A Simple Constraint Solver in Action Rules for the CP’05 Solver Competition

In response to the call for solvers by the CPAI’05 solver competition organizing committee, we implemented a constraint solver, called Mosar, using action rules (AR) in B-Prolog [3] particularly for this competition. The problem format used in the competition is very different from those used by practical solvers. For example, constraints are extensional numeric relations not implicit relations, and only support and conflict constraints are allowed. Our goal was to build a working solver very quickly (in one or two days) rather than to implement a winning solver. Therefore, no optimization is implemented at all. Mosar is a very simple solver. It performs forward checking for unary constraints, maintains a weak-form of interval consistency for non-binary support constraints, and maintains arc-consistency for binary support constraints. Thanks to the high descriptive power of action rules, the solver, which consists of only about 200 lines of AR and Prolog code, was completed in only one day. As there is no solver of this type available for comparison, the performance of Mosar is unclear. Mosar has successfully solved most of the sample benchmarks including the quasi-group, Domino, and all-interval series problems collected by the organizing committee. This note describes the solver and reports on the performance. It also gives several optimizations that could lead to better performance. The source code of the solver is available at: www.probp.com/mosar.