dolfin team mailing list archive

[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

On 8 June 2011 14:18, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
> On 8 June 2011 14:08, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>> On 8 June 2011 13:55, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>>> On 8 June 2011 13:46, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
>>>> On 8 June 2011 13:31, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>>>>> On 8 June 2011 13:11, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
>>>>>> On 8 June 2011 12:11, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>>>>>>> Done and checked in. If someone updates FFC to support this, we can
>>>>>>> hopefully close this bug.
>>>>>>
>>>>>> I'm not sure this is enough to handle the bug. If you look at the
>>>>>> output of printing M in the example code I posted you'll see that the
>>>>>> list tensor contains component '7'. This is what you'll get from
>>>>>> calling self.component(), but the
>>>>>> TT.symmetry() only contains:
>>>>>> {(2,): (1,), (6,): (5,)}
>>>>>>
>>>>>> Is there some function we need to call first to map the component '7'
>>>>>> --> '6', before looking at symmetries to map '6' --> '5'?
>>>>>> Doing so will get us into trouble with mapping '3' --> '2' since
>>>>>> symmetry will map that to '1'.
>>>>>> The TT element has 2 x 3 sub elements due to symmetry.
>>>>>
>>>>> The 7 is an index into the value index space of the coefficient and is
>>>>> correct. It has nothing (directly) to do with subelement indexing. I
>>>>> think you're assuming a closer relation between them than there is?
>>>>> Let me try to clear it up...
>>>>>
>>>>> The value index space is contiguous from the point of view of UFL
>>>>> expressions, but has holes when symmetries are considered. The
>>>>> noncontiguous value index space will then need to be mapped to a
>>>>> contiguous subelement space by associating each value index that is
>>>>> not in the symmetry mapping with a subelement index.
>>>>>
>>>>> 1) We have a component/value index
>>>>> 2) We map that value index to another value index using a symmetry
>>>>> mapping (e.g. 6->5 and 7->7 in your example)
>>>>> 3) We map from the noncontiguous value index space to the contiguous
>>>>> subelement index space
>>>>>
>>>>> Clear as mud? :)
>>>>
>>>> Yes, but since we only deal with the (sub)elements that are actually
>>>> present in FFC, it's really inconvenient that we can't get a mapping
>>>> from the component to the subelement.
>>>> I somehow suspected the FiniteElement.extract_component() to do this,
>>>> but it turns out not to be the case.
>>>>
>>>>> UFL handles (2) for you only when you apply expand_indices.
>>>>>
>>>>> FFC will have to handle (3) when generating code, it doesn't touch
>>>>> anything UFL needs to know about. I'll see if I can whip up a quick
>>>>> utility function for it though.
>>>>
>>>> That would really be nice.
>>>
>>> Done :) The latest patch contains code and test showing usage.
>>>
>>> But maybe it should be integrated into extract_component, I'll take a
>>> look at that now that I'm into this.
>>
>> I applied the symmetry mapping inside extract_component for tensor elements,
>> that way you shouldn't have to do the symmetry mapping in addition to
>> calling extract_component. The numbering utility I checked in could still
>> come in handy though, so I'll let it stay.
>
> OK, will you add the numbering utility function in extract_component such that
> calling this function will return the contiguous component and element?

Hm... I'd rather keep extract_component as it is, returning
the value component tuple and not the element numbering.

Just do
v, s = build_component_numbering(e.value_shape(), e.symmetry())
sc, sube = e.extract_component(c)
sub_element_index = v[sc]

Martin