* Computes the optimal k (number of hashes per element inserted in Bloom filter), given the
   * expected insertions and total number of bits in the Bloom filter.
   *
   * <p>See http://en.wikipedia.org/wiki/File:Bloom_filter_fp_probability.svg for the formula.
   *
   * @param n expected insertions (must be positive)
   * @param m total number of bits in Bloom filter (must be positive)
   */
  @VisibleForTesting