1. Game Theory Conflict Robin Burke GAM 224 Fall 2005

2. Outline Admin Quiz Game Theory Utility theory Zero-sum and non-zero sum games Decision Trees Degenerate strategies Conflict Types of goals Systems of conflict

3. Admin Due Monday Rule paper Hard copy in class turnitin.com Assassin starts midnight Friday pick up player kits from Prof. Wilcox or wait until Monday

4. Ernest Adams Talk 10:30 – 12 noon go to talk and write reaction paper substitute for any other reaction paper Game design workshop 1 – 4 pm CTI students only, preference to GAM majors work with other students / faculty apply his techniques to develop game ideas personal feedback from the expert For workshop send email to jtrinkle@cs.depaul.edu must attend talk to go to workshop

5. Quiz 30 minutes

6. Game Theory A branch of economics Studies rational choice in a adversarial environment Assumptions rational actors complete knowledge in its classic formulation known probabilities of outcomes known utility functions

7. Utility Theory Utility theory a single scale value with each outcome Different actors may have different utility valuations but all have the same scale

8. Expected Utility Expected utility what is the likely outcome of a set of outcomes each with different utility values Example Bet $5 if a player rolls 7 or 11, lose $2 otherwise Should you take this bet?

9. How to evaluate Expected Utility for each outcome reward * probability (1/6) * 5 + (1/18) * 5 + (7/9) (-2) = -2/9 Meaning If you made this bet 1000 times, you would probably end up $222 poorer. Doesn't say anything about how a given trial will end up Probability says nothing about the single case

10. Game Theory Examine strategies based on expected utility The idea a rational player will choose the strategy with the best expected utility

11. Example Non-probabilistic Cake slicing Two players cutter chooser Cutter's Utility Choose bigger piece Choose smaller piece Cut cake evenly ½ - a bit½ + a bit Cut unevenly Small pieceBig piece

12. Rationality each player will take highest utility option taking into account the other player's likely behavior In example if cutter cuts unevenly he might like to end up in the lower right but the other player would never do that -10 if the current cuts evenly, he will end up in the upper left this is a stable outcome neither player has an incentive to deviate Both Utilities Choose bigger piece Choose smaller piece Cut cake evenly (-1, +1)(+1, -1) Cut unevenly (-10, +10)(+10, -10)

13. Zero-sum Note for every outcome the total utility for all players is zero Zero-sum game something gained by one player is lost by another zero-sum games are guaranteed to have a winning strategy a correct way to play the game Makes the game not very interesting to play to study, maybe

14. Non-zero sum A game in which there are non- symmetric outcomes better or worse for both players Classic example Prisoner's Dilemma Hold OutConfess Hold Out[-1, -1][-3, 0] Confess[0, -3][-5, -5]

15. Degenerate Strategy A winning strategy is also called a degenerate strategy Because it means the player doesn't have to think there is a "right" way to play Problem game stops presenting a challenge players will find degenerate strategies if they exist

16. Nash Equilibrium Sometimes there is a "best" solution Even when there is no dominant one A Nash equilibrium is a strategy in which no player has an incentive to deviate because to do so gives the other an advantage Creator John Nash Jr "A Beautiful Mind" Nobel Prize 1994

17. Classic Examples Car Dealers Why are they always next to each other? Why aren't they spaced equally around town? Optimal in the sense of not drawing customers to the competition Equilibrium because to move away from the competitor is to cede some customers to it

18. Prisoner's Dilemma Nash Equilibrium Confess Because in each situation, the prisoner can improve his outcome by confessing Solution iteration communication commitment

19. Rock-Paper-Scissors Player 2 RockPaperScissors Player 1Rock[0,0][-1, +1][+1, -1] Paper[+1, -1][0,0][-1, +1] Scissors[-1, +1][+1, -1][0,0]

20. No dominant strategy Meaning there is no single preferred option for either player Best strategy (single iteration) choose randomly "mixed strategy"

21. Mixed Strategy Important goal in game design Player should feel all of the options are worth using none are dominated by any others Rock-Paper-Scissors dynamic is often used to achieve this Example Warcraft II Archers > Knights Knights > Footmen Footmen > Archers must have a mixed army

22. Mixed Strategy 2 Other ways to achieve mixed strategy Ignorance If the player can't determine the dominance of a strategy a mixed approach will be used (but players will eventually figure it out!) Cost Dominance is reduced if the cost to exercise the option is increased or cost to acquire it Rarity Mixture is required if the dominant strategy can only be used periodically or occasionally Payoff/Probability Environment Mixture is required if the probabilities or payoffs change throughout the game

23. Mixed Strategy 3 In a competitive setting mixed strategy may be called for even when there is a dominant strategy Example Football third down / short yardage highest utility option running play best chance of success lowest cost of failure But if your opponent assumes this defense adjusts to prevent a run increasing the payoff of a long pass But if you are behind in a close game and time is short payoff changes because an incomplete pass stops the clock opponent may not defend the run

24. Degeneracies Are not always obvious May be contingent on game state

25. Example Liar's Dice roll the dice in a cup state the "poker hand" you have rolled stated hand must be higher than the opponent's previous roll opponent can either accept the roll, and take his turn, or say "Liar", and look at the dice if the description is correct opponent pays $1 if the description is a lie player pays $1

26. Lie or Not Lie Make outcome chart for next player assume the roll is not good enough Roller lie or not lie Next player accept or doubt

27. Expectation Knowledge the opponent knows more than just this the opponent knows the previous roll that the player must beat probability of lying

28. Note The opponent will never lie about a better roll Outcome cannot be improved by doing so The opponent cannot tell the truth about a worse roll Illegal under the rules

29. Expected Utility What is the expected utility of the doubting strategy? P(worse) - P(better) When P(worse) is greater than 0.5 doubt Probabilities pair or better: 95% 2 pair or better: 71% 3 of a kind or better: 25% So start to doubt somewhere in the middle of the two- pair range maybe 4s-over-1s

30. BUT There is something we are ignoring

31. Repeated Interactions Roll 1 Roll 2 Roll 1 accept Win accept doubt TruthLie Lose doubt LieTruth doubt TruthLie doubt accept Roll 2

32. Decision Tree Examines game interactions over time Each node Is a unique game state Player choices create branches Leaves end of game (win/lose) Important concept for design usually at abstract level question can the player get stuck? Example tic-tac-toe

33. Future Cost There is a cost to "accept" I may be incurring some future cost because I may have to lie and get caught To compare doubting and accepting we have to look at the possible futures of the game In any case the game becomes degenerate the player must doubt

34. Conflict Somewhat obvious "artificial conflict" is part of the definition

35. Struggle to achieve a goal Single player vs. single player Chess, Boxing, Warcraft II Group vs. group Basketball, Soccer, Battlefield 1942 One against many Tag, Most action-adventure and FPS games Every man for himself Marathon, Risk, Mario Kart

36. Struggle to achieve a goal Single player vs. computer Tetris Group of single players vs. game Blackjack Group against the game Lord of the Rings board game Cooperative mode in Star Wars Battlefront, etc.

37. Game goal May not be fixed May not be obvious from the game's premise Single game may support multiple goals from the player's point of view

38. Example: Centipede Single player mode Do well against the game Beat your personal best Get on the high score list Two player mode Beat your opponent Get on high score list Compete against friend to get on the high score list

39. Example: Joust Single player mode Players compare scores Two player mode One player against the other Battle mode Cooperation mode players vs. computer

40. Example: Gauntlet Single player mode Two player mode Cannot attack each other Real time display of scores Players compete for “food” Allow most needy to eat or be greedy Compete for spending money

41. Competitive vs. cooperative All games are competitive Can they be cooperative at the same time? Adherence to the Magic Circle Players on a Basketball team Conflict within a cooperative framework

42. Systems of conflict Games exhibit conflict at multiple levels Local goals contribute to larger goals Conflicts at different levels work together to create a system the interrelatedness of the conflicts determines if any given conflict seems meaningful

43. Example: Legend of Zelda: The Wind Waker Largest conflict evil sorcerer, Gannon Intermediate conflict defeat level Local conflict solve puzzle defeat particular enemy master particular skill

44. Wind Waker, cont'd Conflict system each level is crafted to introduce new skills which are essential to succeeding in this and later levels each level features enemies with certain common characteristics essential to defeating the boss Game logic links conflicts hero must collect pearls to defeat Gannon pearls are obtained by defeating a boss enemy boss enemies are found at the innermost room of each dungeon exploration of the dungeon requires defeating enemies and solving puzzles exploration also requires weapons, tools and power-ups items found by exploring the dungeon

45. Conflict system may fail Game lacks meaningful play if players do not perceive the links between conflicts Example Kingdom Hearts goal is to protect various "worlds" travel between worlds is done through a 3-D space shooting game no coherence between this local conflict and the global one it feels tacked on and rather pointless

46. Cooperative games? "New Games" Movement Dragon conflict between head and tail mediated by players with uncertain stakes Stand-up conflict with gravity / stability UN Food Force game mission = help refugees conflict with terrain, elements, limited budgets, logistics

47. Monday Analysis Case Study Legend of Zelda: Wind Waker