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| """ | |
| Utilities for connectivity package | |
| """ | |
| import networkx as nx | |
| __all__ = ["build_auxiliary_node_connectivity", "build_auxiliary_edge_connectivity"] | |
| def build_auxiliary_node_connectivity(G): | |
| r"""Creates a directed graph D from an undirected graph G to compute flow | |
| based node connectivity. | |
| For an undirected graph G having `n` nodes and `m` edges we derive a | |
| directed graph D with `2n` nodes and `2m+n` arcs by replacing each | |
| original node `v` with two nodes `vA`, `vB` linked by an (internal) | |
| arc in D. Then for each edge (`u`, `v`) in G we add two arcs (`uB`, `vA`) | |
| and (`vB`, `uA`) in D. Finally we set the attribute capacity = 1 for each | |
| arc in D [1]_. | |
| For a directed graph having `n` nodes and `m` arcs we derive a | |
| directed graph D with `2n` nodes and `m+n` arcs by replacing each | |
| original node `v` with two nodes `vA`, `vB` linked by an (internal) | |
| arc (`vA`, `vB`) in D. Then for each arc (`u`, `v`) in G we add one | |
| arc (`uB`, `vA`) in D. Finally we set the attribute capacity = 1 for | |
| each arc in D. | |
| A dictionary with a mapping between nodes in the original graph and the | |
| auxiliary digraph is stored as a graph attribute: D.graph['mapping']. | |
| References | |
| ---------- | |
| .. [1] Kammer, Frank and Hanjo Taubig. Graph Connectivity. in Brandes and | |
| Erlebach, 'Network Analysis: Methodological Foundations', Lecture | |
| Notes in Computer Science, Volume 3418, Springer-Verlag, 2005. | |
| https://doi.org/10.1007/978-3-540-31955-9_7 | |
| """ | |
| directed = G.is_directed() | |
| mapping = {} | |
| H = nx.DiGraph() | |
| for i, node in enumerate(G): | |
| mapping[node] = i | |
| H.add_node(f"{i}A", id=node) | |
| H.add_node(f"{i}B", id=node) | |
| H.add_edge(f"{i}A", f"{i}B", capacity=1) | |
| edges = [] | |
| for source, target in G.edges(): | |
| edges.append((f"{mapping[source]}B", f"{mapping[target]}A")) | |
| if not directed: | |
| edges.append((f"{mapping[target]}B", f"{mapping[source]}A")) | |
| H.add_edges_from(edges, capacity=1) | |
| # Store mapping as graph attribute | |
| H.graph["mapping"] = mapping | |
| return H | |
| def build_auxiliary_edge_connectivity(G): | |
| """Auxiliary digraph for computing flow based edge connectivity | |
| If the input graph is undirected, we replace each edge (`u`,`v`) with | |
| two reciprocal arcs (`u`, `v`) and (`v`, `u`) and then we set the attribute | |
| 'capacity' for each arc to 1. If the input graph is directed we simply | |
| add the 'capacity' attribute. Part of algorithm 1 in [1]_ . | |
| References | |
| ---------- | |
| .. [1] Abdol-Hossein Esfahanian. Connectivity Algorithms. (this is a | |
| chapter, look for the reference of the book). | |
| http://www.cse.msu.edu/~cse835/Papers/Graph_connectivity_revised.pdf | |
| """ | |
| if G.is_directed(): | |
| H = nx.DiGraph() | |
| H.add_nodes_from(G.nodes()) | |
| H.add_edges_from(G.edges(), capacity=1) | |
| return H | |
| else: | |
| H = nx.DiGraph() | |
| H.add_nodes_from(G.nodes()) | |
| for source, target in G.edges(): | |
| H.add_edges_from([(source, target), (target, source)], capacity=1) | |
| return H | |