Laurent Doyen LSV, ENS Cachan & CNRS

1. Partial-Observation Stochastic Games: How to Win when Belief Fails Laurent DoyenLSV, ENS Cachan & CNRS & Krishnendu ChatterjeeIST Austria Luxembourg, December 8th, 2011

2. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: got_lock++; } 4: if (got_lock != 0) { 5: unlock (); } 6: got_lock--; } while (*); } void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

3. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: got_lock++; } 4: if (got_lock != 0) { 5: unlock (); } 6: got_lock--; } while (*); } void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; } Wrong!

4. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: s0 | s1 | inc | dec; } 4: if (got_lock != 0) { 5: unlock (); } 6: s0 | s1 | inc | dec; } while (*); } s0 | s1 | inc | dec; s0 | s1 | inc | dec; s0 ≡ got_lock = 0 s1 ≡ got_lock = 1 Inc ≡ got_lock++ dec ≡ got_lock-- void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

5. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: s0 | s1 | inc | dec; } 4: if (got_lock != 0) { 5: unlock (); } 6: s0 | s1 | inc | dec; } while (*); } s0 | s1 | inc | dec; s0 | s1 | inc | dec; Repair/synthesis as a game: • System vs. Environment • Turn-based game graph • ω-regular objective void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

6. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: s0 | s1 | inc | dec; } 4: if (got_lock != 0) { 5: unlock (); } 6: s0 | s1 | inc | dec; } while (*); } s0 | s1 | inc | dec; s0 | s1 | inc | dec; void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

7. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: s0 | s1 | inc | dec; } 4: if (got_lock != 0) { 5: unlock (); } 6: s0 | s1 | inc | dec; } while (*); } s0 | s1 | inc | dec; s0 | s1 | inc | dec; • A winning strategy may use the value of L to update got_lock accrodingly: • if L==0 then play s0 (got_lock = 0) • if L==1 then play s1 (got_lock = 1) void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

8. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: s0 | s1 | inc | dec; } 4: if (got_lock != 0) { 5: unlock (); } 6: s0 | s1 | inc | dec; } while (*); } s0 | s1 | inc | dec; s0 | s1 | inc | dec; Repair/synthesis as a game of partial observation: visible hidden void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; } States that differ only by the value of L have the same observation

9. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: s0 | s1 | inc | dec; } 4: if (got_lock != 0) { 5: unlock (); } 6: s0 | s1 | inc | dec; } while (*); } s0 | s1 | inc | dec; s0 | s1 | inc | dec; void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

10. Example void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: s0 | s1 | inc | dec; } 4: if (got_lock != 0) { 5: unlock (); } 6: s0 | s1 | inc | dec; } while (*); } s1; s0; void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

11. Partial-Observation Stochastic Games

12. Examples • Poker • - partial-observation • - stochastic

13. Examples • Poker • - partial-observation • - stochastic • Bonneteau

14. Bonneteau 2 black card, 1 red card Initially, all are face down Goal: find the red card

15. Bonneteau 2 black card, 1 red card Initially, all are face down Goal: find the red card • Rules: • Player 1 points a card • Player 2 flips one remaining black card • Player 1 may change his mind, wins if pointed card is red

16. Bonneteau 2 black card, 1 red card Initially, all are face down Goal: find the red card • Rules: • Player 1 points a card • Player 2 flips one remaining black card • Player 1 may change his mind, wins if pointed card is red

17. Bonneteau 2 black card, 1 red card Initially, all are face down Goal: find the red card • Rules: • Player 1 points a card • Player 2 flips one remaining black card • Player 1 may change his mind, wins if pointed card is red

18. Bonneteau: Game Model

19. Bonneteau: Game Model

20. Game Model

21. Game Model

22. Game Model

23. Game Model

24. Game Model

25. Observations (for player 1)

26. Observations (for player 1)

27. Observations (for player 1)

28. Observations (for player 1)

29. Observations (for player 1)

30. Observation-based strategy This strategy is observation-based, e.g. after it plays

31. Observation-based strategy This strategy is observation-based, e.g. after it plays

32. Optimal ? This strategy is winning with probability 2/3

33. Game Model • This game is: • turn-based • (almost) non-stochastic • player 2 has perfect observation

34. Interaction General case: concurrent & stochastic Player 1’s move Player 2’s move Players choose their moves simultaneously and independently

35. Interaction -player games General case: concurrent & stochastic Probability distribution on successor state Player 1’s move Player 2’s move

36. Interaction Turn-based games Special cases: • player-1 state • player-2 state

37. Partial-observation General case: 2-sided partial observation Two partitions and Observations: partitions induced by coloring

38. Partial-observation General case: 2-sided partial observation Two partitions and Observations: partitions induced by coloring Player 2’s view Player 1’s view

39. Partial-observation Observations: partitions induced by coloring Special case: 1-sided partial observation or

40. Strategies & objective A strategy for Player is a function that maps histories (sequences of observations) to probability distribution over actions.

41. Strategies & objective A strategy for Player is a function that maps histories (sequences of observations) to probability distribution over actions. randomized History-depedent

42. Strategies & objective A strategy for Player is a function that maps histories (sequences of observations) to probability distribution over actions. Reachability objective: Winning probability of :

43. Qualitative analysis The following problem is undecidable: (already for probabilistic automata [Paz71]) Decide if there exists a strategy for player 1 that is winning with probability at least ½. [Paz71] Paz. Introduction to Probabilistic Automata. Academic Press 1971.

44. Qualitative analysis The following problem is undecidable: (already for probabilistic automata [Paz71]) Decide if there exists a strategy for player 1 that is winning with probability at least ½. Qualitative analysis: • Almost-sure: … winning with probability 1 • Positive: … winning with probability > 0

45. Applications in verification tank control • Control with inaccurate digital sensors • multi-process control with private variables • multi-agent protocols • planning with uncertainty/unknown void main () { int got_lock = 0; do { 1: if (*) { 2: lock (); 3: got_lock++; } 4: if (got_lock != 0) { 5: unlock (); } 6: got_lock--; } while (*); } void lock () { assert(L == 0); L = 1; } void unlock () { assert(L == 1); L = 0; }

46. Example 1 Player 1 partial, player 2 perfect

47. Example 1 Player 1 partial, player 2 perfect No pure strategy of Player 1 is winning with probability 1(example from [CDHR06]).

48. Example 1 Player 1 partial, player 2 perfect No pure strategy of Player 1 is winning with probability 1(example from [CDHR06]).

49. Example 1 Player 1 partial, player 2 perfect Player 1 wins with probability 1, and needs randomization Belief-based-only randomized strategies are sufficient

50. Example 2 Player 1 partial, player 2 perfect