Hitting Set Heuristics for Overlapping Landmarks in Satisficing Planning

Landmarks are a core component of LAMA, a state-of-the-art satisficing planning system based on heuristic search. It uses landmarks to estimate the goal distance by summing up the costs of their cheapest achievers. This procedure ignores synergies between different landmarks: The cost of an action is counted multiple times if it is the cheapest achiever of several landmarks. Common admissible landmark heuristics tackle this problem by underapproximating the cost of a minimum hitting set of the landmark achievers. We suggest to overapproximate it by computing suboptimal hitting sets instead if admissibility is not a requirement. As our heuristics consider synergies between landmarks, we further propose to relax certain restrictions LAMA imposes on the number of landmarks and synergies between them. Our experimental evaluation shows a reasonable increase in the number of landmarks that leads to better guidance when used with our new heuristics.