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.flink.translation.functions;
  
  import java.util.ArrayList;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Objects;
  import java.util.concurrent.atomic.AtomicBoolean;
  import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
  import org.apache.beam.sdk.transforms.windowing.PaneInfo;
  import org.apache.beam.sdk.values.KV;
  import org.apache.beam.sdk.values.WindowedValue;
  import org.apache.beam.sdk.values.WindowedValues;
  import org.apache.beam.sdk.values.WindowingStrategy;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterables;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterators;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.PeekingIterator;
  import org.apache.flink.api.common.functions.GroupReduceFunction;
  import org.apache.flink.util.Collector;
  import org.joda.time.Instant;
  
  /**
   * Reduce function for non-merging GBK implementation. Implementation tries to return non-iterable
   * results when possible, so we do not have to materialize all values for a single key in memory.
   *
   * @param <K> Key type.
   * @param <InputT> Input type.
   */
  @SuppressWarnings({"keyfor", "nullness"}) // TODO(https://github.com/apache/beam/issues/20497)
  public class FlinkNonMergingReduceFunction<K, InputT>
      implements GroupReduceFunction<
          WindowedValue<KV<K, InputT>>, WindowedValue<KV<K, Iterable<InputT>>>> {
  
    private static class OnceIterable<T> implements Iterable<T> {
  
      private final Iterator<T> iterator;
  
      private final AtomicBoolean used = new AtomicBoolean(false);
  
      OnceIterable(Iterator<T> iterator) {
        this.iterator = iterator;
      }
  
      @Override
      public Iterator<T> iterator() {
        if (used.compareAndSet(false, true)) {
          return iterator;
        }
        throw new IllegalStateException(
            "GBK result is not re-iterable. You can enable re-iterations by setting '--reIterableGroupByKeyResult'.");
      }
    }
  
    private final WindowingStrategy<?, ?> windowingStrategy;
    private final boolean reIterableResult;
  
    public FlinkNonMergingReduceFunction(
        WindowingStrategy<?, ?> windowingStrategy, boolean reIterableResult) {
      this.windowingStrategy = windowingStrategy;
      this.reIterableResult = reIterableResult;
    }
  
    @Override
    public void reduce(
        Iterable<WindowedValue<KV<K, InputT>>> input,
        Collector<WindowedValue<KV<K, Iterable<InputT>>>> coll) {
      final PeekingIterator<WindowedValue<KV<K, InputT>>> iterator =
          Iterators.peekingIterator(input.iterator());
      final WindowedValue<KV<K, InputT>> first = iterator.peek();
      final BoundedWindow window = Iterables.getOnlyElement(first.getWindows());
      @SuppressWarnings("unchecked")
      final Instant outputTimestamp = first.getTimestamp();
      final Instant combinedTimestamp =
          windowingStrategy.getTimestampCombiner().assign(window, outputTimestamp);
      final Iterable<InputT> values;
      if (reIterableResult) {
        final List<InputT> lst = new ArrayList<>();
        iterator.forEachRemaining(wv -> lst.add(wv.getValue().getValue()));
        values = lst;
      } else {
        values =
            new OnceIterable<>(
                Iterators.transform(
                    iterator,
                    (WindowedValue<KV<K, InputT>> wv) ->
                        Objects.requireNonNull(wv).getValue().getValue()));
      }
      WindowedValues.builder(first)
          .withValue(KV.of(first.getValue().getKey(), values))
          .setReceiver(coll::collect)
          .setPaneInfo(PaneInfo.ON_TIME_AND_ONLY_FIRING)
          .setTimestamp(combinedTimestamp)
          .output();
    }
  }
  