Conflict-Directed Backjumping Revisited

In recent years, many improvements to backtracking algorithms for solving constraint satisfaction problems have been proposed. The techniques for improving backtracking algorithms can be conveniently classiied as look-ahead schemes and look-back schemes. Unfortunately , look-ahead and look-back schemes are not entirely orthogonal as it has been observed empirically that the enhancement of look-ahead techniques is sometimes counter-productive to the eeects of look-back techniques. In this paper, we focus on the relationship between the two most important look-ahead techniques|using a variable ordering heuris-tic and maintaining a level of local consistency during the backtracking search|and the look-back technique of connict-directed backjumping (CBJ). We show that there exists a \perfect" dynamic variable ordering such that CBJ becomes redundant. We also show theoretically that as the level of local consistency that is maintained in the backtracking search is increased, the less that backjumping will be an improvement. Our theoretical results partially explain why a backtracking algorithm doing more in the look-ahead phase cannot beneet more from the backjumping look-back scheme. Finally, we show empirically that adding CBJ to a backtracking algorithm that maintains generalized arc consistency (GAC), an algorithm that we refer to as GAC-CBJ, can still provide orders of magnitude speedups. Our empirical results contrast with Bessi ere and R egin's conclusion (1996) that CBJ is useless to an algorithm that maintains arc consistency.