libgraph

Documentation

About

This library provides:

• An implementation of a graph datastructure, Graph, designed for both directed and undirected graphs. The API supports undirected graphs, but I'm still getting the tests updated to cover properties of undirected graphs.
• A priority queue implementation PriorityQueue, oriented towards graphs (it prioritizes lower integer values over high), it is the fastest priority queue I know of which allows arbitrary priorities, and is more or less at parity with pqueue3 from the pqueue library, which supports priorities from 0 to 65535.
• An idiomatic Elixir API for creating, modifying, and querying it's graph structure. Creating and modifying a graph can be done in a single pipeline, and all queries take a Graph as their first parameter (one of my complaints with :digraph is that there is some inconsistency with the API between :digraph and :digraph_utils for no apparent reason).
• Two "Reducer" implementations for mapping/reducing over a graph. I am trying to figure out the best way to make these extendable and part of the API, so that you can drop in your own shortest path algorithms, etc - but I have yet to come up with an approach that feels good on that front.
• A Serializer behaviour, for defining custom serialization of graphs, with a Graphviz DOT format serializer provided out of the box.

It is backed by a large suite of tests, including several QuickCheck properties for the graph model. It's API shares some similarity with :digraph, but diverges in favor of a more idiomatic Elixir interface. In addition, over time I'm adding new functions to query the graph in ways not previously supported via :digraph, and introducing support for classifying a graph as undirected if so desired, so that queries over such graphs become easier.

Installation

If available in Hex, the package can be installed by adding libgraph to your list of dependencies in mix.exs:

def deps do
  [{:libgraph, "~> 0.7"}]
end

Rationale

The original motiviation for me to start working on this library is the fact that :digraph requires a minimum of 3 ETS tables per graph, and up to 6 depending on the operations you are performing on the graph. If you are working with a lot of graphs concurrently, as I am, this means you can find yourself in a situation where you hit the system limit for the maximum number of ETS table, and bring your system down. Seeing as how it is ridiculous that trying to use a simple graph could potentially kill my system, and not wanting to hack around the problem, I decided to see if I could build an alternative which was competitive performance-wise, without requiring any ETS tables at all.

The result turned out better than I hoped - it is possible to build a graph datastructure without ETS that is both equally performant (and in many of my benchmarks, better performing), and supports all of the same functionality.

Additionally, I also had a few other things I wanted to address:

• Inconsistency with argument order in the API between :digraph and :digraph_utils
• The fact that there are two modules to work with the same datastructure to begin with, and trying to remember what lives where.
• The lack of extensibility, for example, there is no API with which you can implement your own traversal algorithms. This means you are stuck with whatever way the Erlang maintainers decided was ideal, regardless of whether it suits your use case or not. A great example is single-source shortest path algorithms, where you may want a simple breadth-first search, or perhaps you want to use Dijkstra's algorithm - you are stuck with just one approach with :digraph, which as I understand it, is a breadth-first search.
• :digraph as the name implies, only supports directed graphs
• :digraph graphs are unweighted, with no way to supported weighted graphs
• :digraph graphs are not "inspect-friendly", you get a tuple with the underlying ETS table ids, but that's it, not necessarily a big deal, but it's nice for playing around in the shell if you can see how your code affects the structure.

My speculation as to why :digraph is the way it is, is that when :digraph was originally written, there was no efficient key/value datastructure in Erlang that could support large numbers of keys. At that time, maps weren't even a speck in the eye of the language maintainers. Even after the initial introduction of maps in OTP 18, maps still weren't efficient enough to work with large numbers of keys. It wasn't until OTP 19 that the performance of maps with millions of keys became reasonable. So, it's not that :digraph sucks - it was the best possible implementation at the time; but now that the language has come so far, we can take advantage of some of the new hotness and reinvent it from the ground up :).

Benchmarks

Feel free to take a look under the bench folder in the project root. There a few benchmarks I threw together to keep an eye on a few key areas I wanted to ensure parity with :digraph on. You can run them yourself as well, but I would encourage you to use them as a template to construct a benchmark based on your own use case, and compare them that way, as it will give you a better basis to make your decision on. However, if you do find that libgraph is behind :digraph with a benchmark, please let me know so that I can improve the library!

NOTE: While this library is primarily focused on the Graph data structure it defines, it also contains an implementation of a priority queue (you can find it under the PriorityQueue module), designed for use with graphs specifically, as it considers lower integer values higher priority, which is perfect for the kinds of graph algorithms you need a priority queue for.

Roadmap

Please open an issue if you have a feature request!

License

MIT (See the LICENSE file)