Fixed database typo and removed unnecessary class identifier.

venv/Lib/site-packages/networkx/algorithms/regular.py

@@ -0,0 +1,191 @@
+"""Functions for computing and verifying regular graphs."""
+import networkx as nx
+from networkx.utils import not_implemented_for
+
+__all__ = ["is_regular", "is_k_regular", "k_factor"]
+
+
+def is_regular(G):
+    """Determines whether the graph ``G`` is a regular graph.
+
+    A regular graph is a graph where each vertex has the same degree. A
+    regular digraph is a graph where the indegree and outdegree of each
+    vertex are equal.
+
+    Parameters
+    ----------
+    G : NetworkX graph
+
+    Returns
+    -------
+    bool
+        Whether the given graph or digraph is regular.
+
+    """
+    n1 = nx.utils.arbitrary_element(G)
+    if not G.is_directed():
+        d1 = G.degree(n1)
+        return all(d1 == d for _, d in G.degree)
+    else:
+        d_in = G.in_degree(n1)
+        in_regular = all(d_in == d for _, d in G.in_degree)
+        d_out = G.out_degree(n1)
+        out_regular = all(d_out == d for _, d in G.out_degree)
+        return in_regular and out_regular
+
+
+@not_implemented_for("directed")
+def is_k_regular(G, k):
+    """Determines whether the graph ``G`` is a k-regular graph.
+
+    A k-regular graph is a graph where each vertex has degree k.
+
+    Parameters
+    ----------
+    G : NetworkX graph
+
+    Returns
+    -------
+    bool
+        Whether the given graph is k-regular.
+
+    """
+    return all(d == k for n, d in G.degree)
+
+
+@not_implemented_for("directed")
+@not_implemented_for("multigraph")
+def k_factor(G, k, matching_weight="weight"):
+    """Compute a k-factor of G
+
+    A k-factor of a graph is a spanning k-regular subgraph.
+    A spanning k-regular subgraph of G is a subgraph that contains
+    each vertex of G and a subset of the edges of G such that each
+    vertex has degree k.
+
+    Parameters
+    ----------
+    G : NetworkX graph
+      Undirected graph
+
+    weight: string, optional (default='weight')
+       Edge data key corresponding to the edge weight.
+       Used for finding the max-weighted perfect matching.
+       If key not found, uses 1 as weight.
+
+    Returns
+    -------
+    G2 : NetworkX graph
+        A k-factor of G
+
+    References
+    ----------
+    .. [1] "An algorithm for computing simple k-factors.",
+       Meijer, Henk, Yurai Núñez-Rodríguez, and David Rappaport,
+       Information processing letters, 2009.
+    """
+
+    from networkx.algorithms.matching import max_weight_matching
+    from networkx.algorithms.matching import is_perfect_matching
+
+    class LargeKGadget:
+        def __init__(self, k, degree, node, g):
+            self.original = node
+            self.g = g
+            self.k = k
+            self.degree = degree
+
+            self.outer_vertices = [(node, x) for x in range(degree)]
+            self.core_vertices = [(node, x + degree) for x in range(degree - k)]
+
+        def replace_node(self):
+            adj_view = self.g[self.original]
+            neighbors = list(adj_view.keys())
+            edge_attrs = list(adj_view.values())
+            for (outer, neighbor, edge_attrs) in zip(
+                self.outer_vertices, neighbors, edge_attrs
+            ):
+                self.g.add_edge(outer, neighbor, **edge_attrs)
+            for core in self.core_vertices:
+                for outer in self.outer_vertices:
+                    self.g.add_edge(core, outer)
+            self.g.remove_node(self.original)
+
+        def restore_node(self):
+            self.g.add_node(self.original)
+            for outer in self.outer_vertices:
+                adj_view = self.g[outer]
+                for neighbor, edge_attrs in list(adj_view.items()):
+                    if neighbor not in self.core_vertices:
+                        self.g.add_edge(self.original, neighbor, **edge_attrs)
+                        break
+            g.remove_nodes_from(self.outer_vertices)
+            g.remove_nodes_from(self.core_vertices)
+
+    class SmallKGadget:
+        def __init__(self, k, degree, node, g):
+            self.original = node
+            self.k = k
+            self.degree = degree
+            self.g = g
+
+            self.outer_vertices = [(node, x) for x in range(degree)]
+            self.inner_vertices = [(node, x + degree) for x in range(degree)]
+            self.core_vertices = [(node, x + 2 * degree) for x in range(k)]
+
+        def replace_node(self):
+            adj_view = self.g[self.original]
+            for (outer, inner, (neighbor, edge_attrs)) in zip(
+                self.outer_vertices, self.inner_vertices, list(adj_view.items())
+            ):
+                self.g.add_edge(outer, inner)
+                self.g.add_edge(outer, neighbor, **edge_attrs)
+            for core in self.core_vertices:
+                for inner in self.inner_vertices:
+                    self.g.add_edge(core, inner)
+            self.g.remove_node(self.original)
+
+        def restore_node(self):
+            self.g.add_node(self.original)
+            for outer in self.outer_vertices:
+                adj_view = self.g[outer]
+                for neighbor, edge_attrs in adj_view.items():
+                    if neighbor not in self.core_vertices:
+                        self.g.add_edge(self.original, neighbor, **edge_attrs)
+                        break
+            self.g.remove_nodes_from(self.outer_vertices)
+            self.g.remove_nodes_from(self.inner_vertices)
+            self.g.remove_nodes_from(self.core_vertices)
+
+    # Step 1
+    if any(d < k for _, d in G.degree):
+        raise nx.NetworkXUnfeasible("Graph contains a vertex with degree less than k")
+    g = G.copy()
+
+    # Step 2
+    gadgets = []
+    for node, degree in list(g.degree):
+        if k < degree / 2.0:
+            gadget = SmallKGadget(k, degree, node, g)
+        else:
+            gadget = LargeKGadget(k, degree, node, g)
+        gadget.replace_node()
+        gadgets.append(gadget)
+
+    # Step 3
+    matching = max_weight_matching(g, maxcardinality=True, weight=matching_weight)
+
+    # Step 4
+    if not is_perfect_matching(g, matching):
+        raise nx.NetworkXUnfeasible(
+            "Cannot find k-factor because no perfect matching exists"
+        )
+
+    for edge in g.edges():
+        if edge not in matching and (edge[1], edge[0]) not in matching:
+            g.remove_edge(edge[0], edge[1])
+
+    for gadget in gadgets:
+        gadget.restore_node()
+
+    return g