Gapotchenko.FX.Math.Topology 2022.2.7

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.NET 5.0 .NET Core 2.0 .NET Standard 2.0 .NET Framework 4.6
dotnet add package Gapotchenko.FX.Math.Topology --version 2022.2.7
NuGet\Install-Package Gapotchenko.FX.Math.Topology -Version 2022.2.7
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<PackageReference Include="Gapotchenko.FX.Math.Topology" Version="2022.2.7" />
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add Gapotchenko.FX.Math.Topology --version 2022.2.7
#r "nuget: Gapotchenko.FX.Math.Topology, 2022.2.7"
#r directive can be used in F# Interactive and Polyglot Notebooks. Copy this into the interactive tool or source code of the script to reference the package.
// Install Gapotchenko.FX.Math.Topology as a Cake Addin
#addin nuget:?package=Gapotchenko.FX.Math.Topology&version=2022.2.7

// Install Gapotchenko.FX.Math.Topology as a Cake Tool
#tool nuget:?package=Gapotchenko.FX.Math.Topology&version=2022.2.7


The module provides data structures and primitives for working with abstract topologies.

In its classical meaning, the science of topology is concerned with the properties of geometric objects. However, such a definition may be considered as too narrow by some. Math has no boundaries, so topology should not be viewed as limited to just one unique flavor of objects (geometric, physical); it can really be anything.



Graph<T> provided by Gapotchenko.FX.Math.Topology represents a strongly-typed directional graph of objects. The objects correspond to mathematical abstractions called graph vertices and each of the related pairs of vertices is called an edge. A graph can be viewed as a structure that contains two sets: set of vertices and set of edges. Vertices define "what" graph contains and edges define "how" those vertices are connected.

Let's take a look at the simplest graph that contains just two vertices:

using Gapotchenko.FX.Math.Topology;

var g = new Graph<int>
    Vertices = { 1, 2 }

If we could visualize that graph then it would look like this:

Simple graph with two isolated vertices

Now let's add one more vertex 3 plus an edge that goes from vertex 1 to vertex 2:

var g = new Graph<int>
    Vertices = { 1, 2, 3 },
    Edges = { (1, 2) }  // <-- an edge has (from, to) notation

Our new graph looks like this:

Simple graph with three vertices and one edge

The vertices already defined in edges can be omitted for brevity:

var h = new Graph<int>
    Vertices = { 3 },
    Edges = { (1, 2) }

Console.WriteLine(g.GraphEquals(h)); // will print "True"

It is worth mentioning that the graph provides its vertices as an ISet<T>, so the usual operations on a set apply to the vertices as well:

var g = new Graph<int>
    Vertices = { 3 },
    Edges = { (1, 2) }

g.Vertices.UnionWith(new[] { 3, 4, 5 });

The example above produces the following graph:

Simple graph with five vertices and one edge

The same ISet<T> model applies to the graph edges: they are treated as a set too.


Now once we have the basics in place, let's take a look at graph operations. Consider the graph:

var g = new Graph<int>
    Edges =
        (7, 5), (7, 6),
        (6, 3), (6, 4),
        (5, 2), (5, 4),
        (3, 1),
        (2, 1),
        (1, 0)

which looks like this:

Graph with eight vertices and nine edges

Let's transpose the graph (i.e. reverse the direction of its edges):

var h = g.GetTransposition();

Transposed graph h renders as:

Transposed graph with eight vertices and nine edges

Note that graph h is a new instance of Graph<T>.

But what if we want to transpose the graph g in place? Every graph operation has a corresponding in-place variant, so for transposition it will be:


In this way, a developer can freely choose between immutable, mutable, or combined data models when working on a particular task at hand.

Graph transposition is just one example but there are plenty of other operations available. They all work in the same manner and follow the same model:

Operation Description Immutable Function In-Place Method
Transposition Reverses the direction of all edges in the graph. GetTransposition Transpose
Transitive reduction Prunes the transitive relations that have shorter paths. GetTransitiveReduction ReduceTransitions
Reflexive reduction Removes the loops (also called self-loops or buckles). GetReflexiveReduction ReduceReflexes
Subgraph Produces a vertex-induced or edge-induced subgraph. GetSubgraph Subgraph
Intersection Produces a graph containing vertices and edges that are present in both the current and a specified graphs. Intersect IntersectWith
Union Produces a graph containing all vertices and edges that are present in the current graph, in the specified graph, or in both. Union UnionWith
Exception Produces a graph containing vertices and edges that are present in the current graph but not in the specified graph. Except ExceptWith

Topological Sorting

Topological sort of a graph is a linear ordering of its vertices such that for every directed edge uv, u comes before v in the ordering.

A classic example of topological sorting is to schedule a sequence of jobs (or tasks) according to their dependencies. The jobs are represented by vertices, and there is an edge from x to y if job x must be completed before job y can be started. Performing topological sort on such a graph would give us an order in which to perform the jobs.

Let's take a look at example graph:

Graph with eight vertices and nine edges

Let's assume that vertices represent the jobs, and edges define the dependencies between them. In this way, job 0 depends on job 1 and thus cannot be started unless job 1 is finished. In turn, job 1 cannot be started unless jobs 2 and 3 are finished. And so on. In what order should the jobs be executed?

To answer that question, let's use OrderTopologically method:

using Gapotchenko.FX.Math.Topology;

// Define a graph according to the diagram.
var g = new Graph<int>
    Edges =
        (7, 5), (7, 6),
        (6, 3), (6, 4),
        (5, 2), (5, 4),
        (3, 1),
        (2, 1),
        (1, 0)

// Sort the graph topologically.
var ordering = g.OrderTopologically();

// Print the results.
Console.WriteLine(string.Join(", ", ordering));

The resulting sequence of jobs is:

7, 5, 6, 2, 3, 4, 1, 0

OrderTopologically method can only work on directed acyclic graphs. If the graph contains a cycle then CircularDependencyException is raised.

Stable Topological Sort of a Graph

Graph is a data structure similar to a set: it does not guarantee to preserve the order in which the elements were added. As a result, topological sorting may return different orderings for otherwise equal graphs.

To overcome that limitation, it may be beneficial to use the topological sorting with a subsequent ordering by some other criteria. Such approach makes the topological sorting stable. It can be achieved by leveraging the standard IOrderedEnumerable<T> LINQ semantics of the operation, like so:


Stable Topological Sort of a Sequence

Sorting a sequence of elements in topological order is another play on topological sorting idea.

Say we have a sequence of elements {A, B, C, D, E, F}. Some elements depend on others:

  • A depends on B
  • B depends on D

Objective: sort the sequence so that its elements are ordered according to their dependencies. The resulting sequence should have a minimal edit distance to the original one. In other words, sequence should be topologically sorted while preserving the original order of elements whenever it is possible.

Gapotchenko.FX.Math.Topology provides an extension method for IEnumerable<T> that allows to achieve that:

using Gapotchenko.FX.Math.Topology;

string seq = "ABCDEF";

// Dependency function.
static bool df(char a, char b) =>
    (a + " depends on " + b) switch
        "A depends on B" or
        "B depends on D" => true,
        _ => false

var ordering = seq.OrderTopologicallyBy(x => x, df);
Console.WriteLine(string.Join(", ", ordering));  // <- prints "D, B, A, C, E, F"

Unlike its graph sibling, OrderTopologicallyBy method tolerates circular dependencies by ignoring them. They are resolved according to the original order of elements in the sequence.

OrderTopologicallyBy method allows a subseqent sorting by following the standard IOrderedEnumerable<T> LINQ convention:


Commonly Used Types

  • Gapotchenko.FX.Math.Topology.Graph

Other Modules

Let's continue with a look at some other modules provided by Gapotchenko.FX:

Or look at the full list of modules.

Product Compatible and additional computed target framework versions.
.NET net5.0 is compatible.  net5.0-windows was computed.  net6.0 is compatible.  net6.0-android was computed.  net6.0-ios was computed.  net6.0-maccatalyst was computed.  net6.0-macos was computed.  net6.0-tvos was computed.  net6.0-windows was computed.  net7.0 is compatible.  net7.0-android was computed.  net7.0-ios was computed.  net7.0-maccatalyst was computed.  net7.0-macos was computed.  net7.0-tvos was computed.  net7.0-windows was computed. 
.NET Core netcoreapp2.0 is compatible.  netcoreapp2.1 is compatible.  netcoreapp2.2 was computed.  netcoreapp3.0 is compatible.  netcoreapp3.1 was computed. 
.NET Standard netstandard2.0 is compatible.  netstandard2.1 is compatible. 
.NET Framework net46 is compatible.  net461 was computed.  net462 was computed.  net463 was computed.  net47 was computed.  net471 is compatible.  net472 is compatible.  net48 was computed.  net481 was computed. 
MonoAndroid monoandroid was computed. 
MonoMac monomac was computed. 
MonoTouch monotouch was computed. 
Tizen tizen40 was computed.  tizen60 was computed. 
Xamarin.iOS xamarinios was computed. 
Xamarin.Mac xamarinmac was computed. 
Xamarin.TVOS xamarintvos was computed. 
Xamarin.WatchOS xamarinwatchos was computed. 
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Version Downloads Last updated
2022.2.7 352 5/1/2022
2022.2.5 311 5/1/2022
2022.1.4 318 4/6/2022
2021.2.21 364 1/21/2022
2021.2.20 353 1/17/2022