PRA*: a memory-limited heuristic search procedure for the Connection Machine

A variant of A* search designed to run on the massively parallel SIMD (single-instruction-stream, multiple-data-steam) Connection Machine is described. The algorithm is designed to run in a limited memory; a retraction technique allows nodes with poor heuristic values to be removed from the open list until such time as they may need reexpansion if more promising paths fail. The algorithm, called PRA* (for parallel retraction A*), takes maximum advantage of the SIMD design of the Connection Machine and is guaranteed to return an optimal path when an admissible heuristic is used. Results comparing PRA* to R. Korf's IDA* (see Artif. Intell. J., vol.27, 1985) for the 15 puzzle show significantly fewer node expansions for PRA*.<
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