code: static int optimalNumOfHashFunctions(long expectEntries, long bitSize) {
  		return Math.max(1, (int) Math.round((double) bitSize / expectEntries * Math.log(2)));
  	}
docs: compute the optimal hash function number with given input entries and bits size, which would
  make the false positive probability lowest.
  
  @param expectEntries
  @param bitSize
  @return hash function number