# The C++ kernel¶

In this section of the documentation you will find information about the computational kernel of DREAM. The kernel consists of two C++ libraries, named libfvm and libdream, which implement all physics and key algorithms of the code. While these libraries constitute most of what we consider to be “the code”, the average user cannot directly interact with these libraries but must rather do so through either the dreami interface, and/or the Python interface. This documentation is therefore intended for people who will work directly with code in the computational kernel of DREAM, and not for people who merely intend to use the code for simulations.

Separation into two libraries: In order to keep the code modular and allow some code to be reused in future, unrelated applications, the kernel code is separated into two libraries, called libdream and libfvm. The distinction made between what goes into which library is somewhat arbitrary, but the general idea is that code which is very specific to the operation of the DREAM code specifically should go into libdream, while libfvm should contain code which could be used by any PDE solver. This distinction can also be phrased as “libdream implements the physics, while libfvm implements the mathematics”. Perhaps more importantly, libfvm should be completely independent of any other DREAM module, whereas libdream necessarily depends heavily on libfvm.

## DREAM — the physics library¶

The DREAM library implements the actual physics simulated by DREAM. It also contains the C++ API necessary to run simulations. If you would like to build your own interface to DREAM, then you should interface with libdream.

In addition to implementing calculations of collision frequencies, ionization rates and the Fokker-Planck equation, libdream contains the linear and non-linear solvers, the time stepping algorithms, as well as the initialization logic.

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## FVM — the mathematics library¶

The FVM library implements various helper classes which are used by the DREAM library. Perhaps most importantly, the FVM library implements the Grid class and its children, as well as discretizations for the finite volume method (which is the source of the library’s name).

While libfvm is an essential dependency for the DREAM library, libfvm itself is intended to note depend on any other DREAM modules. This allows its code to be reused in future, non-DREAM related applications.

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