Interfaces for a new function

An aggregation function needs to implement one of the following interfaces:

de.uniol.inf.is.odysseus.aggregation.functions.IIncrementalAggregationFunction
An incremental aggregation functions gets a notification when an element gets valid (enters the window) or gets invalid (leaves the window). You have to hold a state to return the current aggregation value on request.
- void addNew(T newElement);
  This method is called when a new element gets valid. You should add this value to the state.
  For example AVG function: State holds sum and count. sum += newElement and count++
- void removeOutdated(Collection<T> outdatedElements, T trigger, PointInTime pointInTime);
  This method is called when a set of elements get invalid. You should remove these values from the state.
  For example AVG function: For each element in outdatedElements: sum -= element and count--
- Object[] evalute(T trigger, PointInTime pointInTime);
  This method is called when a new aggregation value should returned.
  For example AVG function: return sum/count
de.uniol.inf.is.odysseus.aggregation.functions.INonIncrementalAggregationFunction
An non-incremental function gets a set of all elements in the current window. It does not have to hold a state.
- Object[] evaluate(Collection<T> elements, T trigger, PointInTime pointInTime);
  This method is called when a new aggregation value should returned. elements holds all elements in the window that starts at pointInTime.

In de.uniol.inf.is.odysseus.aggregation.functions there are also abstract classes.

Function Registry and Function Factories

Implement de.uniol.inf.is.odysseus.aggregation.functions.factory.IAggregationFunctionFactory to allow Odysseus to create a new function.
Add OSGi service to add function to the registry (de.uniol.inf.is.odysseus.aggregation.AggregationFunctionRegistry).