Recent Advances in Constraints

Volume 2627 of the series Lecture Notes in Computer Science pp 15-30


CGRASS: A System for Transforming Constraint Satisfaction Problems

  • Alan M. FrischAffiliated withAI Group, Dept. Computer Science, University of York
  • , Ian MiguelAffiliated withAI Group, Dept. Computer Science, University of York
  • , Toby WalshAffiliated withCork Constraint Computation Center, University College Cork

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Experts at modelling constraint satisfaction problems (CSPs) carefully choose model transformations to reduce greatly the amount of effort that is required to solve a problem by systematic search. It is a considerable challenge to automate such transformations and to identify which transformations are useful. Transformations include adding constraints that are implied by other constraints, adding constraints that eliminate symmetrical solutions, removing redundant constraints and replacing constraints with their logical equivalents. This paper describes the CGRASS (Constraint Generation And Symmetry-breaking) system that can improve a problem model by automatically performing transformations of these kinds. We focus here on transforming individual CSP instances. Experiments on the Golomb ruler problem suggest that producing good problem formulations solely by transforming problem instances is, generally, infeasible. We argue that, in certain cases, it is better to transform the problem class than individual instances and, furthermore, it can sometimes be better to transform formulations of a problem that are more abstract than a CSP.