Application of pigeon nest principle in Lucene minshouldmatchsumscorer

 

@Override
public int nextDoc() throws IOException {
  // We are moving to the next doc ID, so scorers in 'lead' need to go in
  // 'tail'. If there is not enough space in 'tail', then we take the least
  // costly scorers and advance them.
  for (DisiWrapper s = lead; s != null; s = s.next) {
    final DisiWrapper evicted = insertTailWithOverFlow(s);
    if (evicted != null) {
      if (evicted.doc == doc) {
        evicted.doc = evicted.iterator.nextDoc();
      } else {
        evicted.doc = evicted.iterator.advance(doc + 1);
      }
      head.add(evicted);
    }
  }

  setDocAndFreq();
  // It would be correct to return doNextCandidate() at this point but if you
  // call nextDoc as opposed to advance, it probably means that you really
  // need the next match. Returning 'doc' here would lead to a similar
  // iteration over sub postings overall except that the decision making would
  // happen at a higher level where more abstractions are involved and
  // benchmarks suggested it causes a significant performance hit.
  return doNext();
}

freq+tailSize >= minShouldMatch To ensure the head It won't spill  

private int doNext() throws IOException {
  while (freq < minShouldMatch) {
    assert freq > 0;
    if (freq + tailSize >= minShouldMatch) {
      // a match on doc is still possible, try to
      // advance scorers from the tail
      advanceTail();
    } else {
      // no match on doc is possible anymore, move to the next potential match
      pushBackLeads();
      setDocAndFreq();
    }
  }

  return doc;
}

org.apache.lucene.codecs.lucene80.Lucene80DocValuesProducer

        @Override
        public long nextOrd() throws IOException {
          if (set == false) {
            final int index = disi.index();
            final long start = addresses.get(index);
            this.start = start + 1;
            end = addresses.get(index + 1L);
            set = true;
            return ords.get(start);
          } else if (start == end) {
            return NO_MORE_ORDS;
          } else {
            return ords.get(start++);
          }
        }

        @Override
        public int ordValue() {
          return (int) ords.get(disi.index());
        }

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