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A module that is everything required to understands Druid Segments
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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.druid.query.topn;

import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import org.apache.druid.query.Result;
import org.apache.druid.query.aggregation.AggregatorFactory;
import org.apache.druid.query.dimension.DimensionSpec;
import org.apache.druid.segment.column.ColumnType;
import org.joda.time.DateTime;

import java.util.Arrays;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.PriorityQueue;

/**
 */
public class TopNLexicographicResultBuilder implements TopNResultBuilder
{
  private static final int LOOP_UNROLL_COUNT = 8;

  private final DateTime timestamp;
  private final DimensionSpec dimSpec;
  private final String previousStop;
  private final Comparator comparator;
  private final String[] aggFactoryNames;
  private final PriorityQueue pQueue;
  private final int threshold;

  public TopNLexicographicResultBuilder(
      DateTime timestamp,
      DimensionSpec dimSpec,
      int threshold,
      String previousStop,
      final Comparator comparator,
      List aggFactories
  )
  {
    this.timestamp = timestamp;
    this.dimSpec = dimSpec;
    this.previousStop = previousStop;
    this.comparator = comparator;
    this.aggFactoryNames = TopNQueryQueryToolChest.extractFactoryName(aggFactories);
    this.threshold = threshold;
    this.pQueue = new PriorityQueue<>(
        threshold + 1,
        (o1, o2) -> comparator.compare(o2.getDimValue(), o1.getDimValue())
    );
  }

  @Override
  public TopNResultBuilder addEntry(
      Comparable dimValueObj,
      Object dimValIndex,
      Object[] metricVals
  )
  {
    final String dimValue = Objects.toString(dimValueObj, null);
    final Map metricValues = Maps.newHashMapWithExpectedSize(metricVals.length + 1);

    if (shouldAdd(dimValue)) {
      metricValues.put(dimSpec.getOutputName(), dimValueObj);
      final int extra = metricVals.length % LOOP_UNROLL_COUNT;
      switch (extra) {
        case 7:
          metricValues.put(aggFactoryNames[6], metricVals[6]);
          // fall through
        case 6:
          metricValues.put(aggFactoryNames[5], metricVals[5]);
          // fall through
        case 5:
          metricValues.put(aggFactoryNames[4], metricVals[4]);
          // fall through
        case 4:
          metricValues.put(aggFactoryNames[3], metricVals[3]);
          // fall through
        case 3:
          metricValues.put(aggFactoryNames[2], metricVals[2]);
          // fall through
        case 2:
          metricValues.put(aggFactoryNames[1], metricVals[1]);
          // fall through
        case 1:
          metricValues.put(aggFactoryNames[0], metricVals[0]);
      }
      for (int i = extra; i < metricVals.length; i += LOOP_UNROLL_COUNT) {
        metricValues.put(aggFactoryNames[i + 0], metricVals[i + 0]);
        // LGTM.com flags this, but it's safe
        // because we know "metricVals.length - extra" is a multiple of LOOP_UNROLL_COUNT.
        metricValues.put(aggFactoryNames[i + 1], metricVals[i + 1]); // lgtm [java/index-out-of-bounds]
        metricValues.put(aggFactoryNames[i + 2], metricVals[i + 2]); // lgtm [java/index-out-of-bounds]
        metricValues.put(aggFactoryNames[i + 3], metricVals[i + 3]); // lgtm [java/index-out-of-bounds]
        metricValues.put(aggFactoryNames[i + 4], metricVals[i + 4]); // lgtm [java/index-out-of-bounds]
        metricValues.put(aggFactoryNames[i + 5], metricVals[i + 5]); // lgtm [java/index-out-of-bounds]
        metricValues.put(aggFactoryNames[i + 6], metricVals[i + 6]); // lgtm [java/index-out-of-bounds]
        metricValues.put(aggFactoryNames[i + 7], metricVals[i + 7]); // lgtm [java/index-out-of-bounds]
      }

      pQueue.add(new DimValHolder.Builder().withDimValue(dimValue, ColumnType.STRING).withMetricValues(metricValues).build());
      if (pQueue.size() > threshold) {
        pQueue.poll();
      }
    }

    return this;
  }

  @Override
  public TopNResultBuilder addEntry(DimensionAndMetricValueExtractor dimensionAndMetricValueExtractor)
  {
    Object dimensionValueObj = dimensionAndMetricValueExtractor.getDimensionValue(dimSpec.getOutputName());
    String dimensionValue = Objects.toString(dimensionValueObj, null);

    if (shouldAdd(dimensionValue)) {
      pQueue.add(
          new DimValHolder.Builder().withDimValue(dimensionValue, ColumnType.STRING)
                                    .withMetricValues(dimensionAndMetricValueExtractor.getBaseObject())
                                    .build()
      );
      if (pQueue.size() > threshold) {
        pQueue.poll();
      }
    }
    return this;
  }

  @Override
  public Iterator getTopNIterator()
  {
    return pQueue.iterator();
  }

  @Override
  public Result build()
  {
    // Pull out top aggregated values
    final DimValHolder[] holderValueArray = pQueue.toArray(new DimValHolder[0]);
    Arrays.sort(
        holderValueArray,
        (o1, o2) -> comparator.compare(o1.getDimValue(), o2.getDimValue())
    );
    return new Result<>(
        timestamp,
        TopNResultValue.create(Lists.transform(Arrays.asList(holderValueArray), DimValHolder::getMetricValues))
    );
  }

  private boolean shouldAdd(String dimName)
  {
    final boolean belowThreshold = pQueue.size() < threshold;
    final boolean belowMax = belowThreshold
                             || comparator.compare(pQueue.peek().getTopNMetricVal(), dimName) < 0;
    // Only add if dimName is after previousStop
    return belowMax && (previousStop == null || comparator.compare(dimName, previousStop) > 0);
  }

}