On the Complexity of HTN Plan Verification and Its Implications for Plan Recognition

Book One

Volume

Proceedings of the International Conference on Automated Planning and Scheduling, 25

Technical Papers

In classical planning it is easy to verify if a given sequence of actions is a solution to a planning problem. It has to be checked whether the actions are applicable in the given order and if a goal state is reached after executing them. In this paper we show that verifying whether a plan is a solution to an HTN planning problem is much harder. More specifically, we prove that this problem is NP-complete, even for very simple HTN planning problems. Furthermore, this problem remains NP-complete if an executable sequence of tasks is already provided. HTN-like hierarchical structures are commonly used to represent plan libraries in plan and goal recognition. By applying our result to plan and goal recognition we provide insight into its complexity.

10.1609/icaps.v25i1.13728