Indexing Finite-State Automata Using Forward-Stable Partitions

TL;DR

This paper introduces a new class of partial preorders for indexing finite-state automata, which can be computed efficiently and often have smaller width than previous methods, improving pattern matching performance.

Contribution

The paper proposes a novel class of partial preorders that are computable in polynomial time and have smaller or equal width compared to existing preorders, enhancing automaton indexing.

Findings

New partial preorders can be computed in polynomial time.

Their width is never larger than Cotumaccio's preorders.

Existence of automata where these preorders are linearly smaller.

Abstract

An index on a finite-state automaton is a data structure able to locate specific patterns on the automaton's paths and consequently on the regular language accepted by the automaton itself. Cotumaccio and Prezza [SODA '21], introduced a data structure able to solve pattern matching queries on automata, generalizing the famous FM-index for strings of Ferragina and Manzini [FOCS '00]. The efficiency of their index depends on the width of a particular partial order of the automaton's states, the smaller the width of the partial order, the faster is the index. However, computing the partial order of minimal width is NP-hard. This problem was mitigated by Cotumaccio [DCC '22], who relaxed the conditions on the partial order, allowing it to be a partial preorder. This relaxation yields the existence of a unique partial preorder of minimal width that can be computed in polynomial time. In the paper at hand, we present a new class of partial preorders and show that they have the following useful properties: (i) they can be computed in polynomial time, (ii) their width is never larger than the width of Cotumaccio's preorders, and (iii) there exist infinite classes of automata on which the width of Cotumaccio's pre-order is linearly larger than the width of our preorder.