DataContainer: Fix Offset Read

[ax3l]

Proposed fix for #254 on which the offset >0 during reads could never be determined.

@Flamefire maybe you want to take a look? I would still like to add a test for that, but did not dig deeper.

[ax3l]

RT test verifies that https://github.com/ComputationalRadiationPhysics/picongpu/issues/1544 is fixed with this PR

a test that failed before in libSplash itself would still be awesome

[ax3l]

Also shouldn't the initial size by zero instead of (1,1,1)?

I wouldn't change that for now, I don't have the internal definitions of libSplash in mind right now and it does not cause problems ;)

[ax3l]

@Flamefire do you have any idea on a test? I tried to reproduce the splash2txt problem above in the libSplash tests yesterday but found no way to add it (honestly, I was suprised it was not covered yet by them already... maybe because the tests don't use several subdomains in may cases...?)

[Flamefire]

Add a single subdomain with a non-zero offset. Should be enough to trigger the problem. A test case I'd use would be to add a subdomain with non-zero offset, than 3 more that extend the extents to either side and both sides (offset and size)

[ax3l]

The examples that are already checked in always use a non-zero offset and still don't trigger it 😕

[Flamefire]

Thats impossible as min(0, x) = 0 which makes it a bigger size and wrong offset as it should be. Then what exactly is the behaviour you'd want to see triggered?

[ax3l]

Ideally I want to see a == NULL of a getIndex() of a DataContainer for a non-zero offset on read, the same issue we saw in splashtxt: https://github.com/ComputationalRadiationPhysics/picongpu/issues/1544

(before this patch, this should fail)

[Flamefire]

getIndex is a badly named function IMO. But you probably meant getElement(). Idea which is similar to splash2txt:

• Add Subdomain(size[10,1,1], offset[20,0,0])
• Get size of Domain-> returns 30
• getElement(0..9) -> return elements
• getElement(10..29) -> returns NULL

After fix:

• Add Subdomain(size[10,1,1], offset[20,0,0])
• Get size of Domain-> returns 10
• getElement(0..9) -> return elements

[ax3l]

Added a test that triggered the bug to libSplash and fixed it with the proposed fix to verify :)