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Source code for networkx.algorithms.bipartite.matching

# matching.py - bipartite graph maximum matching algorithms
#
# Copyright 2015 Jeffrey Finkelstein <jeffrey.finkelstein@gmail.com>.
#
# This file is part of NetworkX.
#
# information.
#
# This module uses material from the Wikipedia article Hopcroft--Karp algorithm
# <https://en.wikipedia.org/wiki/Hopcroft%E2%80%93Karp_algorithm>, accessed on
# January 3, 2015, which is released under the Creative Commons
# pseudocode, which has been translated into the corresponding Python code.
#
# Portions of this module use code from David Eppstein's Python Algorithms and
# Data Structures (PADS) library, which is dedicated to the public domain (for
"""Provides functions for computing a maximum cardinality matching in a
bipartite graph.

If you don't care about the particular implementation of the maximum matching
algorithm, simply use the :func:maximum_matching. If you do care, you can
import one of the named maximum matching algorithms directly.

For example, to find a maximum matching in the complete bipartite graph with
two vertices on the left and three vertices on the right:

>>> import networkx as nx
>>> G = nx.complete_bipartite_graph(2, 3)
>>> left, right = nx.bipartite.sets(G)
>>> list(left)
[0, 1]
>>> list(right)
[2, 3, 4]
>>> nx.bipartite.maximum_matching(G)
{0: 2, 1: 3, 2: 0, 3: 1}

The dictionary returned by :func:maximum_matching includes a mapping for
vertices in both the left and right vertex sets.

"""
import collections
import itertools

from networkx.algorithms.bipartite import sets as bipartite_sets
import networkx as nx

__all__ = ['maximum_matching', 'hopcroft_karp_matching', 'eppstein_matching',
'to_vertex_cover']

INFINITY = float('inf')

[docs]def hopcroft_karp_matching(G, top_nodes=None):
"""Returns the maximum cardinality matching of the bipartite graph G.

Parameters
----------
G : NetworkX graph

Undirected bipartite graph

top_nodes : container

Container with all nodes in one bipartite node set. If not supplied
it will be computed. But if more than one solution exists an exception
will be raised.

Returns
-------
matches : dictionary

The matching is returned as a dictionary, matches, such that
matches[v] == w if node v is matched to node w. Unmatched
nodes do not occur as a key in mate.

Raises
------
AmbiguousSolution : Exception

Raised if the input bipartite graph is disconnected and no container
with all nodes in one bipartite set is provided. When determining
the nodes in each bipartite set more than one valid solution is
possible if the input graph is disconnected.

Notes
-----

This function is implemented with the Hopcroft--Karp matching algorithm
<https://en.wikipedia.org/wiki/Hopcroft%E2%80%93Karp_algorithm>_ for
bipartite graphs.

See :mod:bipartite documentation <networkx.algorithms.bipartite>
for further details on how bipartite graphs are handled in NetworkX.

--------

eppstein_matching

References
----------
.. [1] John E. Hopcroft and Richard M. Karp. "An n^{5 / 2} Algorithm for
Maximum Matchings in Bipartite Graphs" In: **SIAM Journal of Computing**
2.4 (1973), pp. 225--231. <https://doi.org/10.1137/0202019>.

"""
# First we define some auxiliary search functions.
#
# If you are a human reading these auxiliary search functions, the "global"
# variables leftmatches, rightmatches, distances, etc. are defined
# below the functions, so that they are initialized close to the initial
# invocation of the search functions.
for v in left:
if leftmatches[v] is None:
distances[v] = 0
queue.append(v)
else:
distances[v] = INFINITY
distances[None] = INFINITY
while queue:
v = queue.popleft()
if distances[v] < distances[None]:
for u in G[v]:
if distances[rightmatches[u]] is INFINITY:
distances[rightmatches[u]] = distances[v] + 1
queue.append(rightmatches[u])
return distances[None] is not INFINITY

def depth_first_search(v):
if v is not None:
for u in G[v]:
if distances[rightmatches[u]] == distances[v] + 1:
if depth_first_search(rightmatches[u]):
rightmatches[u] = v
leftmatches[v] = u
return True
distances[v] = INFINITY
return False
return True

# Initialize the "global" variables that maintain state during the search.
left, right = bipartite_sets(G, top_nodes)
leftmatches = {v: None for v in left}
rightmatches = {v: None for v in right}
distances = {}
queue = collections.deque()

# Implementation note: this counter is incremented as pairs are matched but
# it is currently not used elsewhere in the computation.
num_matched_pairs = 0
for v in left:
if leftmatches[v] is None:
if depth_first_search(v):
num_matched_pairs += 1

# Strip the entries matched to None.
leftmatches = {k: v for k, v in leftmatches.items() if v is not None}
rightmatches = {k: v for k, v in rightmatches.items() if v is not None}

# At this point, the left matches and the right matches are inverses of one
# another. In other words,
#
#     leftmatches == {v, k for k, v in rightmatches.items()}
#
# Finally, we combine both the left matches and right matches.
return dict(itertools.chain(leftmatches.items(), rightmatches.items()))

[docs]def eppstein_matching(G, top_nodes=None):
"""Returns the maximum cardinality matching of the bipartite graph G.

Parameters
----------
G : NetworkX graph

Undirected bipartite graph

top_nodes : container

Container with all nodes in one bipartite node set. If not supplied
it will be computed. But if more than one solution exists an exception
will be raised.

Returns
-------
matches : dictionary

The matching is returned as a dictionary, matching, such that
matching[v] == w if node v is matched to node w. Unmatched
nodes do not occur as a key in mate.

Raises
------
AmbiguousSolution : Exception

Raised if the input bipartite graph is disconnected and no container
with all nodes in one bipartite set is provided. When determining
the nodes in each bipartite set more than one valid solution is
possible if the input graph is disconnected.

Notes
-----

This function is implemented with David Eppstein's version of the algorithm
Hopcroft--Karp algorithm (see :func:hopcroft_karp_matching), which
originally appeared in the Python Algorithms and Data Structures library
(PADS) <http://www.ics.uci.edu/~eppstein/PADS/ABOUT-PADS.txt>_.

See :mod:bipartite documentation <networkx.algorithms.bipartite>
for further details on how bipartite graphs are handled in NetworkX.

--------

hopcroft_karp_matching

"""
# Due to its original implementation, a directed graph is needed
# so that the two sets of bipartite nodes can be distinguished
left, right = bipartite_sets(G, top_nodes)
G = nx.DiGraph(G.edges(left))
# initialize greedy matching (redundant, but faster than full search)
matching = {}
for u in G:
for v in G[u]:
if v not in matching:
matching[v] = u
break
while True:
# structure residual graph into layers
# pred[u] gives the neighbor in the previous layer for u in U
# preds[v] gives a list of neighbors in the previous layer for v in V
# unmatched gives a list of unmatched vertices in final layer of V,
# and is also used as a flag value for pred[u] when u is in the first
# layer
preds = {}
unmatched = []
pred = {u: unmatched for u in G}
for v in matching:
del pred[matching[v]]
layer = list(pred)

# repeatedly extend layering structure by another pair of layers
while layer and not unmatched:
newLayer = {}
for u in layer:
for v in G[u]:
if v not in preds:
newLayer.setdefault(v, []).append(u)
layer = []
for v in newLayer:
preds[v] = newLayer[v]
if v in matching:
layer.append(matching[v])
pred[matching[v]] = v
else:
unmatched.append(v)

# did we finish layering without finding any alternating paths?
if not unmatched:
unlayered = {}
for u in G:
# TODO Why is extra inner loop necessary?
for v in G[u]:
if v not in preds:
unlayered[v] = None
# TODO Originally, this function returned a three-tuple:
#
#     return (matching, list(pred), list(unlayered))
#
# For some reason, the documentation for this function
# indicated that the second and third elements of the returned
# three-tuple would be the vertices in the left and right vertex
# sets, respectively, that are also in the maximum independent set.
# However, what I think the author meant was that the second
# element is the list of vertices that were unmatched and the third
# element was the list of vertices that were matched. Since that
# seems to be the case, they don't really need to be returned,
# since that information can be inferred from the matching
# dictionary.

# All the matched nodes must be a key in the dictionary
for key in matching.copy():
matching[matching[key]] = key
return matching

# recursively search backward through layers to find alternating paths
# recursion returns true if found path, false otherwise
def recurse(v):
if v in preds:
L = preds.pop(v)
for u in L:
if u in pred:
pu = pred.pop(u)
if pu is unmatched or recurse(pu):
matching[v] = u
return True
return False

for v in unmatched:
recurse(v)

def _is_connected_by_alternating_path(G, v, matched_edges, unmatched_edges,
targets):
"""Returns True if and only if the vertex v is connected to one of
the target vertices by an alternating path in G.

An *alternating path* is a path in which every other edge is in the
specified maximum matching (and the remaining edges in the path are not in
the matching). An alternating path may have matched edges in the even
positions or in the odd positions, as long as the edges alternate between
'matched' and 'unmatched'.

G is an undirected bipartite NetworkX graph.

v is a vertex in G.

matched_edges is a set of edges present in a maximum matching in G.

unmatched_edges is a set of edges not present in a maximum
matching in G.

targets is a set of vertices.

"""
def _alternating_dfs(u, along_matched=True):
"""Returns True if and only if u is connected to one of the
targets by an alternating path.

u is a vertex in the graph G.

If along_matched is True, this step of the depth-first search
will continue only through edges in the given matching. Otherwise, it
will continue only through edges *not* in the given matching.

"""
if along_matched:
edges = itertools.cycle([matched_edges, unmatched_edges])
else:
edges = itertools.cycle([unmatched_edges, matched_edges])
visited = set()
stack = [(u, iter(G[u]), next(edges))]
while stack:
parent, children, valid_edges = stack[-1]
try:
child = next(children)
if child not in visited:
if ((parent, child) in valid_edges
or (child, parent) in valid_edges):
if child in targets:
return True
stack.append((child, iter(G[child]), next(edges)))
except StopIteration:
stack.pop()
return False

# Check for alternating paths starting with edges in the matching, then
# check for alternating paths starting with edges not in the
# matching.
return (_alternating_dfs(v, along_matched=True) or
_alternating_dfs(v, along_matched=False))

def _connected_by_alternating_paths(G, matching, targets):
"""Returns the set of vertices that are connected to one of the target
vertices by an alternating path in G or are themselves a target.

An *alternating path* is a path in which every other edge is in the
specified maximum matching (and the remaining edges in the path are not in
the matching). An alternating path may have matched edges in the even
positions or in the odd positions, as long as the edges alternate between
'matched' and 'unmatched'.

G is an undirected bipartite NetworkX graph.

matching is a dictionary representing a maximum matching in G, as
returned by, for example, :func:maximum_matching.

targets is a set of vertices.

"""
# Get the set of matched edges and the set of unmatched edges. Only include
# one version of each undirected edge (for example, include edge (1, 2) but
# not edge (2, 1)). Using frozensets as an intermediary step we do not
# require nodes to be orderable.
edge_sets = {frozenset((u, v)) for u, v in matching.items()}
matched_edges = {tuple(edge) for edge in edge_sets}
unmatched_edges = {(u, v) for (u, v) in G.edges()
if frozenset((u, v)) not in edge_sets}

return {v for v in G if v in targets or
_is_connected_by_alternating_path(G, v, matched_edges,
unmatched_edges, targets)}

[docs]def to_vertex_cover(G, matching, top_nodes=None):
"""Returns the minimum vertex cover corresponding to the given maximum
matching of the bipartite graph G.

Parameters
----------

G : NetworkX graph

Undirected bipartite graph

matching : dictionary

A dictionary whose keys are vertices in G and whose values are the
distinct neighbors comprising the maximum matching for G, as returned
by, for example, :func:maximum_matching. The dictionary *must*
represent the maximum matching.

top_nodes : container

Container with all nodes in one bipartite node set. If not supplied
it will be computed. But if more than one solution exists an exception
will be raised.

Returns
-------

vertex_cover : :class:set

The minimum vertex cover in G.

Raises
------
AmbiguousSolution : Exception

Raised if the input bipartite graph is disconnected and no container
with all nodes in one bipartite set is provided. When determining
the nodes in each bipartite set more than one valid solution is
possible if the input graph is disconnected.

Notes
-----

This function is implemented using the procedure guaranteed by Konig's
theorem
<https://en.wikipedia.org/wiki/K%C3%B6nig%27s_theorem_%28graph_theory%29>_,
which proves an equivalence between a maximum matching and a minimum vertex
cover in bipartite graphs.

Since a minimum vertex cover is the complement of a maximum independent set
for any graph, one can compute the maximum independent set of a bipartite
graph this way:

>>> import networkx as nx
>>> G = nx.complete_bipartite_graph(2, 3)
>>> matching = nx.bipartite.maximum_matching(G)
>>> vertex_cover = nx.bipartite.to_vertex_cover(G, matching)
>>> independent_set = set(G) - vertex_cover
>>> print(list(independent_set))
[2, 3, 4]

See :mod:bipartite documentation <networkx.algorithms.bipartite>
for further details on how bipartite graphs are handled in NetworkX.

"""
# This is a Python implementation of the algorithm described at
# <https://en.wikipedia.org/wiki/K%C3%B6nig%27s_theorem_%28graph_theory%29#Proof>.
L, R = bipartite_sets(G, top_nodes)
# Let U be the set of unmatched vertices in the left vertex set.
unmatched_vertices = set(G) - set(matching)
U = unmatched_vertices & L
# Let Z be the set of vertices that are either in U or are connected to U
# by alternating paths.
Z = _connected_by_alternating_paths(G, matching, U)
# At this point, every edge either has a right endpoint in Z or a left
# endpoint not in Z. This gives us the vertex cover.
return (L - Z) | (R & Z)

#: Returns the maximum cardinality matching in the given bipartite graph.
#:
#: This function is simply an alias for :func:hopcroft_karp_matching.
maximum_matching = hopcroft_karp_matching