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Summary: Nondeterministic Streaming String Transducers
Rajeev Alur and Jyotirmoy V. Deshmukh
Dept. of Computer and Information Science,
University of Pennsylvania.
{alur,djy}@cis.upenn.edu
Abstract. We introduce nondeterministic streaming string transducers
(nssts) a new computational model that can implement MSO-definable
relations between strings. An nsst makes a single left-to-right pass on
the input string and uses a finite set of string variables to compute the
output. In each step, it reads one input symbol, and updates its string
variables in parallel with a copyless assignment. We show that nsst are
closed under sequential composition and that their expressive power co-
incides with that of nondeterministic MSO-definable transductions. Fur-
ther, we identify the class of functional nssts; such an nsst allows non-
deterministic transitions, but for every successful run on a given input
generates the same output string. We show that deciding functionality of
an arbitrary nsst is decidable with pspace complexity, while the equiv-
alence problem for functional nssts is pspace-complete. We also show
that checking if the set of outputs of an nsst is contained within the set
of outputs of a finite number of dssts is decidable in pspace.
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