code: int frequency(long hash)
      {
          long start = (hash & 3) << 2;
  
          int frequency =                 countOf(start, 0, indexOf(hash, 0));
          frequency = Math.min(frequency, countOf(start, 1, indexOf(hash, 1)));
          frequency = Math.min(frequency, countOf(start, 2, indexOf(hash, 2)));
          return      Math.min(frequency, countOf(start, 3, indexOf(hash, 3)));
      }
docs: Returns the estimated number of occurrences of an element, up to the maximum (15).
  
  @param hash the hash code of the element to count occurrences of
  @return the estimated number of occurrences of the element; possibly zero but never negative