Short and Long Supports for Constraint Propagation
  Special-purpose constraint propagation algorithms frequently make implicit
use of short supports -- by examining a subset of the variables, they can infer
support (a justification that a variable-value pair may still form part of an
assignment that satisfies the constraint) for all other variables and values
and save substantial work -- but short supports have not been studied in their
own right. The two main contributions of this paper are the identification of
short supports as important for constraint propagation, and the introduction of
HaggisGAC, an efficient and effective general purpose propagation algorithm for
exploiting short supports. Given the complexity of HaggisGAC, we present it as
an optimised version of a simpler algorithm ShortGAC. Although experiments
demonstrate the efficiency of ShortGAC compared with other general-purpose
propagation algorithms where a compact set of short supports is available, we
show theoretically and experimentally that HaggisGAC is even better. We also
find that HaggisGAC performs better than GAC-Schema on full-length supports. We
also introduce a variant algorithm HaggisGAC-Stable, which is adapted to avoid
work on backtracking and in some cases can be faster and have significant
reductions in memory use. All the proposed algorithms are excellent for
propagating disjunctions of constraints. In all experiments with disjunctions
we found our algorithms to be faster than Constructive Or and GAC-Schema by at
least an order of magnitude, and up to three orders of magnitude.
