A Direct Reduction from Stochastic Parity Games to Simple Stochastic Games

TL;DR

This paper presents a new polynomial-time reduction from stochastic parity games to simple stochastic games, simplifying the relationship between these models and confirming their computational complexity class.

Contribution

It introduces a direct, gadget-based polynomial reduction from SPGs to SSGs, removing the need for intermediate models and formalizing their complexity relationship.

Findings

Reduction is polynomial under binary encoding.

Confirms NP ∩ coNP complexity of SPGs.

Provides a new understanding of parity and reachability objectives.

Abstract

Significant progress has been recently achieved in developing efficient solutions for simple stochastic games (SSGs), focusing on reachability objectives. While reductions from stochastic parity games (SPGs) to SSGs have been presented in the literature through the use of multiple intermediate game models, a direct and simple reduction has been notably absent. This paper introduces a novel and direct polynomial-time reduction from quantitative SPGs to quantitative SSGs. By leveraging a gadget-based transformation that effectively removes the priority function, we construct an SSG that simulates the behavior of a given SPG. We formally establish the correctness of our direct reduction. Furthermore, we demonstrate that under binary encoding this reduction is polynomial, thereby directly corroborating the known $\textbf{NP}\,\mathbf{\cap}\,\textbf{coNP}$ complexity of SPGs and providing new understanding in the relationship between parity and reachability objectives in turn-based stochastic games.