Time-Bounded Best-First Search

Abstract

Time-Bounded A* (TBA*) is a single-agent deterministic search algorithm that expands states of a graph in the same order as A* does, but that unlike A* interleaves search and action execution. Although the idea underlying TBA* can be generalized to other single-agent deterministic search algorithms, little is known about the impact on performance that would result from using algorithms other than A*. In this paper we propose Time-Bounded Best-First Search (TB-BFS) a generalization of the time-bounded approach to any best-first search algorithm. Furthermore, we propose restarting strategies that allow TB-BFS to solve search problems in dynamic environments. In static environments, we prove that the resulting framework allows agents to always find a solution if such a solution exists, and prove cost bounds for the solutions returned by Time-Bounded Weighted A* (TB-WA*). We evaluate the performance of TB-WA* and Time-Bounded Greedy Best-First Search (TB-GBFS). We show that in pathfinding applications in static domains, TB-WA* and TB-GBFS are not only faster than TBA* but also find significantly better solutions in terms of cost. In the context of videogame pathfinding, TB-WA* and TB-GBFS perform fewer undesired movements than TBA*. Restarting TB-WA* was also evaluated in dynamic pathfinding random maps, where we also observed improved performance compared to restarting TBA*. Our experimental results seem consistent with theoretical bounds.