Yet Another Hierarchical State Machine
Stefan Heinzmann · 2004
Most of you are probably familiar with state machines. Finite state machines (FSMs) are widely used in both digital electronics and programming. Applications include communication protocols, industrial control or GUI programming. UML includes a more flexible variant in the form of statechart diagrams. Here, states can contain other states, making the corresponding state machine hierarchical. Hierarchical state machines (HSMs) can be converted to ordinary (flat) state machines, so most implementors concentrate on the implementation of FSMs, and many implementations exist. The conversion however tends to lose the original structure of the HSM, making it difficult to make the connection between a statechart and its implementation. Clearly, it would be nicer if the HSM could be implemented directly in source code. Direct implementations of HSMs seem to be comparatively rarely published. Miro Samek provides a prominent example in his book [1]. See the sidebar (next page) for some more information about his approach, which provided the motivation for the approach I will present here. UML tools are available that generate source code directly from statechart diagrams (example: Rhapsody [5]). More information about HSMs and Statecharts can be found in [1],[2],[3] and – of course – through