Martin Kronegger

The Fourth Answer Set Programming Competition: Preliminary Report

Answer Set Programming is a well-established paradigm of declarative programming in close relationship with other declarative formalisms such as SAT Modulo Theories, Constraint Handling Rules, PDDL and many others. Since its first informal editions, ASP systems are compared in the nowadays customary ASP Competition. The fourth ASP Competition, held in 2012/2013, is the sequel to previous editions and it was jointly organized by University of Calabria Italy and the Vienna University of Technology Austria. Participants competed on a selected collection of benchmark problems, taken from a variety of research areas and real world applications. The Competition featured two tracks: the Model& Solve Track, held on an open problem encoding, on an open language basis, and open to any kind of system based on a declarative specification paradigm; and the System Track, held on the basis of fixed, public problem encodings, written in a standard ASP language.

Conformant Planning as a Case Study of Incremental QBF Solving

We consider planning with uncertainty in the initial state as a case study of incremental quantified Boolean formula (QBF) solving. We report on experiments with a workflow to incrementally encode a planning instance into a sequence of QBFs. To solve this sequence of successively constructed QBFs, we use our general-purpose incremental QBF solver DepQBF. Since the generated QBFs have many clauses and variables in common, our approach avoids redundancy both in the encoding phase and in the solving phase. Experimental results show that incremental QBF solving outperforms non-incremental QBF solving. Our results are the first empirical study of incremental QBF solving in the context of planning and motivate its use in other application domains.

Conformant planning as a case study of incremental QBF solving

We consider planning with uncertainty in the initial state as a case study of incremental quantified Boolean formula (QBF) solving. We report on experiments with a workflow to incrementally encode a planning instance into a sequence of QBFs. To solve this sequence of successively constructed QBFs, we use our general-purpose incremental QBF solver DepQBF. Since the generated QBFs have many clauses and variables in common, our approach avoids redundancy both in the encoding phase as well as in the solving phase. We also present experiments with incremental preprocessing techniques that are based on blocked clause elimination (QBCE). QBCE allows to eliminate certain clauses from a QBF in a satisfiability preserving way. We implemented the QBCE-based techniques in DepQBF in three variants: as preprocessing, as inprocessing (which extends preprocessing by taking into account variable assignments that were fixed by the QBF solver), and as a novel dynamic approach where QBCE is tightly integrated in the solving process. For DepQBF, experimental results show that incremental QBF solving with incremental QBCE outperforms incremental QBF solving without QBCE, which in turn outperforms nonincremental QBF solving. For the first time we report on incremental QBF solving with incremental QBCE as inprocessing. Our results are the first empirical study of incremental QBF solving in the context of planning and motivate its use in other application domains.

VCWC: A Versioning Competition Workflow Compiler

System competitions evaluate solvers and compare state-of-the-art implementations on benchmark sets in a dedicated and controlled computing environment, usually comprising of multiple machines. Recent initiatives such as [6] aim at establishing best practices in computer science evaluations, especially identifying measures to be taken for ensuring repeatability, excluding common pitfalls, and introducing appropriate tools. For instance, Asparagus [1] focusses on maintaining benchmarks and instances thereof. Other known tools such as Runlim http://fmv.jku.at/runlim/ and Runsolver [12] help to limit resources and measure CPU time and memory usage of solver runs. Other systems are tailored at specific needs of specific communities: the not publicly accessible ASP Competition evaluation platform for the 3rd ASP Competition 2011 [4] implements a framework for running a ASP competition. Another more general platform is StarExec [13], which aims at providing a generic framework for competition maintainers. The last two systems are similar in spirit, but each have restrictions that reduce the possibility of general usage: the StarExec platform does not provide support for generic solver input and has no scripting support, while the ASP Competition evaluation platform has no support for fault-tolerant execution of instance runs.Moreover, benchmark statistics and ranking can only be computed after all solver runs for all benchmark instances have been completed.

A SAT-Based Debugging Tool for State Machines and Sequence Diagrams

An effective way to model message exchange in complex settings is to use UML sequence diagrams in combination with state machine diagrams. A natural question that arises in this context is whether these two views are consistent, i.e., whether a desired or forbidden scenario modeled in the sequence diagram can be or cannot be executed by the state machines.In case of an inconsistency, a concrete communication trace of the state machines can give valuable information for debugging purposes on the model level.This trace either hints to a message in the sequence diagram where the communication between the state machines fails, or describes a concrete forbidden communication trace between the state machines.To detect and explain such inconsistencies, we propose a novel SAT-based formalization which can be solved automatically by an off-the-shelf SAT solver. To this end, we present the formal and technical foundations needed for the SAT-encoding, and an implementation inside the Eclipse Modeling Framework (EMF). We evaluate the effectiveness of our approach using grammar-based fuzzing.

Intra- and interdiagram consistency checking of behavioral multiview models

Multiview modeling languages like UML are a very powerful tool to deal with the ever increasing complexity of modern software systems. By splitting the description of a system into different views-the diagrams in the case of UML-system properties relevant for a certain development activity are highlighted while other properties are hidden. This multiview approach has many advantages for the human modeler, but at the same time it is very susceptible to various kinds of defects that may be introduced during the development process. Besides defects which relate only to one view, it can also happen that two different views, which are correct if considered independently, contain inconsistent information when combined. Such inconsistencies between different views usually indicate a defect in the model and can be critical if they propagate up to the executable system.In this paper, we present an approach to formally verify the reachability of a global state of a set of communicating UML state machines, i.e., we present a solution for an intradiagram consistency checking problem. We then extend this approach to solve an interdiagram consistency checking problem. In particular, we verify whether the message exchange modeled in a UML sequence diagram conforms to a set of communicating state machines.For solving both kinds of problems, we proceed as follows. As a first step, we formalize the semantics of UML state machines and of UML sequence diagrams. In the second step, we build upon this formal semantics and encode both verification tasks as decision problems of propositional logic (SAT) allowing the use of efficient SAT technology. We integrate both approaches in a graphical modeling environment, enabling modelers to use formal verification techniques without any special background knowledge. We experimentally evaluate the scalability of our approach. HighlightsFormalization of the semantics of a modeling language similar to UML.Consistency checking via SAT encodings.Direct integration of verification in modeling environment.

Parameterized Complexity of Asynchronous Border Minimization

Microarrays are research tools used in gene discovery as well as disease and cancer diagnostics. Two prominent but challenging problems related to microarrays are the Border Minimization Problem (BMP) and the Border Minimization Problem with given placement (P-BMP). In this paper we investigate the parameterized complexity of natural variants of BMP and P-BMP, termed \(\mathrm{BMP}^e\) and \(\hbox {P-BMP}^{e}\) respectively, under several natural parameters. We show that \(\mathrm{BMP}^e\) and \(\hbox {P-BMP}^{e}\) are in FPT under the following two combinations of parameters: (\(1\)) the size of the alphabet (\(c\)), the maximum length of a sequence (string) in the input (\(\ell \)) and the number of rows of the microarray (\(r\)); and, (\(2\)) the size of the alphabet and the size of the border length (\(o\)). Furthermore, \(\hbox {P-BMP}^{e}\) is in FPT when parameterized by \(c\) and \(\ell \). We complement our tractability results with corresponding hardness results.