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# Damn Cool Algorithms: Levenshtein Automata

blog.notdot.net/2010/07/Damn-Cool-Algorithms-Levenshtein-Automata, posted Mar '22 by peter in development nlp reference text toread

The basic insight behind Levenshtein automata is that it's possible to construct a Finite state automaton that recognizes exactly the set of strings within a given Levenshtein distance of a target word. We can then feed in any word, and the automaton will accept or reject it based on whether the Levenshtein distance to the target word is at most the distance specified when we constructed the automaton. Further, due to the nature of FSAs, it will do so in O(n) time with the length of the string being tested. Compare this to the standard Dynamic Programming Levenshtein algorithm, which takes O(mn) time, where m and n are the lengths of the two input words! It's thus immediately apparrent that Levenshtein automaton provide, at a minimum, a faster way for us to check many words against a single target word and maximum distance - not a bad improvement to start with!

Of course, if that were the only benefit of Levenshtein automata, this would be a short article. There's much more to come, but first let's see what a Levenshtein automaton looks like, and how we can build one.