[petsc-dev] TSSolve web page JED and EMIL READ THIS!

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

>    I note that with the current code if you run TSSolve() with
>    TS_EXACTFINALTIME_INTERPOLATE and then another TSSolve to continue
>    solving for later time it actually uses the "interpolated" value as
>    the starting point for the next solve (and not the final solution
>    of the previous TSSolve()). Is this not intended? If it is ok then
>    we simplify the code by simply interpolating the final solution
>    into ts->vec_sol (and hence not even save the final "solution") and
>    simplify the code and the API. If it is suppose to restart from the
>    previous final solution (not interpolated) then the code needs to
>    be fixed up.

I don't know how to make semantics that would ignore the interpolated
vector passed as u and continue from the stored state without being
disastrously confusing.  I think if the user wants to call TSSolve in a
loop, they should use TS_EXACTFINALTIME_STEPOVER and call
TSInterpolateSolution themselves.

>   An alternative is to say that TSSolve() always returns the solution
>   at a time step and require the user to call something like
>   TSInterpolateSolution(ts,w); to get the solution at the interpolated
>   time?

If we thought that TS_EXACTFINALTIME_INTERPOLATE was the only mode for
reaching an exact time, I would be in favor of this.  But one might want
to compare TS_EXACTFINALTIME_INTERPOLATE with
TS_EXACTFINALTIME_MATCHSTEP and it's wasteful to call
TSInterpolateSolution and ugly logic to check the TSExactFinalTimeOption
and do something different in that case.
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