Decomposition and tractability in qualitative spatial and temporal reasoning

Constraint networks in qualitative spatial and temporal reasoning (QSTR) typically feature variables defined on infinite domains. Mainstream algorithms for deciding network consistency are based on searching for network refinements whose consistency is known to be tractable, either directly or by using a SAT solver. Consequently, these algorithms treat all networks effectively as complete graphs, and are not directly amenable to complexity bounds based on network structure, such as measured by treewidth, that are well known in the finite-domain case. The present paper makes two major contributions, spanning both theory and practice. First, we identify a sufficient condition under which consistency can be decided in polynomial time for networks of bounded treewidth in QSTR, and show that this condition is satisfied by a range of calculi including the Interval Algebra, Rectangle Algebra, Block Algebra, RCC8, and RCC5. Second, we apply the techniques used in establishing these results to a SAT encoding of QSTR, and obtain a new, more compact encoding which is also guaranteed to be solvable in polynomial time for networks of bounded treewidth, and which leads to a significant advance of the state of the art in solving the hardest benchmark problems.