/*
 * 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.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.apache.beam.runners.core.SideInputReader;
import org.apache.beam.sdk.options.PipelineOptions;
import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
import org.apache.beam.sdk.transforms.windowing.PaneInfo;
import org.apache.beam.sdk.transforms.windowing.TimestampCombiner;
import org.apache.beam.sdk.transforms.windowing.WindowFn;
import org.apache.beam.sdk.util.WindowedValue;
import org.apache.beam.sdk.values.KV;
import org.apache.beam.sdk.values.WindowingStrategy;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Iterables;
import org.apache.flink.api.java.tuple.Tuple2;
import org.apache.flink.util.Collector;
import org.joda.time.Instant;

/**
 * A Flink combine runner that builds a map of merged windows and produces output after seeing all
 * input. This is similar to what{@link org.apache.beam.runners.core.ReduceFnRunner} does.
 */
public class HashingFlinkCombineRunner<K, InputT, AccumT, OutputT, W extends BoundedWindow>
    extends AbstractFlinkCombineRunner<K, InputT, AccumT, OutputT, W> {

  @Override
  public void combine(
      FlinkCombiner<K, InputT, AccumT, OutputT> flinkCombiner,
      WindowingStrategy<Object, W> windowingStrategy,
      SideInputReader sideInputReader,
      PipelineOptions options,
      Iterable<WindowedValue<KV<K, InputT>>> elements,
      Collector<WindowedValue<KV<K, OutputT>>> out)
      throws Exception {

    @SuppressWarnings("unchecked")
    TimestampCombiner timestampCombiner = windowingStrategy.getTimestampCombiner();
    WindowFn<Object, W> windowFn = windowingStrategy.getWindowFn();

    // Flink Iterable can be iterated over only once.
    List<WindowedValue<KV<K, InputT>>> inputs = new ArrayList<>();
    Iterables.addAll(inputs, elements);

    Set<W> windows = collectWindows(inputs);
    Map<W, W> windowToMergeResult = mergeWindows(windowingStrategy, windows);

    // Combine all windowedValues into map
    Map<W, Tuple2<AccumT, Instant>> mapState = new HashMap<>();
    Iterator<WindowedValue<KV<K, InputT>>> iterator = inputs.iterator();
    WindowedValue<KV<K, InputT>> currentValue = iterator.next();
    K key = currentValue.getValue().getKey();
    do {
      for (BoundedWindow w : currentValue.getWindows()) {
        @SuppressWarnings("unchecked")
        W currentWindow = (W) w;
        W mergedWindow = windowToMergeResult.get(currentWindow);
        mergedWindow = mergedWindow == null ? currentWindow : mergedWindow;
        Set<W> singletonW = Collections.singleton(mergedWindow);
        Tuple2<AccumT, Instant> accumAndInstant = mapState.get(mergedWindow);
        if (accumAndInstant == null) {
          AccumT accumT =
              flinkCombiner.firstInput(
                  key, currentValue.getValue().getValue(), options, sideInputReader, singletonW);
          Instant windowTimestamp =
              timestampCombiner.assign(
                  mergedWindow, windowFn.getOutputTime(currentValue.getTimestamp(), mergedWindow));
          accumAndInstant = new Tuple2<>(accumT, windowTimestamp);
          mapState.put(mergedWindow, accumAndInstant);
        } else {
          accumAndInstant.f0 =
              flinkCombiner.addInput(
                  key,
                  accumAndInstant.f0,
                  currentValue.getValue().getValue(),
                  options,
                  sideInputReader,
                  singletonW);
          accumAndInstant.f1 =
              timestampCombiner.combine(
                  accumAndInstant.f1,
                  timestampCombiner.assign(
                      mergedWindow,
                      windowingStrategy
                          .getWindowFn()
                          .getOutputTime(currentValue.getTimestamp(), mergedWindow)));
        }
      }
      if (iterator.hasNext()) {
        currentValue = iterator.next();
      } else {
        break;
      }
    } while (true);

    // Output the final value of combiners
    for (Map.Entry<W, Tuple2<AccumT, Instant>> entry : mapState.entrySet()) {
      AccumT accumulator = entry.getValue().f0;
      Instant windowTimestamp = entry.getValue().f1;
      out.collect(
          WindowedValue.of(
              KV.of(
                  key,
                  flinkCombiner.extractOutput(
                      key,
                      accumulator,
                      options,
                      sideInputReader,
                      Collections.singleton(entry.getKey()))),
              windowTimestamp,
              entry.getKey(),
              PaneInfo.NO_FIRING));
    }
  }

  private Map<W, W> mergeWindows(WindowingStrategy<Object, W> windowingStrategy, Set<W> windows)
      throws Exception {
    WindowFn<Object, W> windowFn = windowingStrategy.getWindowFn();

    if (windowingStrategy.getWindowFn().isNonMerging()) {
      // Return an empty map, indicating that every window is not merged.
      return Collections.emptyMap();
    }

    Map<W, W> windowToMergeResult = new HashMap<>();
    windowFn.mergeWindows(new MergeContextImpl(windowFn, windows, windowToMergeResult));
    return windowToMergeResult;
  }

  private class MergeContextImpl extends WindowFn<Object, W>.MergeContext {

    private Set<W> windows;
    private Map<W, W> windowToMergeResult;

    MergeContextImpl(WindowFn<Object, W> windowFn, Set<W> windows, Map<W, W> windowToMergeResult) {
      windowFn.super();
      this.windows = windows;
      this.windowToMergeResult = windowToMergeResult;
    }

    @Override
    public Collection<W> windows() {
      return windows;
    }

    @Override
    public void merge(Collection<W> toBeMerged, W mergeResult) throws Exception {
      for (W w : toBeMerged) {
        windowToMergeResult.put(w, mergeResult);
      }
    }
  }

  private Set<W> collectWindows(Iterable<WindowedValue<KV<K, InputT>>> values) {
    Set<W> windows = new HashSet<>();
    for (WindowedValue<?> value : values) {
      for (BoundedWindow untypedWindow : value.getWindows()) {
        @SuppressWarnings("unchecked")
        W window = (W) untypedWindow;
        windows.add(window);
      }
    }
    return windows;
  }
}
