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.linq4j.tree;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.ArrayList;
  import java.util.List;
  
  /**
   * Holds context for evaluating expressions.
   */
  class Evaluator {
    final List<ParameterExpression> parameters = new ArrayList<>();
    final List<@Nullable Object> values = new ArrayList<>();
  
    Evaluator() {
    }
  
    void push(ParameterExpression parameter, @Nullable Object value) {
      parameters.add(parameter);
      values.add(value);
    }
  
    void pop(int n) {
      while (n > 0) {
        parameters.remove(parameters.size() - 1);
        values.remove(values.size() - 1);
        --n;
      }
    }
  
    @Nullable Object peek(ParameterExpression param) {
      for (int i = parameters.size() - 1; i >= 0; i--) {
        if (parameters.get(i) == param) {
          return values.get(i);
        }
      }
      throw new RuntimeException("parameter " + param + " not on stack");
    }
  
    @Nullable Object evaluate(Node expression) {
      return ((AbstractNode) expression).evaluate(this);
    }
  
    void clear() {
      parameters.clear();
      values.clear();
    }
  }
  