spork-master/src/org/apache/pig/impl/plan/PlanVisitor.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.pig.impl.plan;

import java.util.Deque;
import java.util.LinkedList;

/**
 * A visitor mechanism for navigating and operating on a plan of 
 * Operators.  This class contains the logic to traverse the plan.  It does not
 * visit individual nodes.  That is left to implementing classes (such as
 * LOVisitor).
 *
 */
abstract public class PlanVisitor <O extends Operator, P extends OperatorPlan<O>> {

    protected P mPlan;

    /**
     * Guaranteed to always point to the walker currently being used.
     */
    protected PlanWalker<O, P> mCurrentWalker;

    private Deque<PlanWalker<O, P>> mWalkers;

    /**
     * Entry point for visiting the plan.
     * @throws VisitorException if an error is encountered while visiting.
     */
    public void visit() throws VisitorException {
        mCurrentWalker.walk(this);
    }

    public P getPlan() {
        return mPlan;
    }

    /**
     * @param plan OperatorPlan this visitor will visit.
     * @param walker PlanWalker this visitor will use to traverse the plan.
     */
    protected PlanVisitor(P plan, PlanWalker<O, P> walker) {
        mPlan = plan;
        mCurrentWalker = walker;
        mWalkers = new LinkedList<PlanWalker<O, P>>();
    }

    /**
     * Push the current walker onto the stack of saved walkers and begin using
     * the newly passed walker as the current walker.
     * @param walker new walker to set as the current walker.
     */
    protected void pushWalker(PlanWalker<O, P> walker) {
        mWalkers.push(mCurrentWalker);
        mCurrentWalker = walker;
    }

    /**
     * Pop the next to previous walker off of the stack and set it as the current
     * walker.  This will drop the reference to the current walker.
     * @throws VisitorException if there are no more walkers on the stack.  In
     * this case the current walker is not reset.
     */
    protected void popWalker() throws VisitorException {
        if (mWalkers.isEmpty()) {
            throw new VisitorException("No more walkers to pop.");
        }
        mCurrentWalker = mWalkers.pop();
    }
}
