Controller synthesis techniques based on symbolic abstractions appeal by producing correct-by-design controllers, under intricate behavioural constraints. Yet, being relations between abstract states and inputs, such controllers are immense in size, which makes them futile for embedded platforms. Control-synthesis tools such as PESSOA, SCOTS, and CoSyMA tackle the problem by storing controllers as binary decision diagrams (BDDs). However, due to redundantly keeping multiple inputs per-state, the resulting controllers are still too large. In this work, we first show that choosing an optimal controller determinization is an NPcomplete problem. Further, we consider the previously known controller determinization technique and discuss its weaknesses. We suggest several new approaches to the problem, based on greedy algorithms, symbolic regression, and (muli-terminal) BDDs. Finally, we empirically compare the techniques and show that some of the new algorithms can produce up to ≈ 85% smaller controllers than those obtained with the previous technique.
Preliminaries
Minimum set coverThe minimum set cover problem (MSC) is formulated as:Problem 2.1 (MSC). Given a set X and a cover {S j } j∈I , i.e. X ⊆ j∈I S j , where |X|, |I| < ∞, find the smallest subcover I * ⊆ I : X ⊆ j∈I * S j .Both, the decision and selection versions of (MSC, are known to be NPcomplete. The first approximate poly-nomial-time solution for MSC was given by [12]. Later, [5] suggested an approximate poly-nomial-time solution for the generalized "minimum set weight cover problem" (MWSC); which extends MSC by that each set S k is assigned a weight s k ≥ 0 and the question is to find the smallest sub-cover with the minimum total weight. According to [6], the Chvátal's algorithm time complexity is: O (|I| · |X| · min (|I|, |X|)).1 Instead of storing the control law as an explicit map, we search for a symbolic function that for a given state computes the input value.2 Up to the found optimal BDD variable reordering.