Right Branches Matter in Failure-based Variable Ordering Heuristics

Failure-based variable ordering heuristics (VOH) are efficient general-purpose search heuristics for solving constraint satisfaction problems (CSP). They learn from the failures detected during the search and select the variables that are most likely to fail. The current failure-based VOHs, i.e. the failure-rate-based (FRBA) and failure-length-based (FLBA), focus on only the failures detected in left branches. In this paper, we investigate how the failure information from right branches affects the performance of the failure-based VOHs. Four strategies utilizing the failure information of right branches are proposed to refine the failure-based VOHs. Our experiments performed with the benchmark instances used in the recent MiniZinc challenges show that utilizing the failures detected in right branches enhances the performance of the failure-based VOHs. The refined version combining all the proposed strategies generally gets the best performance. It demonstrates remarkable superiority over several general-purpose VOHs, including activity-based search, conflict-history search, refined weighted degree, pick/dom, and the existing FRBA, which are considered state-of-the-art. Our study demonstrates that right branches matter in failure-based VOHs.