State model of gravitational dynamics codes. The diagram gives the state that a gravitational dynamics code can be in. Transitions can be effected explicitly (by calling the corresponding function, e.g. initialize_code from “start” to “init”), or implicitly (e.g. calling get_position to get the position of a particle is only allowed in the “run” state – the framework will call the necessary functions to get in that state, which will be a difference sequence of functions depending on the starting state).