External stats change

[MaxOstrowski]

Merging UserStatistics and ClaspStatistics for uniform C interface preparation.
In this way the clingo statistics interface can be reused and only one interface has to exist.

[BenKaufmann]

Hi Max,

I really like the idea of combining the interfaces. However, one golden rule of interface design is that interfaces should be easy to use correctly and hard to use incorrectly. Imo, your solution is too easy to use incorrectly. Most clasp statistics for example are read-only StatisticObjects and calling add on a key belonging to such a statistic will trigger undefined behaviour - in the best case you'll get an immediate seg fault but worse consequences are easily possible. To make the interface at least safely usable the following is needed:

• a function extensible(key) indicating whether or not key can be extended
• a (checked) precondition for add(key, ...) enforcing that extensible(key) == true
• a root-key for which extensible(key) == true

The implementation of the combined interface in clasp should probably also avoid storing (user) statistics twice (once in the map and once in the objects vector).

I'll try to implement something like that over the weekend...

[BenKaufmann]

Hi Max,
I updated #23 accordingly. Please have a look.