The Impact of Literal Sorting on Cardinality Constraint Encodings

Joseph E. Reeves, João Filipe, Min-Chien Hsu, Ruben Martins, Marijn J. H. Heule

Abstract: The effectiveness of satisfiability solvers strongly depends on the quality of the encoding of a given problem into conjunctive normal form. Cardinality constraints are prevalent in numerous problems, prompting the development and study of various types of encoding. We present a novel approach to optimizing cardinality constraint encodings by exploring the impact of literal orderings within the constraints. By strategically placing related literals nearby each other, the encoding generates auxiliary variables in a hierarchical structure, enabling the solver to reason more abstractly about groups of related literals. Unlike conventional metrics such as formula size or propagation strength, our method leverages structural properties of the formula to redefine the roles of auxiliary variables to enhance the solver's learning capabilities. The experimental evaluation on benchmarks from the maximum satisfiability competition demonstrates that literal orderings can be more influential than the choice of the encoding type. Our literal ordering technique improves solver performance across various encoding techniques, underscoring the robustness of our approach.