dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Oct 23, 2008 at 10:22:54AM +0200, Martin Sandve Alnæs wrote:
> 2008/10/23 Garth N. Wells <gnw20@xxxxxxxxx>:
> >
> >
> > Martin Sandve Alnæs wrote:
> >>
> >> 2008/10/22 Anders Logg <logg@xxxxxxxxx>:
> >>>
> >>> On Wed, Oct 22, 2008 at 10:00:07PM +0200, Martin Sandve Alnæs wrote:
> >>>>
> >>>> 2008/10/22 Anders Logg <logg@xxxxxxxxx>:
> >>>>>
> >>>>> On Wed, Oct 22, 2008 at 07:45:03PM +0100, Garth N. Wells wrote:
> >>>>>>
> >>>>>> Anders Logg wrote:
> >>>>>>>
> >>>>>>> On Wed, Oct 22, 2008 at 08:24:18PM +0200, Anders Logg wrote:
> >>>>>>>>
> >>>>>>>> On Wed, Oct 22, 2008 at 07:05:09PM +0100, Garth N. Wells wrote:
> >>>>>>>>>
> >>>>>>>>> Anders Logg wrote:
> >>>>>>>>>>
> >>>>>>>>>> On Wed, Oct 22, 2008 at 12:08:43PM +0100, Garth N. Wells wrote:
> >>>>>>>>>>>
> >>>>>>>>>>> The thread on the new Function design has digressed from the
> >>>>>>>>>>> immediate
> >>>>>>>>>>> issue, so I'm restarting it.
> >>>>>>>>>>>
> >>>>>>>>>>> The issue is how to deal with user defined functions in C++. What
> >>>>>>>>>>> if we
> >>>>>>>>>>> have a design such that:
> >>>>>>>>>>>
> >>>>>>>>>>> - All Functions must have a FunctionSpace
> >>>>>>>>>>
> >>>>>>>>>> Yes.
> >>>>>>>>>>
> >>>>>>>>>>> - A FunctionSpace does not have to be complete (a complete
> >>>>>>>>>>> FunctionSpace
> >>>>>>>>>>> having a Mesh, a FiniteElement and a DofMap). As a minimum
> >>>>>>>>>>> requirement,
> >>>>>>>>>>> and FunctionSpace must have a Mesh.
> >>>>>>>>>>
> >>>>>>>>>> I think it should always need to be complete. We can avoid a lot
> >>>>>>>>>> of
> >>>>>>>>>> trouble if we make firm requirements: a Function is always
> >>>>>>>>>> associated
> >>>>>>>>>> with a FunctionSpace and the FunctionSpace is always completely
> >>>>>>>>>> defined.
> >>>>>>>>>>
> >>>>>>>>>>> - A FiniteElement and/or DofMap can be attached to a
> >>>>>>>>>>> FunctionSpace after
> >>>>>>>>>>> its creation
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> Related to the functions Function::interpolate(double*
> >>>>>>>>>>> coefficients, ..)
> >>>>>>>>>>> for interpolating Functions on cells
> >>>>>>>>>>>
> >>>>>>>>>>> - The new Function::interpolate functions do not take a
> >>>>>>>>>>> FiniteElement as
> >>>>>>>>>>> an argument, so it is not possible to interpolate a function in a
> >>>>>>>>>>> different space. Is it desirable to allow Functions from one
> >>>>>>>>>>> space to be
> >>>>>>>>>>> interpolated in another?
> >>>>>>>>>>
> >>>>>>>>>> Yes, it's desirable. This can be done globally when needed:
> >>>>>>>>>>
> >>>>>>>>>>  Function v(V);
> >>>>>>>>>>  Vector coefficients;
> >>>>>>>>>>  v.interpolate(coefficients, W);
> >>>>>>>>>>
> >>>>>>>>>> This will compute the global coefficient vector for v on W.
> >>>>>>>>>>
> >>>>>>>>>> This is currently implemented with the assumption that the meshes
> >>>>>>>>>> for
> >>>>>>>>>> V and W are the same but could quite easily be extended to
> >>>>>>>>>> non-matching meshes now that functions can be evaluated at
> >>>>>>>>>> arbitrary
> >>>>>>>>>> points (using GTS).
> >>>>>>>>>>
> >>>>>>>>>>> If we do this, would:
> >>>>>>>>>>>
> >>>>>>>>>>> - The above allow FiniteElement types to be checked at runtime
> >>>>>>>>>>> for
> >>>>>>>>>>> consistency (the FiniteElement passed to Function::interpolate
> >>>>>>>>>>> should be
> >>>>>>>>>>> the same as the Functions own FiniteElement for discrete
> >>>>>>>>>>> Functions. This
> >>>>>>>>>>> is what we did with the old design.)>
> >>>>>>>>>>>
> >>>>>>>>>>> - The above deal with the issue of user-defined functions which
> >>>>>>>>>>> have a a
> >>>>>>>>>>> FunctionSpace but no FiniteElement?
> >>>>>>>>>>>
> >>>>>>>>>>> - The above deal with special functions, like the mesh size h?
> >>>>>>>>>>
> >>>>>>>>>> Here's my suggestion for how to handle initialization of Functions
> >>>>>>>>>> in C++.
> >>>>>>>>>> There is no need for a circular dependency. First some simple
> >>>>>>>>>> facts:
> >>>>>>>>>>
> >>>>>>>>>>  1. The constructor of a Form may require one or more Functions
> >>>>>>>>>>  2. The constructor of a Function requires a FunctionSpace
> >>>>>>>>>>
> >>>>>>>>>> From this it follows that we must do something like
> >>>>>>>>>>
> >>>>>>>>>>  Function f(V);
> >>>>>>>>>>  Poisson::BilinearForm a;
> >>>>>>>>>>  Poisson::LinearForm L(f);
> >>>>>>>>>>
> >>>>>>>>>> The question is now how to initialize the FunctionSpace V. My
> >>>>>>>>>> suggestion would be to extend the code generation to generate code
> >>>>>>>>>> for
> >>>>>>>>>> creating the FunctionSpace(s), just like we do already when we
> >>>>>>>>>> generate UFC code + some extra code for defining the Form classes
> >>>>>>>>>> (when using -l dolfin in FFC). This does not make FFC
> >>>>>>>>>> DOLFIN-specific
> >>>>>>>>>> or DOLFIN FFC-specific. One can still use other form compilers,
> >>>>>>>>>> but the
> >>>>>>>>>> interface will not be as nice.
> >>>>>>>>>>
> >>>>>>>>>> So, one would do something like this:
> >>>>>>>>>>
> >>>>>>>>>>  #include "Poisson.h"
> >>>>>>>>>>
> >>>>>>>>>>  int main()
> >>>>>>>>>>  {
> >>>>>>>>>>    Mesh mesh("mesh.xml");
> >>>>>>>>>>    Poisson::TestSpace V(mesh);
> >>>>>>>>>
> >>>>>>>>> Why 'TestSpace'?
> >>>>>>>>>
> >>>>>>>>> Also, when many functions are present, how would we
> >>>>>>>>> identify each one? Could FFC create a class
> >>>>>>>>> Poisson::FunctionSpace::foo
> >>>>>>>>> when 'foo' is the name of the function in the FFC input?
> >>>>>>>>
> >>>>>>>> We could let FFC create a number of function space classes:
> >>>>>>>>
> >>>>>>>>  Poisson::TestSpace
> >>>>>>>>  Poisson::TrialSpace
> >>>>>>>>  Poisson::FunctionSpace_0
> >>>>>>>>  Poisson::FunctionSpace_1
> >>>>>>>>  ...
> >>>>>>>>
> >>>>>>>> or even
> >>>>>>>>
> >>>>>>>>  Poisson::v::FunctionSpace
> >>>>>>>>  Poisson::u::FunctionSpace
> >>>>>>>>  Poisson::f::FunctionSpace
> >>>>>>>>  Poisson::g::FunctionSpace
> >>>>>>>>
> >>>>>>>> (but I'm not sure it looks very nice).
> >>>>>>>>
> >>>>>>>> A complication I didn't think of before is how to handle the case
> >>>>>>>> when
> >>>>>>>> all spaces are the same (to get reuse of dofmaps). One option would
> >>>>>>>> be
> >>>>>>>> to either generate just one class if all spaces are the same, and
> >>>>>>>> otherwise generate separate clases like above. So either one does
> >>>>>>>>
> >>>>>>>>  Poisson::FunctionSpace V(mesh);
> >>>>>>>>  Function f(V);
> >>>>>>>>  Function g(V);
> >>>>>>>>  Poisson::LinearForm(f, g);
> >>>>>>>>
> >>>>>>>> or
> >>>>>>>>
> >>>>>>>>  Poisson::FunctionSpace_0 V(mesh);
> >>>>>>>>  Poisson::FunctionSpace_1 W(mesh);
> >>>>>>>>  Function f(V);
> >>>>>>>>  Function g(W);
> >>>>>>>>  Poisson::LinearForm(f, g);
> >>>>>>>>
> >>>>>>>>>>    Function f(V);
> >>>>>>>>>>    Poisson::BilinearForm a;
> >>>>>>>>>>    Poisson::LinearForm L(f);
> >>>>>>>>>>    ...
> >>>>>>>>>>  }
> >>>>>>>>>>
> >>>>>>>>>> First V, then f, then L.
> >>>>>>>>>>
> >>>>>>>>> Would this work for all the Functions in SpecialFunction.h?
> >>>>>>>>
> >>>>>>>> I haven't thought about it, but yes I guess it would.
> >>>>>>>>
> >>>>>>>> We can provide a set of predefined FunctionSpaces (like DG on
> >>>>>>>> triangles and tetrahedra).
> >>>>>>>
> >>>>>>> I'm moving the discussion of the removal of DofMapSet over here since
> >>>>>>> it's related.
> >>>>>>>
> >>>>>>> If a Form should own a set of FunctionSpaces (replacing the current
> >>>>>>> DofMapSet), that would prevent reuse of FunctionSpaces (and DofMaps)
> >>>>>>> across multiple Forms.
> >>>>>>>
> >>>>>>> Another option would be to always require that a Form is initialized
> >>>>>>> with one or more FunctionSpaces. This makes sense and is in agreement
> >>>>>>> with the requirement of a FunctionSpace when initializing a Function.
> >>>>>>>
> >>>>>>> It would then be
> >>>>>>>
> >>>>>>>  PoissonFunctionSpace V(mesh);
> >>>>>>>  Function f(V);
> >>>>>>>  Function g(V);
> >>>>>>>  PoissonBilinearForm a(V, V);
> >>>>>>>  PoissonLinearForm L(V, f, g);
> >>>>>>>
> >>>>>>> Then it's possible to reuse V across multiple forms (even forms not
> >>>>>>> included in Poisson.h).
> >>>>>>>
> >>>>>> Could work. To keep things clear cut, I would use
> >>>>>>
> >>>>>>    PoissonFunctionSpace V(mesh);
> >>>>>>    Array<FunctionSpace*> VxV(V, V);
> >>>>>>    PoissonBilinearForm a(VxV);  // Might also nee a shared_ptr version
> >>>>>>    PoissonLinearForm L(V, f, g);
> >>>>>>
> >>>>>> The first argument to a Form defines the test and/or trial spaces, the
> >>>>>> other arguments are the Functions in the Form.
> >>>>>>
> >>>>>> Garth
> >>>>>
> >>>>> That looks like a complication. The constructor of a Form takes a
> >>>>> variable number of arguments anyway, so it's no big deal to require
> >>>>> one FunctionSpace for a linear form and two for a bilinear form.
> >>>>> It also looks nicer (avoiding Array and pointers).
> >>>>>
> >>>> The good thing about it is that it allows building the argument list
> >>>> dynamically depending on input parameters.
> >>>
> >>> When is that necessary?
> >>>
> >>> If it's needed it's simple enough to generate code for both: one
> >>> constructor that takes (V, V) and one that takes VxV.
> >>
> >> An example would be changing material models in an otherwise
> >> fixed application. Of course, one could probably handle that with
> >> an application-specific "form factory" function or something.
> >> But since it's trivial to do, having both would be nice.
> >>
> >
> > It seems that we've converged on some details of the new Function design. I
> > don't think we need to address the FFC generation of helper code for
> > creating FunctionSpaces first up because we can patch a FunctionSpace
> > together with what we have. Same goes for extracting finite elements and dof
> > maps using nice names. What would be nice straight away is an extension of
> > the generated dolfin::Form code that takes FunctionSpaces as arguments. Just
> > need to figure out to work with  vectors or arrays of shared_ptr in Form.h
> > for storing the FunctionSpaces.
> >
> > Garth
> 
> Since a FunctionSpace is likely to be shared between forms or other
> parts of the application, it seems a good candidate for using shared_ptr.
> However, then the syntax "Form a(V, V)" won't work.

Why not? I thought this would be exactly the thing that shared
pointers were good at.

-- 
Anders

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