[Dsol-development] Confused about DESS implementation

[R.T.H.Chin <r.t...@tb...>]

Hi DSOL team,

Not that I need it right now, actually I don't even use it and should 
focus on other stuff right now, but studying the DSOL code I'm kind of 
confused about the DESS implementation.

Suppose I use the DSOL application, so then I am actually using the 
Animator as a delayed DEVSDESS simulator. Looking at the code of the 
Animator I see two nested loops in the run() method:

1-the outer loop which just keeps going for as long as the simulation is 
not finished
2-the inner loop which runs until simulatorTime + timeStep

The loop 2 handles all DEVS-events (event.execute() ). Every time such 
an event is executed a TIME_CHANGED_EVENT is fired. After loop 2 another 
TIME_CHANGED_EVENT is fired.

Suppose my model uses both discrete events and differential equations. 
Alse suppose that for a specific timestep four events were executed, 
then 4+1=5 TIME_CHANGED_EVENTS are fired in loop 2.

The class 
nl.tudelft.simulation.dsol.formalisms.dess.DifferentialEquation is a 
listener and listens to TIME_CHANGED_EVENTs in the notify() method. A 
differential equation is solved every time a TIME_CHANGED_EVENT is 
fired. So in my example that means that the differential equation is 
solved 5 times:

public synchronized void notify(final EventInterface event) {
    ...
    this.previousY = integrateY(this.simulator.getSimulatorTime(),
                    this.previousX, this.previousY);
    ...
    this.previousX = this.simulator.getSimulatorTime();
}

This is kind of confusing because:
1 Shouldn't a differential equation only be solved at specified timeSteps?
2 The constructor of DifferentialEquation allows setting a timeStep, but 
what is the use of that if the equations are solved at TIME_CHANGED_EVENTS?
3 What if you set a timestep of DifferentialEquation in the constructor 
which differs from the timeStep in Animator: are the results still correct?
4 What is you have two differential equations which should be solved at 
two different timesteps (e.g. a high frequency motion and a low 
frequency motion)?
5 What if you use variable timestep algorithms to optimize the timestep 
/ minimize the error?

Maybe I didn't understand the code, and I didn't make a test, but it 
seems kind of odd.
Greetings,
Roy