220 lines
6.3 KiB
Python
220 lines
6.3 KiB
Python
"""
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Recognition Tests
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=================
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A *forest* is an acyclic, undirected graph, and a *tree* is a connected forest.
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Depending on the subfield, there are various conventions for generalizing these
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definitions to directed graphs.
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In one convention, directed variants of forest and tree are defined in an
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identical manner, except that the direction of the edges is ignored. In effect,
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each directed edge is treated as a single undirected edge. Then, additional
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restrictions are imposed to define *branchings* and *arborescences*.
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In another convention, directed variants of forest and tree correspond to
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the previous convention's branchings and arborescences, respectively. Then two
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new terms, *polyforest* and *polytree*, are defined to correspond to the other
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convention's forest and tree.
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Summarizing::
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+-----------------------------+
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| Convention A | Convention B |
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+=============================+
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| forest | polyforest |
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| tree | polytree |
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| branching | forest |
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| arborescence | tree |
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+-----------------------------+
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Each convention has its reasons. The first convention emphasizes definitional
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similarity in that directed forests and trees are only concerned with
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acyclicity and do not have an in-degree constraint, just as their undirected
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counterparts do not. The second convention emphasizes functional similarity
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in the sense that the directed analog of a spanning tree is a spanning
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arborescence. That is, take any spanning tree and choose one node as the root.
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Then every edge is assigned a direction such there is a directed path from the
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root to every other node. The result is a spanning arborescence.
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NetworkX follows convention "A". Explicitly, these are:
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undirected forest
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An undirected graph with no undirected cycles.
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undirected tree
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A connected, undirected forest.
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directed forest
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A directed graph with no undirected cycles. Equivalently, the underlying
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graph structure (which ignores edge orientations) is an undirected forest.
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In convention B, this is known as a polyforest.
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directed tree
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A weakly connected, directed forest. Equivalently, the underlying graph
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structure (which ignores edge orientations) is an undirected tree. In
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convention B, this is known as a polytree.
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branching
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A directed forest with each node having, at most, one parent. So the maximum
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in-degree is equal to 1. In convention B, this is known as a forest.
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arborescence
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A directed tree with each node having, at most, one parent. So the maximum
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in-degree is equal to 1. In convention B, this is known as a tree.
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For trees and arborescences, the adjective "spanning" may be added to designate
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that the graph, when considered as a forest/branching, consists of a single
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tree/arborescence that includes all nodes in the graph. It is true, by
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definition, that every tree/arborescence is spanning with respect to the nodes
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that define the tree/arborescence and so, it might seem redundant to introduce
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the notion of "spanning". However, the nodes may represent a subset of
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nodes from a larger graph, and it is in this context that the term "spanning"
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becomes a useful notion.
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"""
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import networkx as nx
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__all__ = ["is_arborescence", "is_branching", "is_forest", "is_tree"]
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@nx.utils.not_implemented_for("undirected")
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def is_arborescence(G):
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"""
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Returns True if `G` is an arborescence.
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An arborescence is a directed tree with maximum in-degree equal to 1.
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Parameters
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----------
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G : graph
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The graph to test.
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Returns
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-------
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b : bool
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A boolean that is True if `G` is an arborescence.
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Notes
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-----
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In another convention, an arborescence is known as a *tree*.
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See Also
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--------
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is_tree
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"""
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return is_tree(G) and max(d for n, d in G.in_degree()) <= 1
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@nx.utils.not_implemented_for("undirected")
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def is_branching(G):
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"""
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Returns True if `G` is a branching.
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A branching is a directed forest with maximum in-degree equal to 1.
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Parameters
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----------
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G : directed graph
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The directed graph to test.
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Returns
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-------
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b : bool
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A boolean that is True if `G` is a branching.
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Notes
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-----
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In another convention, a branching is also known as a *forest*.
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See Also
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--------
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is_forest
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"""
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return is_forest(G) and max(d for n, d in G.in_degree()) <= 1
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def is_forest(G):
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"""
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Returns True if `G` is a forest.
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A forest is a graph with no undirected cycles.
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For directed graphs, `G` is a forest if the underlying graph is a forest.
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The underlying graph is obtained by treating each directed edge as a single
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undirected edge in a multigraph.
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Parameters
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----------
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G : graph
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The graph to test.
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Returns
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-------
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b : bool
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A boolean that is True if `G` is a forest.
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Notes
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-----
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In another convention, a directed forest is known as a *polyforest* and
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then *forest* corresponds to a *branching*.
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See Also
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--------
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is_branching
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"""
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if len(G) == 0:
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raise nx.exception.NetworkXPointlessConcept("G has no nodes.")
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if G.is_directed():
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components = (G.subgraph(c) for c in nx.weakly_connected_components(G))
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else:
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components = (G.subgraph(c) for c in nx.connected_components(G))
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return all(len(c) - 1 == c.number_of_edges() for c in components)
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def is_tree(G):
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"""
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Returns True if `G` is a tree.
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A tree is a connected graph with no undirected cycles.
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For directed graphs, `G` is a tree if the underlying graph is a tree. The
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underlying graph is obtained by treating each directed edge as a single
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undirected edge in a multigraph.
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Parameters
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----------
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G : graph
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The graph to test.
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Returns
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-------
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b : bool
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A boolean that is True if `G` is a tree.
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Notes
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-----
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In another convention, a directed tree is known as a *polytree* and then
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*tree* corresponds to an *arborescence*.
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See Also
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--------
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is_arborescence
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"""
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if len(G) == 0:
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raise nx.exception.NetworkXPointlessConcept("G has no nodes.")
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if G.is_directed():
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is_connected = nx.is_weakly_connected
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else:
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is_connected = nx.is_connected
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# A connected graph with no cycles has n-1 edges.
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return len(G) - 1 == G.number_of_edges() and is_connected(G)
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