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Heuristics in Search-Space

Heuristics in Search-Space. CSE 574 April 11, 2003 Dan Weld. Schedule. 3. TEMPORAL Partial-O Graphplan Forward-chaining Stochastic 4. UNCERTAINTY. 1. BASICS Intro Graphplan SATplan State-space Refinement 2. SPEEDUP EBL & DDB Heuristic Gen. Long paper!. More Administrivia.

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Heuristics in Search-Space

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  1. Heuristics in Search-Space CSE 574 April 11, 2003 Dan Weld

  2. Schedule 3. TEMPORAL • Partial-O • Graphplan • Forward-chaining • Stochastic 4. UNCERTAINTY 1. BASICS • Intro • Graphplan • SATplan • State-space • Refinement 2. SPEEDUP • EBL & DDB • Heuristic Gen • Long paper!

  3. More Administrivia • Mailing List • Reviews due by 11am • No class Fri 4/18 • Experimenting with Planners • Context • Basis for Projects

  4. Paper: Main Points • Avoid duplicate work computing heuristics • Pregenerate in forward sweep; search backward • Interpretation of graphplan • Tradeoff for heuristics: admissible?

  5. Regression Search

  6. Experiments • What were they answering? • Weak support for main points • Do you believe GP analysis? • Why is HSPr faster? • Heuristic calculation or backwards search? • Presentation: • Table vs. graph • Speedup ratio (how compared to 85%)

  7. Weaknesses • Experiments • Needed an example • More discussion of mutex tradeoffs • Algo finds “most” mutexes • Memory usage is a problem (but why?) • Hill climbing with w=5

  8. Greedy BFS • F(n) = g(n) + W h(n) • Empirical studies 1979, 1989 on 8 puz, TSP • Increasing W => speeds search • => less optimal solutions • Proof: if W1 • Then |solution|/|optimal|  W • Proof: abstract tree, uniform branch, 1 goal • Then W=1 gives fastest solution • Optimality is a bonus • Contradiction?!

  9. Future Work • Study graphplan in state-space framework • Project idea: Use Hg with IDA* • Recast other planners as HSP • Analysis of where and why HSPr fails • Empirical Comparison • A*, IDA*, BFS, and HSPr

  10. Future Work 2Derivation of better heuristics • Keep some delete effects • Sum vs. max heuristic • Connection to parallel actions • Probabilistic estimate of step reuse • Non admissible, but more accurate? • Can we bound the amount of step sharing?

  11. Bounding Step Sharing • Build bipartite “support” graph • Compute “max non-mutex outdegree” • Good project? A1 A1 A2 A2 Mutex A3 A3 A4 A4 Eff Pre

  12. McDermott’s Grid World

  13. Future Work 3 • Other points on mutex-computation spectrum • HSPr mutexes are limited to pairs of atoms • Do not consider actions directly • Could ADDs help state enumeration problem

  14. Future Work 4Progression / Regression • Why not generate heuristics backwards? • For what domains would this be faster? • Advantages? • Anytime planning

  15. Future Work 5Incomplete Info / Temporal • What is state space? • How to regress?

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