Energy Parity Games
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Energy Parity Games. Laurent Doyen LSV, ENS Cachan & CNRS Krishnendu Chatterjee IST Austria ICALP’10 & GT-Jeux’10. Games for system design. input. output. Spec: φ ( input,output ). System. Verification: check if a given system is correct  reduces to graph searching.

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Energy parity games

Energy Parity Games

Laurent Doyen

LSV, ENS Cachan & CNRS

Krishnendu ChatterjeeIST Austria

ICALP’10 & GT-Jeux’10


Energy parity games

Games for system design

input

output

Spec: φ(input,output)

System

  • Verification: check if a given system is correct

    •  reduces to graph searching


Energy parity games

Games for system design

?

input

output

Spec: φ(input,output)

  • Verification: check if a given system is correct

    •  reduces to graph searching

  • Synthesis : construct a correct system

    •  reduces to game solving – finding a winning strategy


Energy parity games

Games for system design

?

input

output

Spec: φ(input,output)

  • Verification: check if a given system is correct

    •  reduces to graph searching

  • Synthesis : construct a correct system

    •  reduces to game solving – finding a winning strategy

  • … in a game played on a graph:

    • - two players: System vs. Environment

    • - turn-based, infinite number of rounds

    • - winning condition given by the spec


Energy parity games

Two-player games on graphs


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Play:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

Strategies = recipe to extend the play prefix

Player 1:

outcome of two strategies is a play

Player 2:


Energy parity games

Two-player games

Player 1 (good guy)

Player 2 (bad guy)

  • Turn-based

  • Infinite

  • Winning condition ?

Strategies = recipe to extend the play prefix

Player 1:

outcome of two strategies is a play

Player 2:


Energy parity games

Two-player games on graphs

Who wins ?

Qualitative

Parity games

ω-regular specifications

(reactivity, liveness,…)


Energy parity games

Two-player games on graphs

Who wins ?

Qualitative

Quantitative

Parity games

Energy games

ω-regular specifications

(reactivity, liveness,…)

Mean-payoff games

Resource-constrainedspecifications


Energy parity games

Energy games


Energy parity games

Energy games

Energy game:

Positive and negative weights(encoded in binary)


Energy parity games

Energy games

Energy game:

Positive and negative weights (encoded in binary)

Play:

3 3 4 4 -1 …

Energy level: 3, 3, 4, 4, -1,…(sum of weights)


Energy parity games

Energy games

Energy game:

Positive and negative weights (encoded in binary)

Play:

3 3 4 4 -1 …

3

Energy level: 3, 3, 4, 4, -1,…

Initial credit

A play is winning if the energy level is always nonnegative.

“Never exhaust the resource (memory, battery, …)”(equivalent to a game on a one-counter automaton)


Energy parity games

Energy games

Initial credit problem:

Decide if there exist an initial credit c0 and a strategy of player 1 to maintain the energy level always nonnegative.

Player 1 (good guy)


Energy parity games

Energy games

Initial credit problem:

Decide if there exist an initial credit c0 and a strategy of player 1 to maintain the energy level always nonnegative.

For energy games, memoryless strategies suffice.

Player 1 (good guy)


Energy parity games

Energy games

A memoryless strategy is winning if all cycles are nonnegative when is fixed.

Initial credit problem:

Decide if there exist an initial credit c0 and a strategy of player 1 to maintain the energy level always nonnegative.


Energy parity games

Energy games

A memoryless strategy is winning if all cycles are nonnegative when is fixed.

Initial credit problem:

Decide if there exist an initial credit c0 and a strategy of player 1 to maintain the energy level always nonnegative.

See [CdAHS03, BFLMS08]


Energy parity games

Energy games

A memoryless strategy is winning if all cycles are nonnegative when is fixed.

Initial credit problem:

Decide if there exist an initial credit c0 and a strategy of player 1 to maintain the energy level always nonnegative.

See [CdAHS03, BFLMS08]


Energy parity games

Energy games

A memoryless strategy is winning if all cycles are nonnegative when is fixed.

c0=0

c0=15

Initial credit problem:

Decide if there exist an initial credit c0 and a strategy of player 1 to maintain the energy level always nonnegative.

c0=0

c0=0

c0=10

c0=0

Minimum initial credit can be

computed in

c0=5


Energy parity games

Mean-payoff games

Mean-payoff value of a play = limit-average of the visited weights

Optimal mean-payoff value can be achieved with a memoryless strategy.

Decision problem:

Decide if there exists a strategy for player 1 to ensure mean-payoff value ≥ 0.


Energy parity games

Mean-payoff games

Mean-payoff value of a play = limit-average of the visited weights

Optimal mean-payoff value can be achieved with a memoryless strategy.

Decision problem:

Decide if there exists a strategy for player 1 to ensure mean-payoff value ≥ 0.


Energy parity games

Mean-payoff games

Memoryless strategy σ ensures nonnegative mean-payoff value

iff

all cycles are nonnegative in Gσ

iff

memoryless strategy σ is winning in energy game

Mean-payoff games with threshold 0 are equivalent to energy games. [BFLMS08]


Energy parity games

Parity games


Energy parity games

Parity games

Parity game:

integer priority on states


Energy parity games

Parity games

Parity game:

integer priority on states

Play:

5, 5, 5, 1, 0, 2, 4, 5, 5, 3…

A play is winning if the least priority visited infinitely often is even.

“Canonical representation of ω-regular specifications ” (e.g. all requests are eventually granted – G[r -> Fg])


Energy parity games

Parity games

Decision problem:

Decide if there exists a winning strategy of player 1 for parity condition.

Player 1 (good guy)


Energy parity games

Parity games

Decision problem:

Decide if there exists a winning strategy of player 1 for parity condition.

For parity games, memoryless strategies suffice.

Player 1 (good guy)


Energy parity games

Parity games

A memoryless strategy is winning if all cycles are even when is fixed.

Decision problem:

Decide if there exists a winning strategy of player 1 for parity condition.


Energy parity games

Parity games

A memoryless strategy is winning if all cycles are even when is fixed.

Decision problem:

Decide if there exists a winning strategy of player 1 for parity condition.

See [EJ91]


Energy parity games

Summary

Energy games - “never exhaust the resource”

Parity game – “always eventually do something useful”


Energy parity games

Energy parity games


Energy parity games

Two-player games on graphs

Qualitative

Quantitative

Parity games

Energy games

ω-regular specifications

(reactivity, liveness,…)

Mean-payoff games

Mixed qualitative-quantitative

Energy parity games

(Mean-payoff parity games have been studied in Lics’05)


Energy parity games

Energy parity games

“never exhaust the resource”Energy parity games: and “always eventually do something useful”

Decision problem:

Decide the existence of a finite initial credit sufficient to win.


Energy parity games

Energy parity games

“never exhaust the resource”Energy parity games: and “always eventually do something useful”


Energy parity games

Summary

Energy games - “never exhaust the resource”

Parity game – “always eventually do something useful”


Energy parity games

Energy parity games

“never exhaust the resource”Energy parity games: and “always eventually do something useful”

Decision problem:

Decide the existence of a finite initial credit sufficient to win.

Energy games – a story of cycles

Parity games – a story of cycles

Energy parity games - ?


Energy parity games

A story of cycles

A one-player energy Büchi game


Energy parity games

A story of cycles

A good cycle ?

good for parity

good for energy


Energy parity games

A story of cycles

Winning strategy:

1. reach and repeat positive cycle to increase energy; 2. visit priority 0; 3. goto 1;


Energy parity games

A story of cycles

Winning strategy:

1. reach and repeat O(2nW) times the positive cycle; 2. visit priority 0; 3. goto 1;

requires exponential memory !


Energy parity games

Complexity


Energy parity games

A story of cycles

In energy parity games, memoryless strategies are sufficient for Player 2.

(the minimum initial credit is same against memoryless Player 2)

Proof


Energy parity games

A story of cycles

In energy parity games, memoryless strategies are sufficient for Player 2.

(the minimum initial credit is same against memoryless Player 2)

Proof

Preliminary fact: under optimal strategy, if we start in q, then energy in q is always greater than on first visit to q.


Energy parity games

A story of cycles

cl

cr

‘left’ game

‘right’ game

In energy parity games, memoryless strategies are sufficient for Player 2.

(the minimum initial credit is same against memoryless Player 2)

  • Proof

  • induction on player 2 states

  • if player 1 wins against all memoryless strategies,then player 1 wins against all arbitrary strategies.

max(cl,cr)


Energy parity games

Complexity


Energy parity games

Complexity


Energy parity games

A story of cycles

A good cycle ?

good for parity

good for energy


Energy parity games

Good-for-energy strategies

A good-for-energy strategy is a winning strategy in the following cycle-forming game:

Cycle-forming game: play the game until a cycle is formed

Player 1 wins if energy of the cycle is positive, or energy is 0 and least priority is even.

If Player 1 wins in an energy parity game, then a memoryless good-for-energy strategy exists.

(not iff)


Energy parity games

Structure of strategies

Attr1(0)

0

  • If least priority is 0

Win

  • Play good-for-energy. Either energy stabilizes and then least priority is even,or energy (strictly) increases.

  • When energy is high enough (+2nW):

  • 2a. If (and while) game is in Q \ Attr(0), play a winning strategy in subgame defined by Q \ Attr(0).

  • 2b. Whenever game is in Attr(0), reach 0 and start over.


Energy parity games

Structure of strategies

  • If least priority is 1

Attr2(1)

1

Win


Energy parity games

Structure of strategies

Attr2(1)

1

W1

  • If least priority is 1

  • Game can be partitioned into winning regions and their attractor.

  • Winning strategy combines subgame winning strategies and reachability strategies.


Energy parity games

Structure of strategies

Attr2(1)

1

W1

Attr1(W1)

  • If least priority is 1

W2

Attr1(W2)

W3

Attr1(W3)

  • Game can be partitionned into winning regions and their attractor.

  • Winning strategy combines subgame winning stragies and reachability strategies.

Corollary: memoryless strategies are sufficient in coBüchi energy games.


Energy parity games

An NP solution

Attr1(0)

0

Assume NP-algorithm for d-1 priorities

NP-algorithm for d priorities:

- guess the winning set and good-for-energy strategy

- compute 0-attractor, and solve subgame in NP

least priority 0

Cd(n) ≤ p(n) + Cd-1(n-1)


Energy parity games

An NP solution

Attr2(1)

1

Assume NP-algorithm for d-1 priorities

NP-algorithm for d priorities:

- guess the winning set and partition

- compute 1-attractor, and solve subgames in NP

least priority 1

W1

Attr1(W1)

Cd(n) ≤ p(n) + Cd-1(n1) + … + Cd-1(nk)

≤ p(n) + Cd-1(n1+ … + nk)

≤ p(n) + Cd-1(n-1)

W2

Attr1(W2)

W3

Attr1(W3)


Energy parity games

Complexity


Energy parity games

Algorithm

Algorithm for solving energy parity games ?

Determine good-for-energy winning states – a story of cycles

Good-for-energy:

All cycles are either >0, or =0 and even

Reduction to (pure) energy games with modified weights:

cycles =0 and even >0cycles =0 and odd  <0other cycles remain >0 or <0


Energy parity games

Algorithm

Δenergy

1/n

1

3

0

2

0

4

priority

-1/n2

Positive increment for transitions from even states

Negative increment for transitions from odd states

Reduction to energy games with modified weights:

cycles =0 and even >0cycles =0 and odd  <0other cycles remain >0 or <0


Energy parity games

Algorithm

Δenergy

1/n

1

3

0

2

0

4

priority

-1/n2

Positive increment for transitions from even states

Negative increment for transitions from odd states

Increment is exponential (in nb. of priorities)


Energy parity games

Algorithm

  • Algorithm for solving energy parity games ?

  • Determine good-for-energy winning states

  • by solving modified-energy game in O(E.Qd+2.W)

  • Recursive fixpoint algorithm, flavour of McNaughton-Zielonka

Note: a reduction to parity games (making energy explicit) would give complexity O(E.(Q2W)d).


Energy parity games

Relationship with mean-payoff parity games


Energy parity games

Two-player games on graphs

Qualitative

Quantitative

Parity games

Energy games

Mean-payoff games

Mixed qualitative-quantitative

Energy parity games

Mean-payoff parity games


Energy parity games

Mean-payoff

Mean-payoff value of a play = limit-average of the visited weights

Optimal mean-payoff value can be achieved with a memoryless strategy.

Decision problem:

Decide if there exists a strategy for player 1 to ensure mean-payoff value ≥ 0.


Energy parity games

Mean-payoff games

Memoryless strategy σ ensures nonnegative mean-payoff value

iff

all cycles are nonnegative in Gσ

iff

memoryless strategy σ is winning in energy game

Mean-payoff games with threshold 0 are equivalent to energy games. [BFLMS08]

(relies on memoryless strategies)


Energy parity games

Mean-payoff parity

Mean-payoff parity games [CHJ05]

Objective:

- satisfy parity condition

- maximize mean-payoff

Infinite memory may be necessary !

However, finite-memory ε-optimal strategies exist.


Energy parity games

Mean-payoff parity

Mean-payoff parity games are polynomially equivalent to energy parity games.

Reduction idea:

from MPP game, construct EP game by incrementing all weights by ε=1/(n+1)

If Player 1 wins MPP ≥0, then finite-memory ε-optimal strategy exists, which is winning in EP game.

If Player 1 wins EP game, then he wins MPP ≥ -ε and then also MPP ≥0 since the value in MPP has denominator ≤n.


Energy parity games

Complexity

By-product: a conceptually simple algorithm for mean-payoff parity games.


Energy parity games

The end

Thank you !

Questions ?


Energy parity games

References


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