Fixed-Parameter Algorithms for Graph Constraint Logic

TL;DR

This paper proves that non-deterministic constraint logic (NCL), a model capturing extsc{PSPACE} complexity, is fixed-parameter tractable when parameterized by certain graph features, with some parameters admitting linear kernels.

Contribution

It establishes fixed-parameter tractability of NCL for various parameters, showing that NCL is as economical as possible for capturing extsc{PSPACE}.

Findings

NCL is fixed-parameter tractable for parameters like weight-one edges and extsc{and} vertices.

Linear kernels are obtained for parameters such as weight-one edges and extsc{and} vertices.

NCL remains expressive under these parameter restrictions, preserving extsc{PSPACE} completeness.

Abstract

Non-deterministic constraint logic (NCL) is a simple model of computation based on orientations of a constraint graph with edge weights and vertex demands. NCL captures \PSPACE\xspace and has been a useful tool for proving algorithmic hardness of many puzzles, games, and reconfiguration problems. In particular, its usefulness stems from the fact that it remains \PSPACE-complete even under severe restrictions of the weights (e.g., only edge-weights one and two are needed) and the structure of the constraint graph (e.g., planar \textsc{and/or}\xspace graphs of bounded bandwidth). While such restrictions on the structure of constraint graphs do not seem to limit the expressiveness of NCL, the building blocks of the constraint graphs cannot be limited without losing expressiveness: We consider as parameters the number of weight-one edges and the number of weight-two edges of a constraint graph, as well as the number of \textsc{and}\xspace or \textsc{or}\xspace vertices of an \textsc{and/or}\xspace constraint graph. We show that NCL is fixed-parameter tractable (FPT) for any of these parameters. In particular, for NCL parameterized by the number of weight-one edges or the number of \textsc{and}\xspace vertices, we obtain a linear kernel. It follows that, in a sense, NCL as introduced by Hearn and Demaine is defined in the most economical way for the purpose of capturing \PSPACE.