Caching of partial results in skfuzzy.control

[JDWarner]

Execution of the fuzzy system should only recalculate needed quantities given changed values. Tracking which inputs have changed, along with the inbuilt networkx graph, makes this possible.

[jsexauer]

I'm thinking that in order to do this well, we need to have a stronger separation between the "state" of the controller and the "definition" of the controller.

Some easy examples of state are the input and output crip-values and variable membership-values.

Some easy examples of definition are the variables/terms themselves, and the rules. If any of these things change, you've arguably made a completely different controller, and as such, we shouldn't need to worry about caching.

Some more difficult choices are like membership functions. These are usually pretty static, but there are some control design which vary the membership functions between iterations (for example, using particle-swarm optimization to optimize the location of the various terms).

Still mulling over what the best way to do this is. I welcome your thoughts.

[JDWarner]

Agreed. Changes in the overall structure/definition of the controller should trigger a full recalculation of the system.

The main reason I had caching at all was to make repeated calls to an established system as efficient as possible. I think changes to membership functions could in theory be tracked and flagged without requiring the whole system to be recalculated.

However, all of this comes at the cost of major additional complexity, computational overhead, and maintenance burden. It seemed like a good idea, and still does - in theory - but I don't want this to get in our way.

Let's put caching on the back burner for now and revisit once more confident in the API and overarching structure.

[JDWarner]

This was partially addressed by #94, but only complete calculated results were referenced. We probably could do more, but I don't honestly see the need right now.

Closing for the moment; if the current solution ends up not being performant enough we can reassess and reopen.