/**
* Template method, which can be configured by overriding the three
* primitive operations below.
*/
private Assignment recursiveBackTrackingSearch(CSP csp,
Assignment assignment)
{
Assignment result = null;
if (assignment.isComplete(csp.getVariables()))
{
result = assignment;
}
else
{
Variable var = selectUnassignedVariable(assignment, csp);
foreach (Object value in orderDomainValues(var, assignment, csp))
{
assignment.setAssignment(var, value);
fireStateChanged(assignment, csp);
if (assignment.isConsistent(csp.getConstraints(var)))
{
DomainRestoreInfo info = inference(var, assignment, csp);
if (!info.isEmpty())
fireStateChanged(csp);
if (!info.isEmptyDomainFound())
{
result = recursiveBackTrackingSearch(csp, assignment);
if (result != null)
break;
}
info.restoreDomains(csp);
}
assignment.removeAssignment(var);
}
}
return result;
}
private Assignment generateRandomAssignment(CSP csp) {
Assignment assignment = new Assignment();
foreach (Variable var in csp.getVariables())
{
Object randomValue = Util.selectRandomlyFromList(csp.getDomain(var)
.asList());
assignment.setAssignment(var, randomValue);
}
return assignment;
}
private List<Variable> getConflictedVariables(Assignment assignment, CSP csp) {
List<Variable> result = new List<Variable>();
foreach (Constraint constraint in csp.getConstraints())
{
if (!constraint.isSatisfiedWith(assignment))
foreach (Variable var in constraint.getScope())
if (!result.contains(var))
result.Add(var);
}
return result;
}
/**
* Primitive operation, selecting a not yet assigned variable.
*/
protected override Variable selectUnassignedVariable(Assignment assignment, CSP csp)
{
switch (selectionStrategy)
{
case MRV:
return applyMRVHeuristic(csp, assignment).get(0);
case MRV_DEG:
List<Variable> vars = applyMRVHeuristic(csp, assignment);
return applyDegreeHeuristic(vars, assignment, csp).get(0);
default:
foreach (Variable var in csp.getVariables())
{
if (!(assignment.hasAssignmentFor(var)))
return var;
}
}
return null;
}
protected void fireStateChanged(Assignment assignment, CSP csp)
{
foreach (CSPStateListener listener in listeners)
listener.stateChanged(assignment.copy(), csp.copyDomains());
}
private bool revise(Variable xi, Variable xj, Constraint constraint,
CSP csp, DomainRestoreInfo info)
{
bool revised = false;
Assignment assignment = new Assignment();
foreach (Object iValue in csp.getDomain(xi))
{
assignment.setAssignment(xi, iValue);
bool consistentExtensionFound = false;
foreach (Object jValue in csp.getDomain(xj))
{
assignment.setAssignment(xj, jValue);
if (constraint.isSatisfiedWith(assignment))
{
consistentExtensionFound = true;
break;
}
}
if (!consistentExtensionFound)
{
info.storeDomainFor(xi, csp.getDomain(xi));
csp.removeValueFromDomain(xi, iValue);
revised = true;
}
}
return revised;
}
/**
* Primitive operation, selecting a not yet assigned variable. This default
* implementation just selects the first in the ordered list of variables
* provided by the CSP.
*/
protected Variable selectUnassignedVariable(Assignment assignment, CSP csp)
{
foreach (Variable var in csp.getVariables())
{
if (!(assignment.hasAssignmentFor(var)))
return var;
}
return null;
}
/**
* Primitive operation, which tries to prune out values from the CSP which
* are not possible anymore when extending the given assignment to a
* solution. This default implementation just leaves the original CSP as it
* is.
*
* @return An object which provides informations about (1) whether changes
* have been performed, (2) possibly inferred empty domains , and
* (3) how to restore the domains.
*/
protected DomainRestoreInfo inference(Variable var, Assignment assignment,
CSP csp)
{
return new DomainRestoreInfo().compactify();
}
// //////////////////////////////////////////////////////////////
// heuristics for selecting the next unassigned variable and domain ordering
/** : the minimum-remaining-values heuristic. */
private List<Variable> applyMRVHeuristic(CSP csp, Assignment assignment)
{
List<Variable> result = new List<Variable>();
int mrv = int.MAX_VALUE;
foreach (Variable var in csp.getVariables())
{
if (!assignment.hasAssignmentFor(var))
{
int num = csp.getDomain(var).Count;
if (num <= mrv)
{
if (num < mrv)
{
result.clear();
mrv = num;
}
result.Add(var);
}
}
}
return result;
}
public override bool isSatisfiedWith(Assignment assignment)
{
Object value1 = assignment.getAssignment(var1);
return value1 == null || !value1.Equals(assignment.getAssignment(var2));
}
/**
* Primitive operation, ordering the domain values of the specified
* variable.
*/
protected override Iterable? orderDomainValues(Variable var,
Assignment assignment, CSP csp)
{
if (!isLCVHeuristicEnabled)
{
return csp.getDomain(var);
}
else
{
return applyLeastConstrainingValueHeuristic(var, csp);
}
}
private bool revise(Variable var, Constraint constraint,
Assignment assignment, CSP csp, DomainRestoreInfo info)
{
bool revised = false;
foreach (Object value in csp.getDomain(var))
{
assignment.setAssignment(var, value);
if (!constraint.isSatisfiedWith(assignment))
{
info.storeDomainFor(var, csp.getDomain(var));
csp.removeValueFromDomain(var, value);
revised = true;
}
assignment.removeAssignment(var);
}
return revised;
}
// //////////////////////////////////////////////////////////////
// inference algorithms
/** : forward checking. */
private DomainRestoreInfo doForwardChecking(Variable var,
Assignment assignment, CSP csp)
{
DomainRestoreInfo result = new DomainRestoreInfo();
foreach (Constraint constraint in csp.getConstraints(var))
{
List<Variable> scope = constraint.getScope();
if (scope.Count == 2)
{
foreach (Variable neighbor in constraint.getScope())
{
if (!assignment.hasAssignmentFor(neighbor))
{
if (revise(neighbor, constraint, assignment, csp,
result))
{
if (csp.getDomain(neighbor).isEmpty())
{
result.setEmptyDomainFound(true);
return result;
}
}
}
}
}
}
return result;
}
private int countLostValues(Variable var, Object value, CSP csp)
{
int result = 0;
Assignment assignment = new Assignment();
assignment.setAssignment(var, value);
foreach (Constraint constraint in csp.getConstraints(var))
{
Variable neighbor = csp.getNeighbor(var, constraint);
foreach (Object nValue in csp.getDomain(neighbor))
{
assignment.setAssignment(neighbor, nValue);
if (!constraint.isSatisfiedWith(assignment))
{
++result;
}
}
}
return result;
}
/** : the degree heuristic. */
private List<Variable> applyDegreeHeuristic(List<Variable> vars,
Assignment assignment, CSP csp)
{
List<Variable> result = new List<Variable>();
int maxDegree = int.MIN_VALUE;
foreach (Variable var in vars)
{
int degree = 0;
foreach (Constraint constraint in csp.getConstraints(var))
{
Variable neighbor = csp.getNeighbor(var, constraint);
if (!assignment.hasAssignmentFor(neighbor)
&& csp.getDomain(neighbor).Count > 1)
++degree;
}
if (degree >= maxDegree)
{
if (degree > maxDegree)
{
result.clear();
maxDegree = degree;
}
result.Add(var);
}
}
return result;
}
private int countConflicts(Assignment assignment,
List<Constraint> constraints) {
int result = 0;
foreach (Constraint constraint in constraints)
if (!constraint.isSatisfiedWith(assignment))
result++;
return result;
}
public Assignment copy()
{
Assignment copy = new Assignment();
foreach (Variable var in variables)
{
copy.setAssignment(var, variableToValue.get(var));
}
return copy;
}
/**
* Primitive operation, ordering the domain values of the specified
* variable. This default implementation just takes the default order
* provided by the CSP.
*/
protected Iterable? orderDomainValues(Variable var,
Assignment assignment, CSP csp)
{
return csp.getDomain(var);
}
private Object getMinConflictValueFor(Variable var, Assignment assignment,
CSP csp) {
List<Constraint> constraints = csp.getConstraints(var);
Assignment duplicate = assignment.copy();
int minConflict = int.MAX_VALUE;
List<Object> resultCandidates = new List<Object>();
foreach (Object value in csp.getDomain(var))
{
duplicate.setAssignment(var, value);
int currConflict = countConflicts(duplicate, constraints);
if (currConflict <= minConflict) {
if (currConflict < minConflict) {
resultCandidates.clear();
minConflict = currConflict;
}
resultCandidates.Add(value);
}
}
if (!resultCandidates.isEmpty())
return Util.selectRandomlyFromList(resultCandidates);
else
return null;
}
/**
* Primitive operation, which tries to prune out values from the CSP which
* are not possible anymore when extending the given assignment to a
* solution.
*
* @return An object which provides informations about (1) whether changes
* have been performed, (2) possibly inferred empty domains , and
* (3) how to restore the domains.
*/
protected override DomainRestoreInfo inference(Variable var, Assignment assignment,
CSP csp)
{
switch (inferenceStrategy)
{
case FORWARD_CHECKING:
return doForwardChecking(var, assignment, csp);
case AC3:
return new AC3Strategy().reduceDomains(var, assignment
.getAssignment(var), csp);
default:
return new DomainRestoreInfo().compactify();
}
}
