Warning

This documents an unmaintained version of NetworkX. Please upgrade to a maintained version and see the current NetworkX documentation.

# Source code for networkx.algorithms.swap

# -*- coding: utf-8 -*-
"""Swap edges in a graph.
"""
#    Aric Hagberg <hagberg@lanl.gov>
#    Dan Schult <dschult@colgate.edu>
#    Pieter Swart <swart@lanl.gov>
from __future__ import division

import math
import random

import networkx as nx

__author__ = "\n".join(['Aric Hagberg (hagberg@lanl.gov)',
'Pieter Swart (swart@lanl.gov)',
'Dan Schult (dschult@colgate.edu)',
'Joel Miller (joel.c.miller.research@gmail.com)',
'Ben Edwards'])

__all__ = ['double_edge_swap',
'connected_double_edge_swap']

[docs]def double_edge_swap(G, nswap=1, max_tries=100):
"""Swap two edges in the graph while keeping the node degrees fixed.

A double-edge swap removes two randomly chosen edges u-v and x-y
and creates the new edges u-x and v-y::

u--v            u  v
becomes  |  |
x--y            x  y

If either the edge u-x or v-y already exist no swap is performed
and another attempt is made to find a suitable edge pair.

Parameters
----------
G : graph
An undirected graph

nswap : integer (optional, default=1)
Number of double-edge swaps to perform

max_tries : integer (optional)
Maximum number of attempts to swap edges

Returns
-------
G : graph
The graph after double edge swaps.

Notes
-----
Does not enforce any connectivity constraints.

The graph G is modified in place.
"""
if G.is_directed():
raise nx.NetworkXError(\
"double_edge_swap() not defined for directed graphs.")
if nswap>max_tries:
raise nx.NetworkXError("Number of swaps > number of tries allowed.")
if len(G) < 4:
raise nx.NetworkXError("Graph has less than four nodes.")
# Instead of choosing uniformly at random from a generated edge list,
# this algorithm chooses nonuniformly from the set of nodes with
# probability weighted by degree.
n=0
swapcount=0
keys,degrees=zip(*G.degree().items()) # keys, degree
cdf=nx.utils.cumulative_distribution(degrees)  # cdf of degree
while swapcount < nswap:
#        if random.random() < 0.5: continue # trick to avoid periodicities?
# pick two random edges without creating edge list
# choose source node indices from discrete distribution
(ui,xi)=nx.utils.discrete_sequence(2,cdistribution=cdf)
if ui==xi:
continue # same source, skip
u=keys[ui] # convert index to label
x=keys[xi]
# choose target uniformly from neighbors
v=random.choice(list(G[u]))
y=random.choice(list(G[x]))
if v==y:
continue # same target, skip
if (x not in G[u]) and (y not in G[v]): # don't create parallel edges
G.remove_edge(u,v)
G.remove_edge(x,y)
swapcount+=1
if n >= max_tries:
e=('Maximum number of swap attempts (%s) exceeded '%n +
'before desired swaps achieved (%s).'%nswap)
raise nx.NetworkXAlgorithmError(e)
n+=1
return G

[docs]def connected_double_edge_swap(G, nswap=1, _window_threshold=3):
"""Attempts the specified number of double-edge swaps in the graph G.

A double-edge swap removes two randomly chosen edges (u, v) and (x,
y) and creates the new edges (u, x) and (v, y)::

u--v            u  v
becomes  |  |
x--y            x  y

If either (u, x) or (v, y) already exist, then no swap is performed
so the actual number of swapped edges is always *at most* nswap.

Parameters
----------
G : graph
An undirected graph

nswap : integer (optional, default=1)
Number of double-edge swaps to perform

_window_threshold : integer

The window size below which connectedness of the graph will be checked
after each swap.

The "window" in this function is a dynamically updated integer that
represents the number of swap attempts to make before checking if the
graph remains connected. It is an optimization used to decrease the
running time of the algorithm in exchange for increased complexity of
implementation.

If the window size is below this threshold, then the algorithm checks
after each swap if the graph remains connected by checking if there is a
path joining the two nodes whose edge was just removed. If the window
size is above this threshold, then the algorithm performs do all the
swaps in the window and only then check if the graph is still connected.

Returns
-------
int
The number of successful swaps

Raises
------

NetworkXError

If the input graph is not connected, or if the graph has fewer than four
nodes.

Notes
-----

The initial graph G must be connected, and the resulting graph is
connected. The graph G is modified in place.

References
----------
..  C. Gkantsidis and M. Mihail and E. Zegura,
The Markov chain simulation method for generating connected
power law random graphs, 2003.
http://citeseer.ist.psu.edu/gkantsidis03markov.html
"""
if not nx.is_connected(G):
raise nx.NetworkXError("Graph not connected")
if len(G) < 4:
raise nx.NetworkXError("Graph has less than four nodes.")
n = 0
swapcount = 0
deg = G.degree()
# Label key for nodes
dk = list(deg.keys())
cdf = nx.utils.cumulative_distribution(list(G.degree().values()))
window = 1
while n < nswap:
wcount = 0
swapped = []
# If the window is small, we just check each time whether the graph is
# connected by checking if the nodes that were just separated are still
# connected.
if window < _window_threshold:
# This Boolean keeps track of whether there was a failure or not.
fail = False
while wcount < window and n < nswap:
# Pick two random edges without creating the edge list. Choose
# source nodes from the discrete degree distribution.
(ui, xi) = nx.utils.discrete_sequence(2, cdistribution=cdf)
# If the source nodes are the same, skip this pair.
if ui == xi:
continue
# Convert an index to a node label.
u = dk[ui]
x = dk[xi]
# Choose targets uniformly from neighbors.
v = random.choice(G.neighbors(u))
y = random.choice(G.neighbors(x))
# If the target nodes are the same, skip this pair.
if v == y:
continue
if x not in G[u] and y not in G[v]:
G.remove_edge(u, v)
G.remove_edge(x, y)
swapped.append((u, v, x, y))
swapcount += 1
n += 1
# If G remains connected...
if nx.has_path(G, u, v):
wcount += 1
# Otherwise, undo the changes.
else:
G.remove_edge(u, x)
G.remove_edge(v, y)
swapcount -= 1
fail = True
# If one of the swaps failed, reduce the window size.
if fail:
window = int(math.ceil(window / 2))
else:
window += 1
# If the window is large, then there is a good chance that a bunch of
# swaps will work. It's quicker to do all those swaps first and then
# check if the graph remains connected.
else:
while wcount < window and n < nswap:
# Pick two random edges without creating the edge list. Choose
# source nodes from the discrete degree distribution.
(ui, xi) = nx.utils.discrete_sequence(2, cdistribution=cdf)
# If the source nodes are the same, skip this pair.
if ui == xi:
continue
# Convert an index to a node label.
u = dk[ui]
x = dk[xi]
# Choose targets uniformly from neighbors.
v = random.choice(G.neighbors(u))
y = random.choice(G.neighbors(x))
# If the target nodes are the same, skip this pair.
if v == y:
continue
if x not in G[u] and y not in G[v]:
G.remove_edge(u, v)
G.remove_edge(x, y)
swapped.append((u, v, x, y))
swapcount += 1
n += 1
wcount += 1
# If the graph remains connected, increase the window size.
if nx.is_connected(G):
window += 1
# Otherwise, undo the changes from the previous window and decrease
# the window size.
else:
while swapped:
(u, v, x, y) = swapped.pop()