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.adapter.mongodb;
  
  import org.apache.calcite.plan.Convention;
  import org.apache.calcite.plan.RelOptTable;
  import org.apache.calcite.rel.RelNode;
  import org.apache.calcite.rex.RexBuilder;
  import org.apache.calcite.runtime.PairList;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * Relational expression that uses Mongo calling convention.
   */
  public interface MongoRel extends RelNode {
    void implement(Implementor implementor);
  
    /** Calling convention for relational operations that occur in MongoDB. */
    Convention CONVENTION = new Convention.Impl("MONGO", MongoRel.class);
  
    /** Callback for the implementation process that converts a tree of
     * {@link MongoRel} nodes into a MongoDB query. */
    class Implementor {
      final PairList<@Nullable String, String> list = PairList.of();
      final RexBuilder rexBuilder;
      @Nullable RelOptTable table;
      @Nullable MongoTable mongoTable;
  
      public Implementor(RexBuilder rexBuilder) {
        this.rexBuilder = rexBuilder;
      }
  
      public void add(@Nullable String findOp, String aggOp) {
        list.add(findOp, aggOp);
      }
  
      public void visitChild(int ordinal, RelNode input) {
        assert ordinal == 0;
        ((MongoRel) input).implement(this);
      }
    }
  }
  