[petsc-dev] BlockGetIndices and GetBlockIndices

[Smith, Barry F.]

> On Oct 16, 2019, at 2:50 AM, Pierre Jolivet via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> Hello,
> I’m trying to understand what is the rationale for naming a function ISBlockGetIndices and another ISLocalToGlobalMappingGetBlockIndices (BlockGet vs. GetBlock).

   ISBlockGetIndices returns the indices for each block; it is method that only applies too the  ISBlock subclass. Given its purpose its name is perfect.

   ISLocalToGlobalMapping is a class that has direct support for a block size (unlike IS which has no concept of a block size, it is an abstraction of any collection of integers).

   ISLocalToGlobalMappingGetBlockIndices is then a method on ISLocalToGlobalMapping that gives you the block indices (one index per block). Given its purpose its name is also perfect; the method is getblockindices()   Note there is also ISLocalToGlobalMappingGetIndices(). 

  Though IS and ISLocalToGlobalMapping both begin with IS they are really not closely related.

   

> Also, it looks to me like the implementation of ISBlockGetIndices is somehow less versatile than ISLocalToGlobalMappingGetBlockIndices.
> Indeed, I may call ISLocalToGlobalMappingGetBlockIndices with an underlying non-block IS, while I can’t for ISBlockGetIndices, so a check must be performed by the user, e.g., https://www.mcs.anl.gov/petsc/petsc-current/src/vec/is/is/examples/tutorials/ex3.c.html#line58 or this (IMHO) ugly code duplication https://www.mcs.anl.gov/petsc/petsc-dev/src/vec/is/utils/isltog.c.html#line322.
> 
> Thoughts and/or comments? Would it make sense to add an ISGetBlockIndices/ISRestoreBlockIndices or would that be too confusing for the user?

   For non ISBlock subclass of IS I don't know what ISGetBlockIndices is suppose to even mean. 

> 
> Thanks,
> Pierre