WarpX Structure

Repo Organization

All the WarpX source code is located in Source/. All sub-directories have a pretty straightforward name. The PIC loop is part of the WarpX class, in function WarpX::Evolve implemented in Source/WarpXEvolve.cpp. The core of the PIC loop (i.e., without diagnostics etc.) is in WarpX::OneStep_nosub (when subcycling is OFF) or WarpX::OneStep_sub1 (when subcycling is ON, with method 1). Here is a visual representation of the repository structure.

Code organization

The main WarpX class is WarpX, implemented in Source/WarpX.cpp.

Build System

WarpX uses the CMake build system generator. 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.

For experienced developers, we also support AMReX’ GNUmake build script collection. The file Make.package in each sub-folder has the same purpose as the CMakeLists.txt file, please add new .cpp files to both dirs.

C++ Includes

All WarpX header files need to be specified relative to the Source/ directory.

e.g. #include "Utils/WarpXConst.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 implementation 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 WarpX forward declaration headers have the suffix *_fwd.H, while in AMReX they have the suffix *Fwd.H. The include order (see PR #874 and PR #1947) and proper quotation marks are:

In a MyName.cpp file:

#include "MyName.H" (its header) then
(further) WarpX header files #include "..." then
WarpX forward declaration header files #include "..._fwd.H"
AMReX header files #include <...> then
AMReX forward declaration header files #include <...Fwd.H> then
PICSAR header files #include <...> then
other third party includes #include <...> then
standard library includes, e.g. #include <vector>

In a MyName.H file:

#include "MyName_fwd.H" (the corresponding forward declaration header, if it exists) then
WarpX header files #include "..." then
WarpX forward declaration header files #include "..._fwd.H"
AMReX header files #include <...> then
AMReX forward declaration header files #include <...Fwd.H> then
PICSAR header files #include <...> then
other third party includes #include <...> then
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, PR #1947, the LLVM guidelines, and include-what-you-use.

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:

#ifndef MY_CLASS_FWD_H
#define MY_CLASS_FWD_H

class MyClass;

#endif // MY_CLASS_FWD_H

MyClass.H:

#ifndef MY_CLASS_H
#define MY_CLASS_H

#include "MyClass_fwd.H"
#include "someHeader.H"

class MyClass {
    void stuff ();
};

#endif // MY_CLASS_H

MyClass.cpp:

#include "MyClass.H"

class MyClass {
    void stuff () { /* stuff */ }
};

Usage: in SomeType.H

#ifndef SOMETYPE_H
#define SOMETYPE_H

#include "MyClass_fwd.H" // all info we need here
#include <memory>

struct SomeType {
    std::unique_ptr<MyClass> p_my_class;
};

#endif // SOMETYPE_H

Usage: in somewhere.cpp

#include "SomeType.H"
#include "MyClass.H"  // because we call "stuff()" we really need
                      // to know the full declaration of MyClass

void somewhere ()
{
    SomeType s;
    s.p_my_class = std::make_unique<MyClass>();
    s.p_my_class->stuff();
}

All files that only need to know the type SomeType from SomeType.H but do not access the implementation details of MyClass will benefit from improved compilation times.