Prefix Sorting DFAs: a Recursive Algorithm

Abstract: In the past thirty years, numerous algorithms for building the suffix array of a string have been proposed. In 2021, the notion of suffix array was extended from strings to DFAs, and it was shown that the resulting data structure can be built in $O(m^2 + n^{5/2})$ time, where $n$ is the number of states and $m$ is the number of edges [SODA 2021]. Recently, algorithms running in $O(mn)$ and $O(n^2\log n)$ time have been described [CPM 2023]. In this paper, we improve the previous bounds by proposing an $O(n^2)$ recursive algorithm inspired by Farach's algorithm for building a suffix tree [FOCS 1997]. To this end, we provide insight into the rich lexicographic and combinatorial structure of a graph, so contributing to the fascinating journey which might lead to solve the long-standing open problem of building the suffix tree of a graph.