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Track B: Automata, Logic, Semantics, and Theory of Programming

**Published in:** LIPIcs, Volume 229, 49th International Colloquium on Automata, Languages, and Programming (ICALP 2022)

We study synthesis of reactive systems interacting with environments using an infinite data domain. A popular formalism for specifying and modelling such systems is register automata and transducers. They extend finite-state automata by adding registers to store data values and to compare the incoming data values against stored ones. Synthesis from nondeterministic or universal register automata is undecidable in general. However, its register-bounded variant, where additionally a bound on the number of registers in a sought transducer is given, is known to be decidable for universal register automata which can compare data for equality, i.e., for data domain (ℕ, =). This paper extends the decidability border to the domain (ℕ, <) of natural numbers with linear order. Our solution is generic: we define a sufficient condition on data domains (regular approximability) for decidability of register-bounded synthesis. The condition is satisfied by natural data domains like (ℕ, <). It allows one to use simple language-theoretic arguments and avoid technical game-theoretic reasoning. Further, by defining a generic notion of reducibility between data domains, we show the decidability of synthesis in the domain (ℕ^d, <^d) of tuples of numbers equipped with the component-wise partial order and in the domain (Σ^*,≺) of finite strings with the prefix relation.

Léo Exibard, Emmanuel Filiot, and Ayrat Khalimov. A Generic Solution to Register-Bounded Synthesis with an Application to Discrete Orders. In 49th International Colloquium on Automata, Languages, and Programming (ICALP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 229, pp. 122:1-122:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{exibard_et_al:LIPIcs.ICALP.2022.122, author = {Exibard, L\'{e}o and Filiot, Emmanuel and Khalimov, Ayrat}, title = {{A Generic Solution to Register-Bounded Synthesis with an Application to Discrete Orders}}, booktitle = {49th International Colloquium on Automata, Languages, and Programming (ICALP 2022)}, pages = {122:1--122:19}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-235-8}, ISSN = {1868-8969}, year = {2022}, volume = {229}, editor = {Boja\'{n}czyk, Miko{\l}aj and Merelli, Emanuela and Woodruff, David P.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2022.122}, URN = {urn:nbn:de:0030-drops-164634}, doi = {10.4230/LIPIcs.ICALP.2022.122}, annote = {Keywords: Synthesis, Register Automata, Transducers, Ordered Data Domains} }

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**Published in:** LIPIcs, Volume 187, 38th International Symposium on Theoretical Aspects of Computer Science (STACS 2021)

Register automata are finite automata equipped with a finite set of registers in which they can store data, i.e. elements from an unbounded or infinite alphabet. They provide a simple formalism to specify the behaviour of reactive systems operating over data ω-words. We study the synthesis problem for specifications given as register automata over a linearly ordered data domain (e.g. (ℕ, ≤) or (ℚ, ≤)), which allow for comparison of data with regards to the linear order. To that end, we extend the classical Church synthesis game to infinite alphabets: two players, Adam and Eve, alternately play some data, and Eve wins whenever their interaction complies with the specification, which is a language of ω-words over ordered data. Such games are however undecidable, even when the specification is recognised by a deterministic register automaton. This is in contrast with the equality case, where the problem is only undecidable for nondeterministic and universal specifications.
Thus, we study one-sided Church games, where Eve instead operates over a finite alphabet, while Adam still manipulates data. We show they are determined, and deciding the existence of a winning strategy is in ExpTime, both for ℚ and ℕ. This follows from a study of constraint sequences, which abstract the behaviour of register automata, and allow us to reduce Church games to ω-regular games. Lastly, we apply these results to the transducer synthesis problem for input-driven register automata, where each output data is restricted to be the content of some register, and show that if there exists an implementation, then there exists one which is a register transducer.

Léo Exibard, Emmanuel Filiot, and Ayrat Khalimov. Church Synthesis on Register Automata over Linearly Ordered Data Domains. In 38th International Symposium on Theoretical Aspects of Computer Science (STACS 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 187, pp. 28:1-28:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{exibard_et_al:LIPIcs.STACS.2021.28, author = {Exibard, L\'{e}o and Filiot, Emmanuel and Khalimov, Ayrat}, title = {{Church Synthesis on Register Automata over Linearly Ordered Data Domains}}, booktitle = {38th International Symposium on Theoretical Aspects of Computer Science (STACS 2021)}, pages = {28:1--28:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-180-1}, ISSN = {1868-8969}, year = {2021}, volume = {187}, editor = {Bl\"{a}ser, Markus and Monmege, Benjamin}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2021.28}, URN = {urn:nbn:de:0030-drops-136735}, doi = {10.4230/LIPIcs.STACS.2021.28}, annote = {Keywords: Synthesis, Church Game, Register Automata, Transducers, Ordered Data Words} }

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**Published in:** LIPIcs, Volume 140, 30th International Conference on Concurrency Theory (CONCUR 2019)

Traditional synthesis algorithms return, given a specification over finite sets of input and output Boolean variables, a finite-state transducer all whose computations satisfy the specification. Many real-life systems have an infinite state space. In particular, behaviors of systems with a finite control yet variables that range over infinite domains, are specified by automata with infinite alphabets. A register automaton has a finite set of registers, and its transitions are based on a comparison of the letters in the input with these stored in its registers. Unfortunately, reasoning about register automata is complex. In particular, the synthesis problem for specifications given by register automata, where the goal is to generate correct register transducers, is undecidable.
We study the synthesis problem for systems with a bounded number of registers. Formally, the register-bounded realizability problem is to decide, given a specification register automaton A over infinite input and output alphabets and numbers k_s and k_e of registers, whether there is a system transducer T with at most k_s registers such that for all environment transducers T' with at most k_e registers, the computation T|T', generated by the interaction of T with T', satisfies the specification A. The register-bounded synthesis problem is to construct such a transducer T, if exists. The bounded setting captures better real-life scenarios where bounds on the systems and/or its environment are known. In addition, the bounds are the key to new synthesis algorithms, and, as recently shown in [A. Khalimov et al., 2018], they lead to decidability. Our contributions include a stronger specification formalism (universal register parity automata), simpler algorithms, which enable a clean complexity analysis, a study of settings in which both the system and the environment are bounded, and a study of the theoretical aspects of the setting; in particular, the differences among a fixed, finite, and infinite number of registers, and the determinacy of the corresponding games.

Ayrat Khalimov and Orna Kupferman. Register-Bounded Synthesis. In 30th International Conference on Concurrency Theory (CONCUR 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 140, pp. 25:1-25:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{khalimov_et_al:LIPIcs.CONCUR.2019.25, author = {Khalimov, Ayrat and Kupferman, Orna}, title = {{Register-Bounded Synthesis}}, booktitle = {30th International Conference on Concurrency Theory (CONCUR 2019)}, pages = {25:1--25:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-121-4}, ISSN = {1868-8969}, year = {2019}, volume = {140}, editor = {Fokkink, Wan and van Glabbeek, Rob}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2019.25}, URN = {urn:nbn:de:0030-drops-109277}, doi = {10.4230/LIPIcs.CONCUR.2019.25}, annote = {Keywords: Synthesis, Register Automata, Register Transducers} }

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