VHPOP: Versatile Heuristic Partial Order Planner (1106.4868v1)

Abstract: VHPOP is a partial order causal link (POCL) planner loosely based on UCPOP. It draws from the experience gained in the early to mid 1990's on flaw selection strategies for POCL planning, and combines this with more recent developments in the field of domain independent planning such as distance based heuristics and reachability analysis. We present an adaptation of the additive heuristic for plan space planning, and modify it to account for possible reuse of existing actions in a plan. We also propose a large set of novel flaw selection strategies, and show how these can help us solve more problems than previously possible by POCL planners. VHPOP also supports planning with durative actions by incorporating standard techniques for temporal constraint reasoning. We demonstrate that the same heuristic techniques used to boost the performance of classical POCL planning can be effective in domains with durative actions as well. The result is a versatile heuristic POCL planner competitive with established CSP-based and heuristic state space planners.

• The paper details the adaptation of the additive heuristic for plan space planning, enabling the reuse of actions to enhance evaluation efficiency.
• The paper introduces diverse flaw selection strategies, including conflict-driven methods, to identify and resolve inconsistencies early in the planning process.
• The paper demonstrates the integration of temporal planning using simple temporal networks, broadening VHPOP's applicability to both classical and durative action domains.

An Overview of VHPOP: Versatile Heuristic Partial Order Planner

The paper "VHPOP: Versatile Heuristic Partial Order Planner" authored by Håkan L. S. Younes and Reid G. Simmons, presents a detailed exposition of VHPOP, a planner founded on the partial order causal link (POCL) methodology. This work revitalizes the POCL planning paradigm, which had seen a decline in focus with the rise of CSP-based and heuristic state space planners. VHPOP is enhanced with innovative heuristic strategies and supports planning with durative actions, demonstrating its competitiveness in various domains.

Key Contributions

1. Adaptation of the Additive Heuristic: The paper details the adaptation of the additive heuristic for plan space planning, considering the potential reuse of actions within a plan. This adaptation accounts for positive interactions that result in more efficient plan evaluations.
2. Novel Flaw Selection Strategies: VHPOP introduces a diverse set of flaw selection strategies, extending beyond the traditional strategies like DUnf and LCFR. Notably, it incorporates, among others, conflict-driven strategies that prioritize resolving unsafe conditions, thereby uncovering inconsistencies early in the planning process.
3. Improvements in Planning with Durative Actions: By integrating simple temporal networks (STNs) for managing temporal constraints, VHPOP extends its capability to handle durative actions efficiently. This demonstrates its adaptability across both classical and temporal domains.
4. Concurrent Use of Multiple Strategies: The planner employs multiple flaw selection strategies concurrently in a round-robin manner, harnessing the strengths of different approaches to improve the number of solvable problems.

Implications and Future Prospects

The research indicates a renewed potential for POCL planners, particularly when integrated with modern heuristic techniques. VHPOP's adaptability to temporal planning expands the practical applicability of POCL methods beyond traditional domains. The concurrent use of different flaw selection strategies presents a promising direction for further research, suggesting that meta-planning approaches could significantly enhance planner performance.

For future developments, enhancing VHPOP's efficiency remains a priority. The aspect of speed, particularly when using lifted actions, could be optimized further, potentially through improved code for reachability analysis or a more intelligent scheduling of flaw selection strategies specific to problem features. Moreover, integrating additional plan ranking heuristics, such as the FF heuristic, could bridge the performance gap in plan execution time, especially in domains with durative actions.

Conclusion

The paper re-establishes the viability of POCL planners like VHPOP in contemporary planning tasks, particularly in handling complex domains with durative actions. It challenges the previous assumption of POCL's obsolescence by demonstrating its competitiveness against CSP-based and state space planners. VHPOP exemplifies how leveraging a combination of classical and novel heuristic strategies can enhance a planner's capability to tackle a diverse range of problems efficiently. This synthesis of techniques not only addresses contemporary planning needs but also sets a foundation for continued evolution in heuristic-based planning methods.