Parity game

Unsolved problem in computer science: Can parity games be solved in polynomial time? (more unsolved problems in computer science)

Solving a parity game played on a finite graph means deciding, for a given starting position, which of the two players has a winning strategy. It has been shown that this problem is in NP and co-NP, more precisely UP and co-UP,[6] as well as in QP (quasipolynomial time).[7] It remains an open question whether this decision problem is solvable in PTime.

Given that parity games are history-free determined, solving a given parity game is equivalent to solving the following simple looking graph-theoretic problem. Given a finite colored directed bipartite graph with n vertices ${\displaystyle V=V_{0}\cup V_{1}}$, and V colored with colors from 1 to m, is there a choice function selecting a single out-going edge from each vertex of ${\displaystyle V_{0}}$, such that the resulting subgraph has the property that in each cycle the largest occurring color is even.

Zielonka outlined a recursive algorithm that solves parity games. Let ${\displaystyle G=(V,V_{0},V_{1},E,\Omega )}$ be a parity game, where ${\displaystyle V_{0}}$ resp. ${\displaystyle V_{1}}$ are the sets of nodes belonging to player 0 resp. 1, ${\displaystyle V=V_{0}\cup V_{1}}$ is the set of all nodes, ${\displaystyle E\subseteq V\times V}$ is the total set of edges, and ${\displaystyle \Omega :V\rightarrow \mathbb {N} }$ is the priority assignment function.

Zielonka's algorithm is based on the notation of attractors. Let ${\displaystyle U\subseteq V}$ be a set of nodes and ${\displaystyle i=0,1}$ be a player. The i-attractor of U is the least set of nodes ${\displaystyle Attr_{i}(U)}$ containing U such that i can force a visit to U from every node in ${\displaystyle Attr_{i}(U)}$. It can be defined by a fix-point computation:

${\displaystyle {\begin{aligned}Attr_{i}(U)^{0}&:=U\\Attr_{i}(U)^{j+1}&:=Attr_{i}(U)^{j}\cup \{v\in V_{i}\mid \exists (v,w)\in E:w\in Attr_{i}(U)^{j}\}\cup \{v\in V_{1-i}\mid \forall (v,w)\in E:w\in Attr_{i}(U)^{j}\}\\Attr_{i}(U)&:=\bigcup _{j=0}^{\infty }Attr_{i}(U)^{j}\end{aligned}}}$

In other words, one starts with the initial set U. Then, for each step (${\displaystyle Attr_{i}(U)^{j+1}}$) one adds all nodes belonging to player 0 that can reach the previous set (${\displaystyle Attr_{i}(U)^{j}}$) with a single edge and all nodes belonging to player 1 that must reach the previous set (${\displaystyle Attr_{i}(U)^{j}}$) no matter which edge player 1 takes.

Zielonka's algorithm is based on a recursive descent on the number of priorities. If the maximal priority is 0, it is immediate to see that player 0 wins the whole game (with an arbitrary strategy). Otherwise, let p be the largest one and let ${\displaystyle i=p{\bmod {2}}}$ be the player associated with the priority. Let ${\displaystyle U=\{v\mid \Omega (v)=p\}}$ be the set of nodes with priority p and let ${\displaystyle A=Attr_{i}(U)}$ be the corresponding attractor of player i. Player i can now ensure that every play that visits A infinitely often is won by player i.

Consider the game ${\displaystyle G'=G\setminus A}$ in which all nodes and affected edges of A are removed. We can now solve the smaller game ${\displaystyle G'}$ by recursion and obtain a pair of winning sets ${\displaystyle W'_{i},W'_{1-i}}$. If ${\displaystyle W'_{1-i}}$ is empty, then so is ${\displaystyle W_{1-i}}$ for the game G, because player ${\displaystyle 1-i}$ can only decide to escape from ${\displaystyle W_{i}'}$ to A which also results in a win for player i.

Otherwise, if ${\displaystyle W'_{1-i}}$ is not empty, we only know for sure that player ${\displaystyle 1-i}$ can win on ${\displaystyle W'_{1-i}}$ as player i cannot escape from ${\displaystyle W'_{1-i}}$ to A (since A is an i-attractor). We therefore compute the attractor ${\displaystyle B=Attr_{1-i}(W'_{1-i})}$ and remove it from G to obtain the smaller game ${\displaystyle G''=G\setminus B}$. We again solve it by recursion and obtain a pair of winning sets ${\displaystyle W''_{i},W''_{1-i}}$. It follows that ${\displaystyle W_{i}=W''_{i}}$ and ${\displaystyle W_{1-i}=W''_{1-i}\cup B}$.

In simple pseudocode, the algorithm might be expressed as this:

function ${\displaystyle solve(G)}$
    p := maximal priority in G
    if ${\displaystyle p=0}$
        return ${\displaystyle W_{0},W_{1}:=V,\{\}}$
    else
        U := nodes in G with priority p
        ${\displaystyle i:=p{\bmod {2}}}$
        ${\displaystyle A:=Attr_{i}(U)}$
        ${\displaystyle W_{0}',W_{1}':=solve(G\setminus A)}$
        if ${\displaystyle W_{i}'=V}$
            return ${\displaystyle W_{i},W_{1-i}:=V,\{\}}$
        ${\displaystyle B:=Attr_{1-i}(W_{1-i}')}$
        ${\displaystyle W_{0}'',W_{1}'':=solve(G\setminus B)}$
        return ${\displaystyle W_{i},W_{1-i}:=W_{i}'',W_{1-i}''\cup B}$

A slight modification of the above game, and the related graph-theoretic problem, makes solving the game NP-hard. The modified game has the Rabin acceptance condition. Specifically, in the above bipartite graph scenario, the problem now is to determine if there is a choice function selecting a single out-going edge from each vertex of V₀, such that the resulting subgraph has the property that in each cycle (and hence each strongly connected component) it is the case that there exists an i and a node with color 2i, and no node with color 2i + 1...

Note that as opposed to parity games, this game is no longer symmetric with respect to players 0 and 1.

Most common applications of parity game solving.

Despite its interesting complexity theoretic status, parity game solving can be seen as the algorithmic backend to problems in automated verification and controller synthesis. The model-checking problem for the modal μ-calculus for instance is known to be equivalent to parity game solving. Also, decision problems like validity or satisfiability for modal logics can be reduced to parity game solving.

• Erich Grädel, Phokion G. Kolaitis, Leonid Libkin, Maarten Marx, Joel Spencer, Moshe Y. Vardi, Yde Venema, Scott Weinstein (2007). Finite model theory and its applications. Springer. ISBN 978-3-540-00428-8.CS1 maint: multiple names: authors list (link)

• E. Grädel, W. Thomas, T. Wilke (Eds.) : Automata, Logics, and Infinite Games, Springer LNCS 2500 (2003), ISBN 3-540-00388-6
• W. Zielonka : Infinite games on finitely coloured graphs with applications to automata on infinite tree, TCS, 200(1-2):135-183, 1998

Two state-of-the-art parity game solving toolsets are the following:

• PGSolver Collection
• Oink