When there are multiple combinations to choose from, a look-ahead mechanism will choose an optimal combination to satisfy constraints and minimize cost. The following package states influence the cost calculation for a given combination:

installed

selected (for installation)

not selected (for installation)

In cost calculations, virtual packages by themselves are considered to cost nothing since they do not directly install anything. It is the dependencies of a virtual package that contribute to it's cost.

Combinations that include packages from the "installed" or "selected" categories are less costly than those that include packages from the "not selected" category. When a package is chosen for installation, it transitions to the "selected" state. This state change propagates to the cost calculations of later decisions, influencing later decisions to be consistent with earlier decisions. This feedback mechanism serves to propagate constraints and can influence the modeling process to converge on a more optimal final state.