# C++ User Interface

## Meep Design

An overview of Meep's inner workings is summarized in Computer Physics Communications, vol. 181, issue 3, pp. 687-702 (2010). This page is a supplement which provides a description of the source code.

### Data Structures and Chunks

Meep employs several data structures declared in meep.hpp. The principal data structure element is the chunk. A chunk is a contiguous rectangular portion of the computational grid. For example, when Meep runs on a parallel system, each process gets one or more disjoint chunks of the grid. There are several different types of chunks:

• fields and fields_chunks
• structure and structure_chunks
• dft and dft_chunks

As an example, the fields class encapsulates the simulated fields over the entire grid, and one of its members is an array of fields_chunk variables that the grid is divided into. The fields_chunk variables are where the field is actually stored. Every parallel process has a nearly-identical fields variable with a nearly-identical list of chunks. Chunks on one process which have been assigned to another process do not store their fields arrays, and are just placeholders.

If a given material or field is not present in a given chunk, it need not be stored. For this reason, the PML boundary regions are separated into their own chunks, even on one processor, in order that the extra data for PML need not be stored for the whole grid.

In the future, we may implement support for different chunks with different resolution, to allow nonuniform spatial resolution.

Similarly for structure and structure_chunks, except that it is only for materials parameters such as epsilon, etc. and not for simulated fields.

dft_chunk stores accumulated Fourier-transformed fields corresponding to a given chunk.

### grid_volume and volume

The volume class declared in meep/vec.hpp represents a rectangular box-like region, parallel to the $XYZ$ axes, in "continuous space" — i.e. the corners can be at any points, not necessarily grid points. This is used, for example, whenever you want to specify the integral of some quantity (e.g., flux, energy) in a box-like region, and Meep interpolates from the grid as necessary to give an illusion of continuity.

The grid_volume class declared in meep/vec.hpp is a box of pixels. It stores the resolution, the number of pixels in each direction, the origin, etcetera. Given a grid_volume, there are functions to get the volume corresponding to the bounding box, etcetera. There is a grid_volume object associated with the whole computational grid, and with each chunk in the grid. There are various tricky aspects to the grid_volume. One is associated with the Yee grid: it has to know about different field components stored at different points. Another is associated with the fact that boundary conditions, not only the overall grid boundaries but also boundaries between chunks, are handled by an extra layer of "not-owned" pixels around the boundaries. So each chunk's grid_volume has "owned" grid points that the chunk is responsible for updating, and "not-owned" grid points that are updated using the boundary conditions. And thanks to the Yee grid which complicates everything in FDTD, unfortunately, the set of owned and not-owned coordinates is different for each field component. The grid_volume class keeps track of all this.

### File Organization

The Meep C++ source code is organized into the following files in the src/ directory. For the Scheme interface and how it communicates with the back-end C++, look up the source code in the libctl/ directory under the Meep package root directory.

The following table briefly describes what is in each .cpp file:

meep/vec.hpp Declares geometry-related classes like vec, ivec, grid_volume, volume and related utility functions.
meep/mympi.hpp Declares functions for initializing the meep application, cleanup, and data exchange accounting for the presence or absence of MPI. These functions present a unified interface to the rest of the application.
meep.hpp All public classes likes fields, fields_chunks, structure, structure_chunks, src_time, continuous_src_time, material_function, h5_file, polarizability_identifier etc.
meep_internals.hpp Hosts declarations for classes like polarizability, polarization, src_vol, and bandsdata. Also defines macros for frequently-used loop constructs like DOCMP that are internal to Meep implementation.
bicgstab.hpp Declares functions related to an implementation of an iterative solver for non-symmetric linear operators based on a generalization of the stabilized biconjugate-gradient (BiCGSTAB) algorithm proposed by van der Vorst (and described in the book "Templates for the Solution of Linear Systems" by Barrett et al.

The following table briefly describes what is in each .cpp file:

Source File Description
polarization.cpp Implement member functions for the polarization and polarizability classes declared in meep_internals.hpp
bicgstab.cpp Implements the solver described against bicgstab.hpp (see above)

### Functionality Organization

Functionality Location
Material dispersion polarization.cpp, update_from_e.cpp, and friends.
Vectors, volumes etc. meep/vec.hpp, vec.cpp
Geometric objects handled by libctl functions in libctl's geom.c, called from the Scheme front-end (not handled by Meep)
Fields: initialization, cleanup, chunking, stepping-plan, (dis)affiliation with sources, polarizabilities etc. fields.cpp
Structure: initialization, cleanup, chunking, material parameters, boundary conditions etc. structure.cpp
MPI interface meep/mympi.hpp, mympi.cpp

## Deprecated Interfaces

Beware that some of the interfaces in the source code and in the old manual are now deprecated, as they have been superseded by newer features and may be removed at some point.

In particular, you should probably avoid:

• The monitor_point class. Just declare an array to store the fields you want, get them with fields::get_field, and analyze them with do_harminv. Or, to accumulate the DFT as you run, use the dft_chunk class via fields::add_dft.
• Slice and EPS output. This has been superseded by HDF5 output, which is much more flexible and efficient.
• Grace-based plotting in grace.cpp; a more Unix-like philosophy is to output data in a standard format (e.g. HDF5 or comma-separated values) and then import it however you like into the plotting program of your choice.