Issue |
RAIRO-Theor. Inf. Appl.
Volume 34, Number 4, July/August 2000
|
|
---|---|---|
Page(s) | 257 - 277 | |
DOI | https://doi.org/10.1051/ita:2000116 | |
Published online | 15 April 2002 |
Computing ϵ-Free NFA from Regular Expressions in O(n log2(n)) Time
1
Institut für Informatik,
Universität Stuttgart,
Breitwiesenstr. 20-22, 70565 Stuttgart, Germany.
2
LIAFA, Université Paris VII, 2 place Jussieu, Case
7014, 75251 Paris
Cedex 05, France; e-mail: muscholl@liafa.jussieu.fr
3
Institut für Informatik,
Universität Stuttgart,
Breitwiesenstr. 20-22, 70565 Stuttgart, Germany. LIAFA, Université Paris VII, 2 place Jussieu, Case
7014, 75251 Paris
Cedex 05, France; e-mail: muscholl@liafa.jussieu.fr
Received:
31
August
1999
Accepted:
7
August
2000
The standard procedure to transform a regular expression of size n to an ϵ-free nondeterministic finite automaton yields automata with O(n) states and O(n2) transitions. For a long time this was supposed to be also the lower bound, but a result by Hromkovic et al. showed how to build an ϵ-free NFA with only O(n log2(n)) transitions. The current lower bound on the number of transitions is Ω(n log(n)). A rough running time estimation for the common follow sets (CFS) construction proposed by Hromkovič et al. yields a cubic algorithm. In this paper we present a sequential algorithm for the CFS construction which works in time O(n log(n) + size of the output). On a CREW PRAM the CFS construction can be performed in time O(log(n)) using O(n + (size of the output)/log(n)) processors. We also present a simpler proof of the lower bound on the number of transitions.
Mathematics Subject Classification: 68Q45 / 68Q25 / 68W01 / 68W10
Key words: Epsilon-free nondeterministic automata / regular expressions / common follow sets construction.
© EDP Sciences, 2000
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