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.elasticsearch;
  
  import org.apache.calcite.rex.RexCall;
  import org.apache.calcite.rex.RexLiteral;
  import org.apache.calcite.rex.RexVisitorImpl;
  import org.apache.calcite.sql.fun.SqlStdOperatorTable;
  
  /**
   * Visitor that extracts the actual field name from an item expression.
   */
  class MapProjectionFieldVisitor extends RexVisitorImpl<String> {
  
    static final MapProjectionFieldVisitor INSTANCE = new MapProjectionFieldVisitor();
  
    private MapProjectionFieldVisitor() {
      super(true);
    }
  
    @Override public String visitCall(RexCall call) {
      if (call.op == SqlStdOperatorTable.ITEM) {
        return ((RexLiteral) call.getOperands().get(1)).getValueAs(String.class);
      }
      return super.visitCall(call);
    }
  }
  