.. _developers-repo-structure:

pyAMReX Structure
=================

Repo Organization
-----------------

The pyAMReX source code is located in ``src/`` and contains pybind11 C++ code, exposing C++ objects and functions to Python.
All sub-directories have a pretty straightforward name, mirroring the `AMReX Src/ structure <https://github.com/AMReX-Codes/amrex/tree/development/Src>`__.

Additionally, the ``src/amrex/`` directory contains Python import modules and extensions written in pyre Python.


Code organization
-----------------

The main pyAMReX import module(s) are ``amrex.space1d``, ``amrex.space2d`` and ``amrex.space3d``, implemented in ``src/pyAMReX.cpp``.
This import calls helper functions to initialize all other Python objects on import.

Build System
------------

pyAMReX uses the :ref:`CMake build system generator <building-cmake>`.
Each sub-folder contains a file ``CMakeLists.txt`` with the names of the source files (``.cpp``) that are added to the build.
Do not list header files (``.H``) here.

C++ Includes
------------

All pyAMReX header files need to be specified relative to the ``src/`` directory.

- e.g. ``#include "pyAMReX.H"``
- files in the same directory as the including header-file can be included with ``#include "FileName.H"``

By default, in a ``MyName.cpp`` source file we do not include headers already included in ``MyName.H``. Besides this exception, if a function or a class
is used in a source file, the header file containing its declaration must be included, unless the inclusion of a facade header is more appropriate. This is
sometimes the case for AMReX headers. For instance ``AMReX_GpuLaunch.H`` is a façade header for ``AMReX_GpuLaunchFunctsC.H`` and ``AMReX_GpuLaunchFunctsG.H``, which
contain respectively the CPU and the GPU implemetation of some methods, and which should not be included directly.
Whenever possible, forward declarations headers are included instead of the actual headers, in order to save compilation time (see dedicated section below). In pyAMReX forward
declaration headers have the suffix ``*_fwd.H``, while in AMReX they have the suffix ``*Fwd.H``.
The include order (see `PR #874 <https://github.com/ECP-WarpX/WarpX/pull/874#issuecomment-607038803>`__ and `PR #1947 <https://github.com/ECP-WarpX/WarpX/pull/1947>`__) and `proper quotation marks <https://gcc.gnu.org/onlinedocs/cpp/Include-Syntax.html>`__ are:

In a ``MyName.cpp`` file:

1. ``#include "MyName.H"`` (its header) then
2. (further) pyAMReX header files ``#include "..."`` then
3. pyAMReX forward declaration header files ``#include "..._fwd.H"``
4. AMReX header files ``#include <...>`` then
5. AMReX forward declaration header files ``#include <...Fwd.H>`` then
6. other third party includes ``#include <...>`` then
7. standard library includes, e.g. ``#include <vector>``

In a ``MyName.H`` file:

1. ``#include "MyName_fwd.H"`` (the corresponding forward declaration header, if it exists) then
2. pyAMReX header files ``#include "..."`` then
3. pyAMReX forward declaration header files ``#include "..._fwd.H"``
4. AMReX header files ``#include <...>`` then
5. AMReX forward declaration header files ``#include <...Fwd.H>`` then
6. other third party includes ``#include <...>`` then
7. standard library includes, e.g. ``#include <vector>``

Each of these groups of header files should ideally be sorted alphabetically, and a blank line should be placed between the groups.

For details why this is needed, please see `PR #874 <https://github.com/ECP-WarpX/WarpX/pull/874#issuecomment-607038803>`_, `PR #1947 <https://github.com/ECP-WarpX/WarpX/pull/1947>`_, the `LLVM guidelines <https://llvm.org/docs/CodingStandards.html#include-style>`_, and `include-what-you-use <https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/WhyIWYU.md>`_.

Forward Declaration Headers
---------------------------
Forward declarations can be used when a header file needs only to know that a given class exists, without any further detail (e.g., when only a pointer to an instance of
that class is used). Forward declaration headers are a convenient way to organize forward declarations. If a forward declaration is needed for a given class ``MyClass``, declared in ``MyClass.H``,
the forward declaration should appear in a header file named ``MyClass_fwd.H``, placed in the same folder containing ``MyClass.H``. As for regular header files, forward declaration headers must have
include guards. Below we provide a simple example:

``MyClass_fwd.H``:

.. code-block:: cpp

   #ifndef MY_CLASS_FWD_H
   #define MY_CLASS_FWD_H

   class MyClass;

   #endif //MY_CLASS_FWD_H

``MyClass.H``:

.. code-block:: cpp

   #ifndef MY_CLASS_H
   #define MY_CLASS_H

   #include "MyClass_fwd.H"
   #include "someHeader.H"
   class MyClass{/* stuff */};

   #endif //MY_CLASS_H

``MyClass.cpp``:

.. code-block:: cpp

   #include "MyClass.H"
   class MyClass{/* stuff */};

Usage: in ``SimpleUsage.H``

.. code-block:: cpp

   #include "MyClass_fwd.H"
   #include <memory>

   /* stuff */
   std::unique_ptr<MyClass> p_my_class;
   /* stuff */