404 lines
12 KiB
Python
404 lines
12 KiB
Python
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""" Fast approximation for node connectivity
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"""
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import itertools
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from operator import itemgetter
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import networkx as nx
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__all__ = [
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"local_node_connectivity",
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"node_connectivity",
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"all_pairs_node_connectivity",
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]
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INF = float("inf")
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def local_node_connectivity(G, source, target, cutoff=None):
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"""Compute node connectivity between source and target.
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Pairwise or local node connectivity between two distinct and nonadjacent
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nodes is the minimum number of nodes that must be removed (minimum
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separating cutset) to disconnect them. By Menger's theorem, this is equal
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to the number of node independent paths (paths that share no nodes other
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than source and target). Which is what we compute in this function.
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This algorithm is a fast approximation that gives an strict lower
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bound on the actual number of node independent paths between two nodes [1]_.
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It works for both directed and undirected graphs.
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Parameters
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----------
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G : NetworkX graph
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source : node
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Starting node for node connectivity
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target : node
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Ending node for node connectivity
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cutoff : integer
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Maximum node connectivity to consider. If None, the minimum degree
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of source or target is used as a cutoff. Default value None.
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Returns
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-------
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k: integer
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pairwise node connectivity
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Examples
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--------
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>>> # Platonic octahedral graph has node connectivity 4
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>>> # for each non adjacent node pair
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>>> from networkx.algorithms import approximation as approx
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>>> G = nx.octahedral_graph()
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>>> approx.local_node_connectivity(G, 0, 5)
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4
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Notes
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-----
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This algorithm [1]_ finds node independents paths between two nodes by
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computing their shortest path using BFS, marking the nodes of the path
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found as 'used' and then searching other shortest paths excluding the
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nodes marked as used until no more paths exist. It is not exact because
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a shortest path could use nodes that, if the path were longer, may belong
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to two different node independent paths. Thus it only guarantees an
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strict lower bound on node connectivity.
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Note that the authors propose a further refinement, losing accuracy and
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gaining speed, which is not implemented yet.
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See also
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--------
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all_pairs_node_connectivity
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node_connectivity
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References
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----------
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.. [1] White, Douglas R., and Mark Newman. 2001 A Fast Algorithm for
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Node-Independent Paths. Santa Fe Institute Working Paper #01-07-035
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http://eclectic.ss.uci.edu/~drwhite/working.pdf
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"""
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if target == source:
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raise nx.NetworkXError("source and target have to be different nodes.")
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# Maximum possible node independent paths
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if G.is_directed():
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possible = min(G.out_degree(source), G.in_degree(target))
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else:
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possible = min(G.degree(source), G.degree(target))
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K = 0
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if not possible:
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return K
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if cutoff is None:
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cutoff = INF
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exclude = set()
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for i in range(min(possible, cutoff)):
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try:
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path = _bidirectional_shortest_path(G, source, target, exclude)
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exclude.update(set(path))
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K += 1
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except nx.NetworkXNoPath:
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break
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return K
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def node_connectivity(G, s=None, t=None):
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r"""Returns an approximation for node connectivity for a graph or digraph G.
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Node connectivity is equal to the minimum number of nodes that
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must be removed to disconnect G or render it trivial. By Menger's theorem,
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this is equal to the number of node independent paths (paths that
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share no nodes other than source and target).
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If source and target nodes are provided, this function returns the
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local node connectivity: the minimum number of nodes that must be
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removed to break all paths from source to target in G.
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This algorithm is based on a fast approximation that gives an strict lower
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bound on the actual number of node independent paths between two nodes [1]_.
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It works for both directed and undirected graphs.
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Parameters
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----------
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G : NetworkX graph
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Undirected graph
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s : node
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Source node. Optional. Default value: None.
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t : node
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Target node. Optional. Default value: None.
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Returns
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-------
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K : integer
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Node connectivity of G, or local node connectivity if source
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and target are provided.
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Examples
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--------
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>>> # Platonic octahedral graph is 4-node-connected
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>>> from networkx.algorithms import approximation as approx
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>>> G = nx.octahedral_graph()
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>>> approx.node_connectivity(G)
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4
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Notes
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-----
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This algorithm [1]_ finds node independents paths between two nodes by
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computing their shortest path using BFS, marking the nodes of the path
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found as 'used' and then searching other shortest paths excluding the
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nodes marked as used until no more paths exist. It is not exact because
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a shortest path could use nodes that, if the path were longer, may belong
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to two different node independent paths. Thus it only guarantees an
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strict lower bound on node connectivity.
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See also
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--------
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all_pairs_node_connectivity
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local_node_connectivity
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References
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----------
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.. [1] White, Douglas R., and Mark Newman. 2001 A Fast Algorithm for
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Node-Independent Paths. Santa Fe Institute Working Paper #01-07-035
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http://eclectic.ss.uci.edu/~drwhite/working.pdf
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"""
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if (s is not None and t is None) or (s is None and t is not None):
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raise nx.NetworkXError("Both source and target must be specified.")
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# Local node connectivity
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if s is not None and t is not None:
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if s not in G:
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raise nx.NetworkXError(f"node {s} not in graph")
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if t not in G:
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raise nx.NetworkXError(f"node {t} not in graph")
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return local_node_connectivity(G, s, t)
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# Global node connectivity
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if G.is_directed():
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connected_func = nx.is_weakly_connected
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iter_func = itertools.permutations
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def neighbors(v):
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return itertools.chain(G.predecessors(v), G.successors(v))
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else:
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connected_func = nx.is_connected
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iter_func = itertools.combinations
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neighbors = G.neighbors
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if not connected_func(G):
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return 0
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# Choose a node with minimum degree
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v, minimum_degree = min(G.degree(), key=itemgetter(1))
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# Node connectivity is bounded by minimum degree
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K = minimum_degree
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# compute local node connectivity with all non-neighbors nodes
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# and store the minimum
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for w in set(G) - set(neighbors(v)) - {v}:
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K = min(K, local_node_connectivity(G, v, w, cutoff=K))
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# Same for non adjacent pairs of neighbors of v
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for x, y in iter_func(neighbors(v), 2):
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if y not in G[x] and x != y:
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K = min(K, local_node_connectivity(G, x, y, cutoff=K))
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return K
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def all_pairs_node_connectivity(G, nbunch=None, cutoff=None):
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""" Compute node connectivity between all pairs of nodes.
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Pairwise or local node connectivity between two distinct and nonadjacent
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nodes is the minimum number of nodes that must be removed (minimum
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separating cutset) to disconnect them. By Menger's theorem, this is equal
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to the number of node independent paths (paths that share no nodes other
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than source and target). Which is what we compute in this function.
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This algorithm is a fast approximation that gives an strict lower
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bound on the actual number of node independent paths between two nodes [1]_.
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It works for both directed and undirected graphs.
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Parameters
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----------
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G : NetworkX graph
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nbunch: container
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Container of nodes. If provided node connectivity will be computed
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only over pairs of nodes in nbunch.
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cutoff : integer
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Maximum node connectivity to consider. If None, the minimum degree
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of source or target is used as a cutoff in each pair of nodes.
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Default value None.
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Returns
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-------
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K : dictionary
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Dictionary, keyed by source and target, of pairwise node connectivity
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See Also
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--------
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local_node_connectivity
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node_connectivity
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References
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----------
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.. [1] White, Douglas R., and Mark Newman. 2001 A Fast Algorithm for
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Node-Independent Paths. Santa Fe Institute Working Paper #01-07-035
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http://eclectic.ss.uci.edu/~drwhite/working.pdf
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"""
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if nbunch is None:
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nbunch = G
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else:
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nbunch = set(nbunch)
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directed = G.is_directed()
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if directed:
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iter_func = itertools.permutations
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else:
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iter_func = itertools.combinations
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all_pairs = {n: {} for n in nbunch}
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for u, v in iter_func(nbunch, 2):
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k = local_node_connectivity(G, u, v, cutoff=cutoff)
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all_pairs[u][v] = k
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if not directed:
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all_pairs[v][u] = k
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return all_pairs
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def _bidirectional_shortest_path(G, source, target, exclude):
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"""Returns shortest path between source and target ignoring nodes in the
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container 'exclude'.
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Parameters
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----------
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G : NetworkX graph
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source : node
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Starting node for path
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target : node
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Ending node for path
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exclude: container
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Container for nodes to exclude from the search for shortest paths
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Returns
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-------
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path: list
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Shortest path between source and target ignoring nodes in 'exclude'
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Raises
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------
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NetworkXNoPath
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If there is no path or if nodes are adjacent and have only one path
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between them
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Notes
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-----
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This function and its helper are originally from
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networkx.algorithms.shortest_paths.unweighted and are modified to
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accept the extra parameter 'exclude', which is a container for nodes
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already used in other paths that should be ignored.
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References
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----------
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.. [1] White, Douglas R., and Mark Newman. 2001 A Fast Algorithm for
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Node-Independent Paths. Santa Fe Institute Working Paper #01-07-035
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http://eclectic.ss.uci.edu/~drwhite/working.pdf
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"""
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# call helper to do the real work
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results = _bidirectional_pred_succ(G, source, target, exclude)
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pred, succ, w = results
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# build path from pred+w+succ
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path = []
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# from source to w
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while w is not None:
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path.append(w)
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w = pred[w]
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path.reverse()
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# from w to target
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w = succ[path[-1]]
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while w is not None:
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path.append(w)
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w = succ[w]
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return path
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def _bidirectional_pred_succ(G, source, target, exclude):
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# does BFS from both source and target and meets in the middle
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# excludes nodes in the container "exclude" from the search
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if source is None or target is None:
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raise nx.NetworkXException(
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"Bidirectional shortest path called without source or target"
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)
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if target == source:
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return ({target: None}, {source: None}, source)
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# handle either directed or undirected
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if G.is_directed():
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Gpred = G.predecessors
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Gsucc = G.successors
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else:
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Gpred = G.neighbors
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Gsucc = G.neighbors
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# predecesssor and successors in search
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pred = {source: None}
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succ = {target: None}
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# initialize fringes, start with forward
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forward_fringe = [source]
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reverse_fringe = [target]
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level = 0
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while forward_fringe and reverse_fringe:
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# Make sure that we iterate one step forward and one step backwards
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# thus source and target will only trigger "found path" when they are
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# adjacent and then they can be safely included in the container 'exclude'
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level += 1
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if not level % 2 == 0:
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this_level = forward_fringe
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forward_fringe = []
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for v in this_level:
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for w in Gsucc(v):
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if w in exclude:
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continue
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if w not in pred:
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forward_fringe.append(w)
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pred[w] = v
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if w in succ:
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return pred, succ, w # found path
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else:
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this_level = reverse_fringe
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reverse_fringe = []
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for v in this_level:
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for w in Gpred(v):
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if w in exclude:
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continue
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if w not in succ:
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succ[w] = v
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reverse_fringe.append(w)
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if w in pred:
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return pred, succ, w # found path
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raise nx.NetworkXNoPath(f"No path between {source} and {target}.")
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