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;
  
  /**
   * Generates lambda functions used in {@link EnumerableAggregate}.
   *
   * <p>This interface allows a implicit accumulator type variation.
   * ({@code OAccumulate} {@literal ->} {@code TAccumulate})
   *
   * @param <TSource> Type of the enumerable input source
   * @param <TOrigAccumulate> Type of the original accumulator
   * @param <TAccumulate> Type of the varied accumulator
   * @param <TResult> Type of the enumerable output result
   * @param <TKey> Type of the group-by key
   */
  public interface AggregateLambdaFactory<TSource, TOrigAccumulate, TAccumulate,
      TResult, TKey> {
    Function0<TAccumulate> accumulatorInitializer();
  
    Function2<TAccumulate, TSource, TAccumulate> accumulatorAdder();
  
    Function1<TAccumulate, TResult> singleGroupResultSelector(
        Function1<TOrigAccumulate, TResult> resultSelector);
  
    Function2<TKey, TAccumulate, TResult> resultSelector(
        Function2<TKey, TOrigAccumulate, TResult> resultSelector);
  }
  