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Given a directed acyclic graph, simplify straight-line runs into single vertices.

`const simplify = const digraph = let graph = digraph` A -> B B -> C C -> D X -> Y Y -> Z Z -> D D -> U U -> V` let simplified = /* A X ↓ ↓ B Y ↓ ↓ [A, B, C] [X, Y, Z] C Z \___ ___/ \ / \ / Y ---> Y ↓ ↓ D [D, U, V] ↓ U ↓ V*/`

**Note:** passing a graph with cycles to this module will mostly likely result
in an infinite loop. Be sure to remove cycles from your graph before applying
this module.

`Map<Vertex → Set<Edge>> → Edges`

A `Map`

from `Vertex`

(whatever type you provide) to `Edge`

will be defined as `Edges`

.

`{vertices: Set<Vertex>, outgoing: Edges, incoming: Edges} → Graph`

An object with the properties `vertices`

, `outgoing`

, and `incoming`

, whose types are
`Set<Vertex>`

and `Edges`

respectively will be known as a `Graph`

. Incoming `Edges`

will
map a given `Vertex`

instance to every incoming `Edge`

, and outgoing `Edges`

will map `Vertex`

instances to every outgoing `Edge`

. `Edge`

and `Vertex`

types are *user-defined* – that is,
you should provide instructions to this module on how to treat these types.

`Array<Vertex> → DerivedVertex`

`DerivedVertex`

instances represent straight-line runs of `Vertex`

instances from the original
graph. This module will *only* produce `DerivedVertex`

instances.

`{copyEdge: Function?, {s,g}et{From,To}: Function?}? → Interface`

An `Interface`

is defined as an object that optionally defines `copyEdge`

, `getFrom`

, and
`getTo`

properties.

`copyEdge`

takes the original`Edge`

object, and should return a new copy of it.`getFrom`

takes an`Edge`

and returns the source of the edge. If not defined it will treat edges as 2-element arrays, and attempt to take the first element as the source.`getTo`

takes an`Edge`

and returns the destination of the edge. As above, if not defined it will treat edges as a 2-element array and return the second element as the destination.`setFrom`

takes an`Edge`

and a`Vertex`

and should mutate the`Edge`

such that it originates from the vertex.`setTo`

takes an`Edge`

and a`Vertex`

and should mutate the`Edge`

such that it terminates in the vertex.

`simplify(vertices: Set<Vertex>, incoming: Edges,`

`outgoing: Edges, interface: Interface) → Graph`

Given a set of vertices, a map from vertex to incoming edges, a map from vertex to outgoing
edges, and optionally an interface for `Vertex`

and `Edge`

interaction, return a new `Graph`

instance representing a simplified DAG.

MIT