Constrained symbolic search: On mutexes, BDD minimization and more

Symbolic search allows saving large amounts of memory compared to regular explicit-state search algorithms. This is crucial in optimal settings, in which common search algorithms often exhaust the available memory. So far, the most successful uses of symbolic search have been bidirectional blind search and the generation of abstraction heuristics like Pattern Databases. Despite its usefulness, several common techniques in explicit-state search have not been employed in symbolic search. In particular, mutexes and other constraining invariants, techniques that have been proven essential when doing regression, are yet to be exploited in conjunction with BDDs. In this paper we analyze the use of such constraints in symbolic search and its combination with minimization techniques common in BDD manipulation. Experimental results show a significant increase in performance, considerably above the current state of the art in optimal planning.