Constraint Hierarchies in Constraint Logic Programming Languages

Houria is an incremental solver that proposes a new implementation of constraint hierarchies. Houria uses local propagation to maintain sets of required and preferential constraints. It represents constraints between variables by sets of short procedures (methods) and incrementally re-satis es the set of constraints when individual constraints are added and removed. The criteria of comparison used in this solver are global. They allow the comparison of valuations, which are not comparable by local criteria used in existing solvers. The solution found by Houria satis es more constraints than the one produced by other solvers and that for the same over-constrained problems while respecting the semantics of the hierarchy. We also propose an e cient algorithm that integrates the Houria solver in the CLP paradigm.