From Nondeterministic to Multi-Head Deterministic Finite-State Transducers

Martin Raszyk, David Basin, Dmitriy Traytel


Every nondeterministic finite-state automaton is equivalent to a deterministic finite-state automaton. This result does not extend to finite-state transducers—finite-state automata equipped with a one-way output tape. There is a strict hierarchy of languages accepted by one-way deterministic finite-state transducers (1DFTs), one-way nondeterministic finite-state transducers (1NFTs) and two-way non-deterministic finite-state transducers (2NFTs), whereas the two-way deterministic finite-state transducers (2DFTs) accept the same family of languages as their nondeterministic counterparts (2NFTs). We define multi-head one-way deterministic finite-state transducers (mh-1DFTs) as a natural extension of 1DFTs. These transducers have multiple one-way reading heads that move asynchronously over the input word. Our main result is to prove that mh-1DFTs can deterministically express any function defined by a one-way nondeterministic finite-state transducer. Of independent interest, we formulate the all suffix regular matching problem, which is the problem of deciding for each suffix of an input word whether it belongs to a regular language. As part of our proof, we show that a mh-1DFT can solve all suffix regular matching, which has applications, e.g., in runtime verification.

Paper draft