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Qualitative Numeric Planning

Siddharth Srivastava, Shlomo Zilberstein , Neil Immerman University of Massachusetts Amherst Hector Geffner Universitat Pompeu Fabra. Qualitative Numeric Planning. The Story So Far…. Abacus Programs. Finite sets of states & registers Actions with unit increments/decrements.

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Qualitative Numeric Planning

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  1. Siddharth Srivastava, ShlomoZilberstein, Neil Immerman University of Massachusetts Amherst Hector Geffner UniversitatPompeuFabra Qualitative Numeric Planning

  2. The Story So Far… Abacus Programs • Finite sets of states & registers • Actions with unit increments/decrements [Lambek, ‘61]

  3. Abacus Programs • The reachability problem for abacus programs as a method for reasoning about cyclic control flows • But reachability is equivalent to the halting problem for Turing machines …. • Undecidable Approach: identify subclasses or less expressive frameworks … cannot capture TM, but still useful [Srivastava et al., ICAPS-10]

  4. ND Quantitative Planning Problems • Consider situations where • Actions increase or decrease numeric variables by unpredictable amounts • Propositional variables can be added • Plans require cyclic control • E.g., delivery problem with unknown • Fuel • Distances • Quantities of deliverables Driving will use unpredictable amount of fuel

  5. Formulation • X: set of positive valued variables, O: set of actions, I: initial state, G: goal condition • States: numeric assignments to variables • Action effects: • : increases value of variable • : decreases value of variable • Actions may have multiple effects • Action preconditions & goal condition: • or , for some subset of variables Lower bound specific to execution. Need not be known

  6. Example • A1: <> • A2: <> • Initial state: x=10, y=5; Goal: x=0 • No finite acyclic solution! • Solution (intuitive): repeat (until x=0){ repeat (until y=0) { <>} <>}

  7. ND Quantitative Planning Problems: Solutions • Policy: States Actions • Policy trajectory for : • Solution criterion: • Every bounded policy trajectory must terminate at a goal state in finitely many steps. • But how do we express policies? • Cannot map all possible states (real-valued assignments to variables)

  8. Expressing Solutions: Qualitative Formulation • Capture sets of ND numeric planning problems • Abstract/Qualitative states • For each , only record or • Initial state for previous example abstracted to: • Also represents infinitely many other non-zero assignments to and • Qualitative states capture sets of concrete states

  9. Qualitative Formulation • X: Boolean variables; I: initial state, G: goal condition, O: action operators • State = Boolean assignment to each () • Action effects (non-deterministic but finite) • Preconditions & goal condition

  10. Solutions to the Qualitative Problem • Solutions represented as policies over qualitative states • Solution criterion: policy must solve every represented quantitative problem • Termination of all –bounded trajectories for all possible problem instantiations • Goal achievement in all possible problem instantiations

  11. All We Need Are Qualitative Policies • A quantitative policy is essentially qualitative iff: • Maps all states represented by a qualitative state to the same action • Very useful: • Cannot have explicit policy representations over quantitative states anyway Theorem A non-deterministic quantitative planning problem P has a solution policy iff P has a policy that is essentially qualitative

  12. Solution Policy: Example Transition Graph Policy

  13. Qualitative Solution Tests • Can we tell if a policy is correct without ever having to instantiate the problems? • Define the transition graph for a policy: • Nodes = qualitative states • Edge iff • Two aspects of the solution criteria: • Goal-closed – termination possible only at goal states • Traverse the transition graph to check this • Finiteness of all possible instantiated trajectories • ??

  14. Sieve Algorithm for Determining Termination • For every SCC: • Identify edges that cannot be executed infinitely often • Remove them, signifying stage when there executions have been exhausted • Recurse on each resulting SCC • Finally: terminating iff no SCC left on fixed point

  15. Sieve Algorithm: Properties • Completeness: if Sieve algorithm returns non-terminating, an infinite execution is possible • Surprising because of similarity to abacus programs Theorem The sieve algorithm for determining termination of a qualitative policy is sound and complete.

  16. A Generate and Test Planner • Enumerate all possible policies (yes, this is impractical in general!) • But computable! • Check for • Goal-closed (any terminal nodes in transition graph must be goal nodes) • Termination using sieve algorithm

  17. Results • Problems • Nested variables • Snow plow: using snow blower spills snow onto the driveway • Delivery with fuel, unknown number of objects and truck capacities • Trash-collection Solution Time (s)

  18. Future Work • Improve generate and test: • Start with strong cyclic qualitative policies • Introduce constant landmarks/intervals of values • Identify limits of sieve algorithm’s applicability in abacus programs

  19. Conclusions • QNP gives the first framework for planning with loops where termination and correctness are decidable properties • For any class of loops • Any number of unbounded variables

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