/*
 * 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.dataflow;

import java.util.ArrayList;
import java.util.List;
import org.apache.beam.runners.core.construction.PTransformReplacements;
import org.apache.beam.runners.core.construction.SingleInputOutputOverrideFactory;
import org.apache.beam.runners.dataflow.DataflowRunner.StreamingPCollectionViewWriterFn;
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.runners.AppliedPTransform;
import org.apache.beam.sdk.transforms.Combine;
import org.apache.beam.sdk.transforms.Combine.CombineFn;
import org.apache.beam.sdk.transforms.PTransform;
import org.apache.beam.sdk.transforms.ParDo;
import org.apache.beam.sdk.transforms.View.CreatePCollectionView;
import org.apache.beam.sdk.values.PCollection;
import org.apache.beam.sdk.values.PCollectionView;

/**
 * Dataflow streaming overrides for {@link CreatePCollectionView}, specialized for different view
 * types.
 */
class StreamingViewOverrides {
  static class StreamingCreatePCollectionViewFactory<ElemT, ViewT>
      extends SingleInputOutputOverrideFactory<
          PCollection<ElemT>, PCollection<ElemT>, CreatePCollectionView<ElemT, ViewT>> {
    @Override
    public PTransformReplacement<PCollection<ElemT>, PCollection<ElemT>> getReplacementTransform(
        AppliedPTransform<
                PCollection<ElemT>, PCollection<ElemT>, CreatePCollectionView<ElemT, ViewT>>
            transform) {
      StreamingCreatePCollectionView<ElemT, ViewT> streamingView =
          new StreamingCreatePCollectionView<>(transform.getTransform().getView());
      return PTransformReplacement.of(
          PTransformReplacements.getSingletonMainInput(transform), streamingView);
    }

    private static class StreamingCreatePCollectionView<ElemT, ViewT>
        extends PTransform<PCollection<ElemT>, PCollection<ElemT>> {
      private final PCollectionView<ViewT> view;

      private StreamingCreatePCollectionView(PCollectionView<ViewT> view) {
        this.view = view;
      }

      @Override
      public PCollection<ElemT> expand(PCollection<ElemT> input) {
        return input
            .apply(Combine.globally(new Concatenate<ElemT>()).withoutDefaults())
            .apply(ParDo.of(StreamingPCollectionViewWriterFn.create(view, input.getCoder())))
            .apply(CreateDataflowView.forStreaming(view));
      }
    }
  }

  /**
   * Combiner that combines {@code T}s into a single {@code List<T>} containing all inputs.
   *
   * @param <T> the type of elements to concatenate.
   */
  private static class Concatenate<T> extends 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);
    }
  }
}
