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Constraint Satisfaction Introduction to Artificial Intelligence COS302 Michael L. Littman Fall 2001

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Administration HW 1 due, HW 2 assigned (3 question parts!)

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Example CSP 3-coloring: color each node R,B,G. Connected pairs must differ. V3 V4 V2 V6 V5 V1

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Formal Definition Constraint satisfaction problem (CSP): {V, D, C}. V is set of variables V1…Vn. Each variable assigned a value from the domain D. C is set of constraints: each a list of vars, legal assignment set.

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CSP for Graph Problem V = {V1, V2, V3, V4, V5, V6} D = {R, G, B} C = {(V1, V5) : { (B,R), (B,G), (R,B), (R,G), (G,B), (G,R) }, (V2, V5) :{ (B,R), (B,G), (R,B), (R,G), (G,B), (G,R) }, (V2, V5) :{ (B,R), (B,G), (R,B), (R,G), (G,B), (G,R) }, … } … }

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Solve via Search In a state, variables V1 through Vk are assigned and V{k+1} through Vn are unassigned. Start state: All unassigned. G(s): All assigned, constraints satisfied. N(s): One more assigned

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Search Example s 0 = ?????? N(s 0 ) = {R?????, G?????, B?????} What search algorithms could we use? BFS? DFS?

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DFS Looks Dumb V3 V4 V2 V6 V5 V1

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Making DFS Smarter Like A*, we can use a little bit of generic knowledge to guide the search process. Idea? h(s) = infinity if constraints violated by partial assignment Don’t make an assignment if it violates constraints Don’t peek.

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Consistency Checking Still flails a bit… V3 V4 V2 V6 V5 V1 V4 V3 V2

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Forward Checking Track set of (remaining) legal values for each variable. Fail if any variable’s set goes empty. How express this as a heuristic function h(s)?

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Let’s Try It Yay! V3 V4 V2 V6 V5 V1

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Computational Overhead How would you implement forward checking? Could the “empty domain” calculation be performed incrementally?

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Constraint Propagation As values are eliminated from a variable’s domain, this can start a cascade effect on other variables. Expensive operation, so often performed only before search begins. Can be quite powerful…

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Constraint Propagation Solved without search V3 V4 V2 V6 V5 V1

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Example CSPs Graph coloring Real applications include hundreds of thousands of nodesReal applications include hundreds of thousands of nodes VLSI board layout 8 queens, cryptarithmetic Visual scene interpretation: Waltz Minesweeper…

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Minesweeper Example Number is count of bombs in 8 adjacent cells 0 2 1 1 10 00 00 1 1

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Minesweeper CSP V = { V1, …, V8 }, D = {B, S} C = { (V1, V2) : { (B,S), (S,B) }, (V1, V2, V3): {(S,S,B), (S,B,S), (B,S,S)}, …} (V1, V2, V3): {(S,S,B), (S,B,S), (B,S,S)}, …} 0 2 1 1 10 00 00 1 1 V1 V2 V3 V4 V8V7V6V5

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Using CSP to Decide 0 2 1 1 10 00 00 1 1 V1 V2 S V4 V8V7SV5 How can we check if a value is forced?

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Waltz Algorithm Each “Y” intersection can be either concave or convex. Global interpretation is key.

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Scheduling Big, important use of CSPs. Makes multi-million dollar decisions in many industries.Makes multi-million dollar decisions in many industries. Used in space mission planning.Used in space mission planning. Military uses.Military uses. Large state spaces

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Generic CSP Algorithm If all values assigned and no constraints violated, doneIf all values assigned and no constraints violated, done Apply consistency checkingApply consistency checking If deadend, backtrackIf deadend, backtrack Select variable to be assignedSelect variable to be assigned Select value for the variableSelect value for the variable Assign variable and recurseAssign variable and recurse

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Search Heuristics Have some freedom in what variable to assign next Most constrained variableMost constrained variable Most constraining variableMost constraining variable Freedom in value to assign to that variable Least constraining valueLeast constraining value

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Implementing CP Fundamental question: Given a constraint involving variables V1…Vk and possible values P1…Pk, remove impossible valuesGiven a constraint involving variables V1…Vk and possible values P1…Pk, remove impossible values For i = 1 to k for each val in Pi if not possible(V1…V{i-1} { val } V{i+1}…Vk, legalset) Vi -= { val }, restart Return P1…Pk

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What’s “possible”? Possible(V1…Vk, legalset) For l1…lk in legalset fail = 0 For i = 1 to k if li not in Vi, fail = 1 if fail == 0, return TRUE Return FALSE

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Formalizing Other CSPs Cryptograms Paint by Numbers Crossword Puzzles Four on the Floor Battleships

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What to Learn Definition of a CSP: relationship to search. How to formalize problems as CSPs Use of improvements like consistency checking and forward checking

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Homework 2 1.Consider the heuristic for Rush Hour of counting the cars blocking the ice cream truck and adding one. (a) Show this is a relaxation by giving conditions for an illegal move and showing what was eliminated. (b) For the board on the next page, show an optimal sequence of boards en route to the goal. Label each board with the f value from the heuristic. 2.Describe an improved heuristic for Rush Hour. (a) Explain why it is admissible. (b) Is it a relaxation? (c) Label the boards from 1b with the f values from your heuristic.

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Rush Hour Example

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Homework 2 (cont.) 3.Consider the 3-coloring problem on a graph with n nodes with the additional constraint that there not be exactly 2 nodes colored blue. (a) The most direct constraint for this involves all n nodes. How large is the corresponding legal assignment set for this constraint? (b) Explain how to specify this constraint more compactly by breaking it down into a set of simpler constraints. How many constraints do you add and how large is the legal assignment set for each one?

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