An Optimised Algorithm for Determinisation and Completion of Finite Tree Automata.

Determinisation is an important concept in the theory of finite tree automata. However the complexity of the textbook procedure for determinisation is such that it is not viewed as a being a practical procedure for manipulating tree automata, even fairly small ones. The computational problems are exacerbated when an automaton has to be both determinised and completed, for instance to compute the complement of an automaton. In this paper we develop an algorithm for determinisation and completion of finite tree automata, whose worst-case complexity remains unchanged, but which performs dramatically better than existing algorithms in practice. The algorithm is developed in stages by optimising the textbook algorithm. A critical aspect of the algorithm is that the transitions of the determinised automaton are generated in a potentially very compact form called product form, which can often be used directly when manipulating the determinised automaton. The paper contains an experimental evaluation of the algorithm on a large set of tree automata examples. Applications of the algorithm include static analysis of term rewriting systems and logic programs, and checking containment of languages defined by tree automata such as XML schemata.