518 research outputs found
Linear Bounded Composition of Tree-Walking Tree Transducers: Linear Size Increase and Complexity
Compositions of tree-walking tree transducers form a hierarchy with respect
to the number of transducers in the composition. As main technical result it is
proved that any such composition can be realized as a linear bounded
composition, which means that the sizes of the intermediate results can be
chosen to be at most linear in the size of the output tree. This has
consequences for the expressiveness and complexity of the translations in the
hierarchy. First, if the computed translation is a function of linear size
increase, i.e., the size of the output tree is at most linear in the size of
the input tree, then it can be realized by just one, deterministic,
tree-walking tree transducer. For compositions of deterministic transducers it
is decidable whether or not the translation is of linear size increase. Second,
every composition of deterministic transducers can be computed in deterministic
linear time on a RAM and in deterministic linear space on a Turing machine,
measured in the sum of the sizes of the input and output tree. Similarly, every
composition of nondeterministic transducers can be computed in simultaneous
polynomial time and linear space on a nondeterministic Turing machine. Their
output tree languages are deterministic context-sensitive, i.e., can be
recognized in deterministic linear space on a Turing machine. The membership
problem for compositions of nondeterministic translations is nondeterministic
polynomial time and deterministic linear space. The membership problem for the
composition of a nondeterministic and a deterministic tree-walking tree
translation (for a nondeterministic IO macro tree translation) is log-space
reducible to a context-free language, whereas the membership problem for the
composition of a deterministic and a nondeterministic tree-walking tree
translation (for a nondeterministic OI macro tree translation) is possibly
NP-complete
Comparison-Free Polyregular Functions.
This paper introduces a new automata-theoretic class of string-to-string functions with polynomialgrowth. Several equivalent definitions are provided: a machine model which is a restricted variant ofpebble transducers, and a few inductive definitions that close the class of regular functions undercertain operations. Our motivation for studying this class comes from another characterization,which we merely mention here but prove elsewhere, based on a λ-calculus with a linear type system.As their name suggests, these comparison-free polyregular functions form a subclass of polyregularfunctions; we prove that the inclusion is strict. We also show that they are incomparable withHDT0L transductions, closed under usual function composition â but not under a certain âmapâcombinator â and satisfy a comparison-free version of the pebble minimization theorem.On the broader topic of polynomial growth transductions, we also consider the recently introducedlayered streaming string transducers (SSTs), or equivalently k-marble transducers. We prove that afunction can be obtained by composing such transducers together if and only if it is polyregular,and that k-layered SSTs (or k-marble transducers) are closed under âmapâ and equivalent to acorresponding notion of (k + 1)-layered HDT0L systems
Register Transducers Are Marble Transducers
Deterministic two-way transducers define the class of regular functions from words to words. Alur and CernĂœ introduced an equivalent model of transducers with registers called copyless streaming string transducers. In this paper, we drop the âcopylessâ restriction on these machines and show that they are equivalent to two-way transducers enhanced with the ability to drop marks, named âmarblesâ, on the input. We relate the maximal number of marbles used with the amount of register copies performed by the streaming string transducer. Finally, we show that the class membership problems associated with these models are decidable. Our results can be interpreted in terms of program optimization for simple recursive and iterative programs.SCOPUS: cp.pinfo:eu-repo/semantics/publishe
Revisiting the growth of polyregular functions: output languages, weighted automata and unary inputs
Polyregular functions are the class of string-to-string functions definable
by pebble transducers (an extension of finite automata) or equivalently by MSO
interpretations (a logical formalism). Their output length is bounded by a
polynomial in the input length: a function computed by a -pebble transducer
or by a -dimensional MSO interpretation has growth rate .
Boja\'nczyk has recently shown that the converse holds for MSO
interpretations, but not for pebble transducers. We give significantly
simplified proofs of those two results, extending the former to first-order
interpretations by reduction to an elementary property of -weighted
automata. For any , we also prove the stronger statement that there is some
quadratic polyregular function whose output language differs from that of any
-fold composition of macro tree transducers (and which therefore cannot be
computed by any -pebble transducer).
In the special case of unary input alphabets, we show that pebbles
suffice to compute polyregular functions of growth . This is obtained
as a corollary of a basis of simple word sequences whose ultimately periodic
combinations generate all polyregular functions with unary input. Finally, we
study polyregular and polyblind functions between unary alphabets (i.e. integer
sequences), as well as their first-order subclasses.Comment: 27 pages, not submitted ye
Pebble transducers with unary output
Boja\'nczyk recently initiated an intensive study of deterministic pebble
transducers, which are two-way automata that can drop marks (named "pebbles")
on their input word, and produce an output word. They describe functions from
words to words. Two natural restrictions of this definition have been
investigated: marble transducers by Dou\'eneau-Tabot et al., and
comparison-free pebble transducers (that we rename here "blind transducers") by
Nguy\^en et al.
Here, we study the decidability of membership problems between the classes of
functions computed by pebble, marble and blind transducers that produce a unary
output. First, we show that pebble and marble transducers have the same
expressive power when the outputs are unary (which is false over non-unary
outputs). Then, we characterize 1-pebble transducers with unary output that
describe a function computable by a blind transducer, and show that the
membership problem is decidable. These results can be interpreted in terms of
automated simplification of programs.Comment: 39 page
On the growth rate of polyregular functions
We consider polyregular functions, which are certain string-to-string
functions that have polynomial output size. We prove that a polyregular
function has output size if and only if it can be defined by
an MSO interpretation of dimension , i.e. a string-to-string transformation
where every output position is interpreted, using monadic second-order logic
MSO, in some -tuple of input positions. We also show that this
characterization does not extend to pebble transducers, another model for
describing polyregular functions: we show that for every
there is a polyregular function of quadratic output size which needs at least
pebbles to be computed
Pebble Minimization of Polyregular Functions
We show that a polyregular word-to-word function is regular if and only if
its output size is at most linear in its input size. Moreover a polyregular
function can be realized by: a transducer with two pebbles if and only if its
output has quadratic size in its input, a transducer with three pebbles if and
only if its output has cubic size in its input, etc. Moreover the
characterization is decidable and, given a polyregular function, one can
compute a transducer realizing it with the minimal number of pebbles. We apply
the result to mso interpretations from words to words. We show that mso
interpretations of dimension k exactly coincide with k-pebble transductions.Comment: The main result of the article is false. Counterexamples and more can
be found here: arXiv:2301.0923
String-to-String Interpretations With Polynomial-Size Output
String-to-string MSO interpretations are like Courcelle\u27s MSO transductions, except that a single output position can be represented using a tuple of input positions instead of just a single input position. In particular, the output length is polynomial in the input length, as opposed to MSO transductions, which have output of linear length. We show that string-to-string MSO interpretations are exactly the polyregular functions. The latter class has various characterisations, one of which is that it consists of the string-to-string functions recognised by pebble transducers.
Our main result implies the surprising fact that string-to-string MSO interpretations are closed under composition
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