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.sql2rel;
  
  import org.apache.calcite.rel.RelHomogeneousShuttle;
  import org.apache.calcite.rel.RelNode;
  import org.apache.calcite.rel.core.CorrelationId;
  import org.apache.calcite.rex.RexCorrelVariable;
  import org.apache.calcite.rex.RexFieldAccess;
  import org.apache.calcite.rex.RexNode;
  import org.apache.calcite.rex.RexShuttle;
  import org.apache.calcite.rex.RexSubQuery;
  
  import org.checkerframework.checker.initialization.qual.NotOnlyInitialized;
  import org.checkerframework.checker.initialization.qual.UnderInitialization;
  
  /**
   * Shuttle that finds references to a given {@link CorrelationId} within a tree
   * of {@link RelNode}s.
   */
  public abstract class CorrelationReferenceFinder extends RelHomogeneousShuttle {
    @NotOnlyInitialized
    private final MyRexVisitor rexVisitor;
  
    /** Creates CorrelationReferenceFinder. */
    protected CorrelationReferenceFinder() {
      rexVisitor = new MyRexVisitor(this);
    }
  
    protected abstract RexNode handle(RexFieldAccess fieldAccess);
  
    @Override public RelNode visit(RelNode other) {
      RelNode next = super.visit(other);
      return next.accept(rexVisitor);
    }
  
    /**
     * Replaces alternative names of correlation variable to its canonical name.
     */
    private static class MyRexVisitor extends RexShuttle {
      @NotOnlyInitialized
      private final CorrelationReferenceFinder finder;
  
      private MyRexVisitor(@UnderInitialization CorrelationReferenceFinder finder) {
        this.finder = finder;
      }
  
      @Override public RexNode visitFieldAccess(RexFieldAccess fieldAccess) {
        if (fieldAccess.getReferenceExpr() instanceof RexCorrelVariable) {
          return finder.handle(fieldAccess);
        }
        return super.visitFieldAccess(fieldAccess);
      }
  
      @Override public RexNode visitSubQuery(RexSubQuery subQuery) {
        final RelNode r = subQuery.rel.accept(finder); // look inside sub-queries
        if (r != subQuery.rel) {
          subQuery = subQuery.clone(r);
        }
        return super.visitSubQuery(subQuery);
      }
    }
  }
  