Abstract
Parity games play an important role in model checking and synthesis. In their paper, Calude et al. have recently shown that these games can be solved in quasi-polynomial time. We show that their algorithm can be implemented efficiently: we use their data structure as a progress measure, allowing for a backward implementation instead of a complete unravelling of the game. To achieve this, a number of changes have to be made to their techniques, where the main one is to add power to the antagonistic player that allows for determining her rational move without changing the outcome of the game. We provide a first implementation for a quasi-polynomial algorithm, test it on small examples, and provide a number of side results, including minor algorithmic improvements, a quasi-bi-linear complexity in the number of states and edges for a fixed number of colours, matching lower bounds for the algorithm of Calude et al., and a complexity index associated to our approach, which we compare to the recently proposed register index.
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Notes
We recall Definition 1, which state when rules 1 and 2 are defined to be applied.
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Acknowledgements
We thank Xiang Fei Ding for pointing out an error in a previous version of this manuscript. We thank Tin Lok Wong for help with programming and implementation of the algorithms. We thank anonymous reviewers for many helpful suggestions and improvements made to a preliminary submission of this manuscript. Sanjay Jain was supported in part by NUS Grant C252-000-087-001. Further, Sanjay Jain and Frank Stephan were supported in part by the Singapore Ministry of Education Academic Research Fund Tier 2 Grant MOE2016-T2-1-019 / R146-000-234-112. Sven Schewe and Dominik Wojtczak were supported in part by EPSRC Grant EP/M027287/1. Further, John Fearnley, Bart de Keijzer, Sven Schewe and Dominik Wojtczak were supported in part by EPSRC Grant EP/P020909/1.
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Fearnley, J., Jain, S., de Keijzer, B. et al. An ordered approach to solving parity games in quasi-polynomial time and quasi-linear space. Int J Softw Tools Technol Transfer 21, 325–349 (2019). https://doi.org/10.1007/s10009-019-00509-3
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DOI: https://doi.org/10.1007/s10009-019-00509-3