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+"""
+*************
+VF2 Algorithm
+*************
+
+An implementation of VF2 algorithm for graph isomorphism testing.
+
+The simplest interface to use this module is to call the
+:func:`is_isomorphic <networkx.algorithms.isomorphism.is_isomorphic>`
+function.
+
+Introduction
+------------
+
+The GraphMatcher and DiGraphMatcher are responsible for matching
+graphs or directed graphs in a predetermined manner.  This
+usually means a check for an isomorphism, though other checks
+are also possible.  For example, a subgraph of one graph
+can be checked for isomorphism to a second graph.
+
+Matching is done via syntactic feasibility. It is also possible
+to check for semantic feasibility. Feasibility, then, is defined
+as the logical AND of the two functions.
+
+To include a semantic check, the (Di)GraphMatcher class should be
+subclassed, and the
+:meth:`semantic_feasibility <networkx.algorithms.isomorphism.GraphMatcher.semantic_feasibility>`
+function should be redefined.  By default, the semantic feasibility function always
+returns ``True``.  The effect of this is that semantics are not
+considered in the matching of G1 and G2.
+
+Examples
+--------
+
+Suppose G1 and G2 are isomorphic graphs. Verification is as follows:
+
+>>> from networkx.algorithms import isomorphism
+>>> G1 = nx.path_graph(4)
+>>> G2 = nx.path_graph(4)
+>>> GM = isomorphism.GraphMatcher(G1, G2)
+>>> GM.is_isomorphic()
+True
+
+GM.mapping stores the isomorphism mapping from G1 to G2.
+
+>>> GM.mapping
+{0: 0, 1: 1, 2: 2, 3: 3}
+
+
+Suppose G1 and G2 are isomorphic directed graphs.
+Verification is as follows:
+
+>>> G1 = nx.path_graph(4, create_using=nx.DiGraph)
+>>> G2 = nx.path_graph(4, create_using=nx.DiGraph)
+>>> DiGM = isomorphism.DiGraphMatcher(G1, G2)
+>>> DiGM.is_isomorphic()
+True
+
+DiGM.mapping stores the isomorphism mapping from G1 to G2.
+
+>>> DiGM.mapping
+{0: 0, 1: 1, 2: 2, 3: 3}
+
+
+
+Subgraph Isomorphism
+--------------------
+Graph theory literature can be ambiguous about the meaning of the
+above statement, and we seek to clarify it now.
+
+In the VF2 literature, a mapping ``M`` is said to be a graph-subgraph
+isomorphism iff ``M`` is an isomorphism between ``G2`` and a subgraph of ``G1``.
+Thus, to say that ``G1`` and ``G2`` are graph-subgraph isomorphic is to say
+that a subgraph of ``G1`` is isomorphic to ``G2``.
+
+Other literature uses the phrase 'subgraph isomorphic' as in '``G1`` does
+not have a subgraph isomorphic to ``G2``'.  Another use is as an in adverb
+for isomorphic.  Thus, to say that ``G1`` and ``G2`` are subgraph isomorphic
+is to say that a subgraph of ``G1`` is isomorphic to ``G2``.
+
+Finally, the term 'subgraph' can have multiple meanings. In this
+context, 'subgraph' always means a 'node-induced subgraph'. Edge-induced
+subgraph isomorphisms are not directly supported, but one should be
+able to perform the check by making use of
+:func:`line_graph <networkx.generators.line.line_graph>`. For
+subgraphs which are not induced, the term 'monomorphism' is preferred
+over 'isomorphism'.
+
+Let ``G = (N, E)`` be a graph with a set of nodes ``N`` and set of edges ``E``.
+
+If ``G' = (N', E')`` is a subgraph, then:
+    ``N'`` is a subset of ``N`` and
+    ``E'`` is a subset of ``E``.
+
+If ``G' = (N', E')`` is a node-induced subgraph, then:
+    ``N'`` is a subset of ``N`` and
+    ``E'`` is the subset of edges in ``E`` relating nodes in ``N'``.
+
+If ``G' = (N', E')`` is an edge-induced subgraph, then:
+    ``N'`` is the subset of nodes in ``N`` related by edges in ``E'`` and
+    ``E'`` is a subset of ``E``.
+
+If ``G' = (N', E')`` is a monomorphism, then:
+    ``N'`` is a subset of ``N`` and
+    ``E'`` is a subset of the set of edges in ``E`` relating nodes in ``N'``.
+
+Note that if ``G'`` is a node-induced subgraph of ``G``, then it is always a
+subgraph monomorphism of ``G``, but the opposite is not always true, as a
+monomorphism can have fewer edges.
+
+References
+----------
+[1]   Luigi P. Cordella, Pasquale Foggia, Carlo Sansone, Mario Vento,
+      "A (Sub)Graph Isomorphism Algorithm for Matching Large Graphs",
+      IEEE Transactions on Pattern Analysis and Machine Intelligence,
+      vol. 26,  no. 10,  pp. 1367-1372,  Oct.,  2004.
+      http://ieeexplore.ieee.org/iel5/34/29305/01323804.pdf
+
+[2]   L. P. Cordella, P. Foggia, C. Sansone, M. Vento, "An Improved
+      Algorithm for Matching Large Graphs", 3rd IAPR-TC15 Workshop
+      on Graph-based Representations in Pattern Recognition, Cuen,
+      pp. 149-159, 2001.
+      https://www.researchgate.net/publication/200034365_An_Improved_Algorithm_for_Matching_Large_Graphs
+
+See Also
+--------
+:meth:`semantic_feasibility <networkx.algorithms.isomorphism.GraphMatcher.semantic_feasibility>`
+:meth:`syntactic_feasibility <networkx.algorithms.isomorphism.GraphMatcher.syntactic_feasibility>`
+
+Notes
+-----
+
+The implementation handles both directed and undirected graphs as well
+as multigraphs.
+
+In general, the subgraph isomorphism problem is NP-complete whereas the
+graph isomorphism problem is most likely not NP-complete (although no
+polynomial-time algorithm is known to exist).
+
+"""
+
+# This work was originally coded by Christopher Ellison
+# as part of the Computational Mechanics Python (CMPy) project.
+# James P. Crutchfield, principal investigator.
+# Complexity Sciences Center and Physics Department, UC Davis.
+
+import sys
+
+__all__ = ["GraphMatcher", "DiGraphMatcher"]
+
+
+class GraphMatcher:
+    """Implementation of VF2 algorithm for matching undirected graphs.
+
+    Suitable for Graph and MultiGraph instances.
+    """
+
+    def __init__(self, G1, G2):
+        """Initialize GraphMatcher.
+
+        Parameters
+        ----------
+        G1,G2: NetworkX Graph or MultiGraph instances.
+           The two graphs to check for isomorphism or monomorphism.
+
+        Examples
+        --------
+        To create a GraphMatcher which checks for syntactic feasibility:
+
+        >>> from networkx.algorithms import isomorphism
+        >>> G1 = nx.path_graph(4)
+        >>> G2 = nx.path_graph(4)
+        >>> GM = isomorphism.GraphMatcher(G1, G2)
+        """
+        self.G1 = G1
+        self.G2 = G2
+        self.G1_nodes = set(G1.nodes())
+        self.G2_nodes = set(G2.nodes())
+        self.G2_node_order = {n: i for i, n in enumerate(G2)}
+
+        # Set recursion limit.
+        self.old_recursion_limit = sys.getrecursionlimit()
+        expected_max_recursion_level = len(self.G2)
+        if self.old_recursion_limit < 1.5 * expected_max_recursion_level:
+            # Give some breathing room.
+            sys.setrecursionlimit(int(1.5 * expected_max_recursion_level))
+
+        # Declare that we will be searching for a graph-graph isomorphism.
+        self.test = "graph"
+
+        # Initialize state
+        self.initialize()
+
+    def reset_recursion_limit(self):
+        """Restores the recursion limit."""
+        # TODO:
+        # Currently, we use recursion and set the recursion level higher.
+        # It would be nice to restore the level, but because the
+        # (Di)GraphMatcher classes make use of cyclic references, garbage
+        # collection will never happen when we define __del__() to
+        # restore the recursion level. The result is a memory leak.
+        # So for now, we do not automatically restore the recursion level,
+        # and instead provide a method to do this manually. Eventually,
+        # we should turn this into a non-recursive implementation.
+        sys.setrecursionlimit(self.old_recursion_limit)
+
+    def candidate_pairs_iter(self):
+        """Iterator over candidate pairs of nodes in G1 and G2."""
+
+        # All computations are done using the current state!
+
+        G1_nodes = self.G1_nodes
+        G2_nodes = self.G2_nodes
+        min_key = self.G2_node_order.__getitem__
+
+        # First we compute the inout-terminal sets.
+        T1_inout = [node for node in self.inout_1 if node not in self.core_1]
+        T2_inout = [node for node in self.inout_2 if node not in self.core_2]
+
+        # If T1_inout and T2_inout are both nonempty.
+        # P(s) = T1_inout x {min T2_inout}
+        if T1_inout and T2_inout:
+            node_2 = min(T2_inout, key=min_key)
+            for node_1 in T1_inout:
+                yield node_1, node_2
+
+        else:
+            # If T1_inout and T2_inout were both empty....
+            # P(s) = (N_1 - M_1) x {min (N_2 - M_2)}
+            # if not (T1_inout or T2_inout):  # as suggested by  [2], incorrect
+            if 1:  # as inferred from [1], correct
+                # First we determine the candidate node for G2
+                other_node = min(G2_nodes - set(self.core_2), key=min_key)
+                for node in self.G1:
+                    if node not in self.core_1:
+                        yield node, other_node
+
+        # For all other cases, we don't have any candidate pairs.
+
+    def initialize(self):
+        """Reinitializes the state of the algorithm.
+
+        This method should be redefined if using something other than GMState.
+        If only subclassing GraphMatcher, a redefinition is not necessary.
+
+        """
+
+        # core_1[n] contains the index of the node paired with n, which is m,
+        #           provided n is in the mapping.
+        # core_2[m] contains the index of the node paired with m, which is n,
+        #           provided m is in the mapping.
+        self.core_1 = {}
+        self.core_2 = {}
+
+        # See the paper for definitions of M_x and T_x^{y}
+
+        # inout_1[n]  is non-zero if n is in M_1 or in T_1^{inout}
+        # inout_2[m]  is non-zero if m is in M_2 or in T_2^{inout}
+        #
+        # The value stored is the depth of the SSR tree when the node became
+        # part of the corresponding set.
+        self.inout_1 = {}
+        self.inout_2 = {}
+        # Practically, these sets simply store the nodes in the subgraph.
+
+        self.state = GMState(self)
+
+        # Provide a convenient way to access the isomorphism mapping.
+        self.mapping = self.core_1.copy()
+
+    def is_isomorphic(self):
+        """Returns True if G1 and G2 are isomorphic graphs."""
+
+        # Let's do two very quick checks!
+        # QUESTION: Should we call faster_graph_could_be_isomorphic(G1,G2)?
+        # For now, I just copy the code.
+
+        # Check global properties
+        if self.G1.order() != self.G2.order():
+            return False
+
+        # Check local properties
+        d1 = sorted(d for n, d in self.G1.degree())
+        d2 = sorted(d for n, d in self.G2.degree())
+        if d1 != d2:
+            return False
+
+        try:
+            x = next(self.isomorphisms_iter())
+            return True
+        except StopIteration:
+            return False
+
+    def isomorphisms_iter(self):
+        """Generator over isomorphisms between G1 and G2."""
+        # Declare that we are looking for a graph-graph isomorphism.
+        self.test = "graph"
+        self.initialize()
+        yield from self.match()
+
+    def match(self):
+        """Extends the isomorphism mapping.
+
+        This function is called recursively to determine if a complete
+        isomorphism can be found between G1 and G2.  It cleans up the class
+        variables after each recursive call. If an isomorphism is found,
+        we yield the mapping.
+
+        """
+        if len(self.core_1) == len(self.G2):
+            # Save the final mapping, otherwise garbage collection deletes it.
+            self.mapping = self.core_1.copy()
+            # The mapping is complete.
+            yield self.mapping
+        else:
+            for G1_node, G2_node in self.candidate_pairs_iter():
+                if self.syntactic_feasibility(G1_node, G2_node):
+                    if self.semantic_feasibility(G1_node, G2_node):
+                        # Recursive call, adding the feasible state.
+                        newstate = self.state.__class__(self, G1_node, G2_node)
+                        yield from self.match()
+
+                        # restore data structures
+                        newstate.restore()
+
+    def semantic_feasibility(self, G1_node, G2_node):
+        """Returns True if adding (G1_node, G2_node) is semantically feasible.
+
+        The semantic feasibility function should return True if it is
+        acceptable to add the candidate pair (G1_node, G2_node) to the current
+        partial isomorphism mapping.   The logic should focus on semantic
+        information contained in the edge data or a formalized node class.
+
+        By acceptable, we mean that the subsequent mapping can still become a
+        complete isomorphism mapping.  Thus, if adding the candidate pair
+        definitely makes it so that the subsequent mapping cannot become a
+        complete isomorphism mapping, then this function must return False.
+
+        The default semantic feasibility function always returns True. The
+        effect is that semantics are not considered in the matching of G1
+        and G2.
+
+        The semantic checks might differ based on the what type of test is
+        being performed.  A keyword description of the test is stored in
+        self.test.  Here is a quick description of the currently implemented
+        tests::
+
+          test='graph'
+            Indicates that the graph matcher is looking for a graph-graph
+            isomorphism.
+
+          test='subgraph'
+            Indicates that the graph matcher is looking for a subgraph-graph
+            isomorphism such that a subgraph of G1 is isomorphic to G2.
+
+          test='mono'
+            Indicates that the graph matcher is looking for a subgraph-graph
+            monomorphism such that a subgraph of G1 is monomorphic to G2.
+
+        Any subclass which redefines semantic_feasibility() must maintain
+        the above form to keep the match() method functional. Implementations
+        should consider multigraphs.
+        """
+        return True
+
+    def subgraph_is_isomorphic(self):
+        """Returns `True` if a subgraph of ``G1`` is isomorphic to ``G2``.
+
+        Examples
+        --------
+        When creating the `GraphMatcher`, the order of the arguments is important
+
+        >>> G = nx.Graph([("A", "B"), ("B", "C"), ("A", "C")])
+        >>> H = nx.Graph([(0, 1), (1, 2), (0, 2), (1, 3), (0, 4)])
+
+        Check whether a subgraph of G is isomorphic to H:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(G, H)
+        >>> isomatcher.subgraph_is_isomorphic()
+        False
+
+        Check whether a subgraph of H is isomorphic to G:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(H, G)
+        >>> isomatcher.subgraph_is_isomorphic()
+        True
+        """
+        try:
+            x = next(self.subgraph_isomorphisms_iter())
+            return True
+        except StopIteration:
+            return False
+
+    def subgraph_is_monomorphic(self):
+        """Returns `True` if a subgraph of ``G1`` is monomorphic to ``G2``.
+
+        Examples
+        --------
+        When creating the `GraphMatcher`, the order of the arguments is important.
+
+        >>> G = nx.Graph([("A", "B"), ("B", "C")])
+        >>> H = nx.Graph([(0, 1), (1, 2), (0, 2)])
+
+        Check whether a subgraph of G is monomorphic to H:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(G, H)
+        >>> isomatcher.subgraph_is_monomorphic()
+        False
+
+        Check whether a subgraph of H is isomorphic to G:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(H, G)
+        >>> isomatcher.subgraph_is_monomorphic()
+        True
+        """
+        try:
+            x = next(self.subgraph_monomorphisms_iter())
+            return True
+        except StopIteration:
+            return False
+
+    def subgraph_isomorphisms_iter(self):
+        """Generator over isomorphisms between a subgraph of ``G1`` and ``G2``.
+
+        Examples
+        --------
+        When creating the `GraphMatcher`, the order of the arguments is important
+
+        >>> G = nx.Graph([("A", "B"), ("B", "C"), ("A", "C")])
+        >>> H = nx.Graph([(0, 1), (1, 2), (0, 2), (1, 3), (0, 4)])
+
+        Yield isomorphic mappings between ``H`` and subgraphs of ``G``:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(G, H)
+        >>> list(isomatcher.subgraph_isomorphisms_iter())
+        []
+
+        Yield isomorphic mappings  between ``G`` and subgraphs of ``H``:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(H, G)
+        >>> next(isomatcher.subgraph_isomorphisms_iter())
+        {0: 'A', 1: 'B', 2: 'C'}
+
+        """
+        # Declare that we are looking for graph-subgraph isomorphism.
+        self.test = "subgraph"
+        self.initialize()
+        yield from self.match()
+
+    def subgraph_monomorphisms_iter(self):
+        """Generator over monomorphisms between a subgraph of ``G1`` and ``G2``.
+
+        Examples
+        --------
+        When creating the `GraphMatcher`, the order of the arguments is important.
+
+        >>> G = nx.Graph([("A", "B"), ("B", "C")])
+        >>> H = nx.Graph([(0, 1), (1, 2), (0, 2)])
+
+        Yield monomorphic mappings between ``H`` and subgraphs of ``G``:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(G, H)
+        >>> list(isomatcher.subgraph_monomorphisms_iter())
+        []
+
+        Yield monomorphic mappings  between ``G`` and subgraphs of ``H``:
+
+        >>> isomatcher = nx.isomorphism.GraphMatcher(H, G)
+        >>> next(isomatcher.subgraph_monomorphisms_iter())
+        {0: 'A', 1: 'B', 2: 'C'}
+        """
+        # Declare that we are looking for graph-subgraph monomorphism.
+        self.test = "mono"
+        self.initialize()
+        yield from self.match()
+
+    def syntactic_feasibility(self, G1_node, G2_node):
+        """Returns True if adding (G1_node, G2_node) is syntactically feasible.
+
+        This function returns True if it is adding the candidate pair
+        to the current partial isomorphism/monomorphism mapping is allowable.
+        The addition is allowable if the inclusion of the candidate pair does
+        not make it impossible for an isomorphism/monomorphism to be found.
+        """
+
+        # The VF2 algorithm was designed to work with graphs having, at most,
+        # one edge connecting any two nodes.  This is not the case when
+        # dealing with an MultiGraphs.
+        #
+        # Basically, when we test the look-ahead rules R_neighbor, we will
+        # make sure that the number of edges are checked. We also add
+        # a R_self check to verify that the number of selfloops is acceptable.
+        #
+        # Users might be comparing Graph instances with MultiGraph instances.
+        # So the generic GraphMatcher class must work with MultiGraphs.
+        # Care must be taken since the value in the innermost dictionary is a
+        # singlet for Graph instances.  For MultiGraphs, the value in the
+        # innermost dictionary is a list.
+
+        ###
+        # Test at each step to get a return value as soon as possible.
+        ###
+
+        # Look ahead 0
+
+        # R_self
+
+        # The number of selfloops for G1_node must equal the number of
+        # self-loops for G2_node. Without this check, we would fail on
+        # R_neighbor at the next recursion level. But it is good to prune the
+        # search tree now.
+
+        if self.test == "mono":
+            if self.G1.number_of_edges(G1_node, G1_node) < self.G2.number_of_edges(
+                G2_node, G2_node
+            ):
+                return False
+        else:
+            if self.G1.number_of_edges(G1_node, G1_node) != self.G2.number_of_edges(
+                G2_node, G2_node
+            ):
+                return False
+
+        # R_neighbor
+
+        # For each neighbor n' of n in the partial mapping, the corresponding
+        # node m' is a neighbor of m, and vice versa. Also, the number of
+        # edges must be equal.
+        if self.test != "mono":
+            for neighbor in self.G1[G1_node]:
+                if neighbor in self.core_1:
+                    if self.core_1[neighbor] not in self.G2[G2_node]:
+                        return False
+                    elif self.G1.number_of_edges(
+                        neighbor, G1_node
+                    ) != self.G2.number_of_edges(self.core_1[neighbor], G2_node):
+                        return False
+
+        for neighbor in self.G2[G2_node]:
+            if neighbor in self.core_2:
+                if self.core_2[neighbor] not in self.G1[G1_node]:
+                    return False
+                elif self.test == "mono":
+                    if self.G1.number_of_edges(
+                        self.core_2[neighbor], G1_node
+                    ) < self.G2.number_of_edges(neighbor, G2_node):
+                        return False
+                else:
+                    if self.G1.number_of_edges(
+                        self.core_2[neighbor], G1_node
+                    ) != self.G2.number_of_edges(neighbor, G2_node):
+                        return False
+
+        if self.test != "mono":
+            # Look ahead 1
+
+            # R_terminout
+            # The number of neighbors of n in T_1^{inout} is equal to the
+            # number of neighbors of m that are in T_2^{inout}, and vice versa.
+            num1 = 0
+            for neighbor in self.G1[G1_node]:
+                if (neighbor in self.inout_1) and (neighbor not in self.core_1):
+                    num1 += 1
+            num2 = 0
+            for neighbor in self.G2[G2_node]:
+                if (neighbor in self.inout_2) and (neighbor not in self.core_2):
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+            # Look ahead 2
+
+            # R_new
+
+            # The number of neighbors of n that are neither in the core_1 nor
+            # T_1^{inout} is equal to the number of neighbors of m
+            # that are neither in core_2 nor T_2^{inout}.
+            num1 = 0
+            for neighbor in self.G1[G1_node]:
+                if neighbor not in self.inout_1:
+                    num1 += 1
+            num2 = 0
+            for neighbor in self.G2[G2_node]:
+                if neighbor not in self.inout_2:
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+        # Otherwise, this node pair is syntactically feasible!
+        return True
+
+
+class DiGraphMatcher(GraphMatcher):
+    """Implementation of VF2 algorithm for matching directed graphs.
+
+    Suitable for DiGraph and MultiDiGraph instances.
+    """
+
+    def __init__(self, G1, G2):
+        """Initialize DiGraphMatcher.
+
+        G1 and G2 should be nx.Graph or nx.MultiGraph instances.
+
+        Examples
+        --------
+        To create a GraphMatcher which checks for syntactic feasibility:
+
+        >>> from networkx.algorithms import isomorphism
+        >>> G1 = nx.DiGraph(nx.path_graph(4, create_using=nx.DiGraph()))
+        >>> G2 = nx.DiGraph(nx.path_graph(4, create_using=nx.DiGraph()))
+        >>> DiGM = isomorphism.DiGraphMatcher(G1, G2)
+        """
+        super().__init__(G1, G2)
+
+    def candidate_pairs_iter(self):
+        """Iterator over candidate pairs of nodes in G1 and G2."""
+
+        # All computations are done using the current state!
+
+        G1_nodes = self.G1_nodes
+        G2_nodes = self.G2_nodes
+        min_key = self.G2_node_order.__getitem__
+
+        # First we compute the out-terminal sets.
+        T1_out = [node for node in self.out_1 if node not in self.core_1]
+        T2_out = [node for node in self.out_2 if node not in self.core_2]
+
+        # If T1_out and T2_out are both nonempty.
+        # P(s) = T1_out x {min T2_out}
+        if T1_out and T2_out:
+            node_2 = min(T2_out, key=min_key)
+            for node_1 in T1_out:
+                yield node_1, node_2
+
+        # If T1_out and T2_out were both empty....
+        # We compute the in-terminal sets.
+
+        # elif not (T1_out or T2_out):   # as suggested by [2], incorrect
+        else:  # as suggested by [1], correct
+            T1_in = [node for node in self.in_1 if node not in self.core_1]
+            T2_in = [node for node in self.in_2 if node not in self.core_2]
+
+            # If T1_in and T2_in are both nonempty.
+            # P(s) = T1_out x {min T2_out}
+            if T1_in and T2_in:
+                node_2 = min(T2_in, key=min_key)
+                for node_1 in T1_in:
+                    yield node_1, node_2
+
+            # If all terminal sets are empty...
+            # P(s) = (N_1 - M_1) x {min (N_2 - M_2)}
+
+            # elif not (T1_in or T2_in):   # as suggested by  [2], incorrect
+            else:  # as inferred from [1], correct
+                node_2 = min(G2_nodes - set(self.core_2), key=min_key)
+                for node_1 in G1_nodes:
+                    if node_1 not in self.core_1:
+                        yield node_1, node_2
+
+        # For all other cases, we don't have any candidate pairs.
+
+    def initialize(self):
+        """Reinitializes the state of the algorithm.
+
+        This method should be redefined if using something other than DiGMState.
+        If only subclassing GraphMatcher, a redefinition is not necessary.
+        """
+
+        # core_1[n] contains the index of the node paired with n, which is m,
+        #           provided n is in the mapping.
+        # core_2[m] contains the index of the node paired with m, which is n,
+        #           provided m is in the mapping.
+        self.core_1 = {}
+        self.core_2 = {}
+
+        # See the paper for definitions of M_x and T_x^{y}
+
+        # in_1[n]  is non-zero if n is in M_1 or in T_1^{in}
+        # out_1[n] is non-zero if n is in M_1 or in T_1^{out}
+        #
+        # in_2[m]  is non-zero if m is in M_2 or in T_2^{in}
+        # out_2[m] is non-zero if m is in M_2 or in T_2^{out}
+        #
+        # The value stored is the depth of the search tree when the node became
+        # part of the corresponding set.
+        self.in_1 = {}
+        self.in_2 = {}
+        self.out_1 = {}
+        self.out_2 = {}
+
+        self.state = DiGMState(self)
+
+        # Provide a convenient way to access the isomorphism mapping.
+        self.mapping = self.core_1.copy()
+
+    def syntactic_feasibility(self, G1_node, G2_node):
+        """Returns True if adding (G1_node, G2_node) is syntactically feasible.
+
+        This function returns True if it is adding the candidate pair
+        to the current partial isomorphism/monomorphism mapping is allowable.
+        The addition is allowable if the inclusion of the candidate pair does
+        not make it impossible for an isomorphism/monomorphism to be found.
+        """
+
+        # The VF2 algorithm was designed to work with graphs having, at most,
+        # one edge connecting any two nodes.  This is not the case when
+        # dealing with an MultiGraphs.
+        #
+        # Basically, when we test the look-ahead rules R_pred and R_succ, we
+        # will make sure that the number of edges are checked.  We also add
+        # a R_self check to verify that the number of selfloops is acceptable.
+
+        # Users might be comparing DiGraph instances with MultiDiGraph
+        # instances. So the generic DiGraphMatcher class must work with
+        # MultiDiGraphs. Care must be taken since the value in the innermost
+        # dictionary is a singlet for DiGraph instances.  For MultiDiGraphs,
+        # the value in the innermost dictionary is a list.
+
+        ###
+        # Test at each step to get a return value as soon as possible.
+        ###
+
+        # Look ahead 0
+
+        # R_self
+
+        # The number of selfloops for G1_node must equal the number of
+        # self-loops for G2_node. Without this check, we would fail on R_pred
+        # at the next recursion level. This should prune the tree even further.
+        if self.test == "mono":
+            if self.G1.number_of_edges(G1_node, G1_node) < self.G2.number_of_edges(
+                G2_node, G2_node
+            ):
+                return False
+        else:
+            if self.G1.number_of_edges(G1_node, G1_node) != self.G2.number_of_edges(
+                G2_node, G2_node
+            ):
+                return False
+
+        # R_pred
+
+        # For each predecessor n' of n in the partial mapping, the
+        # corresponding node m' is a predecessor of m, and vice versa. Also,
+        # the number of edges must be equal
+        if self.test != "mono":
+            for predecessor in self.G1.pred[G1_node]:
+                if predecessor in self.core_1:
+                    if self.core_1[predecessor] not in self.G2.pred[G2_node]:
+                        return False
+                    elif self.G1.number_of_edges(
+                        predecessor, G1_node
+                    ) != self.G2.number_of_edges(self.core_1[predecessor], G2_node):
+                        return False
+
+        for predecessor in self.G2.pred[G2_node]:
+            if predecessor in self.core_2:
+                if self.core_2[predecessor] not in self.G1.pred[G1_node]:
+                    return False
+                elif self.test == "mono":
+                    if self.G1.number_of_edges(
+                        self.core_2[predecessor], G1_node
+                    ) < self.G2.number_of_edges(predecessor, G2_node):
+                        return False
+                else:
+                    if self.G1.number_of_edges(
+                        self.core_2[predecessor], G1_node
+                    ) != self.G2.number_of_edges(predecessor, G2_node):
+                        return False
+
+        # R_succ
+
+        # For each successor n' of n in the partial mapping, the corresponding
+        # node m' is a successor of m, and vice versa. Also, the number of
+        # edges must be equal.
+        if self.test != "mono":
+            for successor in self.G1[G1_node]:
+                if successor in self.core_1:
+                    if self.core_1[successor] not in self.G2[G2_node]:
+                        return False
+                    elif self.G1.number_of_edges(
+                        G1_node, successor
+                    ) != self.G2.number_of_edges(G2_node, self.core_1[successor]):
+                        return False
+
+        for successor in self.G2[G2_node]:
+            if successor in self.core_2:
+                if self.core_2[successor] not in self.G1[G1_node]:
+                    return False
+                elif self.test == "mono":
+                    if self.G1.number_of_edges(
+                        G1_node, self.core_2[successor]
+                    ) < self.G2.number_of_edges(G2_node, successor):
+                        return False
+                else:
+                    if self.G1.number_of_edges(
+                        G1_node, self.core_2[successor]
+                    ) != self.G2.number_of_edges(G2_node, successor):
+                        return False
+
+        if self.test != "mono":
+            # Look ahead 1
+
+            # R_termin
+            # The number of predecessors of n that are in T_1^{in} is equal to the
+            # number of predecessors of m that are in T_2^{in}.
+            num1 = 0
+            for predecessor in self.G1.pred[G1_node]:
+                if (predecessor in self.in_1) and (predecessor not in self.core_1):
+                    num1 += 1
+            num2 = 0
+            for predecessor in self.G2.pred[G2_node]:
+                if (predecessor in self.in_2) and (predecessor not in self.core_2):
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+            # The number of successors of n that are in T_1^{in} is equal to the
+            # number of successors of m that are in T_2^{in}.
+            num1 = 0
+            for successor in self.G1[G1_node]:
+                if (successor in self.in_1) and (successor not in self.core_1):
+                    num1 += 1
+            num2 = 0
+            for successor in self.G2[G2_node]:
+                if (successor in self.in_2) and (successor not in self.core_2):
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+            # R_termout
+
+            # The number of predecessors of n that are in T_1^{out} is equal to the
+            # number of predecessors of m that are in T_2^{out}.
+            num1 = 0
+            for predecessor in self.G1.pred[G1_node]:
+                if (predecessor in self.out_1) and (predecessor not in self.core_1):
+                    num1 += 1
+            num2 = 0
+            for predecessor in self.G2.pred[G2_node]:
+                if (predecessor in self.out_2) and (predecessor not in self.core_2):
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+            # The number of successors of n that are in T_1^{out} is equal to the
+            # number of successors of m that are in T_2^{out}.
+            num1 = 0
+            for successor in self.G1[G1_node]:
+                if (successor in self.out_1) and (successor not in self.core_1):
+                    num1 += 1
+            num2 = 0
+            for successor in self.G2[G2_node]:
+                if (successor in self.out_2) and (successor not in self.core_2):
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+            # Look ahead 2
+
+            # R_new
+
+            # The number of predecessors of n that are neither in the core_1 nor
+            # T_1^{in} nor T_1^{out} is equal to the number of predecessors of m
+            # that are neither in core_2 nor T_2^{in} nor T_2^{out}.
+            num1 = 0
+            for predecessor in self.G1.pred[G1_node]:
+                if (predecessor not in self.in_1) and (predecessor not in self.out_1):
+                    num1 += 1
+            num2 = 0
+            for predecessor in self.G2.pred[G2_node]:
+                if (predecessor not in self.in_2) and (predecessor not in self.out_2):
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+            # The number of successors of n that are neither in the core_1 nor
+            # T_1^{in} nor T_1^{out} is equal to the number of successors of m
+            # that are neither in core_2 nor T_2^{in} nor T_2^{out}.
+            num1 = 0
+            for successor in self.G1[G1_node]:
+                if (successor not in self.in_1) and (successor not in self.out_1):
+                    num1 += 1
+            num2 = 0
+            for successor in self.G2[G2_node]:
+                if (successor not in self.in_2) and (successor not in self.out_2):
+                    num2 += 1
+            if self.test == "graph":
+                if num1 != num2:
+                    return False
+            else:  # self.test == 'subgraph'
+                if not (num1 >= num2):
+                    return False
+
+        # Otherwise, this node pair is syntactically feasible!
+        return True
+
+    def subgraph_is_isomorphic(self):
+        """Returns `True` if a subgraph of ``G1`` is isomorphic to ``G2``.
+
+        Examples
+        --------
+        When creating the `DiGraphMatcher`, the order of the arguments is important
+
+        >>> G = nx.DiGraph([("A", "B"), ("B", "A"), ("B", "C"), ("C", "B")])
+        >>> H = nx.DiGraph(nx.path_graph(5))
+
+        Check whether a subgraph of G is isomorphic to H:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(G, H)
+        >>> isomatcher.subgraph_is_isomorphic()
+        False
+
+        Check whether a subgraph of H is isomorphic to G:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(H, G)
+        >>> isomatcher.subgraph_is_isomorphic()
+        True
+        """
+        return super().subgraph_is_isomorphic()
+
+    def subgraph_is_monomorphic(self):
+        """Returns `True` if a subgraph of ``G1`` is monomorphic to ``G2``.
+
+        Examples
+        --------
+        When creating the `DiGraphMatcher`, the order of the arguments is important.
+
+        >>> G = nx.DiGraph([("A", "B"), ("C", "B"), ("D", "C")])
+        >>> H = nx.DiGraph([(0, 1), (1, 2), (2, 3), (3, 2)])
+
+        Check whether a subgraph of G is monomorphic to H:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(G, H)
+        >>> isomatcher.subgraph_is_monomorphic()
+        False
+
+        Check whether a subgraph of H is isomorphic to G:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(H, G)
+        >>> isomatcher.subgraph_is_monomorphic()
+        True
+        """
+        return super().subgraph_is_monomorphic()
+
+    def subgraph_isomorphisms_iter(self):
+        """Generator over isomorphisms between a subgraph of ``G1`` and ``G2``.
+
+        Examples
+        --------
+        When creating the `DiGraphMatcher`, the order of the arguments is important
+
+        >>> G = nx.DiGraph([("B", "C"), ("C", "B"), ("C", "D"), ("D", "C")])
+        >>> H = nx.DiGraph(nx.path_graph(5))
+
+        Yield isomorphic mappings between ``H`` and subgraphs of ``G``:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(G, H)
+        >>> list(isomatcher.subgraph_isomorphisms_iter())
+        []
+
+        Yield isomorphic mappings between ``G`` and subgraphs of ``H``:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(H, G)
+        >>> next(isomatcher.subgraph_isomorphisms_iter())
+        {0: 'B', 1: 'C', 2: 'D'}
+        """
+        return super().subgraph_isomorphisms_iter()
+
+    def subgraph_monomorphisms_iter(self):
+        """Generator over monomorphisms between a subgraph of ``G1`` and ``G2``.
+
+        Examples
+        --------
+        When creating the `DiGraphMatcher`, the order of the arguments is important.
+
+        >>> G = nx.DiGraph([("A", "B"), ("C", "B"), ("D", "C")])
+        >>> H = nx.DiGraph([(0, 1), (1, 2), (2, 3), (3, 2)])
+
+        Yield monomorphic mappings between ``H`` and subgraphs of ``G``:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(G, H)
+        >>> list(isomatcher.subgraph_monomorphisms_iter())
+        []
+
+        Yield monomorphic mappings between ``G`` and subgraphs of ``H``:
+
+        >>> isomatcher = nx.isomorphism.DiGraphMatcher(H, G)
+        >>> next(isomatcher.subgraph_monomorphisms_iter())
+        {3: 'A', 2: 'B', 1: 'C', 0: 'D'}
+        """
+        return super().subgraph_monomorphisms_iter()
+
+
+class GMState:
+    """Internal representation of state for the GraphMatcher class.
+
+    This class is used internally by the GraphMatcher class.  It is used
+    only to store state specific data. There will be at most G2.order() of
+    these objects in memory at a time, due to the depth-first search
+    strategy employed by the VF2 algorithm.
+    """
+
+    def __init__(self, GM, G1_node=None, G2_node=None):
+        """Initializes GMState object.
+
+        Pass in the GraphMatcher to which this GMState belongs and the
+        new node pair that will be added to the GraphMatcher's current
+        isomorphism mapping.
+        """
+        self.GM = GM
+
+        # Initialize the last stored node pair.
+        self.G1_node = None
+        self.G2_node = None
+        self.depth = len(GM.core_1)
+
+        if G1_node is None or G2_node is None:
+            # Then we reset the class variables
+            GM.core_1 = {}
+            GM.core_2 = {}
+            GM.inout_1 = {}
+            GM.inout_2 = {}
+
+        # Watch out! G1_node == 0 should evaluate to True.
+        if G1_node is not None and G2_node is not None:
+            # Add the node pair to the isomorphism mapping.
+            GM.core_1[G1_node] = G2_node
+            GM.core_2[G2_node] = G1_node
+
+            # Store the node that was added last.
+            self.G1_node = G1_node
+            self.G2_node = G2_node
+
+            # Now we must update the other two vectors.
+            # We will add only if it is not in there already!
+            self.depth = len(GM.core_1)
+
+            # First we add the new nodes...
+            if G1_node not in GM.inout_1:
+                GM.inout_1[G1_node] = self.depth
+            if G2_node not in GM.inout_2:
+                GM.inout_2[G2_node] = self.depth
+
+            # Now we add every other node...
+
+            # Updates for T_1^{inout}
+            new_nodes = set()
+            for node in GM.core_1:
+                new_nodes.update(
+                    [neighbor for neighbor in GM.G1[node] if neighbor not in GM.core_1]
+                )
+            for node in new_nodes:
+                if node not in GM.inout_1:
+                    GM.inout_1[node] = self.depth
+
+            # Updates for T_2^{inout}
+            new_nodes = set()
+            for node in GM.core_2:
+                new_nodes.update(
+                    [neighbor for neighbor in GM.G2[node] if neighbor not in GM.core_2]
+                )
+            for node in new_nodes:
+                if node not in GM.inout_2:
+                    GM.inout_2[node] = self.depth
+
+    def restore(self):
+        """Deletes the GMState object and restores the class variables."""
+        # First we remove the node that was added from the core vectors.
+        # Watch out! G1_node == 0 should evaluate to True.
+        if self.G1_node is not None and self.G2_node is not None:
+            del self.GM.core_1[self.G1_node]
+            del self.GM.core_2[self.G2_node]
+
+        # Now we revert the other two vectors.
+        # Thus, we delete all entries which have this depth level.
+        for vector in (self.GM.inout_1, self.GM.inout_2):
+            for node in list(vector.keys()):
+                if vector[node] == self.depth:
+                    del vector[node]
+
+
+class DiGMState:
+    """Internal representation of state for the DiGraphMatcher class.
+
+    This class is used internally by the DiGraphMatcher class.  It is used
+    only to store state specific data. There will be at most G2.order() of
+    these objects in memory at a time, due to the depth-first search
+    strategy employed by the VF2 algorithm.
+
+    """
+
+    def __init__(self, GM, G1_node=None, G2_node=None):
+        """Initializes DiGMState object.
+
+        Pass in the DiGraphMatcher to which this DiGMState belongs and the
+        new node pair that will be added to the GraphMatcher's current
+        isomorphism mapping.
+        """
+        self.GM = GM
+
+        # Initialize the last stored node pair.
+        self.G1_node = None
+        self.G2_node = None
+        self.depth = len(GM.core_1)
+
+        if G1_node is None or G2_node is None:
+            # Then we reset the class variables
+            GM.core_1 = {}
+            GM.core_2 = {}
+            GM.in_1 = {}
+            GM.in_2 = {}
+            GM.out_1 = {}
+            GM.out_2 = {}
+
+        # Watch out! G1_node == 0 should evaluate to True.
+        if G1_node is not None and G2_node is not None:
+            # Add the node pair to the isomorphism mapping.
+            GM.core_1[G1_node] = G2_node
+            GM.core_2[G2_node] = G1_node
+
+            # Store the node that was added last.
+            self.G1_node = G1_node
+            self.G2_node = G2_node
+
+            # Now we must update the other four vectors.
+            # We will add only if it is not in there already!
+            self.depth = len(GM.core_1)
+
+            # First we add the new nodes...
+            for vector in (GM.in_1, GM.out_1):
+                if G1_node not in vector:
+                    vector[G1_node] = self.depth
+            for vector in (GM.in_2, GM.out_2):
+                if G2_node not in vector:
+                    vector[G2_node] = self.depth
+
+            # Now we add every other node...
+
+            # Updates for T_1^{in}
+            new_nodes = set()
+            for node in GM.core_1:
+                new_nodes.update(
+                    [
+                        predecessor
+                        for predecessor in GM.G1.predecessors(node)
+                        if predecessor not in GM.core_1
+                    ]
+                )
+            for node in new_nodes:
+                if node not in GM.in_1:
+                    GM.in_1[node] = self.depth
+
+            # Updates for T_2^{in}
+            new_nodes = set()
+            for node in GM.core_2:
+                new_nodes.update(
+                    [
+                        predecessor
+                        for predecessor in GM.G2.predecessors(node)
+                        if predecessor not in GM.core_2
+                    ]
+                )
+            for node in new_nodes:
+                if node not in GM.in_2:
+                    GM.in_2[node] = self.depth
+
+            # Updates for T_1^{out}
+            new_nodes = set()
+            for node in GM.core_1:
+                new_nodes.update(
+                    [
+                        successor
+                        for successor in GM.G1.successors(node)
+                        if successor not in GM.core_1
+                    ]
+                )
+            for node in new_nodes:
+                if node not in GM.out_1:
+                    GM.out_1[node] = self.depth
+
+            # Updates for T_2^{out}
+            new_nodes = set()
+            for node in GM.core_2:
+                new_nodes.update(
+                    [
+                        successor
+                        for successor in GM.G2.successors(node)
+                        if successor not in GM.core_2
+                    ]
+                )
+            for node in new_nodes:
+                if node not in GM.out_2:
+                    GM.out_2[node] = self.depth
+
+    def restore(self):
+        """Deletes the DiGMState object and restores the class variables."""
+
+        # First we remove the node that was added from the core vectors.
+        # Watch out! G1_node == 0 should evaluate to True.
+        if self.G1_node is not None and self.G2_node is not None:
+            del self.GM.core_1[self.G1_node]
+            del self.GM.core_2[self.G2_node]
+
+        # Now we revert the other four vectors.
+        # Thus, we delete all entries which have this depth level.
+        for vector in (self.GM.in_1, self.GM.in_2, self.GM.out_1, self.GM.out_2):
+            for node in list(vector.keys()):
+                if vector[node] == self.depth:
+                    del vector[node]