An nlogn algorithm for hyper-minimizing a (minimized) deterministic automaton

  • Authors:
  • Markus Holzer;Andreas Maletti

  • Affiliations:
  • Institut für Informatik, Universität Giessen, Arndtstr. 2, 35392 Giessen, Germany;Departament de Filologies Romàniques, Universitat Rovira i Virgili, Av. Catalunya 35, 43002 Tarragona, Spain

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2010

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Abstract

We improve a recent result [A. Badr, Hyper-minimization in O(n^2), Internat. J. Found. Comput. Sci. 20 (4) (2009) 735-746] for hyper-minimized finite automata. Namely, we present an O(nlogn) algorithm that computes for a given deterministic finite automaton (dfa) an almost-equivalent dfa that is as small as possible-such an automaton is called hyper-minimal. Here two finite automata are almost-equivalent if and only if the symmetric difference of their languages is finite. In other words, two almost-equivalent automata disagree on acceptance on finitely many inputs. In this way, we solve an open problem stated in [A. Badr, V. Geffert, I. Shipman, Hyper-minimizing minimized deterministic finite state automata, RAIRO Theor. Inf. Appl. 43 (1) (2009) 69-94] and by Badr. Moreover, we show that minimization linearly reduces to hyper-minimization, which shows that the time-bound O(nlogn) is optimal for hyper-minimization. Independently, similar results were obtained in [P. Gawrychowski, A. Jez, Hyper-minimisation made efficient, in: Proc. 34th Int. Symp. Mathematical Foundations of Computer Science, in: LNCS, vol. 5734, Springer, 2009, pp. 356-368].