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# digraph

### MODULE

digraph

### MODULE SUMMARY

Directed Graphs

### DESCRIPTION

The digraph module implements a version of labeled directed graphs. What makes the graphs implemented here non-proper directed graphs is that multiple edges between vertices are allowed. However, the customary definition of directed graphs will be used in the text that follows.

A directed graph (or just "digraph") is a pair (V, E) of a finite set V of vertices and a finite set E of directed edges (or just "edges"). The set of edges E is a subset of V × V (the Cartesian product of V with itself). In this module, V is allowed to be empty; the so obtained unique digraph is called the empty digraph. Both vertices and edges are represented by unique Erlang terms.

Digraphs can be annotated with additional information. Such information may be attached to the vertices and to the edges of the digraph. A digraph which has been annotated is called a labeled digraph, and the information attached to a vertex or an edge is called a label. Labels are Erlang terms.

An edge e = (v, w) is said to emanate from vertex v and to be incident on vertex w. The out-degree of a vertex is the number of edges emanating from that vertex. The in-degree of a vertex is the number of edges incident on that vertex. If there is an edge emanating from v and incident on w, then w is said to be an out-neighbour of v, and v is said to be an in-neighbour of w. A path P from v[1] to v[k] in a digraph (V, E) is a non-empty sequence v[1], v[2], ..., v[k] of vertices in V such that there is an edge (v[i],v[i+1]) in E for 1 <= i < k. The length of the path P is k-1. P is simple if all vertices are distinct, except that the first and the last vertices may be the same. P is a cycle if the length of P is not zero and v[1] = v[k]. A loop is a cycle of length one. A simple cycle is a path that is both a cycle and simple. An acyclic digraph is a digraph that has no cycles.

### EXPORTS

add_edge(G, E, V1, V2, Label) -> edge() | {error, Reason}
add_edge(G, V1, V2, Label) -> edge() | {error, Reason}
add_edge(G, V1, V2) -> edge() | {error, Reason}

Types:

G = digraph()
E = edge()
V1 = V2 = vertex()
Label = label()
Reason = {bad_edge, Path} | {bad_vertex, V}
Path = [vertex()]

add_edge/5 creates (or modifies) the edge E of the digraph G, using Label as the (new) label of the edge. The edge is emanating from V1 and incident on V2. Returns E.

add_edge(G, V1, V2, Label) is equivalent to add_edge(G, E, V1, V2, Label), where E is a created edge. The created edge is represented by the term ['$e' | N], where N is an integer >= 0.

add_edge(G, V1, V2) is equivalent to add_edge(G, V1, V2, []).

If the edge would create a cycle in an acyclic digraph, then {error, {bad_edge, Path}} is returned. If either of V1 or V2 is not a vertex of the digraph G, then {error, {bad_vertex, V}} is returned, V = V1 or V = V2.

add_vertex(G, V, Label) -> vertex()
add_vertex(G, V) -> vertex()
add_vertex(G) -> vertex()

Types:

G = digraph()
V = vertex()
Label = label()

add_vertex/3 creates (or modifies) the vertex V of the digraph G, using Label as the (new) label of the vertex. Returns V.

add_vertex(G, V) is equivalent to add_vertex(G, V, []).

add_vertex/1 creates a vertex using the empty list as label, and returns the created vertex. The created vertex is represented by the term ['$v' | N], where N is an integer >= 0.

del_edge(G, E) -> true

Types:

G = digraph()
E = edge()

Deletes the edge E from the digraph G.

del_edges(G, Edges) -> true

Types:

G = digraph()
Edges = [edge()]

Deletes the edges in the list Edges from the digraph G.

del_path(G, V1, V2) -> true

Types:

G = digraph()
V1 = V2 = vertex()

Deletes edges from the digraph G until there are no paths from the vertex V1 to the vertex V2.

A sketch of the procedure employed: Find an arbitrary simple path v[1], v[2], ..., v[k] from V1 to V2 in G. Remove all edges of G emanating from v[i] and incident to v[i+1] for 1 <= i < k (including multiple edges). Repeat until there is no path between V1 and V2.

del_vertex(G, V) -> true

Types:

G = digraph()
V = vertex()

Deletes the vertex V from the digraph G. Any edges emanating from V or incident on V are also deleted.

del_vertices(G, Vertices) -> true

Types:

G = digraph()
Vertices = [vertex()]

Deletes the vertices in the list Vertices from the digraph G.

delete(G) -> true

Types:

G = digraph()

Deletes the digraph G. This call is important because digraphs are implemented with Ets. There is no garbage collection of Ets tables. The digraph will, however, be deleted if the process that created the digraph terminates.

edge(G, E) -> {E, V1, V2, Label} | false

Types:

G = digraph()
E = edge()
V1 = V2 = vertex()
Label = label()

Returns {E, V1, V2, Label} where Label is the label of the edge E emanating from V1 and incident on V2 of the digraph G. If there is no edge E of the digraph G, then false is returned.

edges(G) -> Edges

Types:

G = digraph()
Edges = [edge()]

Returns a list of all edges of the digraph G, in some unspecified order.

edges(G, V) -> Edges

Types:

G = digraph()
V = vertex()
Edges = [edge()]

Returns a list of all edges emanating from or incident on V of the digraph G, in some unspecified order.

get_cycle(G, V) -> Vertices | false

Types:

G = digraph()
V1 = V2 = vertex()
Vertices = [vertex()]

If there is a simple cycle of length two or more through the vertex V, then the cycle is returned as a list [V, ..., V] of vertices, otherwise if there is a loop through V, then the loop is returned as a list [V]. If there are no cycles through V, then false is returned.

get_path/3 is used for finding a simple cycle through V.

get_path(G, V1, V2) -> Vertices | false

Types:

G = digraph()
V1 = V2 = vertex()
Vertices = [vertex()]

Tries to find a simple path from the vertex V1 to the vertex V2 of the digraph G. Returns the path as a list [V1, ..., V2] of vertices, or false if no simple path from V1 to V2 of length one or more exists.

The digraph G is traversed in a depth-first manner, and the first path found is returned.

get_short_cycle(G, V) -> Vertices | false

Types:

G = digraph()
V1 = V2 = vertex()
Vertices = [vertex()]

Tries to find an as short as possible simple cycle through the vertex V of the digraph G. Returns the cycle as a list [V, ..., V] of vertices, or false if no simple cycle through V exists. Note that a loop through V is returned as the list [V, V].

get_short_path/3 is used for finding a simple cycle through V.

get_short_path(G, V1, V2) -> Vertices | false

Types:

G = digraph()
V1 = V2 = vertex()
Vertices = [vertex()]

Tries to find an as short as possible simple path from the vertex V1 to the vertex V2 of the digraph G. Returns the path as a list [V1, ..., V2] of vertices, or false if no simple path from V1 to V2 of length one or more exists.

The digraph G is traversed in a breadth-first manner, and the first path found is returned.

in_degree(G, V) -> integer()

Types:

G= digraph()
V = vertex()

Returns the in-degree of the vertex V of the digraph G.

in_edges(G, V) -> Edges

Types:

G = digraph()
V = vertex()
Edges = [edge()]

Returns a list of all edges incident on V of the digraph G, in some unspecified order.

in_neighbours(G, V) -> Vertices

Types:

G = digraph()
V = vertex()
Vertices = [vertex()]

Returns a list of all in-neighbours of V of the digraph G, in some unspecified order.

info(G) -> InfoList

Types:

G = digraph()
InfoList = [{cyclicity, Cyclicity}, {memory, NoWords}, {protection, Protection}]
Cyclicity = cyclic | acyclic
Protection = protected | private
NoWords = integer() >= 0

Returns a list of {Tag, Value} pairs describing the digraph G. The following pairs are returned:

- {cyclicity, Cyclicity}, where Cyclicity is cyclic or acyclic, according to the options given to new.

- {memory, NoWords}, where NoWords is the number of words allocated to the ets tables.

- {protection, Protection}, where Protection is protected or private, according to the options given to new.

new() -> digraph()

Equivalent to new([]).

new(Type) -> digraph()

Types:

Type = [cyclic | acyclic | private | protected]

Returns an empty digraph with properties according to the options in Type:

cyclic
  Allow cycles in the digraph (default).
acyclic
  The digraph is to be kept acyclic.
protected
  Other processes can read the digraph (default).
private
  The digraph can be read and modified by the creating process only.

If an unrecognized type option T is given or Type is not a proper list, there will be a badarg exception.

no_edges(G) -> integer() >= 0

Types:

G = digraph()

Returns the number of edges of the digraph G.

no_vertices(G) -> integer() >= 0

Types:

G = digraph()

Returns the number of vertices of the digraph G.

out_degree(G, V) -> integer()

Types:

G = digraph()
V = vertex()

Returns the out-degree of the vertex V of the digraph G.

out_edges(G, V) -> Edges

Types:

G = digraph()
V = vertex()
Edges = [edge()]

Returns a list of all edges emanating from V of the digraph G, in some unspecified order.

out_neighbours(G, V) -> Vertices

Types:

G = digraph()
V = vertex()
Vertices = [vertex()]

Returns a list of all out-neighbours of V of the digraph G, in some unspecified order.

vertex(G, V) -> {V, Label} | false

Types:

G = digraph()
V = vertex()
Label = label()

Returns {V, Label} where Label is the label of the vertex V of the digraph G, or false if there is no vertex V of the digraph G.

vertices(G) -> Vertices

Types:

G = digraph()
Vertices = [vertex()]

Returns a list of all vertices of the digraph G, in some unspecified order.

### See Also

digraph_utils(3), ets(3)