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.state;
  
  import javax.annotation.Nonnull;
  import org.apache.beam.sdk.transforms.Combine.CombineFn;
  
  /**
   * A {@link ReadableState} cell defined by a {@link CombineFn}, accepting multiple input values,
   * combining them as specified into accumulators, and producing a single output value.
   *
   * <p>Implementations of this form of state are expected to implement {@link #add} efficiently, not
   * via a sequence of read-modify-write.
   *
   * @param <InputT> the type of values added to the state
   * @param <AccumT> the type of accumulator
   * @param <OutputT> the type of value extracted from the state
   */
  public interface CombiningState<InputT, AccumT, OutputT> extends GroupingState<InputT, OutputT> {
  
    @Override
    @Nonnull
    OutputT read();
  
    /**
     * Read the merged accumulator for this state cell. It is implied that reading the state involves
     * reading the accumulator, so {@link #readLater} is sufficient to prefetch for this.
     */
    AccumT getAccum();
  
    /**
     * Add an accumulator to this state cell. Depending on implementation this may immediately merge
     * it with the previous accumulator, or may buffer this accumulator for a future merge.
     */
    void addAccum(AccumT accum);
  
    /** Merge the given accumulators according to the underlying {@link CombineFn}. */
    AccumT mergeAccumulators(Iterable<AccumT> accumulators);
  
    @Override
    CombiningState<InputT, AccumT, OutputT> readLater();
  }
  