Re-implementing and Extending a Hybrid SAT-IP Approach to Maximum Satisfiability

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Title: Re-implementing and Extending a Hybrid SAT-IP Approach to Maximum Satisfiability
Author: Saikko, Paul
Contributor: University of Helsinki, Faculty of Science, Department of Computer Science
Publisher: Helsingfors universitet
Date: 2015
Thesis level: master's thesis
Abstract: Real-world optimization problems, such as those found in logistics and bioinformatics, are often NP-hard. Maximum satisfiability (MaxSAT) provides a framework within which many such problems can be efficiently represented. MaxHS is a recent exact algorithm for MaxSAT. It is a hybrid approach that uses a SAT solver to compute unsatisfiable cores and an integer programming (IP) solver to compute minimum-cost hitting sets for the found cores. This thesis analyzes and extends the MaxHS algorithm. To enable this, the algorithm is re-implemented from scratch using the C++ programming language. The resulting MaxSAT solver LMHS recently gained top positions at an international evaluation of MaxSAT solvers. This work looks into various aspects of the MaxHS algorithm and its applications. The impact of different IP solvers on the MaxHS algorithm and the behavior induced by different strategies of postponing IP solver calls is examined. New methods of enhancing the computation of unsatisfiable cores in MaxHS are examined. Fast core extraction through parallelization by partitioning soft clauses is explored. A modification of the final conflict analysis procedure of a SAT solver is used to generate additional cores without additional SAT solver invocations. The use of additional constraint propagation procedures in the SAT solver used by MaxHS is investigated. As a case study, acyclicity constraint propagation is implemented and its effectiveness for bounded treewidth Bayesian network structure learning using MaxSAT is evaluated. The extension of MaxHS to the labeled MaxSAT framework, which allows for more efficient use of preprocessing techniques and group MaxSAT encodings in MaxHS, is discussed. The re-implementation of the MaxHS algorithm, LMHS, also enables incrementality in efficiently adding constraints to a MaxSAT instance during the solving process. As a case study, this incrementality is used in solving subproblems with MaxSAT within GOBNILP, a tool for finding optimal Bayesian network structures.

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