/**
* Makes a CSP consisting of binary constraints arc-consistent.
*
* @return An object which indicates success/failure and contains data to
* undo the operation.
*/
public IInferenceLog <VAR, VAL> apply(CSP <VAR, VAL> csp)
{
ICollection <VAR> queue = CollectionFactory.CreateFifoQueueNoDuplicates <VAR>();
queue.AddAll(csp.getVariables());
DomainLog <VAR, VAL> log = new DomainLog <VAR, VAL>();
reduceDomains(queue, csp, log);
return(log.compactify());
}
/**
* Removes all values from the domain of <code>var</code> which are not consistent with
* <code>constraint</code> and <code>assignment</code>. Modifies the domain log accordingly so
* that all changes can be undone later on.
*/
private bool revise(VAR var, IConstraint <VAR, VAL> constraint, Assignment <VAR, VAL> assignment,
CSP <VAR, VAL> csp, DomainLog <VAR, VAL> log)
{
bool revised = false;
foreach (VAL value in csp.getDomain(var))
{
assignment.add(var, value);
if (!constraint.isSatisfiedWith(assignment))
{
log.storeDomainFor(var, csp.getDomain(var));
csp.removeValueFromDomain(var, value);
revised = true;
}
assignment.remove(var);
}
return(revised);
}
/**
* For efficiency reasons the queue manages updated variables vj whereas the original AC3
* manages neighbor arcs (vi, vj). Constraints which are not binary are ignored.
*/
private void reduceDomains(ICollection <VAR> queue, CSP <VAR, VAL> csp, DomainLog <VAR, VAL> log)
{
while (!queue.IsEmpty())
{
VAR var = queue.Pop();
foreach (IConstraint <VAR, VAL> constraint in csp.getConstraints(var))
{
VAR neighbor = csp.getNeighbor(var, constraint);
if (neighbor != null && revise(neighbor, var, constraint, csp, log))
{
if (csp.getDomain(neighbor).isEmpty())
{
log.setEmptyDomainFound(true);
return;
}
queue.Add(neighbor);
}
}
}
}
/**
* Removes all values from the domains of the neighbor variables of <code>var</code> in the
* constraint graph which are not consistent with the new value for <code>var</code>.
* It is called after <code>assignment</code> has (recursively) been extended with a value
* assignment for <code>var</code>.
*/
public IInferenceLog <VAR, VAL> apply(CSP <VAR, VAL> csp, Assignment <VAR, VAL> assignment, VAR var)
{
DomainLog <VAR, VAL> log = new DomainLog <VAR, VAL>();
foreach (IConstraint <VAR, VAL> constraint in csp.getConstraints(var))
{
VAR neighbor = csp.getNeighbor(var, constraint);
if (neighbor != null && !assignment.contains(neighbor))
{
if (revise(neighbor, constraint, assignment, csp, log))
{
if (csp.getDomain(neighbor).isEmpty())
{
log.setEmptyDomainFound(true);
return(log);
}
}
}
}
return(log);
}
/**
* Reduces the domain of the specified variable to the specified value and
* reestablishes arc-consistency. It is assumed that the provided CSP was
* arc-consistent before the call.
*
* @return An object which indicates success/failure and contains data to
* undo the operation.
*/
public IInferenceLog <VAR, VAL> apply(CSP <VAR, VAL> csp, Assignment <VAR, VAL> assignment, VAR var)
{
Domain <VAL> domain = csp.getDomain(var);
VAL value = assignment.getValue(var);
if (!domain.contains(value))
{
throw new Exception("domain does not contain value");
}
DomainLog <VAR, VAL> log = new DomainLog <VAR, VAL>();
if (domain.size() > 1)
{
ICollection <VAR> queue = CollectionFactory.CreateFifoQueue <VAR>();
queue.Add(var);
log.storeDomainFor(var, domain);
csp.setDomain(var, new Domain <VAL>(value));
reduceDomains(queue, csp, log);
}
return(log.compactify());
}
/**
* Establishes arc-consistency for (xi, xj).
* @return value true if the domain of xi was reduced.
*/
private bool revise(VAR xi, VAR xj, IConstraint <VAR, VAL> constraint, CSP <VAR, VAL> csp, DomainLog <VAR, VAL> log)
{
Domain <VAL> currDomain = csp.getDomain(xi);
ICollection <VAL> newValues = CollectionFactory.CreateQueue <VAL>();
Assignment <VAR, VAL> assignment = new Assignment <VAR, VAL>();
foreach (VAL vi in currDomain)
{
assignment.add(xi, vi);
foreach (VAL vj in csp.getDomain(xj))
{
assignment.add(xj, vj);
if (constraint.isSatisfiedWith(assignment))
{
newValues.Add(vi);
break;
}
}
}
if (newValues.Size() < currDomain.size())
{
log.storeDomainFor(xi, csp.getDomain(xi));
csp.setDomain(xi, new Domain <VAL>(newValues));
return(true);
}
return(false);
}
