Automatic Move Pruning Revisited

In this paper we show that the move pruning method we presented at SoCS last year sometimes prunes all the least-cost paths from one state to another. We present two examples exhibiting this erroneous behaviour--a simple, artificial example and a slightly more complex example that arose in last year's experiments. We then formally prove that a simple modification to our move pruning method makes it "safe," i.e., it will never prune all the least-cost paths between a pair of states. Finally, we present the results of rerunning last year's experiments with this provably safe version of move pruning and show that last year's main conclusions still hold, namely: (1) in domains where there are short redundant sequences move pruning produces substantial speedups in depth-first search, and (2) in domains where the only short redundant sequences are 2-cycles, move pruning is faster than parent pruning by a factor of two or more.