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authorS. Solomon Darnell2025-03-28 21:52:21 -0500
committerS. Solomon Darnell2025-03-28 21:52:21 -0500
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+# See https://github.com/networkx/networkx/pull/1474
+# Copyright 2011 Reya Group <http://www.reyagroup.com>
+# Copyright 2011 Alex Levenson <alex@isnotinvain.com>
+# Copyright 2011 Diederik van Liere <diederik.vanliere@rotman.utoronto.ca>
+"""Functions for analyzing triads of a graph."""
+
+from collections import defaultdict
+from itertools import combinations, permutations
+
+import networkx as nx
+from networkx.utils import not_implemented_for, py_random_state
+
+__all__ = [
+    "triadic_census",
+    "is_triad",
+    "all_triplets",
+    "all_triads",
+    "triads_by_type",
+    "triad_type",
+    "random_triad",
+]
+
+#: The integer codes representing each type of triad.
+#:
+#: Triads that are the same up to symmetry have the same code.
+TRICODES = (
+    1,
+    2,
+    2,
+    3,
+    2,
+    4,
+    6,
+    8,
+    2,
+    6,
+    5,
+    7,
+    3,
+    8,
+    7,
+    11,
+    2,
+    6,
+    4,
+    8,
+    5,
+    9,
+    9,
+    13,
+    6,
+    10,
+    9,
+    14,
+    7,
+    14,
+    12,
+    15,
+    2,
+    5,
+    6,
+    7,
+    6,
+    9,
+    10,
+    14,
+    4,
+    9,
+    9,
+    12,
+    8,
+    13,
+    14,
+    15,
+    3,
+    7,
+    8,
+    11,
+    7,
+    12,
+    14,
+    15,
+    8,
+    14,
+    13,
+    15,
+    11,
+    15,
+    15,
+    16,
+)
+
+#: The names of each type of triad. The order of the elements is
+#: important: it corresponds to the tricodes given in :data:`TRICODES`.
+TRIAD_NAMES = (
+    "003",
+    "012",
+    "102",
+    "021D",
+    "021U",
+    "021C",
+    "111D",
+    "111U",
+    "030T",
+    "030C",
+    "201",
+    "120D",
+    "120U",
+    "120C",
+    "210",
+    "300",
+)
+
+
+#: A dictionary mapping triad code to triad name.
+TRICODE_TO_NAME = {i: TRIAD_NAMES[code - 1] for i, code in enumerate(TRICODES)}
+
+
+def _tricode(G, v, u, w):
+    """Returns the integer code of the given triad.
+
+    This is some fancy magic that comes from Batagelj and Mrvar's paper. It
+    treats each edge joining a pair of `v`, `u`, and `w` as a bit in
+    the binary representation of an integer.
+
+    """
+    combos = ((v, u, 1), (u, v, 2), (v, w, 4), (w, v, 8), (u, w, 16), (w, u, 32))
+    return sum(x for u, v, x in combos if v in G[u])
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def triadic_census(G, nodelist=None):
+    """Determines the triadic census of a directed graph.
+
+    The triadic census is a count of how many of the 16 possible types of
+    triads are present in a directed graph. If a list of nodes is passed, then
+    only those triads are taken into account which have elements of nodelist in them.
+
+    Parameters
+    ----------
+    G : digraph
+       A NetworkX DiGraph
+    nodelist : list
+        List of nodes for which you want to calculate triadic census
+
+    Returns
+    -------
+    census : dict
+       Dictionary with triad type as keys and number of occurrences as values.
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (2, 3), (3, 1), (3, 4), (4, 1), (4, 2)])
+    >>> triadic_census = nx.triadic_census(G)
+    >>> for key, value in triadic_census.items():
+    ...     print(f"{key}: {value}")
+    003: 0
+    012: 0
+    102: 0
+    021D: 0
+    021U: 0
+    021C: 0
+    111D: 0
+    111U: 0
+    030T: 2
+    030C: 2
+    201: 0
+    120D: 0
+    120U: 0
+    120C: 0
+    210: 0
+    300: 0
+
+    Notes
+    -----
+    This algorithm has complexity $O(m)$ where $m$ is the number of edges in
+    the graph.
+
+    For undirected graphs, the triadic census can be computed by first converting
+    the graph into a directed graph using the ``G.to_directed()`` method.
+    After this conversion, only the triad types 003, 102, 201 and 300 will be
+    present in the undirected scenario.
+
+    Raises
+    ------
+    ValueError
+        If `nodelist` contains duplicate nodes or nodes not in `G`.
+        If you want to ignore this you can preprocess with `set(nodelist) & G.nodes`
+
+    See also
+    --------
+    triad_graph
+
+    References
+    ----------
+    .. [1] Vladimir Batagelj and Andrej Mrvar, A subquadratic triad census
+        algorithm for large sparse networks with small maximum degree,
+        University of Ljubljana,
+        http://vlado.fmf.uni-lj.si/pub/networks/doc/triads/triads.pdf
+
+    """
+    nodeset = set(G.nbunch_iter(nodelist))
+    if nodelist is not None and len(nodelist) != len(nodeset):
+        raise ValueError("nodelist includes duplicate nodes or nodes not in G")
+
+    N = len(G)
+    Nnot = N - len(nodeset)  # can signal special counting for subset of nodes
+
+    # create an ordering of nodes with nodeset nodes first
+    m = {n: i for i, n in enumerate(nodeset)}
+    if Nnot:
+        # add non-nodeset nodes later in the ordering
+        not_nodeset = G.nodes - nodeset
+        m.update((n, i + N) for i, n in enumerate(not_nodeset))
+
+    # build all_neighbor dicts for easy counting
+    # After Python 3.8 can leave off these keys(). Speedup also using G._pred
+    # nbrs = {n: G._pred[n].keys() | G._succ[n].keys() for n in G}
+    nbrs = {n: G.pred[n].keys() | G.succ[n].keys() for n in G}
+    dbl_nbrs = {n: G.pred[n].keys() & G.succ[n].keys() for n in G}
+
+    if Nnot:
+        sgl_nbrs = {n: G.pred[n].keys() ^ G.succ[n].keys() for n in not_nodeset}
+        # find number of edges not incident to nodes in nodeset
+        sgl = sum(1 for n in not_nodeset for nbr in sgl_nbrs[n] if nbr not in nodeset)
+        sgl_edges_outside = sgl // 2
+        dbl = sum(1 for n in not_nodeset for nbr in dbl_nbrs[n] if nbr not in nodeset)
+        dbl_edges_outside = dbl // 2
+
+    # Initialize the count for each triad to be zero.
+    census = {name: 0 for name in TRIAD_NAMES}
+    # Main loop over nodes
+    for v in nodeset:
+        vnbrs = nbrs[v]
+        dbl_vnbrs = dbl_nbrs[v]
+        if Nnot:
+            # set up counts of edges attached to v.
+            sgl_unbrs_bdy = sgl_unbrs_out = dbl_unbrs_bdy = dbl_unbrs_out = 0
+        for u in vnbrs:
+            if m[u] <= m[v]:
+                continue
+            unbrs = nbrs[u]
+            neighbors = (vnbrs | unbrs) - {u, v}
+            # Count connected triads.
+            for w in neighbors:
+                if m[u] < m[w] or (m[v] < m[w] < m[u] and v not in nbrs[w]):
+                    code = _tricode(G, v, u, w)
+                    census[TRICODE_TO_NAME[code]] += 1
+
+            # Use a formula for dyadic triads with edge incident to v
+            if u in dbl_vnbrs:
+                census["102"] += N - len(neighbors) - 2
+            else:
+                census["012"] += N - len(neighbors) - 2
+
+            # Count edges attached to v. Subtract later to get triads with v isolated
+            # _out are (u,unbr) for unbrs outside boundary of nodeset
+            # _bdy are (u,unbr) for unbrs on boundary of nodeset (get double counted)
+            if Nnot and u not in nodeset:
+                sgl_unbrs = sgl_nbrs[u]
+                sgl_unbrs_bdy += len(sgl_unbrs & vnbrs - nodeset)
+                sgl_unbrs_out += len(sgl_unbrs - vnbrs - nodeset)
+                dbl_unbrs = dbl_nbrs[u]
+                dbl_unbrs_bdy += len(dbl_unbrs & vnbrs - nodeset)
+                dbl_unbrs_out += len(dbl_unbrs - vnbrs - nodeset)
+        # if nodeset == G.nodes, skip this b/c we will find the edge later.
+        if Nnot:
+            # Count edges outside nodeset not connected with v (v isolated triads)
+            census["012"] += sgl_edges_outside - (sgl_unbrs_out + sgl_unbrs_bdy // 2)
+            census["102"] += dbl_edges_outside - (dbl_unbrs_out + dbl_unbrs_bdy // 2)
+
+    # calculate null triads: "003"
+    # null triads = total number of possible triads - all found triads
+    total_triangles = (N * (N - 1) * (N - 2)) // 6
+    triangles_without_nodeset = (Nnot * (Nnot - 1) * (Nnot - 2)) // 6
+    total_census = total_triangles - triangles_without_nodeset
+    census["003"] = total_census - sum(census.values())
+
+    return census
+
+
+@nx._dispatchable
+def is_triad(G):
+    """Returns True if the graph G is a triad, else False.
+
+    Parameters
+    ----------
+    G : graph
+       A NetworkX Graph
+
+    Returns
+    -------
+    istriad : boolean
+       Whether G is a valid triad
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (2, 3), (3, 1)])
+    >>> nx.is_triad(G)
+    True
+    >>> G.add_edge(0, 1)
+    >>> nx.is_triad(G)
+    False
+    """
+    if isinstance(G, nx.Graph):
+        if G.order() == 3 and nx.is_directed(G):
+            if not any((n, n) in G.edges() for n in G.nodes()):
+                return True
+    return False
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def all_triplets(G):
+    """Returns a generator of all possible sets of 3 nodes in a DiGraph.
+
+    .. deprecated:: 3.3
+
+       all_triplets is deprecated and will be removed in NetworkX version 3.5.
+       Use `itertools.combinations` instead::
+
+          all_triplets = itertools.combinations(G, 3)
+
+    Parameters
+    ----------
+    G : digraph
+       A NetworkX DiGraph
+
+    Returns
+    -------
+    triplets : generator of 3-tuples
+       Generator of tuples of 3 nodes
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (2, 3), (3, 4)])
+    >>> list(nx.all_triplets(G))
+    [(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]
+
+    """
+    import warnings
+
+    warnings.warn(
+        (
+            "\n\nall_triplets is deprecated and will be removed in v3.5.\n"
+            "Use `itertools.combinations(G, 3)` instead."
+        ),
+        category=DeprecationWarning,
+        stacklevel=4,
+    )
+    triplets = combinations(G.nodes(), 3)
+    return triplets
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable(returns_graph=True)
+def all_triads(G):
+    """A generator of all possible triads in G.
+
+    Parameters
+    ----------
+    G : digraph
+       A NetworkX DiGraph
+
+    Returns
+    -------
+    all_triads : generator of DiGraphs
+       Generator of triads (order-3 DiGraphs)
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (2, 3), (3, 1), (3, 4), (4, 1), (4, 2)])
+    >>> for triad in nx.all_triads(G):
+    ...     print(triad.edges)
+    [(1, 2), (2, 3), (3, 1)]
+    [(1, 2), (4, 1), (4, 2)]
+    [(3, 1), (3, 4), (4, 1)]
+    [(2, 3), (3, 4), (4, 2)]
+
+    """
+    triplets = combinations(G.nodes(), 3)
+    for triplet in triplets:
+        yield G.subgraph(triplet).copy()
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def triads_by_type(G):
+    """Returns a list of all triads for each triad type in a directed graph.
+    There are exactly 16 different types of triads possible. Suppose 1, 2, 3 are three
+    nodes, they will be classified as a particular triad type if their connections
+    are as follows:
+
+    - 003: 1, 2, 3
+    - 012: 1 -> 2, 3
+    - 102: 1 <-> 2, 3
+    - 021D: 1 <- 2 -> 3
+    - 021U: 1 -> 2 <- 3
+    - 021C: 1 -> 2 -> 3
+    - 111D: 1 <-> 2 <- 3
+    - 111U: 1 <-> 2 -> 3
+    - 030T: 1 -> 2 -> 3, 1 -> 3
+    - 030C: 1 <- 2 <- 3, 1 -> 3
+    - 201: 1 <-> 2 <-> 3
+    - 120D: 1 <- 2 -> 3, 1 <-> 3
+    - 120U: 1 -> 2 <- 3, 1 <-> 3
+    - 120C: 1 -> 2 -> 3, 1 <-> 3
+    - 210: 1 -> 2 <-> 3, 1 <-> 3
+    - 300: 1 <-> 2 <-> 3, 1 <-> 3
+
+    Refer to the :doc:`example gallery </auto_examples/graph/plot_triad_types>`
+    for visual examples of the triad types.
+
+    Parameters
+    ----------
+    G : digraph
+       A NetworkX DiGraph
+
+    Returns
+    -------
+    tri_by_type : dict
+       Dictionary with triad types as keys and lists of triads as values.
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (1, 3), (2, 3), (3, 1), (5, 6), (5, 4), (6, 7)])
+    >>> dict = nx.triads_by_type(G)
+    >>> dict["120C"][0].edges()
+    OutEdgeView([(1, 2), (1, 3), (2, 3), (3, 1)])
+    >>> dict["012"][0].edges()
+    OutEdgeView([(1, 2)])
+
+    References
+    ----------
+    .. [1] Snijders, T. (2012). "Transitivity and triads." University of
+        Oxford.
+        https://web.archive.org/web/20170830032057/http://www.stats.ox.ac.uk/~snijders/Trans_Triads_ha.pdf
+    """
+    # num_triads = o * (o - 1) * (o - 2) // 6
+    # if num_triads > TRIAD_LIMIT: print(WARNING)
+    all_tri = all_triads(G)
+    tri_by_type = defaultdict(list)
+    for triad in all_tri:
+        name = triad_type(triad)
+        tri_by_type[name].append(triad)
+    return tri_by_type
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def triad_type(G):
+    """Returns the sociological triad type for a triad.
+
+    Parameters
+    ----------
+    G : digraph
+       A NetworkX DiGraph with 3 nodes
+
+    Returns
+    -------
+    triad_type : str
+       A string identifying the triad type
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (2, 3), (3, 1)])
+    >>> nx.triad_type(G)
+    '030C'
+    >>> G.add_edge(1, 3)
+    >>> nx.triad_type(G)
+    '120C'
+
+    Notes
+    -----
+    There can be 6 unique edges in a triad (order-3 DiGraph) (so 2^^6=64 unique
+    triads given 3 nodes). These 64 triads each display exactly 1 of 16
+    topologies of triads (topologies can be permuted). These topologies are
+    identified by the following notation:
+
+    {m}{a}{n}{type} (for example: 111D, 210, 102)
+
+    Here:
+
+    {m}     = number of mutual ties (takes 0, 1, 2, 3); a mutual tie is (0,1)
+              AND (1,0)
+    {a}     = number of asymmetric ties (takes 0, 1, 2, 3); an asymmetric tie
+              is (0,1) BUT NOT (1,0) or vice versa
+    {n}     = number of null ties (takes 0, 1, 2, 3); a null tie is NEITHER
+              (0,1) NOR (1,0)
+    {type}  = a letter (takes U, D, C, T) corresponding to up, down, cyclical
+              and transitive. This is only used for topologies that can have
+              more than one form (eg: 021D and 021U).
+
+    References
+    ----------
+    .. [1] Snijders, T. (2012). "Transitivity and triads." University of
+        Oxford.
+        https://web.archive.org/web/20170830032057/http://www.stats.ox.ac.uk/~snijders/Trans_Triads_ha.pdf
+    """
+    if not is_triad(G):
+        raise nx.NetworkXAlgorithmError("G is not a triad (order-3 DiGraph)")
+    num_edges = len(G.edges())
+    if num_edges == 0:
+        return "003"
+    elif num_edges == 1:
+        return "012"
+    elif num_edges == 2:
+        e1, e2 = G.edges()
+        if set(e1) == set(e2):
+            return "102"
+        elif e1[0] == e2[0]:
+            return "021D"
+        elif e1[1] == e2[1]:
+            return "021U"
+        elif e1[1] == e2[0] or e2[1] == e1[0]:
+            return "021C"
+    elif num_edges == 3:
+        for e1, e2, e3 in permutations(G.edges(), 3):
+            if set(e1) == set(e2):
+                if e3[0] in e1:
+                    return "111U"
+                # e3[1] in e1:
+                return "111D"
+            elif set(e1).symmetric_difference(set(e2)) == set(e3):
+                if {e1[0], e2[0], e3[0]} == {e1[0], e2[0], e3[0]} == set(G.nodes()):
+                    return "030C"
+                # e3 == (e1[0], e2[1]) and e2 == (e1[1], e3[1]):
+                return "030T"
+    elif num_edges == 4:
+        for e1, e2, e3, e4 in permutations(G.edges(), 4):
+            if set(e1) == set(e2):
+                # identify pair of symmetric edges (which necessarily exists)
+                if set(e3) == set(e4):
+                    return "201"
+                if {e3[0]} == {e4[0]} == set(e3).intersection(set(e4)):
+                    return "120D"
+                if {e3[1]} == {e4[1]} == set(e3).intersection(set(e4)):
+                    return "120U"
+                if e3[1] == e4[0]:
+                    return "120C"
+    elif num_edges == 5:
+        return "210"
+    elif num_edges == 6:
+        return "300"
+
+
+@not_implemented_for("undirected")
+@py_random_state(1)
+@nx._dispatchable(preserve_all_attrs=True, returns_graph=True)
+def random_triad(G, seed=None):
+    """Returns a random triad from a directed graph.
+
+    .. deprecated:: 3.3
+
+       random_triad is deprecated and will be removed in version 3.5.
+       Use random sampling directly instead::
+
+          G.subgraph(random.sample(list(G), 3))
+
+    Parameters
+    ----------
+    G : digraph
+       A NetworkX DiGraph
+    seed : integer, random_state, or None (default)
+        Indicator of random number generation state.
+        See :ref:`Randomness<randomness>`.
+
+    Returns
+    -------
+    G2 : subgraph
+       A randomly selected triad (order-3 NetworkX DiGraph)
+
+    Raises
+    ------
+    NetworkXError
+        If the input Graph has less than 3 nodes.
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (1, 3), (2, 3), (3, 1), (5, 6), (5, 4), (6, 7)])
+    >>> triad = nx.random_triad(G, seed=1)
+    >>> triad.edges
+    OutEdgeView([(1, 2)])
+
+    """
+    import warnings
+
+    warnings.warn(
+        (
+            "\n\nrandom_triad is deprecated and will be removed in NetworkX v3.5.\n"
+            "Use random.sample instead, e.g.::\n\n"
+            "\tG.subgraph(random.sample(list(G), 3))\n"
+        ),
+        category=DeprecationWarning,
+        stacklevel=5,
+    )
+    if len(G) < 3:
+        raise nx.NetworkXError(
+            f"G needs at least 3 nodes to form a triad; (it has {len(G)} nodes)"
+        )
+    nodes = seed.sample(list(G.nodes()), 3)
+    G2 = G.subgraph(nodes)
+    return G2