private Assignment <TVar, TVal> RecursiveBacktrack(Assignment <TVar, TVal> currentAssignment, OptimizationProblem <TVar, TVal> problem, CancellationToken cancellationToken)
{
// This is based on an algorithm from
// "Artificial Intelligence: A Modern Approach" 2nd Edition by Stuart Russel and Perter Norvig (page 142).
if (currentAssignment.IsComplete(problem.Variables))
{
return(currentAssignment);
}
cancellationToken.ThrowIfCancellationRequested();
Variable <TVar, TVal> variable = VariableSelectionStrategy.SelectUnassignedVariable(problem.Variables, currentAssignment, new Problem <TVar, TVal>(problem));
IEnumerable <TVal> domain = DomainSortStrategy.GetOrderedDomain(variable, currentAssignment, new Problem <TVar, TVal>(problem));
foreach (var value in domain)
{
var nextAssignment = currentAssignment.Assign(variable, value);
if (this.costFunction.Cost(nextAssignment) >= this.best)
{
continue;
}
if (nextAssignment.IsConsistent(problem.Constraints))
{
var result = RecursiveBacktrack(nextAssignment, problem, cancellationToken);
if (result != null)
{
return(result);
}
}
}
return(null);
}
private async Task <Assignment <Tval> > Backtrack(ConstraintSatisfactionProblem <Tval> csp, Assignment <Tval> assignment)
{
if (assignment.IsComplete(csp.GetVariables()))
{
return(assignment);
}
Variable <Tval> variable = variableSelectionStrategy.SelectUnassignedVariable(csp, assignment);
foreach (Tval value in domainValueSelectionStragety.getOrderedDomainValues(variable, assignment, csp))
{
assignment.Assign(variable, value);
OnVariableAssigned(variable, value);
InferenceResults <Tval> inference = new InferenceResults <Tval>();
inference.StoreDomainForVariable(variable, variable.GetDomain());
variable.UpdateDomain(new Domain <Tval>(value));
// it should already be consistent if it reached this point
// since we make a pre-solving inference
//bool isConsistent = assignment.IsConsistent(csp.GetAllDiffConstraints());
//if (isConsistent)
if (true)
{
inference = await inferenceStrategy.Infer(csp, variable, value, inference);
if (inference.IsAssignmentConsistent())
{
Assignment <Tval> result = await Backtrack(csp, assignment);
if (result != null)
{
return(result);
}
}
}
assignment.RemoveAssignment(variable);
OnAssignmentRemoved(variable, value);
inference.RestoreOldDomains();
}
return(null);
}
