Paul Saikko

LMHS: A SAT-IP Hybrid MaxSAT Solver

We describe LMHS, an open-source weighted partial maximum satisfiability (MaxSAT) solver. LMHS is a hybrid SAT-IP MaxSAT solver that implements the implicit hitting set approach to MaxSAT. On top of the main algorithm, LMHS offers integrated preprocessing, solution enumeration, an incremental API, and the use of a choice of SAT and IP solvers. We describe the main features of LMHS, and give empirical results on the influence of preprocessing and the choice of the underlying SAT and IP solvers on the performance of LMHS.

MaxSAT-Based Cutting Planes for Learning Graphical Models

A way of implementing domain-specific cutting planes in branch-and-cut based Mixed-Integer Programming (MIP) solvers is through solving so-called sub-IPs, solutions of which correspond to the actual cuts. We consider the suitability of using Maximum satisfiability solvers instead of MIP for solving sub-IPs. As a case study, we focus on the problem of learning optimal graphical models, namely, Bayesian and chordal Markov network structures.

Re-using Auxiliary Variables for MaxSAT Preprocessing

Solvers for the maximum satisfiability (MaxSAT) problem -- a well-known optimization variant of Boolean satisfiability (SAT) -- are finding an increasing number of applications. Preprocessing has proven an integral part of the SAT-based approach to efficiently solving various types of real-world problem instances. It was recently shown that SAT preprocessing for MaxSAT becomes more effective by re-using the auxiliary variables introduced in the preprocessing phase directly in the SAT solver within a core-based hybrid MaxSAT solver. We take this idea of re-using auxiliary variables further by identifying them among variables already present in the input MaxSAT instance. Such variables can be re-used already in the preprocessing step, avoiding the introduction of multiple layers of new auxiliary variables in the process. Empirical results show that by detecting auxiliary variables in the input MaxSAT instances can lead to modest additional runtime improvements when applied before preprocessing. Furthermore, we show that by re-using auxiliary variables not only within preprocessing but also as assumptions within the SAT solver of the MaxHS MaxSAT algorithm can alone lead to performance improvements similar to those observed by applying SAT-based preprocessing.

Reduced Cost Fixing in MaxSAT

We investigate utilizing the integer programming (IP) technique of reduced cost fixing to improve maximum satisfiability (MaxSAT) solving. In particular, we show how reduced cost fixing can be used within the implicit hitting set approach (IHS) for solving MaxSAT. Solvers based on IHS have proved to be quite effective for MaxSAT, especially on problems with a variety of clause weights. The unique feature of IHS solvers is that they utilize both SAT and IP techniques. We show how reduced cost fixing can be used in this framework to conclude that some soft clauses can be left falsified or forced to be satisfied without influencing the optimal cost. Applying these forcings simplifies the remaining problem. We provide an extensive empirical study showing that reduced cost fixing employed in this manner can be useful in improving the state-of-the-art in MaxSAT solving especially on hard instances arising from real-world application domains.

Subsumed Label Elimination for Maximum Satisfiability

We propose subsumed label elimination (SLE), a socalled label-based preprocessing technique for the Boolean optimization paradigm of maximum satisfiability (MaxSAT). We formally show that SLE is orthogonal to previously proposed SAT-based preprocessing techniques for MaxSAT in that it can simplify the underlying minimal unsatisfiable core structure of MaxSAT instances. We also formally show that SLE can considerably reduce the number of internal SAT solver calls within modern core-guided MaxSAT solvers. Empirically, we show that combining SLE with SAT-based preprocessing improves the performance of various state-of-the-art MaxSAT solvers on standard industrial weighted partial MaxSAT benchmarks.

Reduced Cost Fixing for Maximum Satisfiability.

Maximum satisfiability (MaxSAT) offers a competitive approach to solving NP-hard real-world optimization problems. While state-of-the-art MaxSAT solvers rely heavily on Boolean satisfiability (SAT) solvers, a recent trend, brought on by MaxSAT solvers implementing the so-called implicit hitting set (IHS) approach, is to integrate techniques from the realm of integer programming (IP) into the solving process. This allows for making use of additional IP solving techniques to further speed up MaxSAT solving. In this line of work, we investigate the integration of the technique of reduced cost fixing from the IP realm into IHS solvers, and empirically show that reduced cost fixing considerable speeds up a state-of-the-art MaxSAT solver implementing the IHS approach.

MaxPre: An Extended MaxSAT Preprocessor

We describe MaxPre, an open-source preprocessor for (weighted partial) maximum satisfiability (MaxSAT). MaxPre implements both SAT-based and MaxSAT-specific preprocessing techniques, and offers solution reconstruction, cardinality constraint encoding, and an API for tight integration into SAT-based MaxSAT solvers.

A Core-Guided Approach to Learning Optimal Causal Graphs

Discovery of causal relations is an important part of data analysis. Recent exact Boolean optimization approaches enable tackling very general search spaces of causal graphs with feedback cycles and latent confounders, simultaneously obtaining high accuracy by optimally combining conflicting independence information in sample data. We propose several domain-specific techniques and integrate them into a core-guided maximum satisfiability solver, thereby speeding up current state of the art in exact search for causal graphs with cycles and latent confounders on simulated and real-world data.