This package implements the Leiden algorithm in C++. It relies on igraph for it to function. Besides the relative flexibility of the implementation, it also scales well, and can be run on graphs of millions of nodes (as long as they can fit in memory). The core class is Optimiser which finds the optimal partition using the Leiden algorithm¹, which is an extension of the Louvain algorithm² for a number of different methods. The methods currently implemented are (1) modularity³, (2) Reichardt and Bornholdt's model using the configuration null model and the Erdös-Rényi null model⁴, (3) the Constant Potts model (CPM) ⁵, (4) Significance ⁶, and finally (5) Surprise ⁷. In addition, it supports multiplex partition optimisation allowing community detection on for example negative links ⁸ or multiple time slices ⁹. There is the possibility of only partially optimising a partition, so that some community assignments remain fixed ¹⁰. It also provides some support for community detection on bipartite graphs.

This package contains the C++ code only. Most people will find it easier to work with the Python interface at https://github.com/vtraag/leidenalg.

The build system uses CMake and follows the prototypical CMake build steps:

1. Get the source code.

   You can download the source code from the latest release at https://github.com/vtraag/libleidenalg/releases. Alternatively, you can clone the repository using git.

2. Create a build directory

   You can create a build directory anywhere. A common location is to create a subdirectory in the source code as

   mkdir build
   

3. Configure the build system

   Assuming you created the build directory as a subdirectory, you can run the following

   cmake ..
   

   Note that the build directory should be your current working directory.

4. Build the library

   cmake --build .
   

5. Install the library

   cmake --build . --target install
   

You can change the installation location of libleidenalg using CMAKE_INSTALL_PREFIX as usual.

You can change whether a static or dynamic library should be built using BUILD_SHARED_LIBS as usual.

This library depends on igraph, which you should install before. See https://igraph.org/c/doc/igraph-Installation.html for more details.

If you have installed igraph in a non-standard location, CMake might not be able to find it automatically. If you use CMAKE_INSTALL_PATH=<dir> to install igraph, you can specify the []CMAKE_PREFIX_PATH=<dir>](https://cmake.org/cmake/help/latest/variable/CMAKE_PREFIX_PATH.html) when configuring libleidenalg to find igraph.

The Optimiser class is responsible for optimising a MutableVertexPartition (possibly multiple in the case of a multiplex approach). The MutableVertexPartition is just a base class, and should be implemented to provide explicit quality function:

• CPMVertexPartition
• ModularityVertexPartition
• RBConfigurationVertexPartition
• RBERVertexPartition
• SignificanceVertexPartition
• SurpriseVertexPartition

Some of these classes depend on intermediate derived classes. The implementation of a quality function in a derived class, essentially comes to down implementing the diff_move and quality, see also CONTRIBUTING.md in this repository.

An igraph_t object from igraph is used to construct a separate Graph object, which can be used to construct a MutableVertexPartition. For example, to find a partition using CPM, you could do the following

    igraph_t g;
    igraph_famous(&g, "Zachary");

    Graph graph(&g);

    CPMVertexPartition part(&graph,
                            0.05 /* resolution */ );

    Optimiser o;

    o.optimise_partition(&part);

In the example directory, we added a complete example, including the CMake build files (please note that this directory is not included in the release source package, and only available on the GitHub repository). In order to compile it, first make sure you have properly installed igraph and libleidenalg. In the example directory, you can then follow the standard CMake routine to build it

mkdir build && cd build
cmake ..
cmake --build .

and run ./example.

Copyright (C) 2020 V.A. Traag, (C) 2022 Andrew Robbins

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.

Please cite the references appropriately in case they are used.