Pruning operators for answer set programming systems

Answer Set Programming (ASP) is a novel paradigm in Logic Programming, which permits solving problems in a simple and highly declarative way. The language of ASP (function-free disjunctive logic programming) is very expressive and supports the representation of problems of high computational complexity (specifically, all problems in the complexity class P2 = NPNP). Importantly, the ASP encoding of a large variety of problems is often very concise, simple, and elegant. In this paper, we explain the computational process performed by ASP systems, with a focus on search space pruning, which is crucial for the efficiency. We present two suitable operators for pruning (Fitting and Well-founded), discuss their peculiarities and differences with respect to efficiency and effectiveness. We design an intelligent strategy for combining the two operators, which exploits the advantages of both. We implement our approach in the ASP system DLV, and perform some experiments. The experiments show interesting results, and evidence how the choice of the pruning operator affects the performances of ASP systems. 1D.E.I.S., University of Calabria, 87030 Rende (CS), Italy. E-mail: [email protected] 2Institut für Informationssysteme, Technische Universität Wien, A-1040 Wien, Austria. E-mail: [email protected] 3Department of Mathematics, University of Calabria, 87030 Rende (CS), Italy. E-mail: [email protected] 4Institut für Informationssysteme, Technische Universität Wien, A-1040 Wien, Austria. E-mail: [email protected] Acknowledgements: This work was supported by FWF (Austrian Science Funds) under the projects Z29-INF and P14781. Copyright c 2001 by the authors 2 INFSYS RR 1843-01-07