content: /*
   * 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.calcite.rel;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * A <code>RelVisitor</code> is a Visitor role in the
   * {@link org.apache.calcite.util.Glossary#VISITOR_PATTERN visitor pattern} and
   * visits {@link RelNode} objects as the role of Element. Other components in
   * the pattern: {@link RelNode#childrenAccept(RelVisitor)}.
   */
  public abstract class RelVisitor {
    //~ Instance fields --------------------------------------------------------
  
    private @Nullable RelNode root;
  
    //~ Methods ----------------------------------------------------------------
  
    /**
     * Visits a node during a traversal.
     *
     * @param node    Node to visit
     * @param ordinal Ordinal of node within its parent
     * @param parent  Parent of the node, or null if it is the root of the
     *                traversal
     */
    public void visit(
        RelNode node,
        int ordinal,
        @Nullable RelNode parent) {
      node.childrenAccept(this);
    }
  
    /**
     * Replaces the root node of this traversal.
     *
     * @param node The new root node
     */
    public void replaceRoot(@Nullable RelNode node) {
      this.root = node;
    }
  
    /**
     * Starts an iteration.
     */
    public @Nullable RelNode go(RelNode p) {
      this.root = p;
      visit(p, 0, null);
      return root;
    }
  }
  