Lecture 5 Constraint Satisfaction by nml23533

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									   Lecture 5: Constraint Satisfaction

                        Definition and examples
   Heshaam Faili
                       Properties of CSP problems
 hfaili@ece.ut.ac.ir            Heuristics
University of Tehran       arc-consistency checking
                                heuristic repair
                            most-constrained first
    Constraint Satisfaction
    Problems
   Many problems can be formulated as finding a
    solution, possibly optimal, that satisfies a set of
    constraints.
        built a time table for the next semester with the
        minimum number of conflicts
       schedule air crew assignments so that pilots don’t fly
        too many hours, have evenly-spaced breaks, …
       find a 0/1 assignment to a propositional formula
       8-queen problem, crypto arithmetic


                                                             2
        CSP as a search problem (1)
   State:
       a set of variables x1, x2, …,xn
       variable domains D1, D2 …Dn
       state vector (x1, x2, x3, … xn) with instantiated and
        uninstantiated variables
   Rule: constraints relating the variables
       xi  xj,  xi = 1, min f(x1, x2, …,xn)
   Start state: vector of uninstantiated variables
   Goal state: instantiated vector that satisfies all
    the constraints
                                                                3
  Examples of CSP formulation
• Cryptoarithmetic problem
  FO RTY               29786         Solution
 +    TEN            +    850        F=2, O=9
                                     R=7, T=8
 +    TEN            +    850        Y=6, E=5
  S I XTY              31486         N=0, I=1, X=4

• 8-queens problem     • Time-table schedule
                               Mon Tue Wed Thu Fri
                           8
                          9
                         10
                         11
                                                     4
CSP and Constraint Graph




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CSP as a search problem (2)
   Simplest form: discrete and finite domain
   For n variables and domain size d the possible assignments are O(dn)
   Boolean CSP: domains are boolean, like 3SAT , special case of NP-
    complete
   Infinite domain: no exact solution can be found
        Linear constraints
        Non-linear constraints
   Continuous domains:
        The best-known category is linear programming
        Constraints are linear inequalities forming a convex region
   Constraints:
        Unary: involve only one variable: can be eliminated by remove any value
         violate the constraint from the domain of that variable
        Binary: involves two variables (like 8-queen)
        Higher-order : like crypt arithmetic



                                                                                   6
    Higher-order Constraints
   Can be represented by a constraints hyper-graph
   prove, every higher-order, finite-domain constraint can be
    reduced to a set of binary constraints if enough auxiliary
    variables are introduced.




                                                            7
Backtracking in CSP
   For n variables with d value in each domain
        Branching factor for level 1 is nd, level 2 is (n-1)d, level 3 is (n-2)d,
         …
        Tree has n!dn leaves while there are only dn possible assignments
   Commutativity: if the order of application of any given set of
    actions has no effect on the outcome.
   CSPs are commutative
        all CSP search algorithms generate successors by considering
         possible assignments for only a single variable at each node in the
         search tree
        In root node of Australia region coloring: we choice between SA =
         red, SA = green, and SA = blue, and we never choose between SA
         = red and WA = blue.
        The number of leaves are dn



                                                                                 8
CSP as a search problem
   Search strategy
    1. pick an uninstantiated variable, pick a value from its
      domain, check if all constraints are satisfied
    2. if constraints are satisfied, continue search
      recursively
    3. else, backtrack: go back to previous decision and
      make another choice
   Size of search tree:     bd
       d = n, number of variables
       b = max Di when only one variable at a time is
                                                           9
Backtracking Search




                      10
CSP search example




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CSP search example
 C1 C2 C3                 
   S E N D
 +
   M O R E
                         M=1
 M O N E Y
                   O=0   O=2     O=3


             N=2
                          S32D
      E=3                 10R3
                                   Conflict!   12
                         1023Y
CSP problem characteristics (1)
   Special case of a search problem
   Domains can be discrete or continuous
   The order in which the constraints are
    satisfied does not matter.
   The order in which variables and their
    values are picked makes a big difference!


                                          13
     CSP problem characteristics (2)
   The goal test is decomposed into a set of
    constraints on variables, rather than a
    single black box
   When sets of variables are independent
    (no constraints between them) the
    problem is decomposable and sub-
    problems can be solved independently.
   A each step we must check for
    consistency. We need constraint
    propagation methods!                        14
CSP problem characteristics (3)
   Answering the following questions are
    important key:
       Which variable should be assigned next, and in
        what order should its values be tried?
       What are the implications of the current variable
        assignments for the other unassigned variables?
       When a path fails—that is, a state is reached in
        which a variable has no legal values— can the
        search avoid repeating this failure in subsequent
        paths?

                                                            15
Comparison of various CSP algorithms




                                       16
        Variable and value ordering
   Pick values for the minimum remaining
    values (MRV) or most constrained
    variables or first fail principle
       choosing the variable with the fewest "legal" value
       those with smaller domains and which appear in
        more constraints.
       The rationale: those leave fewer choices and focus
        the search
   Improve the search from 3 til 3000 times
    better than simple
                                                              17
Variable and value ordering
   MRV doesn’t help in choosing the first
    variable
       Degree heuristic: attempts to reduce the
        BF in feature by selecting the variables that
        is involved in the largest number of
        constraints on other unassigned variables
            SA on region coloring example



                                                   18
Value ordering
   least-constraining-value: prefers the
    value that rules out the fewest choices for the
    neighboring variables in the constraint
   In the previous coloring problem:
       WA = red and NT = green, Q is better to assign
        red that blue
   if we are trying to find all the solutions to a
    problem, not just the first one, then the
    ordering does not matter because we have to
    consider every value anyway.
                                                         19
Propagating information
through constraints (1)
   Forward checking: delete values from
    domains as the assignments are made.
    When a domain becomes empty, abort.
    Often more efficient than backtracking




                                         20
Forward checking example




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     Propagating information
     through constraints (2)
   Forward Checking: doesn't detect all of inconsistencies.
        WA = red, Q=green => NT = SA = blue (this inconsistency doesn’t
         detected)
   Arc-consistency: a state is arc-consistent if every variable has a
    value in its domain that is consistent with each of the constraints
    of that variable. This is a form of constraint propagation.
   Arc-consistency provides a fast method of constraint propagation
    that is substantially stronger than forward checking.
   arc-consistency is directed arc in constraint graph
   (SA, NSW) is consistent while (NSW,SA) is not consistent
   Complexity: O(n2d3) for binary CSP
        Binary CSP has at most O(n2) arcs
        Each arc (Xi,Xk) can be inserted on the agenda only d times: Xi has at
         most d values to delete
                                                                                  22
        Checking consistency of an arc can be done in O(d2)
Arc-Consistency



      AC-3 can not find harder the
            inconsistency
         (WA=red, NSW=red)
         Need more concepts




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CSP terminology
   Label <xi, a>
   Compound Label { <xi, a> }
   K-Constraint
   Modeling 8-Queen problem:
       Q1,Q2, …,Q8 in [a,b,…,h]
       Qx  Qy , |Qx-Qy|  |x-y|


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Important Concepts
   K-Satisfiable
   K-Consistency
       K=1: node consistency
       K>1: for each K-1 satisfiable variable set can add
        the Kth variable with consistent value and no
        change in the previous K-1 variable values
   K-strongly consistent
                  A {red, green}         3-Consistent
                                         But not 2-Consistent

            C{red}        B{red}
                                                                25
Handling special constraints
   some problem-specific constraints are used much in different
    problems
        AllDiff constraints says that all variables should be different
        If n variables with m values and n>m, the constraints should not
         be satisfied
   Use the following idea to develop a method:
        First, remove any variable in the constraint that has a singleton
         domain, and delete that variable's value from the domains of the
         remaining variables.
        Repeat as long as there are singleton variables.
        If at any point an empty domain is produced or there are more
         variables than domain values left, then an inconsistency has been
         detected.
   By using the above method, the inconsistency (WA=red,
    NSW=red) can be detected.

                                                                             26
Handling special constraints
   Resource constraints: the most important higher-order
    constraints
   Atmost constraints:(A0, PA1, PA2, PA3, PA4).
        Checking the sum of minimum values of current domains (in each step of
         procedure)
   For example, let's suppose there are two flights, 271 and 272,
    for which the planes have capacities 165 and 385, respectively.
    The initial domains for the numbers of passengers on each flight
    are then
        Flight271  [0,165] and Flight272  [0, 385]
   Now suppose we have the additional constraint that the two
    flights together must carry 420 people: Flight271 + Flight272 
    [420,420]. Propagating bounds constraints, we reduce the
    domains to
        Flight271  [35,165] and Flight272  [255, 385]


                                                                                  27
    CSP heuristics: example
C1 C2 C3 C4                  
    S E N D
 +
    M O R E
                            M=1
                                              C1 = 1
 M O N E Y                                    C2 + S + 1 = O
                    O=0     O=2      O=3      C2 + S + 1 = 1
                                              C3 + E + 0 = N
                                              C4 + N + R = E
              N=2

                    Conflict is detected as
       E=3
                    soon as the assignement
                                                           28
                    is made
 Graph of variables
 dependencies
                     Y
Constraints
X+Y=Z                        W
X-Z<7
Y = Z           X
X-W=3                    Z
U-V=0

Domains
DX = {0, 1, 2}           V       U
DY = {0, … 10}
….                                   29
Intelligent backtracking:
looking backward
   chronological backtracking: Simple BT
   More intelligent: use conflict set
   Backjumping method: backtrack to the
    most recent variable in conflict set




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Min-conflict strategy on 8-
queens




                              31
Important concept (constraint graph)
   Graph Order
   Node width
   Graph Order width
   Graph Width
   Value ordering for CSP search in the
    minimum Graph order width
   If the level of strong consistency is greater
    than the width of constraint graph, the search
    for solution tuple is backtrack free

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Search Strategies
(Haralick & Elliot 1980)
   Standard Backtracking
   Fully Looking Forward
   Partial Looking Ahead
   Forward Checking
   Backchecking
   BackMarking


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        CSP problems at large
   An area of great economic importance
       scheduling, production, etc.
   Closely related to optimization problems
       integer programming
       multiobjective optimization
   Large body of literature in different
    areas: AI, CS, Operations Research,
    Mathematics, …
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?
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