This Article 
 Bibliographic References 
 Add to: 
Real-Time Bidirectional Search: Coordinated Problem Solving in Uncertain Situations
June 1996 (vol. 18 no. 6)
pp. 617-628

Abstract—This paper investigates real-time bidirectional search (RTBS) algorithms, where two problem solvers, starting from the initial and goal states, physically move toward each other. To evaluate the RTBS performance, two kinds of algorithms are proposed and are compared to real-time unidirectional search. One is called centralized RTBS where a supervisor always selects the best action from all possible moves of the two problem solvers. The other is called decoupled RTBS where no supervisor exists and the two problem solvers independently select their next moves.

Experiments on mazes and n-puzzles show that 1) in clear situations decoupled RTBS performs better, while in uncertain situations, centralized RTBS becomes more efficient, and that 2) RTBS is more efficient than real-time unidirectional search for 15- and 24-puzzles but not for randomly generated mazes. It will be shown that the selection of the problem solving organization is the selection of the problem space, which determines the baseline of the organizational efficiency; once a difficult problem space is selected, the local coordination among problem solvers hardly overcome the deficit.

[1] F. Chimura and M. Tokoro, "The Trailblazer Search: A New Method for Searching and Capturing Moving Targets," AAAI-94, pp. 1,347-1,352, 1994.
[2] D. de Champeaux and L. Sint, "An Improved Bidirectional Heuristic Search Algorithm," J. ACM, vol. 24, no. 2, pp. 177-191, 1977.
[3] D. de Champeaux, "Bidirectional Heuristic Search Again," J. ACM, vol. 30, no. 1, pp. 22-32, 1983.
[4] E.H. Durfee, V.R. Lesser, and D.D., Corkill, "Coherent Cooperation among Communicating Problem Solvers," IEEE Trans. Computers, vol. 36, pp. 1,275-1,291, 1987.
[5] M.S. Fox, "An Organizational View of Distributed Systems," IEEE Trans. Systems, Man, and Cybernetics, vol. 11, no. 1, pp. 70-80, 1981.
[6] L. Gasser, N. Rouquette, R. Hill, and J. Lieb, "Representing and Using Organizational Knowledge in DAI Systems," L. Gasser and M.N. Huhns, eds., Distributed Artificial Intelligence, vol. 2. London: Pitman, pp. 55-78, 1989.
[7] M.P. Georgeff and A.L. Lansky, "Reactive Reasoning and Planning," AAAI-87, pp. 677-682, 1987.
[8] T. Ishida and R.E. Korf, "Moving Target Search," IJCAI-91, pp. 204-210, 1991.
[9] T. Ishida, "Moving Target Search with Intelligence," AAAI-92, pp. 525-532, 1992.
[10] T. Ishida, L. Gasser, and M. Yokoo, "Organization Self-Design of Distributed Production Systems," IEEE Trans. Knowledge and Data Engineering, vol. 4, no. 2, pp. 123-134, 1992.
[11] T. Ishida, "Towards Organizational Problem Solving," IEEE Int'l Conf. Robotics and Automation, pp. 839-845, 1993.
[12] T. Ishida, "Two is not Always Better than One: Experiences in Real-Time Bidirectional Search," Int'l Conf. Multi-Agent Systems (ICMAS-95), pp. 185-192, 1995.
[13] T. Ishida and R.E. Korf, "A Moving Target Search: A Real-Time Search for Changing Goals," IEEE Trans. Pattern Analysis and Machine Intelligence, vol. 17, no. 6, pp. 609-619, 1995.
[14] R.E. Korf, "Depth-first Iterative-Deepening: An Optimal Admissible Tree Search," Artificial Intelligence, vol. 27, no. 1, pp. 97-109, 1985.
[15] R.E. Korf,“Real-time heuristic search,” Artificial Intelligence, vol. 42, nos. 2-3, pp. 189-211, Mar. 1990.
[16] J.B.H. Kwa, "An Admissible Bidirectional Staged Heuristic Search Algorithm," Artificial Intelligence, vol. 38, pp. 95-109, 1989.
[17] V.R. Lesser, "An Overview of DAI: Viewing Distributed AI as Distributed Search," JSAI J., vol. 5, no. 4, pp. 392-400, 1990.
[18] J. Pearl, Heuristics: Intelligent Search Strategies for Computer Problem Solving, Addison-Wesley, Reading, Mass., 1984.
[19] I. Pohl, "Bi-directional Search," Machine Intelligence, vol. 6, pp. 127-140, 1971.
[20] G. Politowski and I. Pohl, "D-node Retargeting in Bi-directional Heuristic Search," AAAI-84, pp. 274-277, 1984.
[21] M.E. Pollack and M. Ringuette, "Introducing the Tileworld: Experimentally Evaluating Agent Architectures," AAAI-90, pp. 183-189, 1990.
[22] S. Russell and E. Wefald, Do the Right Thing. MIT Press, 1991.

Index Terms:
Search, real-time search, bidirectional search, problem solving, real-time problem solving, organizational problem solving, heuristic depression.
Toru Ishida, "Real-Time Bidirectional Search: Coordinated Problem Solving in Uncertain Situations," IEEE Transactions on Pattern Analysis and Machine Intelligence, vol. 18, no. 6, pp. 617-628, June 1996, doi:10.1109/34.506412
Usage of this product signifies your acceptance of the Terms of Use.