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.RexBuilder;
  import org.apache.calcite.rex.RexCorrelVariable;
  import org.apache.calcite.rex.RexNode;
  import org.apache.calcite.rex.RexShuttle;
  import org.apache.calcite.rex.RexSubQuery;
  
  import com.google.common.collect.ImmutableSet;
  
  import org.checkerframework.checker.initialization.qual.NotOnlyInitialized;
  import org.checkerframework.checker.initialization.qual.UnderInitialization;
  
  /**
   * Rewrites relations to ensure the same correlation is referenced by the same
   * correlation variable.
   */
  public class DeduplicateCorrelateVariables extends RelHomogeneousShuttle {
    @NotOnlyInitialized
    private final RexShuttle dedupRex;
  
    /** Creates a DeduplicateCorrelateVariables. */
    private DeduplicateCorrelateVariables(RexBuilder builder,
        CorrelationId canonicalId, ImmutableSet<CorrelationId> alternateIds) {
      dedupRex =
          new DeduplicateCorrelateVariablesShuttle(builder, canonicalId,
              alternateIds, this);
    }
  
    /**
     * Rewrites a relational expression, replacing alternate correlation variables
     * with a canonical correlation variable.
     */
    public static RelNode go(RexBuilder builder, CorrelationId canonicalId,
        Iterable<? extends CorrelationId> alternateIds, RelNode r) {
      return r.accept(
          new DeduplicateCorrelateVariables(builder, canonicalId,
              ImmutableSet.copyOf(alternateIds)));
    }
  
    @Override public RelNode visit(RelNode other) {
      RelNode next = super.visit(other);
      return next.accept(dedupRex);
    }
  
    /**
     * Replaces alternative names of correlation variable to its canonical name.
     */
    private static class DeduplicateCorrelateVariablesShuttle extends RexShuttle {
      private final RexBuilder builder;
      private final CorrelationId canonicalId;
      private final ImmutableSet<CorrelationId> alternateIds;
      @NotOnlyInitialized
      private final DeduplicateCorrelateVariables shuttle;
  
      private DeduplicateCorrelateVariablesShuttle(RexBuilder builder,
          CorrelationId canonicalId, ImmutableSet<CorrelationId> alternateIds,
          @UnderInitialization DeduplicateCorrelateVariables shuttle) {
        this.builder = builder;
        this.canonicalId = canonicalId;
        this.alternateIds = alternateIds;
        this.shuttle = shuttle;
      }
  
      @Override public RexNode visitCorrelVariable(RexCorrelVariable variable) {
        if (!alternateIds.contains(variable.id)) {
          return variable;
        }
  
        return builder.makeCorrel(variable.getType(), canonicalId);
      }
  
      @Override public RexNode visitSubQuery(RexSubQuery subQuery) {
        if (shuttle != null) {
          RelNode r = subQuery.rel.accept(shuttle); // look inside sub-queries
          if (r != subQuery.rel) {
            subQuery = subQuery.clone(r);
          }
        }
        return super.visitSubQuery(subQuery);
      }
    }
  }
  