Rank functions for stable diagrams

Let $D$ be the diagram of a sufficiently homogeneous model. For types that are realized in this model, we introduce certain rank functions and prove the following assertions: (1) If, for each type, the rank is less than $\infty$ then the diagram is stable; (2) if the diagram $D$ is stable then the set of non-algebraic types of rank less than $\infty$ is large enough.