Abstract
This paper presents a technique for symmetry reduction that adaptively assigns a prefix of variables in a system of constraints so that the generated prefix-assignments are pairwise nonisomorphic under the action of the symmetry group of the system. The technique is based on McKay’s canonical extension framework [J. Algorithms 26 (1998), no. 2, 306–324]. Among key features of the technique are (i) adaptability—the prefix sequence can be user-prescribed and truncated for compatibility with the group of symmetries; (ii) parallelisability—prefix-assignments can be processed in parallel independently of each other; (iii) versatility—the method is applicable whenever the group of symmetries can be concisely represented as the automorphism group of a vertex-colored graph; and (iv) implementability—the method can be implemented relying on a canonical labeling map for vertex-colored graphs as the only nontrivial subroutine. To demonstrate the tentative practical applicability of our technique we have prepared a preliminary implementation and report on a limited set of experiments that demonstrate ability to reduce symmetry on hard instances.
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Notes
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This implementation can be found at https://github.com/pkaski/reduce/.
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Available at https://arxiv.org/abs/1706.08325.
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For conciseness and accessibility, the present conference version develops the method only for variable symmetries, and accordingly the group action (3) acts only on the variables in U and not on the values in R. However, the method does extend to capture symmetries on both variables in U and values in R (essentially by consideration of the Cartesian product \(U\times R\) in place of U), and such an extension will be developed in a full version of this paper.
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Here it should be noted that executing the symmetry reduction carries in itself a nontrivial computational cost. That is, there is a tradeoff between the potential savings in solving the system gained by symmetry reduction versus the cost of performing symmetry reduction. For example, if the instance has no symmetry and \(\varGamma \) is a trivial group, then symmetry reduction merely makes it more costly to solve the system.
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Reduction to vertex-colored graphs is by no means the only possibility to obtain the canonical labeling map to enable (P’), (T1’), and (T2’). Another possibility would be to represent \(\varGamma \) directly as a permutation group and use dedicated permutation-group algorithms [33, 34]. Our present choice of vertex-colored graphs is motivated by easy availability of carefully engineered implementations for working with vertex-colored graphs.
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Acknowledgments
The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement 338077 “Theory and Practice of Advanced Search and Enumeration” (M.K., P.K., and J.K.). We gratefully acknowledge the use of computational resources provided by the Aalto Science-IT project at Aalto University. We thank Tomi Janhunen for useful discussions.
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Junttila, T., Karppa, M., Kaski, P., Kohonen, J. (2017). An Adaptive Prefix-Assignment Technique for Symmetry Reduction. In: Gaspers, S., Walsh, T. (eds) Theory and Applications of Satisfiability Testing – SAT 2017. SAT 2017. Lecture Notes in Computer Science(), vol 10491. Springer, Cham. https://doi.org/10.1007/978-3-319-66263-3_7
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