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.enumerable;
  
  import org.apache.calcite.linq4j.function.Function0;
  import org.apache.calcite.linq4j.function.Function1;
  import org.apache.calcite.linq4j.function.Function2;
  
  import java.util.List;
  
  /**
   * Implementation of {@link AggregateLambdaFactory} that applies a sequence of
   * accumulator adders to input source.
   *
   * @param <TSource> Type of the enumerable input source
   * @param <TAccumulate> Type of the accumulator
   * @param <TResult> Type of the enumerable output result
   * @param <TKey> Type of the group-by key
   */
  public class BasicAggregateLambdaFactory<TSource, TAccumulate, TResult, TKey>
      implements AggregateLambdaFactory<TSource, TAccumulate, TAccumulate, TResult, TKey> {
  
    private final Function0<TAccumulate> accumulatorInitializer;
    private final Function2<TAccumulate, TSource, TAccumulate> accumulatorAdderDecorator;
  
    public BasicAggregateLambdaFactory(
        Function0<TAccumulate> accumulatorInitializer,
        List<Function2<TAccumulate, TSource, TAccumulate>> accumulatorAdders) {
      this.accumulatorInitializer = accumulatorInitializer;
      this.accumulatorAdderDecorator = new AccumulatorAdderSeq(accumulatorAdders);
    }
  
    @Override public Function0<TAccumulate> accumulatorInitializer() {
      return accumulatorInitializer;
    }
  
    @Override public Function2<TAccumulate, TSource, TAccumulate> accumulatorAdder() {
      return accumulatorAdderDecorator;
    }
  
    @Override public Function1<TAccumulate, TResult> singleGroupResultSelector(
        Function1<TAccumulate, TResult> resultSelector) {
      return resultSelector;
    }
  
    @Override public Function2<TKey, TAccumulate, TResult> resultSelector(
        Function2<TKey, TAccumulate, TResult> resultSelector) {
      return resultSelector;
    }
  
    /**
     * Decorator class of a sequence of accumulator adder functions.
     */
    private class AccumulatorAdderSeq
        implements Function2<TAccumulate, TSource, TAccumulate> {
      private final List<Function2<TAccumulate, TSource, TAccumulate>> accumulatorAdders;
  
      AccumulatorAdderSeq(
          List<Function2<TAccumulate, TSource, TAccumulate>> accumulatorAdders) {
        this.accumulatorAdders = accumulatorAdders;
      }
  
      @Override public TAccumulate apply(TAccumulate accumulator, TSource source) {
        TAccumulate result = accumulator;
        for (Function2<TAccumulate, TSource, TAccumulate> accumulatorAdder
            : accumulatorAdders) {
          result = accumulatorAdder.apply(accumulator, source);
        }
        return result;
      }
    }
  }
  