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+"""Algorithms for directed acyclic graphs (DAGs).
+
+Note that most of these functions are only guaranteed to work for DAGs.
+In general, these functions do not check for acyclic-ness, so it is up
+to the user to check for that.
+"""
+
+import heapq
+from collections import deque
+from functools import partial
+from itertools import chain, combinations, product, starmap
+from math import gcd
+
+import networkx as nx
+from networkx.utils import arbitrary_element, not_implemented_for, pairwise
+
+__all__ = [
+    "descendants",
+    "ancestors",
+    "topological_sort",
+    "lexicographical_topological_sort",
+    "all_topological_sorts",
+    "topological_generations",
+    "is_directed_acyclic_graph",
+    "is_aperiodic",
+    "transitive_closure",
+    "transitive_closure_dag",
+    "transitive_reduction",
+    "antichains",
+    "dag_longest_path",
+    "dag_longest_path_length",
+    "dag_to_branching",
+    "compute_v_structures",
+]
+
+chaini = chain.from_iterable
+
+
+@nx._dispatchable
+def descendants(G, source):
+    """Returns all nodes reachable from `source` in `G`.
+
+    Parameters
+    ----------
+    G : NetworkX Graph
+    source : node in `G`
+
+    Returns
+    -------
+    set()
+        The descendants of `source` in `G`
+
+    Raises
+    ------
+    NetworkXError
+        If node `source` is not in `G`.
+
+    Examples
+    --------
+    >>> DG = nx.path_graph(5, create_using=nx.DiGraph)
+    >>> sorted(nx.descendants(DG, 2))
+    [3, 4]
+
+    The `source` node is not a descendant of itself, but can be included manually:
+
+    >>> sorted(nx.descendants(DG, 2) | {2})
+    [2, 3, 4]
+
+    See also
+    --------
+    ancestors
+    """
+    return {child for parent, child in nx.bfs_edges(G, source)}
+
+
+@nx._dispatchable
+def ancestors(G, source):
+    """Returns all nodes having a path to `source` in `G`.
+
+    Parameters
+    ----------
+    G : NetworkX Graph
+    source : node in `G`
+
+    Returns
+    -------
+    set()
+        The ancestors of `source` in `G`
+
+    Raises
+    ------
+    NetworkXError
+        If node `source` is not in `G`.
+
+    Examples
+    --------
+    >>> DG = nx.path_graph(5, create_using=nx.DiGraph)
+    >>> sorted(nx.ancestors(DG, 2))
+    [0, 1]
+
+    The `source` node is not an ancestor of itself, but can be included manually:
+
+    >>> sorted(nx.ancestors(DG, 2) | {2})
+    [0, 1, 2]
+
+    See also
+    --------
+    descendants
+    """
+    return {child for parent, child in nx.bfs_edges(G, source, reverse=True)}
+
+
+@nx._dispatchable
+def has_cycle(G):
+    """Decides whether the directed graph has a cycle."""
+    try:
+        # Feed the entire iterator into a zero-length deque.
+        deque(topological_sort(G), maxlen=0)
+    except nx.NetworkXUnfeasible:
+        return True
+    else:
+        return False
+
+
+@nx._dispatchable
+def is_directed_acyclic_graph(G):
+    """Returns True if the graph `G` is a directed acyclic graph (DAG) or
+    False if not.
+
+    Parameters
+    ----------
+    G : NetworkX graph
+
+    Returns
+    -------
+    bool
+        True if `G` is a DAG, False otherwise
+
+    Examples
+    --------
+    Undirected graph::
+
+        >>> G = nx.Graph([(1, 2), (2, 3)])
+        >>> nx.is_directed_acyclic_graph(G)
+        False
+
+    Directed graph with cycle::
+
+        >>> G = nx.DiGraph([(1, 2), (2, 3), (3, 1)])
+        >>> nx.is_directed_acyclic_graph(G)
+        False
+
+    Directed acyclic graph::
+
+        >>> G = nx.DiGraph([(1, 2), (2, 3)])
+        >>> nx.is_directed_acyclic_graph(G)
+        True
+
+    See also
+    --------
+    topological_sort
+    """
+    return G.is_directed() and not has_cycle(G)
+
+
+@nx._dispatchable
+def topological_generations(G):
+    """Stratifies a DAG into generations.
+
+    A topological generation is node collection in which ancestors of a node in each
+    generation are guaranteed to be in a previous generation, and any descendants of
+    a node are guaranteed to be in a following generation. Nodes are guaranteed to
+    be in the earliest possible generation that they can belong to.
+
+    Parameters
+    ----------
+    G : NetworkX digraph
+        A directed acyclic graph (DAG)
+
+    Yields
+    ------
+    sets of nodes
+        Yields sets of nodes representing each generation.
+
+    Raises
+    ------
+    NetworkXError
+        Generations are defined for directed graphs only. If the graph
+        `G` is undirected, a :exc:`NetworkXError` is raised.
+
+    NetworkXUnfeasible
+        If `G` is not a directed acyclic graph (DAG) no topological generations
+        exist and a :exc:`NetworkXUnfeasible` exception is raised.  This can also
+        be raised if `G` is changed while the returned iterator is being processed
+
+    RuntimeError
+        If `G` is changed while the returned iterator is being processed.
+
+    Examples
+    --------
+    >>> DG = nx.DiGraph([(2, 1), (3, 1)])
+    >>> [sorted(generation) for generation in nx.topological_generations(DG)]
+    [[2, 3], [1]]
+
+    Notes
+    -----
+    The generation in which a node resides can also be determined by taking the
+    max-path-distance from the node to the farthest leaf node. That value can
+    be obtained with this function using `enumerate(topological_generations(G))`.
+
+    See also
+    --------
+    topological_sort
+    """
+    if not G.is_directed():
+        raise nx.NetworkXError("Topological sort not defined on undirected graphs.")
+
+    multigraph = G.is_multigraph()
+    indegree_map = {v: d for v, d in G.in_degree() if d > 0}
+    zero_indegree = [v for v, d in G.in_degree() if d == 0]
+
+    while zero_indegree:
+        this_generation = zero_indegree
+        zero_indegree = []
+        for node in this_generation:
+            if node not in G:
+                raise RuntimeError("Graph changed during iteration")
+            for child in G.neighbors(node):
+                try:
+                    indegree_map[child] -= len(G[node][child]) if multigraph else 1
+                except KeyError as err:
+                    raise RuntimeError("Graph changed during iteration") from err
+                if indegree_map[child] == 0:
+                    zero_indegree.append(child)
+                    del indegree_map[child]
+        yield this_generation
+
+    if indegree_map:
+        raise nx.NetworkXUnfeasible(
+            "Graph contains a cycle or graph changed during iteration"
+        )
+
+
+@nx._dispatchable
+def topological_sort(G):
+    """Returns a generator of nodes in topologically sorted order.
+
+    A topological sort is a nonunique permutation of the nodes of a
+    directed graph such that an edge from u to v implies that u
+    appears before v in the topological sort order. This ordering is
+    valid only if the graph has no directed cycles.
+
+    Parameters
+    ----------
+    G : NetworkX digraph
+        A directed acyclic graph (DAG)
+
+    Yields
+    ------
+    nodes
+        Yields the nodes in topological sorted order.
+
+    Raises
+    ------
+    NetworkXError
+        Topological sort is defined for directed graphs only. If the graph `G`
+        is undirected, a :exc:`NetworkXError` is raised.
+
+    NetworkXUnfeasible
+        If `G` is not a directed acyclic graph (DAG) no topological sort exists
+        and a :exc:`NetworkXUnfeasible` exception is raised.  This can also be
+        raised if `G` is changed while the returned iterator is being processed
+
+    RuntimeError
+        If `G` is changed while the returned iterator is being processed.
+
+    Examples
+    --------
+    To get the reverse order of the topological sort:
+
+    >>> DG = nx.DiGraph([(1, 2), (2, 3)])
+    >>> list(reversed(list(nx.topological_sort(DG))))
+    [3, 2, 1]
+
+    If your DiGraph naturally has the edges representing tasks/inputs
+    and nodes representing people/processes that initiate tasks, then
+    topological_sort is not quite what you need. You will have to change
+    the tasks to nodes with dependence reflected by edges. The result is
+    a kind of topological sort of the edges. This can be done
+    with :func:`networkx.line_graph` as follows:
+
+    >>> list(nx.topological_sort(nx.line_graph(DG)))
+    [(1, 2), (2, 3)]
+
+    Notes
+    -----
+    This algorithm is based on a description and proof in
+    "Introduction to Algorithms: A Creative Approach" [1]_ .
+
+    See also
+    --------
+    is_directed_acyclic_graph, lexicographical_topological_sort
+
+    References
+    ----------
+    .. [1] Manber, U. (1989).
+       *Introduction to Algorithms - A Creative Approach.* Addison-Wesley.
+    """
+    for generation in nx.topological_generations(G):
+        yield from generation
+
+
+@nx._dispatchable
+def lexicographical_topological_sort(G, key=None):
+    """Generate the nodes in the unique lexicographical topological sort order.
+
+    Generates a unique ordering of nodes by first sorting topologically (for which there are often
+    multiple valid orderings) and then additionally by sorting lexicographically.
+
+    A topological sort arranges the nodes of a directed graph so that the
+    upstream node of each directed edge precedes the downstream node.
+    It is always possible to find a solution for directed graphs that have no cycles.
+    There may be more than one valid solution.
+
+    Lexicographical sorting is just sorting alphabetically. It is used here to break ties in the
+    topological sort and to determine a single, unique ordering.  This can be useful in comparing
+    sort results.
+
+    The lexicographical order can be customized by providing a function to the `key=` parameter.
+    The definition of the key function is the same as used in python's built-in `sort()`.
+    The function takes a single argument and returns a key to use for sorting purposes.
+
+    Lexicographical sorting can fail if the node names are un-sortable. See the example below.
+    The solution is to provide a function to the `key=` argument that returns sortable keys.
+
+
+    Parameters
+    ----------
+    G : NetworkX digraph
+        A directed acyclic graph (DAG)
+
+    key : function, optional
+        A function of one argument that converts a node name to a comparison key.
+        It defines and resolves ambiguities in the sort order.  Defaults to the identity function.
+
+    Yields
+    ------
+    nodes
+        Yields the nodes of G in lexicographical topological sort order.
+
+    Raises
+    ------
+    NetworkXError
+        Topological sort is defined for directed graphs only. If the graph `G`
+        is undirected, a :exc:`NetworkXError` is raised.
+
+    NetworkXUnfeasible
+        If `G` is not a directed acyclic graph (DAG) no topological sort exists
+        and a :exc:`NetworkXUnfeasible` exception is raised.  This can also be
+        raised if `G` is changed while the returned iterator is being processed
+
+    RuntimeError
+        If `G` is changed while the returned iterator is being processed.
+
+    TypeError
+        Results from un-sortable node names.
+        Consider using `key=` parameter to resolve ambiguities in the sort order.
+
+    Examples
+    --------
+    >>> DG = nx.DiGraph([(2, 1), (2, 5), (1, 3), (1, 4), (5, 4)])
+    >>> list(nx.lexicographical_topological_sort(DG))
+    [2, 1, 3, 5, 4]
+    >>> list(nx.lexicographical_topological_sort(DG, key=lambda x: -x))
+    [2, 5, 1, 4, 3]
+
+    The sort will fail for any graph with integer and string nodes. Comparison of integer to strings
+    is not defined in python.  Is 3 greater or less than 'red'?
+
+    >>> DG = nx.DiGraph([(1, "red"), (3, "red"), (1, "green"), (2, "blue")])
+    >>> list(nx.lexicographical_topological_sort(DG))
+    Traceback (most recent call last):
+    ...
+    TypeError: '<' not supported between instances of 'str' and 'int'
+    ...
+
+    Incomparable nodes can be resolved using a `key` function. This example function
+    allows comparison of integers and strings by returning a tuple where the first
+    element is True for `str`, False otherwise. The second element is the node name.
+    This groups the strings and integers separately so they can be compared only among themselves.
+
+    >>> key = lambda node: (isinstance(node, str), node)
+    >>> list(nx.lexicographical_topological_sort(DG, key=key))
+    [1, 2, 3, 'blue', 'green', 'red']
+
+    Notes
+    -----
+    This algorithm is based on a description and proof in
+    "Introduction to Algorithms: A Creative Approach" [1]_ .
+
+    See also
+    --------
+    topological_sort
+
+    References
+    ----------
+    .. [1] Manber, U. (1989).
+       *Introduction to Algorithms - A Creative Approach.* Addison-Wesley.
+    """
+    if not G.is_directed():
+        msg = "Topological sort not defined on undirected graphs."
+        raise nx.NetworkXError(msg)
+
+    if key is None:
+
+        def key(node):
+            return node
+
+    nodeid_map = {n: i for i, n in enumerate(G)}
+
+    def create_tuple(node):
+        return key(node), nodeid_map[node], node
+
+    indegree_map = {v: d for v, d in G.in_degree() if d > 0}
+    # These nodes have zero indegree and ready to be returned.
+    zero_indegree = [create_tuple(v) for v, d in G.in_degree() if d == 0]
+    heapq.heapify(zero_indegree)
+
+    while zero_indegree:
+        _, _, node = heapq.heappop(zero_indegree)
+
+        if node not in G:
+            raise RuntimeError("Graph changed during iteration")
+        for _, child in G.edges(node):
+            try:
+                indegree_map[child] -= 1
+            except KeyError as err:
+                raise RuntimeError("Graph changed during iteration") from err
+            if indegree_map[child] == 0:
+                try:
+                    heapq.heappush(zero_indegree, create_tuple(child))
+                except TypeError as err:
+                    raise TypeError(
+                        f"{err}\nConsider using `key=` parameter to resolve ambiguities in the sort order."
+                    )
+                del indegree_map[child]
+
+        yield node
+
+    if indegree_map:
+        msg = "Graph contains a cycle or graph changed during iteration"
+        raise nx.NetworkXUnfeasible(msg)
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def all_topological_sorts(G):
+    """Returns a generator of _all_ topological sorts of the directed graph G.
+
+    A topological sort is a nonunique permutation of the nodes such that an
+    edge from u to v implies that u appears before v in the topological sort
+    order.
+
+    Parameters
+    ----------
+    G : NetworkX DiGraph
+        A directed graph
+
+    Yields
+    ------
+    topological_sort_order : list
+        a list of nodes in `G`, representing one of the topological sort orders
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is not directed
+    NetworkXUnfeasible
+        If `G` is not acyclic
+
+    Examples
+    --------
+    To enumerate all topological sorts of directed graph:
+
+    >>> DG = nx.DiGraph([(1, 2), (2, 3), (2, 4)])
+    >>> list(nx.all_topological_sorts(DG))
+    [[1, 2, 4, 3], [1, 2, 3, 4]]
+
+    Notes
+    -----
+    Implements an iterative version of the algorithm given in [1].
+
+    References
+    ----------
+    .. [1] Knuth, Donald E., Szwarcfiter, Jayme L. (1974).
+       "A Structured Program to Generate All Topological Sorting Arrangements"
+       Information Processing Letters, Volume 2, Issue 6, 1974, Pages 153-157,
+       ISSN 0020-0190,
+       https://doi.org/10.1016/0020-0190(74)90001-5.
+       Elsevier (North-Holland), Amsterdam
+    """
+    if not G.is_directed():
+        raise nx.NetworkXError("Topological sort not defined on undirected graphs.")
+
+    # the names of count and D are chosen to match the global variables in [1]
+    # number of edges originating in a vertex v
+    count = dict(G.in_degree())
+    # vertices with indegree 0
+    D = deque([v for v, d in G.in_degree() if d == 0])
+    # stack of first value chosen at a position k in the topological sort
+    bases = []
+    current_sort = []
+
+    # do-while construct
+    while True:
+        assert all(count[v] == 0 for v in D)
+
+        if len(current_sort) == len(G):
+            yield list(current_sort)
+
+            # clean-up stack
+            while len(current_sort) > 0:
+                assert len(bases) == len(current_sort)
+                q = current_sort.pop()
+
+                # "restores" all edges (q, x)
+                # NOTE: it is important to iterate over edges instead
+                # of successors, so count is updated correctly in multigraphs
+                for _, j in G.out_edges(q):
+                    count[j] += 1
+                    assert count[j] >= 0
+                # remove entries from D
+                while len(D) > 0 and count[D[-1]] > 0:
+                    D.pop()
+
+                # corresponds to a circular shift of the values in D
+                # if the first value chosen (the base) is in the first
+                # position of D again, we are done and need to consider the
+                # previous condition
+                D.appendleft(q)
+                if D[-1] == bases[-1]:
+                    # all possible values have been chosen at current position
+                    # remove corresponding marker
+                    bases.pop()
+                else:
+                    # there are still elements that have not been fixed
+                    # at the current position in the topological sort
+                    # stop removing elements, escape inner loop
+                    break
+
+        else:
+            if len(D) == 0:
+                raise nx.NetworkXUnfeasible("Graph contains a cycle.")
+
+            # choose next node
+            q = D.pop()
+            # "erase" all edges (q, x)
+            # NOTE: it is important to iterate over edges instead
+            # of successors, so count is updated correctly in multigraphs
+            for _, j in G.out_edges(q):
+                count[j] -= 1
+                assert count[j] >= 0
+                if count[j] == 0:
+                    D.append(j)
+            current_sort.append(q)
+
+            # base for current position might _not_ be fixed yet
+            if len(bases) < len(current_sort):
+                bases.append(q)
+
+        if len(bases) == 0:
+            break
+
+
+@nx._dispatchable
+def is_aperiodic(G):
+    """Returns True if `G` is aperiodic.
+
+    A directed graph is aperiodic if there is no integer k > 1 that
+    divides the length of every cycle in the graph.
+
+    Parameters
+    ----------
+    G : NetworkX DiGraph
+        A directed graph
+
+    Returns
+    -------
+    bool
+        True if the graph is aperiodic False otherwise
+
+    Raises
+    ------
+    NetworkXError
+        If `G` is not directed
+
+    Examples
+    --------
+    A graph consisting of one cycle, the length of which is 2. Therefore ``k = 2``
+    divides the length of every cycle in the graph and thus the graph
+    is *not aperiodic*::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 1)])
+        >>> nx.is_aperiodic(DG)
+        False
+
+    A graph consisting of two cycles: one of length 2 and the other of length 3.
+    The cycle lengths are coprime, so there is no single value of k where ``k > 1``
+    that divides each cycle length and therefore the graph is *aperiodic*::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 3), (3, 1), (1, 4), (4, 1)])
+        >>> nx.is_aperiodic(DG)
+        True
+
+    A graph consisting of two cycles: one of length 2 and the other of length 4.
+    The lengths of the cycles share a common factor ``k = 2``, and therefore
+    the graph is *not aperiodic*::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 1), (3, 4), (4, 5), (5, 6), (6, 3)])
+        >>> nx.is_aperiodic(DG)
+        False
+
+    An acyclic graph, therefore the graph is *not aperiodic*::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 3)])
+        >>> nx.is_aperiodic(DG)
+        False
+
+    Notes
+    -----
+    This uses the method outlined in [1]_, which runs in $O(m)$ time
+    given $m$ edges in `G`. Note that a graph is not aperiodic if it is
+    acyclic as every integer trivial divides length 0 cycles.
+
+    References
+    ----------
+    .. [1] Jarvis, J. P.; Shier, D. R. (1996),
+       "Graph-theoretic analysis of finite Markov chains,"
+       in Shier, D. R.; Wallenius, K. T., Applied Mathematical Modeling:
+       A Multidisciplinary Approach, CRC Press.
+    """
+    if not G.is_directed():
+        raise nx.NetworkXError("is_aperiodic not defined for undirected graphs")
+    if len(G) == 0:
+        raise nx.NetworkXPointlessConcept("Graph has no nodes.")
+    s = arbitrary_element(G)
+    levels = {s: 0}
+    this_level = [s]
+    g = 0
+    lev = 1
+    while this_level:
+        next_level = []
+        for u in this_level:
+            for v in G[u]:
+                if v in levels:  # Non-Tree Edge
+                    g = gcd(g, levels[u] - levels[v] + 1)
+                else:  # Tree Edge
+                    next_level.append(v)
+                    levels[v] = lev
+        this_level = next_level
+        lev += 1
+    if len(levels) == len(G):  # All nodes in tree
+        return g == 1
+    else:
+        return g == 1 and nx.is_aperiodic(G.subgraph(set(G) - set(levels)))
+
+
+@nx._dispatchable(preserve_all_attrs=True, returns_graph=True)
+def transitive_closure(G, reflexive=False):
+    """Returns transitive closure of a graph
+
+    The transitive closure of G = (V,E) is a graph G+ = (V,E+) such that
+    for all v, w in V there is an edge (v, w) in E+ if and only if there
+    is a path from v to w in G.
+
+    Handling of paths from v to v has some flexibility within this definition.
+    A reflexive transitive closure creates a self-loop for the path
+    from v to v of length 0. The usual transitive closure creates a
+    self-loop only if a cycle exists (a path from v to v with length > 0).
+    We also allow an option for no self-loops.
+
+    Parameters
+    ----------
+    G : NetworkX Graph
+        A directed/undirected graph/multigraph.
+    reflexive : Bool or None, optional (default: False)
+        Determines when cycles create self-loops in the Transitive Closure.
+        If True, trivial cycles (length 0) create self-loops. The result
+        is a reflexive transitive closure of G.
+        If False (the default) non-trivial cycles create self-loops.
+        If None, self-loops are not created.
+
+    Returns
+    -------
+    NetworkX graph
+        The transitive closure of `G`
+
+    Raises
+    ------
+    NetworkXError
+        If `reflexive` not in `{None, True, False}`
+
+    Examples
+    --------
+    The treatment of trivial (i.e. length 0) cycles is controlled by the
+    `reflexive` parameter.
+
+    Trivial (i.e. length 0) cycles do not create self-loops when
+    ``reflexive=False`` (the default)::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 3)])
+        >>> TC = nx.transitive_closure(DG, reflexive=False)
+        >>> TC.edges()
+        OutEdgeView([(1, 2), (1, 3), (2, 3)])
+
+    However, nontrivial (i.e. length greater than 0) cycles create self-loops
+    when ``reflexive=False`` (the default)::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 3), (3, 1)])
+        >>> TC = nx.transitive_closure(DG, reflexive=False)
+        >>> TC.edges()
+        OutEdgeView([(1, 2), (1, 3), (1, 1), (2, 3), (2, 1), (2, 2), (3, 1), (3, 2), (3, 3)])
+
+    Trivial cycles (length 0) create self-loops when ``reflexive=True``::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 3)])
+        >>> TC = nx.transitive_closure(DG, reflexive=True)
+        >>> TC.edges()
+        OutEdgeView([(1, 2), (1, 1), (1, 3), (2, 3), (2, 2), (3, 3)])
+
+    And the third option is not to create self-loops at all when ``reflexive=None``::
+
+        >>> DG = nx.DiGraph([(1, 2), (2, 3), (3, 1)])
+        >>> TC = nx.transitive_closure(DG, reflexive=None)
+        >>> TC.edges()
+        OutEdgeView([(1, 2), (1, 3), (2, 3), (2, 1), (3, 1), (3, 2)])
+
+    References
+    ----------
+    .. [1] https://www.ics.uci.edu/~eppstein/PADS/PartialOrder.py
+    """
+    TC = G.copy()
+
+    if reflexive not in {None, True, False}:
+        raise nx.NetworkXError("Incorrect value for the parameter `reflexive`")
+
+    for v in G:
+        if reflexive is None:
+            TC.add_edges_from((v, u) for u in nx.descendants(G, v) if u not in TC[v])
+        elif reflexive is True:
+            TC.add_edges_from(
+                (v, u) for u in nx.descendants(G, v) | {v} if u not in TC[v]
+            )
+        elif reflexive is False:
+            TC.add_edges_from((v, e[1]) for e in nx.edge_bfs(G, v) if e[1] not in TC[v])
+
+    return TC
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable(preserve_all_attrs=True, returns_graph=True)
+def transitive_closure_dag(G, topo_order=None):
+    """Returns the transitive closure of a directed acyclic graph.
+
+    This function is faster than the function `transitive_closure`, but fails
+    if the graph has a cycle.
+
+    The transitive closure of G = (V,E) is a graph G+ = (V,E+) such that
+    for all v, w in V there is an edge (v, w) in E+ if and only if there
+    is a non-null path from v to w in G.
+
+    Parameters
+    ----------
+    G : NetworkX DiGraph
+        A directed acyclic graph (DAG)
+
+    topo_order: list or tuple, optional
+        A topological order for G (if None, the function will compute one)
+
+    Returns
+    -------
+    NetworkX DiGraph
+        The transitive closure of `G`
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is not directed
+    NetworkXUnfeasible
+        If `G` has a cycle
+
+    Examples
+    --------
+    >>> DG = nx.DiGraph([(1, 2), (2, 3)])
+    >>> TC = nx.transitive_closure_dag(DG)
+    >>> TC.edges()
+    OutEdgeView([(1, 2), (1, 3), (2, 3)])
+
+    Notes
+    -----
+    This algorithm is probably simple enough to be well-known but I didn't find
+    a mention in the literature.
+    """
+    if topo_order is None:
+        topo_order = list(topological_sort(G))
+
+    TC = G.copy()
+
+    # idea: traverse vertices following a reverse topological order, connecting
+    # each vertex to its descendants at distance 2 as we go
+    for v in reversed(topo_order):
+        TC.add_edges_from((v, u) for u in nx.descendants_at_distance(TC, v, 2))
+
+    return TC
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable(returns_graph=True)
+def transitive_reduction(G):
+    """Returns transitive reduction of a directed graph
+
+    The transitive reduction of G = (V,E) is a graph G- = (V,E-) such that
+    for all v,w in V there is an edge (v,w) in E- if and only if (v,w) is
+    in E and there is no path from v to w in G with length greater than 1.
+
+    Parameters
+    ----------
+    G : NetworkX DiGraph
+        A directed acyclic graph (DAG)
+
+    Returns
+    -------
+    NetworkX DiGraph
+        The transitive reduction of `G`
+
+    Raises
+    ------
+    NetworkXError
+        If `G` is not a directed acyclic graph (DAG) transitive reduction is
+        not uniquely defined and a :exc:`NetworkXError` exception is raised.
+
+    Examples
+    --------
+    To perform transitive reduction on a DiGraph:
+
+    >>> DG = nx.DiGraph([(1, 2), (2, 3), (1, 3)])
+    >>> TR = nx.transitive_reduction(DG)
+    >>> list(TR.edges)
+    [(1, 2), (2, 3)]
+
+    To avoid unnecessary data copies, this implementation does not return a
+    DiGraph with node/edge data.
+    To perform transitive reduction on a DiGraph and transfer node/edge data:
+
+    >>> DG = nx.DiGraph()
+    >>> DG.add_edges_from([(1, 2), (2, 3), (1, 3)], color="red")
+    >>> TR = nx.transitive_reduction(DG)
+    >>> TR.add_nodes_from(DG.nodes(data=True))
+    >>> TR.add_edges_from((u, v, DG.edges[u, v]) for u, v in TR.edges)
+    >>> list(TR.edges(data=True))
+    [(1, 2, {'color': 'red'}), (2, 3, {'color': 'red'})]
+
+    References
+    ----------
+    https://en.wikipedia.org/wiki/Transitive_reduction
+
+    """
+    if not is_directed_acyclic_graph(G):
+        msg = "Directed Acyclic Graph required for transitive_reduction"
+        raise nx.NetworkXError(msg)
+    TR = nx.DiGraph()
+    TR.add_nodes_from(G.nodes())
+    descendants = {}
+    # count before removing set stored in descendants
+    check_count = dict(G.in_degree)
+    for u in G:
+        u_nbrs = set(G[u])
+        for v in G[u]:
+            if v in u_nbrs:
+                if v not in descendants:
+                    descendants[v] = {y for x, y in nx.dfs_edges(G, v)}
+                u_nbrs -= descendants[v]
+            check_count[v] -= 1
+            if check_count[v] == 0:
+                del descendants[v]
+        TR.add_edges_from((u, v) for v in u_nbrs)
+    return TR
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def antichains(G, topo_order=None):
+    """Generates antichains from a directed acyclic graph (DAG).
+
+    An antichain is a subset of a partially ordered set such that any
+    two elements in the subset are incomparable.
+
+    Parameters
+    ----------
+    G : NetworkX DiGraph
+        A directed acyclic graph (DAG)
+
+    topo_order: list or tuple, optional
+        A topological order for G (if None, the function will compute one)
+
+    Yields
+    ------
+    antichain : list
+        a list of nodes in `G` representing an antichain
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is not directed
+
+    NetworkXUnfeasible
+        If `G` contains a cycle
+
+    Examples
+    --------
+    >>> DG = nx.DiGraph([(1, 2), (1, 3)])
+    >>> list(nx.antichains(DG))
+    [[], [3], [2], [2, 3], [1]]
+
+    Notes
+    -----
+    This function was originally developed by Peter Jipsen and Franco Saliola
+    for the SAGE project. It's included in NetworkX with permission from the
+    authors. Original SAGE code at:
+
+    https://github.com/sagemath/sage/blob/master/src/sage/combinat/posets/hasse_diagram.py
+
+    References
+    ----------
+    .. [1] Free Lattices, by R. Freese, J. Jezek and J. B. Nation,
+       AMS, Vol 42, 1995, p. 226.
+    """
+    if topo_order is None:
+        topo_order = list(nx.topological_sort(G))
+
+    TC = nx.transitive_closure_dag(G, topo_order)
+    antichains_stacks = [([], list(reversed(topo_order)))]
+
+    while antichains_stacks:
+        (antichain, stack) = antichains_stacks.pop()
+        # Invariant:
+        #  - the elements of antichain are independent
+        #  - the elements of stack are independent from those of antichain
+        yield antichain
+        while stack:
+            x = stack.pop()
+            new_antichain = antichain + [x]
+            new_stack = [t for t in stack if not ((t in TC[x]) or (x in TC[t]))]
+            antichains_stacks.append((new_antichain, new_stack))
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable(edge_attrs={"weight": "default_weight"})
+def dag_longest_path(G, weight="weight", default_weight=1, topo_order=None):
+    """Returns the longest path in a directed acyclic graph (DAG).
+
+    If `G` has edges with `weight` attribute the edge data are used as
+    weight values.
+
+    Parameters
+    ----------
+    G : NetworkX DiGraph
+        A directed acyclic graph (DAG)
+
+    weight : str, optional
+        Edge data key to use for weight
+
+    default_weight : int, optional
+        The weight of edges that do not have a weight attribute
+
+    topo_order: list or tuple, optional
+        A topological order for `G` (if None, the function will compute one)
+
+    Returns
+    -------
+    list
+        Longest path
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is not directed
+
+    Examples
+    --------
+    >>> DG = nx.DiGraph(
+    ...     [(0, 1, {"cost": 1}), (1, 2, {"cost": 1}), (0, 2, {"cost": 42})]
+    ... )
+    >>> list(nx.all_simple_paths(DG, 0, 2))
+    [[0, 1, 2], [0, 2]]
+    >>> nx.dag_longest_path(DG)
+    [0, 1, 2]
+    >>> nx.dag_longest_path(DG, weight="cost")
+    [0, 2]
+
+    In the case where multiple valid topological orderings exist, `topo_order`
+    can be used to specify a specific ordering:
+
+    >>> DG = nx.DiGraph([(0, 1), (0, 2)])
+    >>> sorted(nx.all_topological_sorts(DG))  # Valid topological orderings
+    [[0, 1, 2], [0, 2, 1]]
+    >>> nx.dag_longest_path(DG, topo_order=[0, 1, 2])
+    [0, 1]
+    >>> nx.dag_longest_path(DG, topo_order=[0, 2, 1])
+    [0, 2]
+
+    See also
+    --------
+    dag_longest_path_length
+
+    """
+    if not G:
+        return []
+
+    if topo_order is None:
+        topo_order = nx.topological_sort(G)
+
+    dist = {}  # stores {v : (length, u)}
+    for v in topo_order:
+        us = [
+            (
+                dist[u][0]
+                + (
+                    max(data.values(), key=lambda x: x.get(weight, default_weight))
+                    if G.is_multigraph()
+                    else data
+                ).get(weight, default_weight),
+                u,
+            )
+            for u, data in G.pred[v].items()
+        ]
+
+        # Use the best predecessor if there is one and its distance is
+        # non-negative, otherwise terminate.
+        maxu = max(us, key=lambda x: x[0]) if us else (0, v)
+        dist[v] = maxu if maxu[0] >= 0 else (0, v)
+
+    u = None
+    v = max(dist, key=lambda x: dist[x][0])
+    path = []
+    while u != v:
+        path.append(v)
+        u = v
+        v = dist[v][1]
+
+    path.reverse()
+    return path
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable(edge_attrs={"weight": "default_weight"})
+def dag_longest_path_length(G, weight="weight", default_weight=1):
+    """Returns the longest path length in a DAG
+
+    Parameters
+    ----------
+    G : NetworkX DiGraph
+        A directed acyclic graph (DAG)
+
+    weight : string, optional
+        Edge data key to use for weight
+
+    default_weight : int, optional
+        The weight of edges that do not have a weight attribute
+
+    Returns
+    -------
+    int
+        Longest path length
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is not directed
+
+    Examples
+    --------
+    >>> DG = nx.DiGraph(
+    ...     [(0, 1, {"cost": 1}), (1, 2, {"cost": 1}), (0, 2, {"cost": 42})]
+    ... )
+    >>> list(nx.all_simple_paths(DG, 0, 2))
+    [[0, 1, 2], [0, 2]]
+    >>> nx.dag_longest_path_length(DG)
+    2
+    >>> nx.dag_longest_path_length(DG, weight="cost")
+    42
+
+    See also
+    --------
+    dag_longest_path
+    """
+    path = nx.dag_longest_path(G, weight, default_weight)
+    path_length = 0
+    if G.is_multigraph():
+        for u, v in pairwise(path):
+            i = max(G[u][v], key=lambda x: G[u][v][x].get(weight, default_weight))
+            path_length += G[u][v][i].get(weight, default_weight)
+    else:
+        for u, v in pairwise(path):
+            path_length += G[u][v].get(weight, default_weight)
+
+    return path_length
+
+
+@nx._dispatchable
+def root_to_leaf_paths(G):
+    """Yields root-to-leaf paths in a directed acyclic graph.
+
+    `G` must be a directed acyclic graph. If not, the behavior of this
+    function is undefined. A "root" in this graph is a node of in-degree
+    zero and a "leaf" a node of out-degree zero.
+
+    When invoked, this function iterates over each path from any root to
+    any leaf. A path is a list of nodes.
+
+    """
+    roots = (v for v, d in G.in_degree() if d == 0)
+    leaves = (v for v, d in G.out_degree() if d == 0)
+    all_paths = partial(nx.all_simple_paths, G)
+    # TODO In Python 3, this would be better as `yield from ...`.
+    return chaini(starmap(all_paths, product(roots, leaves)))
+
+
+@not_implemented_for("multigraph")
+@not_implemented_for("undirected")
+@nx._dispatchable(returns_graph=True)
+def dag_to_branching(G):
+    """Returns a branching representing all (overlapping) paths from
+    root nodes to leaf nodes in the given directed acyclic graph.
+
+    As described in :mod:`networkx.algorithms.tree.recognition`, a
+    *branching* is a directed forest in which each node has at most one
+    parent. In other words, a branching is a disjoint union of
+    *arborescences*. For this function, each node of in-degree zero in
+    `G` becomes a root of one of the arborescences, and there will be
+    one leaf node for each distinct path from that root to a leaf node
+    in `G`.
+
+    Each node `v` in `G` with *k* parents becomes *k* distinct nodes in
+    the returned branching, one for each parent, and the sub-DAG rooted
+    at `v` is duplicated for each copy. The algorithm then recurses on
+    the children of each copy of `v`.
+
+    Parameters
+    ----------
+    G : NetworkX graph
+        A directed acyclic graph.
+
+    Returns
+    -------
+    DiGraph
+        The branching in which there is a bijection between root-to-leaf
+        paths in `G` (in which multiple paths may share the same leaf)
+        and root-to-leaf paths in the branching (in which there is a
+        unique path from a root to a leaf).
+
+        Each node has an attribute 'source' whose value is the original
+        node to which this node corresponds. No other graph, node, or
+        edge attributes are copied into this new graph.
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is not directed, or if `G` is a multigraph.
+
+    HasACycle
+        If `G` is not acyclic.
+
+    Examples
+    --------
+    To examine which nodes in the returned branching were produced by
+    which original node in the directed acyclic graph, we can collect
+    the mapping from source node to new nodes into a dictionary. For
+    example, consider the directed diamond graph::
+
+        >>> from collections import defaultdict
+        >>> from operator import itemgetter
+        >>>
+        >>> G = nx.DiGraph(nx.utils.pairwise("abd"))
+        >>> G.add_edges_from(nx.utils.pairwise("acd"))
+        >>> B = nx.dag_to_branching(G)
+        >>>
+        >>> sources = defaultdict(set)
+        >>> for v, source in B.nodes(data="source"):
+        ...     sources[source].add(v)
+        >>> len(sources["a"])
+        1
+        >>> len(sources["d"])
+        2
+
+    To copy node attributes from the original graph to the new graph,
+    you can use a dictionary like the one constructed in the above
+    example::
+
+        >>> for source, nodes in sources.items():
+        ...     for v in nodes:
+        ...         B.nodes[v].update(G.nodes[source])
+
+    Notes
+    -----
+    This function is not idempotent in the sense that the node labels in
+    the returned branching may be uniquely generated each time the
+    function is invoked. In fact, the node labels may not be integers;
+    in order to relabel the nodes to be more readable, you can use the
+    :func:`networkx.convert_node_labels_to_integers` function.
+
+    The current implementation of this function uses
+    :func:`networkx.prefix_tree`, so it is subject to the limitations of
+    that function.
+
+    """
+    if has_cycle(G):
+        msg = "dag_to_branching is only defined for acyclic graphs"
+        raise nx.HasACycle(msg)
+    paths = root_to_leaf_paths(G)
+    B = nx.prefix_tree(paths)
+    # Remove the synthetic `root`(0) and `NIL`(-1) nodes from the tree
+    B.remove_node(0)
+    B.remove_node(-1)
+    return B
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def compute_v_structures(G):
+    """Yields 3-node tuples that represent the v-structures in `G`.
+
+    .. deprecated:: 3.4
+
+       `compute_v_structures` actually yields colliders. It will be removed in
+       version 3.6. Use `nx.dag.v_structures` or `nx.dag.colliders` instead.
+
+    Colliders are triples in the directed acyclic graph (DAG) where two parent nodes
+    point to the same child node. V-structures are colliders where the two parent
+    nodes are not adjacent. In a causal graph setting, the parents do not directly
+    depend on each other, but conditioning on the child node provides an association.
+
+    Parameters
+    ----------
+    G : graph
+        A networkx `~networkx.DiGraph`.
+
+    Yields
+    ------
+    A 3-tuple representation of a v-structure
+        Each v-structure is a 3-tuple with the parent, collider, and other parent.
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is an undirected graph.
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (0, 4), (3, 1), (2, 4), (0, 5), (4, 5), (1, 5)])
+    >>> nx.is_directed_acyclic_graph(G)
+    True
+    >>> list(nx.compute_v_structures(G))
+    [(0, 4, 2), (0, 5, 4), (0, 5, 1), (4, 5, 1)]
+
+    See Also
+    --------
+    v_structures
+    colliders
+
+    Notes
+    -----
+    This function was written to be used on DAGs, however it works on cyclic graphs
+    too. Since colliders are referred to in the cyclic causal graph literature
+    [2]_ we allow cyclic graphs in this function. It is suggested that you test if
+    your input graph is acyclic as in the example if you want that property.
+
+    References
+    ----------
+    .. [1]  `Pearl's PRIMER <https://bayes.cs.ucla.edu/PRIMER/primer-ch2.pdf>`_
+            Ch-2 page 50: v-structures def.
+    .. [2] A Hyttinen, P.O. Hoyer, F. Eberhardt, M J ̈arvisalo, (2013)
+           "Discovering cyclic causal models with latent variables:
+           a general SAT-based procedure", UAI'13: Proceedings of the Twenty-Ninth
+           Conference on Uncertainty in Artificial Intelligence, pg 301–310,
+           `doi:10.5555/3023638.3023669 <https://dl.acm.org/doi/10.5555/3023638.3023669>`_
+    """
+    import warnings
+
+    warnings.warn(
+        (
+            "\n\n`compute_v_structures` actually yields colliders. It will be\n"
+            "removed in version 3.6. Use `nx.dag.v_structures` or `nx.dag.colliders`\n"
+            "instead.\n"
+        ),
+        category=DeprecationWarning,
+        stacklevel=5,
+    )
+
+    return colliders(G)
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def v_structures(G):
+    """Yields 3-node tuples that represent the v-structures in `G`.
+
+    Colliders are triples in the directed acyclic graph (DAG) where two parent nodes
+    point to the same child node. V-structures are colliders where the two parent
+    nodes are not adjacent. In a causal graph setting, the parents do not directly
+    depend on each other, but conditioning on the child node provides an association.
+
+    Parameters
+    ----------
+    G : graph
+        A networkx `~networkx.DiGraph`.
+
+    Yields
+    ------
+    A 3-tuple representation of a v-structure
+        Each v-structure is a 3-tuple with the parent, collider, and other parent.
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is an undirected graph.
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (0, 4), (3, 1), (2, 4), (0, 5), (4, 5), (1, 5)])
+    >>> nx.is_directed_acyclic_graph(G)
+    True
+    >>> list(nx.dag.v_structures(G))
+    [(0, 4, 2), (0, 5, 1), (4, 5, 1)]
+
+    See Also
+    --------
+    colliders
+
+    Notes
+    -----
+    This function was written to be used on DAGs, however it works on cyclic graphs
+    too. Since colliders are referred to in the cyclic causal graph literature
+    [2]_ we allow cyclic graphs in this function. It is suggested that you test if
+    your input graph is acyclic as in the example if you want that property.
+
+    References
+    ----------
+    .. [1]  `Pearl's PRIMER <https://bayes.cs.ucla.edu/PRIMER/primer-ch2.pdf>`_
+            Ch-2 page 50: v-structures def.
+    .. [2] A Hyttinen, P.O. Hoyer, F. Eberhardt, M J ̈arvisalo, (2013)
+           "Discovering cyclic causal models with latent variables:
+           a general SAT-based procedure", UAI'13: Proceedings of the Twenty-Ninth
+           Conference on Uncertainty in Artificial Intelligence, pg 301–310,
+           `doi:10.5555/3023638.3023669 <https://dl.acm.org/doi/10.5555/3023638.3023669>`_
+    """
+    for p1, c, p2 in colliders(G):
+        if not (G.has_edge(p1, p2) or G.has_edge(p2, p1)):
+            yield (p1, c, p2)
+
+
+@not_implemented_for("undirected")
+@nx._dispatchable
+def colliders(G):
+    """Yields 3-node tuples that represent the colliders in `G`.
+
+    In a Directed Acyclic Graph (DAG), if you have three nodes A, B, and C, and
+    there are edges from A to C and from B to C, then C is a collider [1]_ . In
+    a causal graph setting, this means that both events A and B are "causing" C,
+    and conditioning on C provide an association between A and B even if
+    no direct causal relationship exists between A and B.
+
+    Parameters
+    ----------
+    G : graph
+        A networkx `~networkx.DiGraph`.
+
+    Yields
+    ------
+    A 3-tuple representation of a collider
+        Each collider is a 3-tuple with the parent, collider, and other parent.
+
+    Raises
+    ------
+    NetworkXNotImplemented
+        If `G` is an undirected graph.
+
+    Examples
+    --------
+    >>> G = nx.DiGraph([(1, 2), (0, 4), (3, 1), (2, 4), (0, 5), (4, 5), (1, 5)])
+    >>> nx.is_directed_acyclic_graph(G)
+    True
+    >>> list(nx.dag.colliders(G))
+    [(0, 4, 2), (0, 5, 4), (0, 5, 1), (4, 5, 1)]
+
+    See Also
+    --------
+    v_structures
+
+    Notes
+    -----
+    This function was written to be used on DAGs, however it works on cyclic graphs
+    too. Since colliders are referred to in the cyclic causal graph literature
+    [2]_ we allow cyclic graphs in this function. It is suggested that you test if
+    your input graph is acyclic as in the example if you want that property.
+
+    References
+    ----------
+    .. [1] `Wikipedia: Collider in causal graphs <https://en.wikipedia.org/wiki/Collider_(statistics)>`_
+    .. [2] A Hyttinen, P.O. Hoyer, F. Eberhardt, M J ̈arvisalo, (2013)
+           "Discovering cyclic causal models with latent variables:
+           a general SAT-based procedure", UAI'13: Proceedings of the Twenty-Ninth
+           Conference on Uncertainty in Artificial Intelligence, pg 301–310,
+           `doi:10.5555/3023638.3023669 <https://dl.acm.org/doi/10.5555/3023638.3023669>`_
+    """
+    for node in G.nodes:
+        for p1, p2 in combinations(G.predecessors(node), 2):
+            yield (p1, node, p2)