Landmark-Based Plan Distance Measures for Diverse Planning

Prior approaches to generating diverse plans in domain-independent  planning seek out variations on plan structure such as actions or  causal links used, or states entered.  Measuring such syntactic  differences between plans can be misleading because syntactically  different plans can be semantically identical.  We develop a  landmark-based plan distance measure that captures semantic  differences between plans. The landmark-based distance measure focuses on the disjunctive landmarks  satisfied by each plan.  We develop a  simple algorithm for finding diverse plans that is based upon the LAMA planner.  We illustrate that, in comparison with plan distance  measures,  landmark-based plan distance is  not as susceptible to including irrelevant or redundant actions in  plans to increase plan distance. Through extensive empirical evaluation, we find that  high landmark distance between plans implies high action set  distance, but not vice versa.  Landmark-based plan distance overcomes some of the weaknesses of syntactic plan distance measures and can be used to find plan sets that are both landmark diverse and action set diverse.