Combining rules and constraints for employee timetabling

Employee Timetabling Problems (ETP) are all around us. One possible approach for solving ETPs is to use constraint processing techniques. Another approach is to model human knowledge which is commonly used for solving such problems into knowledge-based systems for timetabling. It is difficult to represent the complex constraints of timetabling explicitly in constraint networks. On the other hand, knowledge-based representations of constraints are implicit and cannot support most of the heuristics of constraint-based processing that have been developed over the last decade. The present article presents an approach to representing and processing employee timetabling problems by a combination of explicit representations of some constraints and rule-based processing with heuristics for generic ETP constraints. This mixed-mode approach has been implemented in the form of a software package for defining and solving real-world ETPs. A general description of the design and organization of this software tool is given. Results for solving a typical real-world employee timetabling problem are presented and a comparison with the use of standard CSP (Constraint Satisfaction Problems) techniques is made.