public String probeLengthsHistogram() {
    final Int2IntMap histo = collectProbeLengthsHistogram();
    final int sum = histo.values().intStream().sum();
    return histo.keySet().intStream()
      .sorted()
      .mapToObj(len -> String.format("%3d: %4d (%4.1f%%)", len, histo.get(len), histo.get(len) * 100.0 / sum))
      .collect(joining("\n"));
  }
public String probeLengthsHistogram()