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www.itu.dk 1 Integrating CSP Decomposition Techniques and BDDs for Compiling Configuration Problems Sathiamoorthy Subbarayan IT University of Copenhagen Denmark CP-AI-OR 2005

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www.itu.dk 2 Configuration An arrangement of parts Valid Configuration- a valid arrangement Valid according to some specified rules NP-Hard problem

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www.itu.dk 3 Examples Bike Car PC Power-Backup Systems Controllers Business Software

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www.itu.dk 4 Problem Representation Constraint Satisfaction Problem (CSP) A set of variables and their domains A set of rules/constraints on variable values Configuration - A legal assignment to all variables

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www.itu.dk 5 Interactive Configurator Interactively guides the user Shows only valid extensions User always in a consistent state

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www.itu.dk 6 Required Functionalities Propagate the user selections Display the valid choices Explain reasons for unselectable or implied options

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www.itu.dk 7 Binary Decision Diagrams A compressed representation of solutions

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www.itu.dk 8 Previous work Compile into a Binary Decision Diagram rules BDD generation BDD lookup offlineonline configurator Compilation NP-hard P BDD Problem! BDD size could explode

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www.itu.dk 9 This work Configuration rules have hierarchies Hierarchies close to tree-shape Tree Decomposition of CSP? Space reduction? (upto 96%) Response time reduction? (upto 200x)

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www.itu.dk 10 CSP to BDD compilation C1 C2 C3C4 C5 C6 C7 bdd CSP BDD

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www.itu.dk 11 Tree Decomposition of CSP C1 C2 C3C4 C5 C6 C7 C1 C2 C3 C4 C5 C6 C7 C3 C6 CSP CSP’ b1b2b3 Tree-of-BDDs

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www.itu.dk 12 Benefit of decomposition BDD b1b2b3 bdd Tree-of-BDDs |b1|+|b2|+|b3| < |bdd| Potential Space Reduction -Online Configurators -Embedded Configurators

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www.itu.dk 13 Required Functionalities Propagate the user selections Display the valid choices Explain reasons for unselectable or implied options

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www.itu.dk 14 Explanations A partial assignment by the user might cause some options unselectable /implied Explanation for such an option is the subset of the partial assignment that makes the option unselectable/implied When user assignments are given priorities, we might need minimum explanations

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www.itu.dk 15 Explanations in single BDD case Variant of Shortest path problem in DAG Assigned variable (v=val) –0 for ’val’ edges –1 for other edges Other variables –0 for their edges Shortest path from root to 1-node corresponds to an explanation

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www.itu.dk 16 Example Assign a=0 Assign b=1 e=1 is implied Explain e=0? 1 1 Explanation b=1 Can also handle priorities and non-Boolean variables

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www.itu.dk 17 Don’t Care variables for Explain In the example, necessary variables are: a,b, and e Variables c and d are hence don’t cares for Explain Abstraction !

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www.itu.dk 18 Explanations in Tree-of-BDDs b1 b2 b3 b1’b2’ b3’ Abstraction Join b’ Explain Explanation

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www.itu.dk 19 Experiments Four Benchmarks –PC –Renault –Two PSR instances 10000 random interactions Comparison of: –Space usage –Average and Worst time for Responses –Average and Worst time for Explanations Tools –CLab: an existing single BDD-based tool –iCoDE: implementation of the tree-of-BDDs

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www.itu.dk 20 Compilation Details Benchmark Tree (sec) Tree Nodes Compile (sec)BDD-nodesPeak BDD-nodes CLabiCoDECLabiCoDECLabiCoDE PC0.04160.110.1916494445880K40K Renault0.257311977455796176022500K80K psr-1-32-121140.464569238917600K psr-1-32-1211429246775221011200K

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www.itu.dk 21 Time Comparisons Response (Propagate+Display) Benchmark Average(sec)Worst(sec) CLabiCoDECLabiCoDE PC0.0040.00010.0500.006 Renault0.0700.00030.4520.020 psr-1-32-10.0160.00100.0570.038 psr-1-32-20.0370.00020.6180.107

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www.itu.dk 22 Time Comparisons Explanation Benchmark Average(sec)Worst(sec) CLabiCoDECLabiCoDE PC0.0040.010 0.030 Renault0.1600.0880.4400.921 psr-1-32-10.0311.2080.0672.155 psr-1-32-20.1225.380.32910.78

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www.itu.dk 23 Conclusion Decomposition results in –Space reduction –Shorter responses Tree-of-BDDs can scale higher than the single BDD approach Future work include: –BDD variable orderings –Multi-valued Decision Diagrams –different decomposition techniques

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