author shellac Mon, 22 Mar 2021 18:12:50 +0000
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```
# See https://github.com/networkx/networkx/pull/1474
# Copyright 2011 Reya Group <http://www.reyagroup.com>
# Copyright 2011 Alex Levenson <alex@isnotinvain.com>
# Copyright 2011 Diederik van Liere <diederik.vanliere@rotman.utoronto.ca>
"""Functions for analyzing triads of a graph."""

from itertools import combinations, permutations
from collections import defaultdict
from random import sample

import networkx as nx
from networkx.utils import not_implemented_for

__all__ = [
"all_triplets",
]

#: The integer codes representing each type of triad.
#:
#: Triads that are the same up to symmetry have the same code.
TRICODES = (
1,
2,
2,
3,
2,
4,
6,
8,
2,
6,
5,
7,
3,
8,
7,
11,
2,
6,
4,
8,
5,
9,
9,
13,
6,
10,
9,
14,
7,
14,
12,
15,
2,
5,
6,
7,
6,
9,
10,
14,
4,
9,
9,
12,
8,
13,
14,
15,
3,
7,
8,
11,
7,
12,
14,
15,
8,
14,
13,
15,
11,
15,
15,
16,
)

#: The names of each type of triad. The order of the elements is
#: important: it corresponds to the tricodes given in :data:`TRICODES`.
"003",
"012",
"102",
"021D",
"021U",
"021C",
"111D",
"111U",
"030T",
"030C",
"201",
"120D",
"120U",
"120C",
"210",
"300",
)

TRICODE_TO_NAME = {i: TRIAD_NAMES[code - 1] for i, code in enumerate(TRICODES)}

def _tricode(G, v, u, w):
"""Returns the integer code of the given triad.

This is some fancy magic that comes from Batagelj and Mrvar's paper. It
treats each edge joining a pair of `v`, `u`, and `w` as a bit in
the binary representation of an integer.

"""
combos = ((v, u, 1), (u, v, 2), (v, w, 4), (w, v, 8), (u, w, 16), (w, u, 32))
return sum(x for u, v, x in combos if v in G[u])

@not_implemented_for("undirected")
"""Determines the triadic census of a directed graph.

The triadic census is a count of how many of the 16 possible types of
triads are present in a directed graph.

Parameters
----------
G : digraph
A NetworkX DiGraph

Returns
-------
census : dict
Dictionary with triad type as keys and number of occurrences as values.

Notes
-----
This algorithm has complexity \$O(m)\$ where \$m\$ is the number of edges in
the graph.

--------

References
----------
algorithm for large sparse networks with small maximum degree,
University of Ljubljana,

"""
# Initialize the count for each triad to be zero.
census = {name: 0 for name in TRIAD_NAMES}
n = len(G)
# m = dict(zip(G, range(n)))
m = {v: i for i, v in enumerate(G)}
for v in G:
vnbrs = set(G.pred[v]) | set(G.succ[v])
for u in vnbrs:
if m[u] <= m[v]:
continue
neighbors = (vnbrs | set(G.succ[u]) | set(G.pred[u])) - {u, v}
if v in G[u] and u in G[v]:
census["102"] += n - len(neighbors) - 2
else:
census["012"] += n - len(neighbors) - 2
for w in neighbors:
if m[u] < m[w] or (
m[v] < m[w] < m[u] and v not in G.pred[w] and v not in G.succ[w]
):
code = _tricode(G, v, u, w)
census[TRICODE_TO_NAME[code]] += 1
#
# Use integer division here, since we know this formula guarantees an
# integral value.
census["003"] = ((n * (n - 1) * (n - 2)) // 6) - sum(census.values())
return census

"""Returns True if the graph G is a triad, else False.

Parameters
----------
G : graph
A NetworkX Graph

Returns
-------
Whether G is a valid triad
"""
if isinstance(G, nx.Graph):
if G.order() == 3 and nx.is_directed(G):
if not any((n, n) in G.edges() for n in G.nodes()):
return True
return False

@not_implemented_for("undirected")
def all_triplets(G):
"""Returns a generator of all possible sets of 3 nodes in a DiGraph.

Parameters
----------
G : digraph
A NetworkX DiGraph

Returns
-------
triplets : generator of 3-tuples
Generator of tuples of 3 nodes
"""
triplets = combinations(G.nodes(), 3)
return triplets

@not_implemented_for("undirected")
"""A generator of all possible triads in G.

Parameters
----------
G : digraph
A NetworkX DiGraph

Returns
-------
"""
triplets = combinations(G.nodes(), 3)
for triplet in triplets:
yield G.subgraph(triplet).copy()

@not_implemented_for("undirected")
"""Returns a list of all triads for each triad type in a directed graph.

Parameters
----------
G : digraph
A NetworkX DiGraph

Returns
-------
tri_by_type : dict
Dictionary with triad types as keys and lists of triads as values.
"""
# num_triads = o * (o - 1) * (o - 2) // 6
tri_by_type = defaultdict(list)
return tri_by_type

@not_implemented_for("undirected")

Parameters
----------
G : digraph
A NetworkX DiGraph with 3 nodes

Returns
-------
A string identifying the triad type

Notes
-----
There can be 6 unique edges in a triad (order-3 DiGraph) (so 2^^6=64 unique
triads given 3 nodes). These 64 triads each display exactly 1 of 16
topologies of triads (topologies can be permuted). These topologies are
identified by the following notation:

{m}{a}{n}{type} (for example: 111D, 210, 102)

Here:

{m}     = number of mutual ties (takes 0, 1, 2, 3); a mutual tie is (0,1)
AND (1,0)
{a}     = number of assymmetric ties (takes 0, 1, 2, 3); an assymmetric tie
is (0,1) BUT NOT (1,0) or vice versa
{n}     = number of null ties (takes 0, 1, 2, 3); a null tie is NEITHER
(0,1) NOR (1,0)
{type}  = a letter (takes U, D, C, T) corresponding to up, down, cyclical
and transitive. This is only used for topologies that can have
more than one form (eg: 021D and 021U).

References
----------
..  Snijders, T. (2012). "Transitivity and triads." University of
Oxford.
"""
raise nx.NetworkXAlgorithmError("G is not a triad (order-3 DiGraph)")
num_edges = len(G.edges())
if num_edges == 0:
return "003"
elif num_edges == 1:
return "012"
elif num_edges == 2:
e1, e2 = G.edges()
if set(e1) == set(e2):
return "102"
elif e1 == e2:
return "021D"
elif e1 == e2:
return "021U"
elif e1 == e2 or e2 == e1:
return "021C"
elif num_edges == 3:
for (e1, e2, e3) in permutations(G.edges(), 3):
if set(e1) == set(e2):
if e3 in e1:
return "111U"
# e3 in e1:
return "111D"
elif set(e1).symmetric_difference(set(e2)) == set(e3):
if {e1, e2, e3} == {e1, e2, e3} == set(G.nodes()):
return "030C"
# e3 == (e1, e2) and e2 == (e1, e3):
return "030T"
elif num_edges == 4:
for (e1, e2, e3, e4) in permutations(G.edges(), 4):
if set(e1) == set(e2):
# identify pair of symmetric edges (which necessarily exists)
if set(e3) == set(e4):
return "201"
if {e3} == {e4} == set(e3).intersection(set(e4)):
return "120D"
if {e3} == {e4} == set(e3).intersection(set(e4)):
return "120U"
if e3 == e4:
return "120C"
elif num_edges == 5:
return "210"
elif num_edges == 6:
return "300"

@not_implemented_for("undirected")
"""Returns a random triad from a directed graph.

Parameters
----------
G : digraph
A NetworkX DiGraph

Returns
-------
G2 : subgraph
A randomly selected triad (order-3 NetworkX DiGraph)
"""
nodes = sample(G.nodes(), 3)
G2 = G.subgraph(nodes)
return G2

"""
@not_implemented_for('undirected')
'''Returns a list of order-3 subgraphs of G that are triadic closures.

Parameters
----------
G : digraph
A NetworkX DiGraph

Returns
-------
closures : list
'''
pass

@not_implemented_for('undirected')
def focal_closures(G, attr_name):
'''Returns a list of order-3 subgraphs of G that are focally closed.

Parameters
----------
G : digraph
A NetworkX DiGraph
attr_name : str
An attribute name

Returns
-------
closures : list
List of triads of G that are focally closed on attr_name
'''
pass

@not_implemented_for('undirected')
'''Returns a list of order-3 subgraphs of G that are stable.

Parameters
----------
G : digraph
A NetworkX DiGraph
crit_func : function
A function that determines if a triad (order-3 digraph) is stable

Returns
-------