// Restore the domains of the variables which where reduced.
public void UndoPropagation(Propagation propagation)
{
foreach (var reduction in propagation.Reductions)
{
Model.Variables[reduction.Variable].Domain.Values.Add(reduction.Value);
}
}
// Forward propagation of the assigned value, based on the type of constraint.
public Propagation PropagateAssignment(Variable variable, int value)
{
var propagation = new Propagation();
propagation.IsValid = true;
// Check each constraint that contains the assigned variable. Can be made
// considerably faster by precomputing which constraint is used in which
// variable, and storing this in the variable.
for (int i = 0; i < Model.Constraints.Length; i++)
{
var constraint = Model.Constraints[i];
if (constraint.ContainsVariable(variable.Id))
{
switch (constraint)
{
case AllDifferentConstraint:
PropagateAllDifferentConstraint(variable, value, constraint, ref propagation);
break;
case MandatoryConstraint:
PropagateMandatoryConstraint(variable, value, constraint, ref propagation);
break;
case OptionalConstraint:
PropagateOptionalConstraint(variable, value, constraint, ref propagation);
break;
case AlternativeConstraint:
PropagateAlternativeConstraint(variable, value, constraint, ref propagation);
break;
case OrConstraint:
PropagateOrConstraint(variable, value, constraint, ref propagation);
break;
case RequiredConstraint:
PropagateRequiredConstraint(variable, value, constraint, ref propagation);
break;
case ExcludeConstraint:
PropagateExcludeConstraint(variable, value, constraint, ref propagation);
break;
default:
Debug.WriteLine($"Constraint of type '{constraint.GetType().Name}' does not support constraint propagation.");
break;
}
if (!propagation.IsValid)
{
return(propagation);
}
}
}
return(propagation);
}
private void PropagateAllDifferentConstraint(Variable variable, int value, Constraint constraint, ref Propagation propagation)
{
// Check all the other variables that are shared in this constraint.
for (int i = 0; i < constraint.Variables.Length; i++)
{
var otherVariable = constraint.Variables[i];
// If the variable is set, and the domain of the other variable contains the
// assigned value.
if (!otherVariable.IsSet && otherVariable.Domain.Values.Contains(value))
{
// Remove the value from the domain of the variable which shares a
// constraint with the assigned variable.
propagation.Add(otherVariable.Id, value);
otherVariable.Domain.Values.Remove(value);
// If the domain is reduced to zero, the propagation resulted in
// an inconsistent model. Set the flag, and return the reduced domains
// so they can be restored.
if (otherVariable.Domain.Values.Count == 0)
{
propagation.IsValid = false;
return;
}
}
}
}
private void PropagateExcludeConstraint(Variable variable, int value, Constraint constraint, ref Propagation propagation)
{
// In this case, if a variable is enabled then we must disable the other variable.
var parent = constraint.Variables[PARENT];
var child = constraint.Variables[CHILD];
var other = variable.Id == parent.Id ? child : parent;
if (!other.IsSet && value == ON && other.Domain.Values.Contains(ON))
{
propagation.Add(other.Id, ON);
other.Domain.Values.Remove(ON);
propagation.IsValid = other.Domain.Values.Count > 0;
}
}
private void PropagateRequiredConstraint(Variable variable, int value, Constraint constraint, ref Propagation propagation)
{
// If the parent is enabled then the child must also be enabled.
var parent = constraint.Variables[PARENT];
var child = constraint.Variables[CHILD];
if (variable.Id == parent.Id && value == ON)
{
if (!child.IsSet && child.Domain.Values.Contains(OFF))
{
propagation.Add(child.Id, OFF);
child.Domain.Values.Remove(OFF);
propagation.IsValid = child.Domain.Values.Count > 0;
}
}
}
private void PropagateOrConstraint(Variable variable, int value, Constraint constraint, ref Propagation propagation)
{
// In the case that the parent is enabled, we don't know which child to enable, so let the backtracker handle that.
// In the case a child is enabled, we can disable the parent.
var parent = constraint.Variables[PARENT];
if (variable.Id != parent.Id && value == ON)
{
// In the case a child is enabled, we must also enable the parent.
if (!parent.IsSet && value == ON && parent.Domain.Values.Contains(OFF))
{
propagation.Add(parent.Id, OFF);
parent.Domain.Values.Remove(OFF);
propagation.IsValid = parent.Domain.Values.Count > 0;
}
}
}
private void PropagateAlternativeConstraint(Variable variable, int value, Constraint constraint, ref Propagation propagation)
{
// In the case that the parent is enabled, we don't know which child to enable, so let the backtracker handle that.
// In the case a child is enabled, we can disable all the other children, because there can only be one enabled.
var parent = constraint.Variables[PARENT];
if (variable.Id != parent.Id && value == ON)
{
for (int i = 1; i < constraint.Variables.Length; i++)
{
var other = constraint.Variables[i];
if (!other.IsSet && other.Id != variable.Id && other.Domain.Values.Contains(1))
{
propagation.Add(other.Id, ON);
other.Domain.Values.Remove(ON);
if (other.Domain.Values.Count == 0)
{
propagation.IsValid = false;
return;
}
}
}
// In the case a child is enabled, we must also enable the parent.
if (!parent.IsSet && value == ON && parent.Domain.Values.Contains(OFF))
{
propagation.Add(parent.Id, OFF);
parent.Domain.Values.Remove(OFF);
propagation.IsValid = parent.Domain.Values.Count > 0;
}
}
}
private void PropagateOptionalConstraint(Variable variable, int value, Constraint constraint, ref Propagation propagation)
{
// The only case that can be handled is when the child is turned on, the parent must also be turned on.
var parent = constraint.Variables[PARENT];
var child = constraint.Variables[CHILD];
if (value == ON && variable.Id == child.Id)
{
if (!parent.IsSet && parent.Domain.Values.Contains(OFF))
{
propagation.Add(parent.Id, OFF);
parent.Domain.Values.Remove(OFF);
propagation.IsValid = parent.Domain.Values.Count > 0;
}
}
}
private void PropagateMandatoryConstraint(Variable variable, int value, Constraint constraint, ref Propagation propagation)
{
// In the case of a mandatory, if a variable is enabled, we also want to enable the other
// variable. This means that if the variable is enabled, we need to remove 0 from the domain of the other variable.
// We can simply swap the on/off by taking the XOR of the value with 1.
value ^= 1;
// Then we find the other variable.
var parent = constraint.Variables[PARENT];
var child = constraint.Variables[CHILD];
var other = variable.Id == parent.Id ? child : parent;
// And finally remove the value from the domain of the other variable.
if (!other.IsSet && other.Domain.Values.Contains(value))
{
propagation.Add(other.Id, value);
other.Domain.Values.Remove(value);
propagation.IsValid = other.Domain.Values.Count > 0;
}
}
