The surprizing complexity of generalized reachability games

TL;DR

This paper investigates the computational complexity and memory requirements of generalized reachability games on graphs, revealing PSPACE-completeness and identifying subclasses with efficient decision procedures.

Contribution

It proves PSPACE-completeness of winner determination, establishes bounds on strategy memory, and identifies subclasses allowing efficient algorithms.

Findings

Deciding the winner is PSPACE-complete.

Memory bounds for winning strategies are established.

Certain subclasses enable efficient winner determination.

Abstract

Games on graphs provide a natural and powerful model for reactive systems. In this paper, we consider generalized reachability objectives, defined as conjunctions of reachability objectives. We first prove that deciding the winner in such games is $\PSPACE$-complete, although it is fixed-parameter tractable with the number of reachability objectives as parameter. Moreover, we consider the memory requirements for both players and give matching upper and lower bounds on the size of winning strategies. In order to allow more efficient algorithms, we consider subclasses of generalized reachability games. We show that bounding the size of the reachability sets gives two natural subclasses where deciding the winner can be done efficiently.