# data/graph

Provides an implementation of directed graphs.

This provides basic algorithms for working with graphs, support for super-vertices, and other nifty features.

`(add-edge! from to)`

*Defined at lib/data/graph.lisp:66:2*

Add an edge `FROM`

one vertex `TO`

another.

`(add-vertex! graph value)`

*Defined at lib/data/graph.lisp:45:2*

Create a vertex with the corresponding `VALUE`

and add it to the `GRAPH`

.

`(condensation in-graph)`

*Defined at lib/data/graph.lisp:145:2*

Compute the condensation of an input graph, `IN-GRAPH`

, replacing all strongly connected
components with a super vertex.

### Example:

```
> (define g (make-graph))
> (let [(a (add-vertex! g :a))
. (b (add-vertex! g :b))
. (c (add-vertex! g :c))]
. (add-edge! a b)
. (add-edge! b a)
. (add-edge! a c))
> (condensation g)
out = «graph: "c" ("b" "a")»
```

`(dominators root)`

*Defined at lib/data/graph.lisp:253:2*

Build the dominators from nodes descended from `ROOT`

.

This uses an adaptation of “`A`

Simple, Fast Dominance Algorithm” by
Keith `D`

. Cooper, Timothy `J`

. Harvey and Ken Kennedy. It does not
compute post order numbers, as this is designed to be generalised for
cyclic graphs.

This returns two objects: a lookup of vertex to its immediate dominator,
and a `DAG`

with edges from dominators to their immediate children.

### Example:

```
> (define g (make-graph))
> (define doms
. (let [(a (add-vertex! g :a))
. (b (add-vertex! g :b))
. (c (add-vertex! g :c))
. (d (add-vertex! g :d))]
. (add-edge! a b)
. (add-edge! a c)
. (add-edge! b c)
. (add-edge! c b)
. (add-edge! b d)
.
. (dominators a)))
> (.> doms (get-vertex g :b))
out = «vertex: "a"»
> (.> doms (get-vertex g :c))
out = «vertex: "a"»
> (.> doms (get-vertex g :d))
out = «vertex: "b"»
```

`(get-vertex graph value)`

*Defined at lib/data/graph.lisp:58:2*

Get the corresponding vertex for this `VALUE`

from the given `GRAPH`

.

`(graph->dot graph name)`

*Defined at lib/data/graph.lisp:28:2*

Convert `GRAPH`

to a string in the `DOT`

format, suitable for consumption
with GraphViz.

`(make-graph)`

*Defined at lib/data/graph.lisp:9:2*

Create a new, empty graph.

`(strongly-connected-components graph)`

*Defined at lib/data/graph.lisp:78:2*

Find all strong components from a `GRAPH`

.

This uses Tarjan’s strongly connected components algorithm, which runs in linear time.

### Example:

```
> (define g (make-graph))
> (let [(a (add-vertex! g :a))
. (b (add-vertex! g :b))
. (c (add-vertex! g :c))]
. (add-edge! a b)
. (add-edge! b a)
. (add-edge! a c))
> (strongly-connected-components g)
out = ((«vertex: "c"») («vertex: "b"» «vertex: "a"»))
```

`(traverse-postorder root visitor)`

*Defined at lib/data/graph.lisp:217:2*

Visit a graph using postorder traversal starting at `ROOT`

, calling
`VISITOR`

for each vertex.

`VISITOR`

should take the form `(lambda (vertex visited))`

, where
`vertex`

is the current vertex and `visited`

is a set of already
visited nodes.

Whilst the vertices are traversed in order, edges may be traversed in a different order for semantically identical graphs. This is especially prevalent when the graph contains cycles.

Note that each vertex will be visited exactly `ONCE`

.

### Example:

```
> (define g (make-graph))
> (let [(a (add-vertex! g :a))
. (b (add-vertex! g :b))
. (c (add-vertex! g :c))]
. (add-edge! a b)
. (add-edge! b c)
. (traverse-postorder a (compose print! pretty)))
«vertex: "c"»
«vertex: "b"»
«vertex: "a"»
out = nil
```

`(traverse-preorder root visitor)`

*Defined at lib/data/graph.lisp:180:2*

Visit a graph using preorder traversal starting at `ROOT`

, calling
`VISITOR`

for each vertex.

`VISITOR`

should take the form `(lambda (vertex visited))`

, where
`vertex`

is the current vertex and `visited`

is a set of already
visited nodes.

Whilst the vertices are traversed in order, edges may be traversed in a different order for semantically identical graphs. This is especially prevalent when the graph contains cycles.

Note that each vertex will be visited exactly `ONCE`

.

### Example:

```
> (define g (make-graph))
> (let [(a (add-vertex! g :a))
. (b (add-vertex! g :b))
. (c (add-vertex! g :c))]
. (add-edge! a b)
. (add-edge! b c)
. (traverse-preorder a (compose print! pretty)))
«vertex: "a"»
«vertex: "b"»
«vertex: "c"»
out = nil
```

## Undocumented symbols

`$graph`

*Defined at lib/data/graph.lisp:9:2*`$vertex`

*Defined at lib/data/graph.lisp:17:2*`(graph? graph)`

*Defined at lib/data/graph.lisp:9:2*`(vertex? vertex)`

*Defined at lib/data/graph.lisp:17:2*