Kahn
An implementation of Kahn's algorithm.
Contents
Introduction
This algorithm will topologically sort a graph, if there are no cycles, otherwise it will report the cycles. The Wikipedia page on topological sorting has a brief explanation.
A graph can be constructed with the fromVertexLiterals() factory method as follows:
const kahn = require('occam-kahn');
const { Graph } = kahn;
const graph = Graph.fromVertexLiterals([
['a', ['b']],
['b', ['c']],
['d', ['c']],
['e', []],
['f', ['g']],
['h', ['g']]
]);
Note that the array of names that is the second element of each literal gives the ancestors of the vertex and not its descendants. This is the preferred method when constructing a dependency tree, because a resource's dependencies are usually stipulated whereas the converse is not usually true.
It is possible to check whether there are any cycles present:
const cyclesPresent = graph.areCyclesPresent();
If there are no cycles present, the topologically sorted vertices of the graph are available:
const topologicallySortedVertices = graph.getTopologicallySortedVertices();
If there are cycles present, they will be amongst the remaining edges:
const remainingEdges = graph.getRemainingEdges();
remainingEdges.forEachEdgeByVertexNames(function(sourceVertexName, targetVertexName) {
...
});
Rather than iterate through the remaining edges and recover the vertex names yourself you can use the forEachRemainingEdgeByVertexNames() method:
graph.forEachRemainingEdgeByVertexNames(function(remainingEdgeSourceVertexName, remainingEdgeTargetVertexName) {
...
});
The algorithm will also leave both the incoming and outgoing edges of the topologically sorted vertices intact and these are available by way of the requisite getters:
const firstTopologicallySortedVertex = first(topologicallySortedVertices),
incomingEdges = firstTopologicallySortedVertex.getIncomingEdges(),
outgoingEdges = firstTopologicallySortedVertex.getOutgoingEdges();
Installation
With npm:
npm install occam-kahn
You can also clone the repository with Git...
git clone git@github.com:occam-proof-assistant/Kahn.git
...and then install the necessary modules with npm from within the project's root directory:
npm install
Building
Automation is done with npm scripts, have a look at the package.json file. The pertinent commands are:
npm run build-debug
npm run watch-debug
Acknowledgements
This implementation was closely based on the following:
https://gist.github.com/Sup3rc4l1fr4g1l1571c3xp14l1d0c10u5/3341dba6a53d7171fe3397d13d00ee3f