LM-BFS: A Framework for Landmarks in Planning

Clemens Büchner, Thomas Keller

Keywords: classical planning, landmarks, heuristic search

Abstract:

It is usually prohibitively expensive to recompute landmarks in every state of a heuristic search algorithm that bases its heuristic estimate on landmarks. Successful planning systems like LAMA compute landmarks just for the initial state instead and only keep track for each state encountered during search which landmarks of the initial state are accepted or required again. The LM-A* algorithm describes a similar idea for cost-optimal heuristic search. However, it lacks support for reasonable orderings, which have recently been shown to be informative when cyclic dependencies between landmarks are exploited in the heuristic computation.

We propose the novel LM-BFS framework for best-first heuristic search with landmark-based heuristics which allows to describe a variety of algorithms in terms of five components: landmark generation, landmark graph progression, landmark graph merging, landmark graph extension, and exploration strategy. We use LM-BFS to show that considering reasonable orderings in the landmark graph progression component leads to flawed landmark graphs in LAMA , and propose a novel landmark graph extension that captures the same information flawlessly. We observe empirically that considering reasonable orderings in the extension rather than the progression component has a positive effect both for LAMA and for cost-optimal planning.