[firedrake] Enforcing slip boundary condition strongly

[David Ham]

I agree. I think that's what I was proposing. Are you on this or shall I do
something?
On Fri, 10 Jul 2015 at 12:14 Mitchell, Lawrence <
lawrence.mitchell at imperial.ac.uk> wrote:

>
> > On 10 Jul 2015, at 11:37, David Ham <David.Ham at imperial.ac.uk> wrote:
> >
> >
> > That's quite yuck, however I can see the attraction as we're kind of
> sort of not mucking with the PyOP2 interface.
> >
> > However the generated code is specific to BCs applied to a particular
> component, which doesn't satisfy Justin's (very standard) use case.
> >
> > I think we need to encode *which* component we're changing into the map
> value. EG.
> >
> > val = -val -(2**(30-i))
> >
> > where i is the component. This can be decoded to zero the correct entry
> in a general purpose way. The downside is that we limit ourselves to about
> half a billion nodes per core. I think that's OK.
>
> OK, so it may be that a node may have more than one component zeroed (e.g.
> at a corner).
>
> So how about this:
>
>
> Reserve top 2 (3?) bits to encode the component:
>
> x = sum(2**idx for idx in indices)
>
> val = val + (x << 29)
>
> Convert to negative
>
> val = -(val + 1)
>
> In the generated code we can then unwind that and pull out the appropriate
> components.
>
> This could work.  They're all pretty ugly solutions, tbf.
>
> Lawrence
>
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