code: public static <T, U> Collector<T, ?, Optional<Long>> denseRankBy(U value, Function<? super T, ? extends U> function, Comparator<? super U> comparator) {
          return Collector.of(
              () -> (TreeSet<U>[]) new TreeSet[1],
              (l, v) -> {
                  if (l[0] == null)
                      l[0] = new TreeSet<>(comparator);
  
                  U u = function.apply(v);
                  if (comparator.compare(value, u) > 0)
                      l[0].add(u);
              },
              (l1, l2) -> {
                  if (l2[0] == null)
                      return l1;
                  else if (l1[0] == null)
                      return l2;
  
                  l1[0].addAll(l2[0]);
                  return l1;
              },
              l -> l[0] == null ? Optional.empty() : Optional.of((long) l[0].size())
          );
      }
docs: Get a {@link Collector} that calculates the derived <code>DENSE_RANK()</code> function given a specific ordering.