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.beam.sdk.extensions.sql.udf;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * An aggregate function that can be executed as part of a SQL query.
   *
   * <p>AggregateFn contains a subset of the functionality of {@code
   * org.apache.beam.sdk.transforms.Combine.CombineFn}.
   *
   * @param <InputT> type of input values
   * @param <AccumT> type of mutable accumulator values
   * @param <OutputT> type of output values
   */
  public interface AggregateFn<
      InputT extends @Nullable Object,
      AccumT extends @Nullable Object,
      OutputT extends @Nullable Object> {
  
    /**
     * Returns a new, mutable accumulator value, representing the accumulation of zero input values.
     */
    AccumT createAccumulator();
  
    /**
     * Adds the given input value to the given accumulator, returning the new accumulator value.
     *
     * @param mutableAccumulator may be modified and returned for efficiency
     * @param input should not be mutated
     */
    AccumT addInput(AccumT mutableAccumulator, InputT input);
  
    /**
     * Returns an accumulator representing the accumulation of all the input values accumulated in the
     * merging accumulators.
     *
     * @param mutableAccumulator This accumulator may be modified and returned for efficiency.
     * @param immutableAccumulators These other accumulators should not be mutated, because they may
     *     be shared with other code and mutating them could lead to incorrect results or data
     *     corruption.
     */
    AccumT mergeAccumulators(AccumT mutableAccumulator, Iterable<AccumT> immutableAccumulators);
  
    /**
     * Returns the output value that is the result of combining all the input values represented by
     * the given accumulator.
     *
     * @param mutableAccumulator can be modified for efficiency
     */
    OutputT extractOutput(AccumT mutableAccumulator);
  }
  