HTN Problem Spaces: Structure, Algorithms, Termination
Ron Alford, Vikas Shivashankar, Ugur Kuter, Dana S. Nau · Proceedings of the International Symposium on Combinatorial Search · 2021
For HTN planning, we formally characterize and classify four kinds of problem spaces in which each node represents a planning problem or subproblem. Two of the problem spaces are searched by current HTN planning algorithms; the other two problem spaces are new.This enables us to provide:Sufficient (and in one case, necessary) conditions for finiteness of each kind of problem space. The conditions can be evaluated up-front to see if an HTN planning problem is finite.Loop-detection tests that can be used in HTN planners to ensure termination when the problem space is finite.A way to compute the correct value for an upper-bound parameter in an HTN-to-PDDL translation algorithm published in IJCAI-2009.Planning algorithms that utilize the two new problem spaces to guarantee termination on broader classes of planning problems than previous HTN planning algorithms.