This private method doesn't need access to any member variables, so it would be better to pull it out of the class and just make it a static function in this file (or if it's useful for other weighting schemes, somewhere they can share it).  That would mean it didn't need to be in the public API header, which is better since it's really just an implementation detail.

Also, method names should be lower case, so `stirling_value(x, y)` not `stirlingValue(x, y)`.