(Smart) Look-Ahead Arc Consistency and the Pursuit of CSP Tractability

Abstract

The constraint satisfaction problem (CSP) can be formulated as the problem of deciding, given a pair (A,B) of relational structures, whether or not there is a homomorphism from A to B. Although the CSP is in general intractable, it may be restricted by requiring the "target structure" B to be fixed; denote this restriction by CSP(B). In recent years, much effort has been directed towards classifying the complexity of all problems CSP(B). The acquisition of CSP(B) tractability results has generally proceeded by isolating a class of relational structures B believed to be tractable, and then demonstrating a polynomial-time algorithm for the class. In this paper, we introduce a new approach to obtaining CSP(B) tractability results: instead of starting with a class of structures, we start with an algorithm called look-ahead arc consistency, and give an algebraic characterization of the structures solvable by our algorithm. This characterization is used both to identify new tractable structures and to give new proofs of known tractable structures.