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![Page 1: SE 420 1 This time: Outline Game playing The minimax algorithm Resource limitations alpha-beta pruning Elements of chance.](https://reader035.fdocuments.net/reader035/viewer/2022062407/56649d315503460f94a09f34/html5/thumbnails/1.jpg)
SE 420 1
This time: Outline
• Game playing• The minimax algorithm• Resource limitations• alpha-beta pruning• Elements of chance
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SE 420 2
What kind of games?
• Abstraction: To describe a game we must capture every relevant aspect of the game. Such as:• Chess• Tic-tac-toe• …
• Accessible environments: Such games are characterized by perfect information
• Search: game-playing then consists of a search through possible game positions
• Unpredictable opponent: introduces uncertainty thus game-playing must deal with contingency problems
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SE 420 3
Searching for the next move
• Complexity: many games have a huge search space• Chess: b = 35, m=100 nodes = 35 100
if each node takes about 1 ns to explorethen each move will take about 10 50 millennia to calculate.
• Resource (e.g., time, memory) limit: optimal solution not feasible/possible, thus must approximate
1. Pruning: makes the search more efficient by discarding portions of the search tree that cannot improve quality result.
2. Evaluation functions: heuristics to evaluate utility of a state without exhaustive search.
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SE 420 4
Two-player games
• A game formulated as a search problem:
• Initial state: ?• Operators: ?• Terminal state: ?• Utility function: ?
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SE 420 5
Two-player games
• A game formulated as a search problem:
• Initial state: board position and turn• Operators: definition of legal moves• Terminal state: conditions for when game is over• Utility function: a numeric value that describes the
outcome of thegame. E.g., -1, 0, 1 for loss, draw, win.(AKA payoff function)
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SE 420 6
Game vs. search problem
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SE 420 7
Example: Tic-Tac-Toe
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SE 420 8
Type of games
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SE 420 9
Type of games
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SE 420 10
The minimax algorithm
• Perfect play for deterministic environments with perfect information
• Basic idea: choose move with highest minimax value= best achievable payoff against best play
• Algorithm: 1. Generate game tree completely
2. Determine utility of each terminal state
3. Propagate the utility values upward in the tree by applying MIN and MAX operators on the nodes in the current level
4. At the root node use minimax decision to select the move with the max (of the min) utility value
• Steps 2 and 3 in the algorithm assume that the opponent will play perfectly.
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SE 420 11
Generate Game Tree
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SE 420 12
Generate Game Tree
x x x
x
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SE 420 13
Generate Game Tree
x
o x x
o
x
o
x o
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SE 420 14
Generate Game Tree
x
o x x
o
x
o
x o
1 ply
1 move
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SE 420 15
A subtree
win
lose
draw
xxo
o
ox
xxo
o
ox
xxo
o
oxx
xxo
o
ox
x x
xxo
o
oxx
xxo
o
oxx
xxo
o
ox
x
xxo
o
ox
x
xxo
o
ox
x
xxo
o
ox
xo
oo oo
o
xxo
o
oxx
o x xx
xxo
o
ox
xo
xxo
o
ox
x
o xxx
oo
ox
xo
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SE 420 16
What is a good move?
win
lose
draw
xxo
o
ox
xxo
o
ox
xxo
o
oxx
xxo
o
ox
x x
xxo
o
oxx
xxo
o
oxx
xxo
o
ox
x
xxo
o
ox
x
xxo
o
ox
x
xxo
o
ox
xo
oo oo
o
xxo
o
oxx
o x xx
xxo
o
ox
xo
xxo
o
ox
x
o xxx
oo
ox
xo
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SE 420 17
Minimax
3 812 4 6 14 252
•Minimize opponent’s chance•Maximize your chance
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SE 420 18
Minimax
3 2
3
2
812 4 6 14 252
MIN
•Minimize opponent’s chance•Maximize your chance
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SE 420 19
Minimax
3
3
2
3
2
812 4 6 14 252
MAX
MIN
•Minimize opponent’s chance•Maximize your chance
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SE 420 20
Minimax
3
3
2
3
2
812 4 6 14 252
MAX
MIN
•Minimize opponent’s chance•Maximize your chance
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SE 420 21
minimax = maximum of the minimum
1st ply
2nd ply
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SE 420 22
Minimax: Recursive implementation
Complete: ?Optimal: ?
Time complexity: ?Space complexity: ?
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SE 420 23
Minimax: Recursive implementation
Complete: Yes, for finite state-spaceOptimal: Yes
Time complexity: O(bm)Space complexity: O(bm) (= DFSDoes not keep all nodes in memory.)
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SE 420 24
1. Move evaluation without complete search
• Complete search is too complex and impractical
• Evaluation function: evaluates value of state using heuristics and cuts off search
• New MINIMAX:• CUTOFF-TEST: cutoff test to replace the termination
condition (e.g., deadline, depth-limit, etc.)• EVAL: evaluation function to replace utility function
(e.g., number of chess pieces taken)
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SE 420 25
Evaluation functions
• Weighted linear evaluation function: to combine n heuristics
f = w1f1 + w2f2 + … + wnfn
E.g, w’s could be the values of pieces (1 for prawn, 3 for bishop etc.)f’s could be the number of type of pieces on the board
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SE 420 26
Note: exact values do not matter
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SE 420 27
Minimax with cutoff: viable algorithm?
Assume we have 100 seconds, evaluate 104 nodes/s; can evaluate 106 nodes/move
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SE 420 28
2. - pruning: search cutoff
• Pruning: eliminating a branch of the search tree from consideration without exhaustive examination of each node
- pruning: the basic idea is to prune portions of the search tree that cannot improve the utility value of the max or min node, by just considering the values of nodes seen so far.
• Does it work? Yes, in roughly cuts the branching factor from b to b resulting in double as far look-ahead than pure minimax
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SE 420 29
- pruning: example
6
6
MAX
6 12 8
MIN
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SE 420 30
- pruning: example
6
6
MAX
6 12 8 2
2MIN
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SE 420 31
- pruning: example
6
6
MAX
6 12 8 2
2
5
5MIN
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SE 420 32
- pruning: example
6
6
MAX
6 12 8 2
2
5
5MIN
Selected move
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SE 420 33
- pruning: general principle
Player
Player
Opponent
Opponent
m
n
v
If > v then MAX will chose m so prune tree under n
Similar for for MIN
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SE 420 34
Properties of -
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SE 420 35
The - algorithm:
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SE 420 36
More on the - algorithm
• Same basic idea as minimax, but prune (cut away) branches of the tree that we know will not contain the solution.
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SE 420 37
More on the - algorithm: start from Minimax
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SE 420 38
Remember: Minimax: Recursive implementation
Complete: Yes, for finite state-spaceOptimal: Yes
Time complexity: O(bm)Space complexity: O(bm) (= DFSDoes not keep all nodes in memory.)
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SE 420 39
More on the - algorithm
• Same basic idea as minimax, but prune (cut away) branches of the tree that we know will not contain the solution.
• Because minimax is depth-first, let’s consider nodes along a given path in the tree. Then, as we go along this path, we keep track of: : Best choice so far for MAX : Best choice so far for MIN
![Page 40: SE 420 1 This time: Outline Game playing The minimax algorithm Resource limitations alpha-beta pruning Elements of chance.](https://reader035.fdocuments.net/reader035/viewer/2022062407/56649d315503460f94a09f34/html5/thumbnails/40.jpg)
SE 420 40
More on the - algorithm: start from Minimax
Note: These are bothLocal variables. At theStart of the algorithm,We initialize them to = - and = +
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SE 420 41
More on the - algorithm
…
MAX
MIN
MAX
= - = +
5 10 6 2 8 7
Min-Value loopsover these
In Min-Value:
= - = 5
= - = 5
= - = 5
Max-Value loopsover these
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SE 420 42
More on the - algorithm
…
MAX
MIN
MAX
= - = +
5 10 6 2 8 7
In Max-Value:
= - = 5
= - = 5
= - = 5
= 5 = +Max-Value loops
over these
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SE 420 43
In Min-Value:More on the - algorithm
…
MAX
MIN
MAX
= - = +
5 10 6 2 8 7 = - = 5
= - = 5
= - = 5
= 5 = +
= 5 = 2End loop and return 5
Min-Value loopsover these
![Page 44: SE 420 1 This time: Outline Game playing The minimax algorithm Resource limitations alpha-beta pruning Elements of chance.](https://reader035.fdocuments.net/reader035/viewer/2022062407/56649d315503460f94a09f34/html5/thumbnails/44.jpg)
SE 420 44
In Max-Value:More on the - algorithm
…
MAX
MIN
MAX
= - = +
5 10 6 2 8 7 = - = 5
= - = 5
= - = 5
= 5 = +
= 5 = 2End loop and return 5
= 5 = +
Max-Value loopsover these
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SE 420 45
Example
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SE 420 46
- algorithm:
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SE 420 47
Solution
NODE TYPE ALPHA BETA SCORE A Max -I +IB Min -I +I C Max -I +I D Min -I +I E Max 10 10 10 D Min -I 10 F Max 11 11 11 D Min -I 10 10 C Max 10 +I G Min 10 +I H Max 9 9 9 G Min 10 9 9 C Max 10 +I 10 B Min -I 10 J Max -I 10 K Min -I 10 L Max 14 14 14 K Min -I 10 10 …
NODE TYPE ALPHA BETA SCORE …J Max 10 10 10 B Min -I 10 10 A Max 10 +I Q Min 10 +I R Max 10 +I S Min 10 +I T Max 5 5 5 S Min 10 5 5 R Max 10 +I V Min 10 +I W Max 4 4 4 V Min 10 4 4 R Max 10 +I 10 Q Min 10 10 10 A Max 10 10 10
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SE 420 48
State-of-the-art for deterministic games
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SE 420 49
Nondeterministic games
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SE 420 50
Algorithm for nondeterministic games
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SE 420 51
Remember: Minimax algorithm
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SE 420 52
Nondeterministic games: the element of chance
3 ?
0.50.5
817
8
?
CHANCE ?
expectimax and expectimin, expected values over all possible outcomes
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SE 420 53
Nondeterministic games: the element of chance
3 50.50.5
817
8
5
CHANCE 4 = 0.5*3 + 0.5*5Expectimax
Expectimin
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SE 420 54
Evaluation functions: Exact values DO matter
Order-preserving transformation do not necessarily behave the same!
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SE 420 55
State-of-the-art for nondeterministic games
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SE 420 56
Summary
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SE 420 57
Exercise: Game Playing
(a) Compute the backed-up values computed by the minimax algorithm. Show your answer by writing values at the appropriate nodes in the above tree.
(b) Compute the backed-up values computed by the alpha-beta algorithm. What nodes will not be examined by the alpha-beta pruning algorithm?
(c) What move should Max choose once the values have been backed-up all the way?
A
B C D
E F G H I J K
L M N O P Q R S T U V W YX
2 3 8 5 7 6 0 1 5 2 8 4 210
Max
Max
Min
Min
Consider the following game tree in which the evaluation function values are shown below each leaf node. Assume that the root node corresponds to the maximizing player. Assume the search always visits children left-to-right.