The IPC 2020 only featured Hierarchical Task Network (HTN) planning, so all tracks were concerned with this hierarchical problem class.
Based on submitted benchmark domains and a questionnaire we offered the following tracks:
- totally ordered HTN problems, recursive
- partially ordered HTN problems, recursive
These are defined as follows:
- total-order: A domain is totally ordered iff the subtasks in all methods and in the initial task network form a sequence, i.e., the declared ordering arranges the tasks in a sequence.
- partial-order: A domain is partially ordered iff the subtasks in all methods and in the initial task network may have any order that can be specified in HDDL. (That formally includes total-order domains as well, though our domains in the partial-order track do not contain totally ordered ones.)
- non-recursive: A domain is non-recursive if there is a total order on its lifted tasks (primitive and abstract) such that for every method that decomposes an abstract task A, the tasks in the methods' task network occur after A in the total order. (Informally, this means that no task may be decomposed into itself.)
- recursive: A domain that is not non-recursive
Originally we also had a totally ordered track without recursion planned, though it was cancelled due to too few planner and domain submissions.
All problems are defined using the standard HDDL (an extension to PDDL that features task hierarchies). Please find more information in the Benchmarks section.
All tracks share common restrictions on the input:
- All preconditions and effects contain only literals (which may be negative), conjunctions, and universal quantifiers (forall),
- all actions have unit-cost, and
- methods may contain preconditions, i.e., a list of literals. A method may only be applied if its method precondition "holds". For totally-ordered HTN planning problems, the precondition has to hold in the state in which the first action (that originates via further decompositions from the method's subtasks) is executed. For partially-ordered HTN planning problems, the precondition has to hold in some state before the first action (that originates via further decompositions from the methods' subtasks) is executed, but after the state resulting from the last necessarily preceding tasks for the tasks contained in the method.