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   * 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.sdk.nexmark.queries;
  
  import org.apache.beam.sdk.nexmark.Monitor;
  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.CategoryPrice;
  import org.apache.beam.sdk.nexmark.model.Event;
  import org.apache.beam.sdk.transforms.DoFn;
  import org.apache.beam.sdk.transforms.Filter;
  import org.apache.beam.sdk.transforms.Mean;
  import org.apache.beam.sdk.transforms.ParDo;
  import org.apache.beam.sdk.transforms.windowing.SlidingWindows;
  import org.apache.beam.sdk.transforms.windowing.Window;
  import org.apache.beam.sdk.values.KV;
  import org.apache.beam.sdk.values.PCollection;
  import org.joda.time.Duration;
  
  /**
   * Query 4, 'Average Price for a Category'. Select the average of the wining bid prices for all
   * closed auctions in each category. In CQL syntax:
   *
   * <pre>{@code
   * SELECT Istream(AVG(Q.final))
   * FROM Category C, (SELECT Rstream(MAX(B.price) AS final, A.category)
   *                   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.category) Q
   * WHERE Q.category = C.id
   * GROUP BY C.id;
   * }</pre>
   *
   * <p>For extra spiciness our implementation differs slightly from the above:
   *
   * <ul>
   *   <li>We select both the average winning price and the category.
   *   <li>We don't bother joining with a static category table, since it's contents are never used.
   *   <li>We only consider bids which are above the auction's reserve price.
   *   <li>We accept the highest-price, earliest valid bid as the winner.
   *   <li>We calculate the averages oven a sliding window of size {@code windowSizeSec} and period
   *       {@code windowPeriodSec}.
   * </ul>
   */
  public class Query4 extends NexmarkQueryTransform<CategoryPrice> {
    private final Monitor<AuctionBid> winningBidsMonitor;
    private final NexmarkConfiguration configuration;
  
    public Query4(NexmarkConfiguration configuration) {
      super("Query4");
      this.configuration = configuration;
      winningBidsMonitor = new Monitor<>(name + ".WinningBids", "winning");
    }
  
    @Override
    public PCollection<CategoryPrice> expand(PCollection<Event> events) {
      PCollection<AuctionBid> winningBids =
          events
              .apply(Filter.by(new AuctionOrBid()))
              // Find the winning bid for each closed auction.
              .apply(new WinningBids(name + ".WinningBids", configuration));
  
      // Monitor winning bids
      winningBids =
          winningBids.apply(name + ".WinningBidsMonitor", winningBidsMonitor.getTransform());
  
      return winningBids
          // Key the winning bid price by the auction category.
          .apply(
              name + ".Rekey",
              ParDo.of(
                  new DoFn<AuctionBid, KV<Long, Long>>() {
                    @ProcessElement
                    public void processElement(ProcessContext c) {
                      Auction auction = c.element().auction;
                      Bid bid = c.element().bid;
                      c.output(KV.of(auction.category, bid.price));
                    }
                  }))
  
          // Re-window so we can calculate a sliding average
          .apply(
              Window.into(
                  SlidingWindows.of(Duration.standardSeconds(configuration.windowSizeSec))
                      .every(Duration.standardSeconds(configuration.windowPeriodSec))))
  
          // Find the average of the winning bids for each category.
          // Make sure we share the work for each category between workers.
          .apply(Mean.<Long, Long>perKey().withHotKeyFanout(configuration.fanout))
  
          // For testing against Query4Model, capture which results are 'final'.
          .apply(
              name + ".Project",
              ParDo.of(
                  new DoFn<KV<Long, Double>, CategoryPrice>() {
                    @ProcessElement
                    public void processElement(ProcessContext c) {
                      c.output(
                          new CategoryPrice(
                              c.element().getKey(),
                              Math.round(c.element().getValue()),
                              c.pane().isLast()));
                    }
                  }));
    }
  }
  