A library for creating generic graph data structures and modifying, analyzing, and visualizing them.

# Features

- Generic vertices of any type, such as
`int`

or`City`

. - Graph traits with corresponding validations, such as cycle checks in acyclic graphs.
- Algorithms for finding paths or components, such as shortest paths or strongly connected components.
- Algorithms for transformations and representations, such as transitive reduction or topological order.
- Algorithms for non-recursive graph traversal, such as DFS or BFS.
- Vertices and edges with optional metadata, such as weights or custom attributes.
- Visualization of graphs using the DOT language and Graphviz.
- Extensive tests with ~90% coverage, and zero dependencies.

Status: Because

`graph`

is in version 0, the public API shouldn't be considered stable.

This README may contain unreleased changes. Check out the latest documentation.

# Getting started

```
go get github.com/dominikbraun/graph
```

# Quick examples

## Create a graph of integers

```
g := graph.New(graph.IntHash)
_ = g.AddVertex(1)
_ = g.AddVertex(2)
_ = g.AddVertex(3)
_ = g.AddVertex(4)
_ = g.AddVertex(5)
_ = g.AddEdge(1, 2)
_ = g.AddEdge(1, 4)
_ = g.AddEdge(2, 3)
_ = g.AddEdge(2, 4)
_ = g.AddEdge(2, 5)
_ = g.AddEdge(3, 5)
```

## Create a directed acyclic graph of integers

```
g := graph.New(graph.IntHash, graph.Directed(), graph.Acyclic())
_ = g.AddVertex(1)
_ = g.AddVertex(2)
_ = g.AddVertex(3)
_ = g.AddVertex(4)
_ = g.AddEdge(1, 2)
_ = g.AddEdge(1, 3)
_ = g.AddEdge(2, 3)
_ = g.AddEdge(2, 4)
_ = g.AddEdge(3, 4)
```

## Create a graph of a custom type

To understand this example in detail, see the concept of hashes.

```
type City struct {
Name string
}
cityHash := func(c City) string {
return c.Name
}
g := graph.New(cityHash)
_ = g.AddVertex(london)
```

## Create a weighted graph

```
g := graph.New(cityHash, graph.Weighted())
_ = g.AddVertex(london)
_ = g.AddVertex(munich)
_ = g.AddVertex(paris)
_ = g.AddVertex(madrid)
_ = g.AddEdge("london", "munich", graph.EdgeWeight(3))
_ = g.AddEdge("london", "paris", graph.EdgeWeight(2))
_ = g.AddEdge("london", "madrid", graph.EdgeWeight(5))
_ = g.AddEdge("munich", "madrid", graph.EdgeWeight(6))
_ = g.AddEdge("munich", "paris", graph.EdgeWeight(2))
_ = g.AddEdge("paris", "madrid", graph.EdgeWeight(4))
```

## Perform a Depth-First Search

This example traverses and prints all vertices in the graph in DFS order.

```
g := graph.New(graph.IntHash, graph.Directed())
_ = g.AddVertex(1)
_ = g.AddVertex(2)
_ = g.AddVertex(3)
_ = g.AddVertex(4)
_ = g.AddEdge(1, 2)
_ = g.AddEdge(1, 3)
_ = g.AddEdge(3, 4)
_ = graph.DFS(g, 1, func(value int) bool {
fmt.Println(value)
return false
})
```

```
1 3 4 2
```

## Find strongly connected components

```
g := graph.New(graph.IntHash)
// Add vertices and edges ...
scc, _ := graph.StronglyConnectedComponents(g)
fmt.Println(scc)
```

```
[[1 2 5] [3 4 8] [6 7]]
```

## Find the shortest path

```
g := graph.New(graph.StringHash, graph.Weighted())
// Add vertices and weighted edges ...
path, _ := graph.ShortestPath(g, "A", "B")
fmt.Println(path)
```

```
[A C E B]
```

## Perform a topological sort

```
g := graph.New(graph.IntHash, graph.Directed(), graph.PreventCycles())
// Add vertices and edges ...
order, _ := graph.TopologicalSort(g)
fmt.Println(order)
```

```
[1 2 3 4 5]
```

## Perform a transitive reduction

```
g := graph.New(graph.StringHash, graph.Directed(), graph.PreventCycles())
// Add vertices and edges ...
_ := graph.TransitiveReduction(g)
```

## Prevent the creation of cycles

```
g := graph.New(graph.IntHash, graph.PreventCycles())
_ = g.AddVertex(1)
_ = g.AddVertex(2)
_ = g.AddVertex(3)
_ = g.AddEdge(1, 2)
_ = g.AddEdge(1, 3)
if err := g.AddEdge(2, 3); err != nil {
panic(err)
}
```

```
panic: an edge between 2 and 3 would introduce a cycle
```

## Visualize a graph using Graphviz

The following example will generate a DOT description for `g`

and write it into the given file.

```
g := graph.New(graph.IntHash, graph.Directed())
_ = g.AddVertex(1)
_ = g.AddVertex(2)
_ = g.AddVertex(3)
_ = g.AddEdge(1, 2)
_ = g.AddEdge(1, 3)
file, _ := os.Create("./mygraph.gv")
_ = draw.DOT(g, file)
```

To generate an SVG from the created file using Graphviz, use a command such as the following:

```
dot -Tsvg -O mygraph.gv
```

## Setting edge attributes

Edges may have one or more attributes which can be used to store metadata. Attributes will be taken into account when visualizing a graph. For example, this edge will be rendered in red color:

```
_ = g.AddEdge(1, 2, graph.EdgeAttribute("color", "red"))
```

To get an overview of all supported attributes, take a look at the DOT documentation.

## Setting vertex attributes

Vertices may have one or more attributes which can be used to store metadata. Attributes will be taken into account when visualizing a graph. For example, this vertex will be rendered in red color:

```
_ = g.AddVertex(1, graph.VertexAttribute("style", "filled"), graph.VertexAttribute("fillcolor", "red"))
```

To get an overview of all supported attributes, take a look at the DOT documentation.

# Concepts

## Hashes

A graph consists of nodes (or vertices) of type `T`

, which are identified by a hash value of type
`K`

. The hash value is obtained using the hashing function passed to `graph.New`

.

### Primitive types

For primitive types such as `string`

or `int`

, you may use a predefined hashing function such as
`graph.IntHash`

– a function that takes an integer and uses it as a hash value at the same time:

```
g := graph.New(graph.IntHash)
```

This also means that only one vertex with a value like

`5`

can exist in the graph if`graph.IntHash`

used.

### Custom types

For storing custom data types, you need to provide your own hashing function. This example function
takes a `City`

and returns the city name as an unique hash value:

```
cityHash := func(c City) string {
return c.Name
}
```

Creating a graph using this hashing function will yield a graph with vertices of type `City`

identified by hash values of type `string`

.

```
g := graph.New(cityHash)
```

## Traits

The behavior of a graph, for example when adding or retrieving edges, depends on its traits. You can set the graph's traits using the functional options provided by this library:

```
g := graph.New(graph.IntHash, graph.Directed(), graph.Weighted())
```

# Documentation

The full documentation is available at pkg.go.dev.