Beyond value iteration for parity games: strategy iteration with universal trees

Koh, Zhuan Khye and Loho, Georg (2022) Beyond value iteration for parity games: strategy iteration with universal trees. In: Szeider, Stefan, Ganian, Robert and Silva, Alexandra, (eds.) Leibniz International Proceedings in Informatics, LIPIcs. Leibniz International Proceedings in Informatics, LIPIcs. Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing. ISBN 9783959772563

Text (LIPIcs-MFCS-2022-63) - Accepted Version Available under License Creative Commons Attribution. Download (806kB)

Abstract

Parity games have witnessed several new quasi-polynomial algorithms since the breakthrough result of Calude et al. (STOC 2017). The combinatorial object underlying these approaches is a universal tree, as identified by Czerwinski et al. (SODA 2019). By proving a quasi-polynomial lower bound on the size of a universal tree, they have highlighted a barrier that must be overcome by all existing approaches to attain polynomial running time. This is due to the existence of worst case instances which force these algorithms to explore a large portion of the tree. As an attempt to overcome this barrier, we propose a strategy iteration framework which can be applied on any universal tree. It is at least as fast as its value iteration counterparts, while allowing one to take bigger leaps in the universal tree. Our main technical contribution is an efficient method for computing the least fixed point of 1-player games. This is achieved via a careful adaptation of shortest path algorithms to the setting of ordered trees. By plugging in the universal tree of Jurdzinski and Lazic (LICS 2017), or the Strahler universal tree of Daviaud et al. (ICALP 2020), we obtain instantiations of the general framework that take time O(mn2 log n log d) and O(mn2 log3 n log d) respectively per iteration.

Item Type:	Book Section
Additional Information:	© 2022 The Author(s).
Divisions:	Mathematics
Subjects:	Q Science > QA Mathematics > QA75 Electronic computers. Computer science
Date Deposited:	30 Sep 2022 12:45
Last Modified:	23 Nov 2024 22:36
URI:	http://eprints.lse.ac.uk/id/eprint/116860

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