Decremental Subset Construction

Abstract

Finite automata are exploited in several domains, including language processing, artificial intelligence, automatic control, and software engineering. Let \({\mathcal {N}}\) be a nondeterministic finite automaton (NFA) and \({\mathcal {D}}\) the deterministic finite automaton (DFA) equivalent to \({\mathcal {N}}\). Assume to decrement \({\mathcal {N}}\) by \(\varDelta {\mathcal {N}}\), thus obtaining the decremented NFA \({\mathcal {N}}' = {\mathcal {N}}\setminus \varDelta {\mathcal {N}}\), where some states and/or transitions are missing. Consider determinizing \({\mathcal {N}}'\). Instead of determinizing \({\mathcal {N}}'\) from scratch using the classical Subset Construction algorithm, we propose Decremental Subset Construction, an algorithm which generates the DFA \({\mathcal {D}}'\) equivalent to \({\mathcal {N}}'\) by updating \({\mathcal {D}}\) based on \({\mathcal {N}}\) and \(\varDelta {\mathcal {N}}\). This way, only the actions necessary to transform \({\mathcal {D}}\) into \({\mathcal {D}}'\) are applied. Although evidence from worst-case complexity analysis indicates that Decremental Subset Construction is not better than Subset Construction, in practice, when \({\mathcal {N}}\) is large and \(\varDelta {{\mathcal {N}}}\) relatively small, Decremental Subset Construction may outperform Subset Construction significantly.