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

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

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

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

Regression Search

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%)

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

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?!

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

Future Work 2 Derivation 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?

Bounding Step Sharing Build bipartite “support” graph Compute “max non-mutex outdegree” Good project? A1A1 A2A2 A3A3 A4A4 A1A1 A2A2 A3A3 A4A4 PreEff Mutex

McDermott’s Grid World

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

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

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