Contributing to libNEGF

How to contribute

The preferred method is to fork the project on and then create a pull request. Your changes should be based on the default branch. Before you start, please read the style convention below.

libNEGF has been contributed by several authors, and a lot of the code is not homogeneous in style. Nevertheless, future development should follow this guideline in order to increase the readability and homogeneity of the project.

Style guide

Line length and indentation

Maximal line length is 100 characters. For lines longer than that, use continuation lines.

Nested blocks are indented by 2 white spaces::

write(*, *) "Nested block follows"
do ii = 1, 100
  write(*, *) "This is the nested block"
  if (ii == 50) then
  write(*, *) "Next nested block"
  end if
end do

Continuation lines are indented by 4 white spaces. Make sure to place continuation characters (&) both at the end of the line as well as at the beginning of the continuation line::
```
call someRoutineWithManyParameters(param1, param2, param3, param4,&
    & param5)
```
Try to break lines at natural places (e.g. at white space characters) and include one white space character after the opening ampersand in the continuation line.
Single line preprocessor directives are indented as normal code::
```
@:ASSERT(someCondition)
call someRoutine(...)
```

Preprocessor block directives (directives with starting and ending constructs) are outdented by 2 characters with respect of the code they enclose. The enclosed code must be aligned as if the preprocessor directives were not present::

call doSomething()
#:if WITH_SCALAPACK
call someRoutineScalapackVersion(...)
#:else
call someRoutineSerialVersion(...)
#:endif

do iKS = 1, nKS
  #:if WITH_SCALAPACK
  call someRoutineScalapackVersion(iKS, ...)
  #:else
  call someRoutineSerialVersion(iKS, ...)
  #:endif
end do

Naming

The naming conventions are similar to those in the Google Style Guide for C++ naming convention with some modifications.

Use lowercase for all Fortran constructs and prefer the end do, end if to enddo and endif::
```
do i = 1, 2
  j = i + 1
end do
```

Variable names follow the snake_case convention::

logical :: has_component

Variable names should be descriptive, excessive use of abbreviations is discouraged. However full lowercase names without lowercase can be used when the name is short and clear. For example::

integer :: numofprms ! Don't do this, multiple abbreviations without separator.
... :: kpoint        ! This is ok
... :: meshgrid      ! This is also ok
... :: save_to_mem   ! Ok, it is obvious that mem stands for memory

Constants (parameters) can be declared using the snake_case convention similar to variables (especially for parameters with limited scope), or can be UPPER_CASE (especially for parameters defined at module level and externally accessible) ::
```
integer, parameter :: max_array_size = 100
integer, parameter :: MAX_ARRAY_SIZE = 100
```
with the exception of the constants used to define the kind parameter for intrinsic types, which should be all lowercase (and short)::
```
integer, parameter :: dp = kind(1.0d0)
real(dp) :: val
```

Subroutine and function names follow also the snake_case notation::

subroutine test_some_functionality()
myValue = get_some_value(...)

Type (object) names are written UpperCamelCase::
```
type :: RealList
type(RealList) :: my_list
```
Type names can be prefixed with a capital 'T' when they need to be clearly distinguished from other components, but it is not demanded necessary::
```
type :: TNegf
:
end type TNegf
:
```
Module names follow snake_case convention::
```
use dftb_common_accuracy
```
Underscores are used for name-spacing only, so the module above would be typically found at the path dftb/common/accuracy.f90. The individual component names (dftb, common, accuracy) may not contain any underscores and must be shorter than 15 characters.
Preprocessor variables and macros follow UPPER_CASE_WITH_UNDERSCORE convention::

#:if WITH_MPI with_mpi = ${FORTRAN_LOGICAL(WITH_MPI)}$ #:endif

White spaces

Please use white spaces to make the code readable.

Avoid trailing whitespaces. They slow down code navigation and most modern editors trim trailing whitespaces by default. Please make sure that your editor does it too. If a file comes with trailing whitespaces, feel free to clean it in a separate commit (see above).

In general, you must use white spaces in following situations:

Around arithmetic operators::
```
2 + 2
```
Around assignment and pointer assignment operators::
```
aa = 3 + 2
window => array(1:3)
```
Around the :: separator in declarations::
```
integer :: ind
```

After commas (,) in general and especially in declarations, calls and lists::

real(wp), allocatable :: array(:)
type, extends(TBaseType) :: TDerivedType
subroutine my_routine(par1, par2)
call my_routine(val1, val2)
print *, 'My value:', val
do ii = 1, 3
array(1:3) = [1, 2, 3]

When separating array indices, when the actual index value for an index contains an expression::
```
my_array(ii + 2, jj) = 12
```

You may omit white space in following cases:

When separating array indices and the actual index values are simple and short (typically two letters) variable names, one or two digit integers or the range operator :::
```
my_array(:,1) = vector
lat_vecs(1,1) = 1.0_wp
my_array(ii,jj) = my_array(jj,ii)
```

You must omit white spaces in following cases:

Around opening and closing braces of any kind::

call my_subroutine(aa, bb)  ! and NOT call my_subroutine( aa, bb )
my_vector(:) = [1, 2, 3]    ! instead of my_vector(:) = [ 1, 2, 3 ]
tmp = 2 * (aa + bb)        ! instead of 2 * ( aa + bb )

Around the equal (=) sign, when passing named arguments to a function or subroutine::
```
call my_subroutine(aa, optional_argument=.true.)
```

Around the power operator::

val = base**power   (instead of val = base ** power)

Avoid white spaces for visual aligning of code, use::

  integer, intent(in) :: num_neighbors
  real(wp), intent(out) :: interaction

instead of::

  integer, intent(in)   :: num_eighbors
  real(wp), intent(out) :: energy

Although latter may look more readable, it makes rather difficult to track real changes in the code with the revision control system. For example when a new line is added to the block making the realignment of previous (but otherwise unchanged) lines necessary ::

  integer, intent(in)             :: num_neighbors
  real(wp), intent(out)           :: energy
  real(wp), intent(out), optional :: forces(:)

the version control system will indicate all of those lines having been modified, although only the alignment (but not the actual instructions) were changed.

Comments

Module, Subroutine and function comments should be consistent with doxygen <http://doxygen.org/>_ / FORD <https://github.com/cmacmackin/ford>_ literate comments for publicly visible interfaces and variables.
Comments are indented to the same position as the code they document::
```
! Take spin degeneracy into account
energy = 2.0_wp * energy
```

Generally, write the comment before the code snippet it documents::

! Loop over all neighbours
do i_neigh = 1, num_neighbours
  :
end do

Try to avoid mixing code and comments within one line as this is often hard to read::
```
bb = 2 * aa   ! this comment should be before the line.
```

Never use multi-line suffix comments, as an indenting editor would mess up the indentation of subsequent lines::

bb = 2 * aa  ! This comment goes over multiple lines, therefore, it
             ! should stay ALWAYS before the code snippet and NOT HERE.

Specifically comment any workarounds, include the compiler name and the version number for which the workaround had to be made. Always use the following pattern, so that searching for workarounds which can be possibly removed is easy::
```
! Workaround: gfortran 4.8
! Finalisation not working, we have to deallocate explicitly
deallocate(myPointer)
```
Comments should always start with one bang only. Comments with two bangs are reserved for source code documentation systems::
```
! This block needs a documentation
do ii = 1, 2
  :
end do
```

If you need a comment for a longer block of code, consider instead packaging that block of code into a properly named function (if the additional function call would be performance critical, write it as an internal procedure)::

some_previous_statement
ind = get_first_non_zero(array)
some_statement_after

instead of ::

some_previous_statement

! Look for the first nonzero element
found = .false.
do ind = 1, size(array)
  if (array(ind) > 0) then
found = .true.
exit
  end if
end do
if (.not. found) then
  ind = 0
end if

some_statement_after

Allocation status

At several places, the allocation status of a variable is used to signal choices about logical flow in the code::

  !> SCC module internal variables
  type(TScc), allocatable :: scc_calc
  .
  .
  .
  if (allocated(scc_calc)) then

  end if

This is to be preferred to the use of additional logical variables if possible.

Part of the reason for this choice is that from Fortran 2008 onwards, optional arguments to subroutines and functions are treated as not-present if not allocated.

Refactoring existing code

Everyone is welcome to refactor existing code and namings, as long as this is harmless from a user perspective (e.g. fixing indentation, whitespaces etc.). However, it is suggested to separate commits related to clean-up and refactoring from the commits containing the actual features.

Impactful refactoring should be done in specific pull requests, to avoid compatibility issues.

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CONTRIBUTING.md

CONTRIBUTING.md

Contributing to libNEGF

How to contribute

Style guide

Line length and indentation

Naming

White spaces

Comments

Allocation status

Refactoring existing code

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CONTRIBUTING.md

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CONTRIBUTING.md

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Contributing to libNEGF

How to contribute

Style guide

Line length and indentation

Naming

White spaces

Comments

Allocation status

Refactoring existing code