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.runners.core;
  
  import java.util.ArrayList;
  import java.util.List;
  import org.apache.beam.sdk.coders.Coder;
  import org.apache.beam.sdk.coders.CoderRegistry;
  import org.apache.beam.sdk.coders.ListCoder;
  import org.apache.beam.sdk.transforms.Combine;
  import org.apache.beam.sdk.transforms.GroupByKey;
  import org.apache.beam.sdk.values.PCollection;
  
  /**
   * Combiner that combines {@code T}s into a single {@code List<T>} containing all inputs.
   *
   * <p>For internal use to translate {@link GroupByKey}. For a large {@link PCollection} this is
   * expected to crash!
   *
   * <p>This is copied from the dataflow runner code.
   *
   * @param <T> the type of elements to concatenate.
   */
  public class Concatenate<T> extends Combine.CombineFn<T, List<T>, List<T>> {
    @Override
    public List<T> createAccumulator() {
      return new ArrayList<>();
    }
  
    @Override
    public List<T> addInput(List<T> accumulator, T input) {
      accumulator.add(input);
      return accumulator;
    }
  
    @Override
    public List<T> mergeAccumulators(Iterable<List<T>> accumulators) {
      List<T> result = createAccumulator();
      for (List<T> accumulator : accumulators) {
        result.addAll(accumulator);
      }
      return result;
    }
  
    @Override
    public List<T> extractOutput(List<T> accumulator) {
      return accumulator;
    }
  
    @Override
    public Coder<List<T>> getAccumulatorCoder(CoderRegistry registry, Coder<T> inputCoder) {
      return ListCoder.of(inputCoder);
    }
  
    @Override
    public Coder<List<T>> getDefaultOutputCoder(CoderRegistry registry, Coder<T> inputCoder) {
      return ListCoder.of(inputCoder);
    }
  }
  