# C++ Developer Information

An overview of Meep's inner workings is provided in Computer Physics Communications, Vol. 181, pp. 687-702, 2010. This page is a supplement which provides a description of the source code.

For additional details, see Chunks and Symmetry

### Data Structures and Chunks

Meep's data structures are defined 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 fields over the entire grid, and one of its members is an array of fields_chunk variables that divides the grid. The fields_chunk variables store the actual field information. 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; they 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 the 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 rectilinear 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. Due 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 core Meep C++ simulation code (all of the physics) is located in the src/ directory, with C++ tests in the tests/ directory. The libmeepgeom/ directory provides a C++ library to specify Meep geometries in terms of a list of geometric objects (spheres, cylinders, boxes) with various material properties (via libctl's geometry library), and is also used by the Python interface.

The Scheme and Python interfaces are found in the scheme/ and python/ directories. Both interfaces use SWIG to generate wrapper code from the C++ header files, but also have hand-written Scheme/Python code to provide a higher-level interface. The libpympb/ directory contains a Python interface to MPB (which may, in the future, be moved to the MPB repository).

The following table briefly describes the purpose of some of the source files:

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.

# Python Developer Information

## Overview

The meep Python package consists of a low-level interface and a high-level interface. The low-level interface is the direct result of running SWIG on the C++ headers.

Next, we compile meep-python.cpp, rename meep.py to __init__.py and put them in a folder called meep. Putting all the code in __init__.py allows us to access the symbols directly from the meep namespace rather than going through an additional module like meep.meep.vec. Now we have a complete Python package.

__init__.py contains "proxy" classes for all public meep objects. They hold a this pointer that dispatches to the appropriate C++ functions in the _meep.so extension module. The interface this package exposes is basically the same as the C++ interface. That is, a simulation written in this low-level Python interface would not look much different from the same simulation written in C++. By implementing a high-level interface on top of the basic SWIG wrappers, we can abstract away many of the low details of setting up a simulation, take advantage of Python language features like keyword arguments, and gain productivity from libraries like NumPy.

## Package Organization

After adding the high-level interface files, the meep package looks like this.

meep
├── __init__.py
├── _meep.so
├── geom.py
├── simulation.py
└── source.py


The Python MPB interface is also included in the meep package. It's constructed in the same manner as the meep package. The low-level interface is in meep/mpb/__init__.py and meep/mpb/_mpb.so, and the high-level interface is in solver.py. Here is a view of the complete package.

meep
├── mpb
│   ├── __init__.py
│   ├── _mpb.so
│   ├── mpb_data.py
│   └── solver.py
├── __init__.py
├── _meep.so
├── geom.py
├── simulation.py
└── source.py


## Description of Files

### meep.i

SWIG interface file for the meep Python module. Includes typemaps, helper functions, and module initialization code. The typemaps mostly call other helper functions defined either in meep.i, or in typemap_utils.cpp (if the function is used in mpb.i too). There are also various py_*_wrap functions that handle calling user defined Python functions from C++. Everything in the %pythoncode block at the end of the file is run once when the meep module is first imported. See the SWIG documenation for more details.

### vec.i

SWIG interface file for vec.hpp. Included into meep.i. SWIG warnings are disabled (if found benign) in this file.

### numpy.i

Typemaps for numpy arrays (taken from the NumPy Github repository). See the documentation for instructions on using these typemaps.

### typemap_utils.cpp

Utility functions for writing SWIG typemaps. Since this file is included into both meep.i and mpb.i, only code that is useful to both interface files should be put here (otherwise the compiler complains about unused functions). Code used only by one interface should be put in the respective .i file. The majority of the code in this file is for converting the Python geometric objects defined in geom.py to C objects. The get_attr_* functions are helpers for getting C versions of attributes on Python objects. The convention in the file is for functions to return 1 on success and 0 on failure so that the top level typemaps in meep.i can be written as:

%typemap(in) type {
if(!py_type_to_type($input, &$1)) {
SWIG_fail;
}
}


Keeping the code within %typemap blocks small is valuable because it gets copied everywhere the typemap is used, which can lead to code bloat.

### geom.py

Pure Python implementations of the geometric objects defined in libctl. The user-defined list of objects (Simulation.geometry) gets converted to C objects when meep.set_materials_from_geometry is called in Simulation._init_structure. The function responsible for this conversion is typemap_utils.cpp:py_list_to_gobj_list. This file also contains classes that represent materials (Medium) and susceptibilities, and cartesian/reciprocal/lattice conversion functions. Note that when adding a class or function to this file, it must also be imported in the %pythoncode block at the end of python/meep.i if you want it to be directly accessible from the meep namespace (i.e., to get meep.MyClass instead of meep.geom.MyClass).

### simulation.py

Holds the Simulation class, which is the primary abstraction of the high-level interface. Minimally, a simulation script amounts to passing the desired keyword arguments to the Simulation contructor and calling the run method on the resulting instance. The various step functions are also included in this file. When adding extra functions or class to this file, an import statement should also be added to the %pythoncode block at the bottom of python/meep.i.

### source.py

Holds classes representing sources, including GaussianSource, ContinuousSource, CustomSource, and EigenModeSource. When adding extra functions or class to this file, an import statement should also be added to the %pythoncode block at the bottom of python/meep.i.

### mpb.i

SWIG interface file for the meep.mpb Python module.

### solver.py

Classes and functions related to the high-level Python interface to MPB. Additional classes or functions in this file should be accompanied by an import statement in the %pythoncode block at the bottom of mpb.i.

### mpb_data.py

Definition of MPBData, a Python class useful for MPB data analysis (documented here). This is is a Python port of the functionality available in the mpb-data command line program originally written in C.

## Testing

The tests for the Python interface are located in python/tests. To run the whole test suite, run make check in the python build tree. During development it is more convenient to run individual tests. This can be accomplished by running python <path_to_test>/test.py MyTestCase.test_method. See the Python unittest framework documentation for more info. If you're using an out-of-tree build (recommended), the development process will look something like this:

cd meep/build/python
export PYTHONPATH=pwd

# After making changes, rebuild the Python package
make
# Run test
python ../../python/tests/my_test.py MyNewTest.test_my_feature