Dear OPM community,
We have been considering how to best refactor the (huge) class
FullyImplicitBlackoilSolver
in such a manner that it can be more easily be extended with new options and
functionality
without copying the whole class (as is currently done to implement the
flow_polymer variant
in opm-polymer).
The goal is simple enough: avoid copying of code between the black-oil and
black-oil-with-polymer
cases, which also will make the code a better base for further extensions. A
first step has been taken:
separating the NewtonSolver functionality into its own class, leaving the rest
of the functionality in the
BlackoilModel class.
Further refactoring of this is faced with some nontrivial technical challenges,
centering on the
State classes. For example, many functions take a State object argument, which
(in the current approach)
are different between models (black-oil and b.o. with polymer for example).
Then that function
cannot be virtual (it would need to take the same argument).
There seems to be two main alternatives:
1. Applying the "curiously recurring template pattern" (it is indeed called
that!). This means
essentially using templates to get what we want, including the ability to
extend models,
reusing most functions and only modifying the few that we want to be different.
This will
all be checked at compile time, so illegal combinations of objects (such as
accidentally
using a State class without a certain member together with a model that
requires that member)
will not even compile. The drawback is that it involves a little more
boilerplate code than regular
inheritance: assuming that we want to an extensible black-oil model and a
polymer model that
extends it we'll have three classes, like this:
template <class Implementation>
class BlackoilModelBase
{
// This is where almost all the functions and data go.
};
class BlackoilModel : public BlackoilModelBase<BlackoilModel>
{
// This contains only type definitions and a constructor.
};
class BlackoilPolymerModel: public BlackoilModelBase<BlackoilPolymerModel>
{
// Contains type definitions and a constructor.
// Contains new functions, as well as new versions of functions from
BlackoilModelBase.
};
I do think that the boilerplate is manageable, and that people will get by just
fine by starting their own
extension by copying an existing extension. (The resulting copied code will not
be that many lines).
This pattern is a fairly common C++ idiom and is used a lot in Dune for example.
2. Using regular inheritance and a generic State class. This means we have a
single, flexible State
class that can be used for all purposes. It should function a bit like a
map<string, vector<double>>,
but not be implemented like that. This solution is most similar to how things
would be done in Matlab,
or other weakly typed languages: any usage errors would not be caught until
runtime. Each access
to the State will be a little more annoying than before, instead of
state.saturation() you would need to
call state.get("saturation") and it could possibly fail.
In summary, none of these solutions are perfect, but both are workable. I have
discussed this with
Andreas, and we agreed to propose alternative 1 as the one we'd like to try out
first. I would like to
hear your opinions!
Atgeirr
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