Activities may be ranked automatically as part of the constraint propagation process.

Ranking a resource constraint first means that the activity corresponding to the resource constraint will be executed before every activity on the resource that has not already been ranked first. Specifying that a resource constraint is not ranked first (that is, IlcResourceConstraint::rankNotFirst) means that the corresponding activity will execute after at least one of the activities that is currently not ranked. A similar (inverse) rule applies to ranking last.

More precisely, the propagation of global constraints (light precedence graph, disjunctive constraint) allows Scheduler Engine to deduce that some activities cannot be first or cannot be last: internal versions of the member functions IlcResourceConstraint::rankNotFirst and IlcResourceConstraint::rankNotLast are called as part of the constraint propagation process.

Ranking facilities are defined only for unary resources with a posted light precedence graph constraint, or state resources with a posted disjunctive constraint. Use the member function IlcResource::hasRankInfo to find out whether or not ranking facilities are available on a given resource

A resource constraint can be ranked if and only if its time extent is IlcTimeExtent::IlcFromStartToEnd.

In addition, if the resource is closed (as defined for the parent class IlcResource) and only one activity can be first, then this activity is necessarily first: an internal version of the member function IlcResourceConstraint::rankFirst is called as part of the constraint propagation process.

Similarly, if the resource is closed and only one activity can be last, then this activity is necessarily last: an internal version of the member function rankLast is called as part of the constraint propagation process.

If the resource is not closed, then ranking an activity to be not first or not last on a resource has no influence on its earliest start time.

The member functions IlcResourceConstraint::isPossibleFirst, IlcResourceConstraint::isPossibleLast, IlcResourceConstraint::isRanked, IlcResourceConstraint::isRankedFirst, and IlcResourceConstraint::isRankedLast let you know whether the activity that corresponds to a given resource constraint can be first, can be last, is already ranked, ranked first or ranked last.

Non-contributing resource constraints (those not posted, with null processing time or with null capacity requirement) are automatically considered as being ranked; therfore, they are not possible first or possible last anymore.

Furthermore, some member functions are available on the class IlcResource to access the last resource constraint that has been ranked first or last (IlcResource::getLastRankedFirstRC, IlcResource::getLastRankedLastRC) as well as to post demons on the detection of new ranked first or last activities on the resource (IlcResource::whenRankedFirstRC, IlcResource::whenRankedLastRC).

Some iterators are available for a chronological traversal of all the activities that have been ranked first or last or for iterating over the resource constraints that are possible first or last (IlcResource::ResourceConstraintIterator, IlcResource::ResourceConstraintDeltaIterator).

See Also

IlcResource, IlcResourceConstraint, IlcTimeExtent, Precedence Graph Constraints, Disjunctive Constraint