/**
   * Accumulates groups for the BlockJoinQuery specified by its slot.
   *
   * @param slot Search query's slot
   * @param offset Parent docs offset
   * @param maxDocsPerGroup Upper bound of documents per group number
   * @param withinGroupOffset Offset within each group of child docs
   * @param withinGroupSort Sort criteria within groups
   * @param fillSortFields Specifies whether to add sort fields or not
   * @return TopGroups for the query specified by slot
   * @throws IOException if there is a low-level I/O error
   */
  @SuppressWarnings({"unchecked", "rawtypes"})
  private TopGroups<Integer> accumulateGroups(
      int slot,
      int offset,
      int maxDocsPerGroup,
      int withinGroupOffset,
      Sort withinGroupSort,
      boolean fillSortFields)
      throws IOException {
    final GroupDocs<Integer>[] groups = new GroupDocs[sortedGroups.length - offset];
    final FakeScorer fakeScorer = new FakeScorer();

    int totalGroupedHitCount = 0;
    // System.out.println("slot=" + slot);

    for (int groupIDX = offset; groupIDX < sortedGroups.length; groupIDX++) {
      final OneGroup og = sortedGroups[groupIDX];
      final int numChildDocs;
      if (slot == -1 || slot >= og.counts.length) {
        numChildDocs = 0;
      } else {
        numChildDocs = og.counts[slot];
      }

      // Number of documents in group should be bounded to prevent redundant memory allocation
      final int numDocsInGroup = Math.max(1, Math.min(numChildDocs, maxDocsPerGroup));
      // System.out.println("parent doc=" + og.doc + " numChildDocs=" + numChildDocs + " maxDocsPG="
      // + maxDocsPerGroup);

      // At this point we hold all docs w/ in each group,
      // unsorted; we now sort them:
      final TopDocsCollector<?> collector;
      if (withinGroupSort == null) {
        // System.out.println("sort by score");
        // Sort by score
        if (!trackScores) {
          throw new IllegalArgumentException(
              "cannot sort by relevance within group: trackScores=false");
        }
        collector = TopScoreDocCollector.create(numDocsInGroup, true);
      } else {
        // Sort by fields
        collector =
            TopFieldCollector.create(
                withinGroupSort, numDocsInGroup, fillSortFields, trackScores, trackMaxScore, true);
      }

      collector.setScorer(fakeScorer);
      collector.setNextReader(og.readerContext);
      for (int docIDX = 0; docIDX < numChildDocs; docIDX++) {
        // System.out.println("docIDX=" + docIDX + " vs " + og.docs[slot].length);
        final int doc = og.docs[slot][docIDX];
        fakeScorer.doc = doc;
        if (trackScores) {
          fakeScorer.score = og.scores[slot][docIDX];
        }
        collector.collect(doc);
      }
      totalGroupedHitCount += numChildDocs;

      final Object[] groupSortValues;

      if (fillSortFields) {
        groupSortValues = new Object[comparators.length];
        for (int sortFieldIDX = 0; sortFieldIDX < comparators.length; sortFieldIDX++) {
          groupSortValues[sortFieldIDX] = comparators[sortFieldIDX].value(og.slot);
        }
      } else {
        groupSortValues = null;
      }

      final TopDocs topDocs = collector.topDocs(withinGroupOffset, numDocsInGroup);

      groups[groupIDX - offset] =
          new GroupDocs<>(
              og.score,
              topDocs.getMaxScore(),
              numChildDocs,
              topDocs.scoreDocs,
              og.doc,
              groupSortValues);
    }

    return new TopGroups<>(
        new TopGroups<>(
            sort.getSort(),
            withinGroupSort == null ? null : withinGroupSort.getSort(),
            0,
            totalGroupedHitCount,
            groups,
            maxScore),
        totalHitCount);
  }