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   * Licensed to the Apache Software Foundation (ASF) under one
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   * distributed with this work for additional information
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   * 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,
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   * See the License for the specific language governing permissions and
   * limitations under the License.
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  package org.apache.beam.sdk.nexmark.queries;
  
  import java.util.ArrayList;
  import java.util.List;
  import org.apache.beam.sdk.nexmark.NexmarkConfiguration;
  import org.apache.beam.sdk.nexmark.model.Auction;
  import org.apache.beam.sdk.nexmark.model.AuctionBid;
  import org.apache.beam.sdk.nexmark.model.Bid;
  import org.apache.beam.sdk.nexmark.model.Event;
  import org.apache.beam.sdk.nexmark.model.SellerPrice;
  import org.apache.beam.sdk.transforms.Combine;
  import org.apache.beam.sdk.transforms.DoFn;
  import org.apache.beam.sdk.transforms.Filter;
  import org.apache.beam.sdk.transforms.ParDo;
  import org.apache.beam.sdk.transforms.windowing.AfterPane;
  import org.apache.beam.sdk.transforms.windowing.GlobalWindows;
  import org.apache.beam.sdk.transforms.windowing.Repeatedly;
  import org.apache.beam.sdk.transforms.windowing.Window;
  import org.apache.beam.sdk.values.KV;
  import org.apache.beam.sdk.values.PCollection;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Lists;
  import org.joda.time.Duration;
  
  /**
   * Query 6, 'Average Selling Price by Seller'. Select the average selling price over the last 10
   * closed auctions by the same seller. In CQL syntax:
   *
   * <pre>{@code
   * SELECT Istream(AVG(Q.final), Q.seller)
   * FROM (SELECT Rstream(MAX(B.price) AS final, A.seller)
   *       FROM Auction A [ROWS UNBOUNDED], Bid B [ROWS UNBOUNDED]
   *       WHERE A.id=B.auction AND B.datetime < A.expires AND A.expires < CURRENT_TIME
   *       GROUP BY A.id, A.seller) [PARTITION BY A.seller ROWS 10] Q
   * GROUP BY Q.seller;
   * }</pre>
   *
   * <p>We are a little more exact with selecting winning bids: see {@link WinningBids}.
   */
  public class Query6 extends NexmarkQueryTransform<SellerPrice> {
  
    private final NexmarkConfiguration configuration;
  
    public Query6(NexmarkConfiguration configuration) {
      super("Query6");
      this.configuration = configuration;
    }
  
    /**
     * Combiner to keep track of up to {@code maxNumBids} of the most recent wining bids and calculate
     * their average selling price.
     */
    private static class MovingMeanSellingPrice extends Combine.CombineFn<Bid, List<Bid>, Long> {
      private final int maxNumBids;
  
      public MovingMeanSellingPrice(int maxNumBids) {
        this.maxNumBids = maxNumBids;
      }
  
      @Override
      public List<Bid> createAccumulator() {
        return new ArrayList<>();
      }
  
      @Override
      public List<Bid> addInput(List<Bid> accumulator, Bid input) {
        accumulator.add(input);
        accumulator.sort(Bid.ASCENDING_TIME_THEN_PRICE);
        if (accumulator.size() > maxNumBids) {
          accumulator.remove(0);
        }
        return accumulator;
      }
  
      @Override
      public List<Bid> mergeAccumulators(Iterable<List<Bid>> accumulators) {
        List<Bid> result = new ArrayList<>();
        for (List<Bid> accumulator : accumulators) {
          result.addAll(accumulator);
        }
        result.sort(Bid.ASCENDING_TIME_THEN_PRICE);
        if (result.size() > maxNumBids) {
          result = Lists.newArrayList(result.listIterator(result.size() - maxNumBids));
        }
        return result;
      }
  
      @Override
      public Long extractOutput(List<Bid> accumulator) {
        if (accumulator.isEmpty()) {
          return 0L;
        }
        long sumOfPrice = 0;
        for (Bid bid : accumulator) {
          sumOfPrice += bid.price;
        }
        return Math.round((double) sumOfPrice / accumulator.size());
      }
    }
  
    @Override
    public PCollection<SellerPrice> expand(PCollection<Event> events) {
      return events
          .apply(Filter.by(new AuctionOrBid()))
          // Find the winning bid for each closed auction.
          .apply(new WinningBids(name + ".WinningBids", configuration))
  
          // Key the winning bid by the seller id.
          .apply(
              name + ".Rekey",
              ParDo.of(
                  new DoFn<AuctionBid, KV<Long, Bid>>() {
                    @ProcessElement
                    public void processElement(ProcessContext c) {
                      Auction auction = c.element().auction;
                      Bid bid = c.element().bid;
                      c.output(KV.of(auction.seller, bid));
                    }
                  }))
  
          // Re-window to update on every wining bid.
          .apply(
              Window.<KV<Long, Bid>>into(new GlobalWindows())
                  .triggering(Repeatedly.forever(AfterPane.elementCountAtLeast(1)))
                  .accumulatingFiredPanes()
                  .withAllowedLateness(Duration.ZERO))
  
          // Find the average of last 10 winning bids for each seller.
          .apply(Combine.perKey(new MovingMeanSellingPrice(10)))
  
          // Project into our datatype.
          .apply(
              name + ".Select",
              ParDo.of(
                  new DoFn<KV<Long, Long>, SellerPrice>() {
                    @ProcessElement
                    public void processElement(ProcessContext c) {
                      c.output(new SellerPrice(c.element().getKey(), c.element().getValue()));
                    }
                  }));
    }
  }
  