Handle time events efficiently

[Martin Sjölund]

The relations

time >= <discrete_expr>
time < <discrete_expr>

can be handled as time events without resorting to crossing functions, thus leading to more efficient simulation code (see Modelica Specification 3.2 rev1, sect. 8.5).

We can look for expressions containing only pre(discrete), constant, or parameter expressions. Then it is always possible to calculate the next exact time of this event indicator, by replacing >= (or <) with == and solving for time.

If time is <= current time, no more events occur, otherwise schedule next time event and deal with it as with sample()

[Martin Sjölund]

Would be good to fix this as soon as possible so we can the new tables implementation. I do believe it is working, but it needs better event detection to work properly.

[Martin Sjölund]

See also #2224

[Francesco Casella]

Changed description to reflect contents more accurately
Merged text with closed duplicate #2224

[Martin Sjölund]

Moved to new milestone 1.9.4

[Martin Sjölund]

Milestone pushed to 1.9.5

[Martin Sjölund]

Milestone renamed

[Martin Sjölund]

Ticket retargeted after milestone closed

[Martin Sjölund]

Milestone moved to 1.12.0 due to 1.11.0 already being released.

[Francesco Casella]

@lochel, do you think you can take care of this for 1.13.0? It is particularly important to handle the simulation of systems with clocked variables efficiently

[Francesco Casella]

A good test case is reported in #2226

model Test1
   discrete Real next_t;
   discrete Real next_t2;
equation
   when time >= pre(next_t) then
     next_t = pre(next_t)+1;
   end when;
   when sample(0.5, 1.0) then
     next_t2 = pre(next_t2)+1;
   end when;
initial equation
   next_t = 1;
   annotation (experiment(StopTime=10));
end Test1;

Time events are generated by the sample() operator, but in fact time events could also be generated for the when time >= pre(next_t) conditions.

This would be very useful to handle delayed transitions in state machine models efficiently.

[Francesco Casella]

Rescheduled to 1.14.0 after 1.13.0 releasee

[Francesco Casella]

I think this definitely needs to be addressed, but not in 1.14.0

[Francesco Casella]

@Karim, @AnHeuermann, decide among yourselves who wants to take care of this