General suffix automaton construction algorithm and space bounds

Suffix automata and factor automata are efficient data structures for representing the full index of a set of strings. They are minimal deterministic automata representing the set of all suffixes or substrings of a set of strings. This paper presents a novel analysis of the size of the suffix automaton or factor automaton of a set of strings. It shows that the suffix automaton or factor automaton of a set of strings U has at most 2Q− 2 states, where Q is the number of nodes of a prefix-tree representing the strings in U . This bound significantly improves over 2‖U‖−1, the bound given by Blumer et al. (1987), where ‖U‖ is the sum of the lengths of all strings in U . More generally, we give novel and general bounds for the size of the suffix or factor automaton of an automaton as a function of the size of the original automaton and the maximal length of a suffix shared by the strings it accepts. We also describe in detail a linear-time algorithm for constructing the suffix automaton S or factor automaton F of U in time O(|S|). Our algorithm applies in fact to any input suffix-unique automaton and strictly generalizes the standard on-line construction of a suffix automaton for a single input string. Our algorithm can also be used straightforwardly to generate the suffix oracle or factor oracle of a set of strings, which has been shown to have various useful properties in string-matching. Our analysis suggests that the use of factor automata of automata can be practical for large-scale applications, a fact that is further supported by the results of our experiments applying factor automata to a music identification task with more than 15,000 songs.