fast forward and pddl: is the computed solution the best?

问题

how can i be sure that the plan, computed by the fast forward planner, is the best of all the possible plans?!

Does exist an automatic tool to solve this problem?!

thanks a lot!

回答1:

If I don't remember it wrong FF is not an optimal planner so you can't be sure if the generated plan is optimal. On the other side FF is fast in generating "good enough" solutions in contrast to optimal planners (cpt4, bjolp, ecc...) which provide optimal plans but much more slowly than satisficing planners.

You can find a list of these planner here: IPC2011 Planners

This is the only way I can imagine to obtain the best plan apart from writing down the full search space and using A* on it.

回答2:

As Demplo already pointed out, FF does not guarantee to find optimal solutions. The reason for it is important to know in case you really want to find optimal solutions:

1. the algorithm it uses (enforced hillclimbing) does not provide optimality guarantees
2. the heuristic it uses (FF heuristic) is also not suited to find optimal solutions.

To "fix" both, one needs to use the A* algorithm in combination with an admissible heuristic.

I recommend to install the well-known Fast Downward Planning System (www.fast-downward.org/), as it supports a large number of different algorithms and heuristics. As algithm, as said, A* should be chosen, and as heuristic any that is admissible. With such a combination, any found plan is an optimal solution to the given problem.

Just a remark (mainly for the search/planning experts): A* is only guaranteed to find optimal solutions if it implements tree search, rather than graph search (i.e., if duplicates are repeatedly expanded). If it implements graph search, it's not sufficient for the heuristic to be admissible, it also needs to be monotone (also called consistent). However, according to http://www.fast-downward.org/Doc/SearchEngine, it implements tree search (I think). Also, there are consistent heuristics, too.

来源：https://stackoverflow.com/questions/18945035/fast-forward-and-pddl-is-the-computed-solution-the-best