1992 年 7 巻 4 号 p. 639-653
STRIPS is well known as one of unidirectional search algorithms in which the search tree is rooted at the goal. In order to avoid the defect, the number of nodes in a search tree grows exponentially with its depth, of STRIPS like some other unidirectional algorithms, many improved algorithms have been proposed. Here we would furthermore introduce a new efficient bidirectional heuristic search algorithm into STRIPS to make its search more efficient. We would like call our new algorithm Bi-STRIPS. Being different from other bidirectional heuristic searches, the forward search, from the initial state, and the backward search, from the goal, of Bi-STRIPS both aim at a set of nodes between the initial state and the goal. Those nodes are called middle goals and any solution path must go througth one of them. By utilizing this kind of bidirectional heuristic search, the two search trees not only will expand less nodes than STRIPS but also can be expanded parallely, and the search time for planning can be reduced. Therefore, efficiency of planning can be improved. We would call an element of a state destructive element if it can be deleted by applying a production rule to the state. Surfacing an element of a state means to search for a new state in which the element will get to be a destructive element. We would call those elements of a state differences between that state and a subgoal if they cannot exist with the subgoal at the same time. A middle goal is such a state in which every element of differences between that state and the present subgoal is surfaced. The forward search tree of our algorithm is a destructive one because a node contains no less surfaced elements than its ancestors. In this paper, we will describe the deduction process of the middle goals and show the algorithms of the main parts of Bi-STRIPS. Some examples also will be shown to clarify the efficiency of our system.
