# Developer Notes¶

## Shape¶

Backends often manipulate tensors of dimension greater than 2. It is awkward to pass fully-specified multi-dimensional arrays using C99 and certain operations will flatten/reshape the tensors for computational convenience. We frequently use comments to document shapes using a lexicographic ordering. For example, the comment

// u has shape [dim, ncomp, Q, nelem]


means that it can be traversed as

for (d=0; d<dim; d++)
for (c=0; c<ncomp; c++)
for (q=0; q<Q; q++)
for (e=0; e<nelem; e++)
u[((d*ncomp + c)*Q + q)*nelem + e] = ...


This ordering is sometimes referred to as row-major or C-style. Note that flattening such as

// u has shape [dim, ncomp, Q*nelem]


and

// u has shape [dim*ncomp, Q, nelem]


are purely implicit – one just indexes the same array using the appropriate convention.

## Internal Layouts¶

Ceed backends are free to use any E-vector and Q-vector data layout, to include never fully forming these vectors, so long as the backend passes the t5** series tests and all examples. There are several common layouts for L-vectors, E-vectors, and Q-vectors, detailed below:

• L-vector layouts

• L-vectors described by a have a layout described by the offsets array and compstride parameter. Data for node i, component j, element k can be found in the L-vector at index offsets[i + k*elemsize] + j*compstride.

• L-vectors described by a strided have a layout described by the strides array. Data for node i, component j, element k can be found in the L-vector at index i*strides[0] + j*strides[1] + k*strides[2].

• E-vector layouts

• If possible, backends should use CeedElemRestrictionSetELayout() to use the t2** tests. If the backend uses a strided E-vector layout, then the data for node i, component j, element k in the E-vector is given by i*layout[0] + j*layout[1] + k*layout[2].

• Backends may choose to use a non-strided E-vector layout; however, the t2** tests will not function correctly in this case and the tests will need to be whitelisted for the backend to pass the test suite.

• Q-vector layouts

• When the size of a field is greater than 1, data for quadrature point i component j can be found in the Q-vector at index i + Q*j. Backends are free to provide the quadrature points in any order.

• When the field has emode CEED_EVAL_GRAD, data for quadrature point i, component j, derivative k can be found in the Q-vector at index i + Q*j + Q*size*k.

• Note that backend developers must take special care to ensure that the data in the Q-vectors for a field with emode CEED_EVAL_NONE is properly ordered when the backend uses different layouts for E-vectors and Q-vectors.

## Backend Inheritance¶

There are three mechanisms by which a Ceed backend can inherit implementation from another Ceed backend. These options are set in the backend initialization routine.

1. Delegation - Developers may use CeedSetDelegate() to set a backend that will provide the implementation of any unimplemented Ceed objects.

2. Object delegation - Developers may use CeedSetObjectDelegate() to set a backend that will provide the implementation of a specific unimplemented Ceed object. Object delegation has higher precedence than delegation.

3. Operator fallback - Developers may use CeedSetOperatorFallbackResource() to set a resource that will provide the implementation of unimplemented methods. A fallback with this resource will only be instantiated if a method is called that is not implemented by the parent . In order to use the fallback mechanism, the parent and fallback resource must use compatible E-vector and Q-vector layouts.

## Clang-tidy¶

make tidy

which uses the clang-tidy utility included in recent releases of Clang. This tool is much slower than actual compilation (make -j8 parallelism helps). To run on a single file, use

make interface/ceed.c.tidy

for example. All issues reported by make tidy should be fixed.

Header inclusion for source files should follow the principal of ‘include what you use’ rather than relying upon transitive #includes to define all symbols.
Every symbol that is used in the source file foo.c should be defined in foo.c, foo.h, or in a header file #included in one of these two locations. Please check your code by running the tool include-what-you-use to see recommendations for changes to your source. Most issues reported by include-what-you-use should be fixed; however this rule is flexible to account for differences in header file organization in external libraries.
Header files should be listed in alphabetical order, with installed headers preceding local headers and ceed headers being listed first.