# -*- encoding: utf-8 -*-
"""
Path Semigroups
"""

# ****************************************************************************
#  Copyright (C) 2012 Jim Stark <jstarx@gmail.com>
#                2013 Simon King <simon.king@uni-jena.de>
#
#  Distributed under the terms of the GNU General Public License (GPL)
#
#    This code is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty
#    of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
#
#  See the GNU General Public License for more details; the full text
#  is available at:
#
#                  https://www.gnu.org/licenses/
# ****************************************************************************

from sage.rings.integer import Integer
from sage.rings.integer_ring import ZZ
from sage.structure.parent import Parent
from sage.structure.unique_representation import UniqueRepresentation
from sage.categories.semigroups import Semigroups
from sage.categories.monoids import Monoids
from sage.categories.infinite_enumerated_sets import InfiniteEnumeratedSets
from sage.categories.finite_enumerated_sets import FiniteEnumeratedSets
from sage.misc.cachefunc import cached_method
from sage.misc.lazy_attribute import lazy_attribute
from .paths import QuiverPath
from .representation import QuiverRep

#########################
# Some auxiliary function to create generating functions to count paths.


class PathSemigroup(UniqueRepresentation, Parent):
    r"""
    The partial semigroup that is given by the directed paths of a quiver,
    subject to concatenation.

    See :mod:`~sage.quivers.representation` for a definition of this
    semigroup and of the notion of a path in a quiver.

    Note that a *partial semigroup* here is defined as a set `G` with a
    partial binary operation `G \times G \to G \cup \{\mbox{None}\}`,
    which is written infix as a `*` sign and satisfies associativity in
    the following sense: If `a`, `b` and `c` are three elements of `G`,
    and if one of the products `(a*b)*c` and `a*(b*c)` exists, then so
    does the other and the two products are equal. A partial semigroup
    is not required to have a neutral element (and this one usually has
    no such element).

    EXAMPLES::

        sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}})
        sage: S = Q.path_semigroup()
        sage: S
        Partial semigroup formed by the directed paths of Multi-digraph on 3 vertices
        sage: S.variable_names()
        ('e_1', 'e_2', 'e_3', 'a', 'b', 'c', 'd')
        sage: S.gens()
        (e_1, e_2, e_3, a, b, c, d)
        sage: S.category()
        Category of finite enumerated semigroups

    In the test suite, we skip the associativity test, as in this example the
    paths used for testing cannot be concatenated::

        sage: TestSuite(S).run(skip=['_test_associativity'])

    If there is only a single vertex, the partial semigroup is a monoid. If
    the underlying quiver has cycles or loops, then the (partial) semigroup
    only is an infinite enumerated set. This time, there is no need to skip
    tests::

        sage: Q = DiGraph({1:{1:['a', 'b', 'c', 'd']}})
        sage: M = Q.path_semigroup()
        sage: M
        Monoid formed by the directed paths of Looped multi-digraph on 1 vertex
        sage: M.category()
        Category of infinite enumerated monoids
        sage: TestSuite(M).run()
    """
    Element = QuiverPath

    @staticmethod
    def __classcall__(cls, Q):
        """
        Normalize the arguments passed to the constructor.

        The normalization consists of making an immutable copy of ``Q``
        that is made weighted.

        INPUT:

        - a :class:`~sage.graphs.digraph.DiGraph`.

        TESTS::

            sage: G1 = DiGraph({1:{2:['a']}})
            sage: G2 = DiGraph({1:{2:['b']}})
            sage: P1 = G1.path_semigroup()
            sage: P2 = G2.path_semigroup()
            sage: G1 == G2 # equality of unweighted graphs ignores edge labels
            True
            sage: P1.quiver() == P2.quiver() # edge labels no longer ignored
            False
            sage: P1 == P2
            False
        """
        # If self is immutable and weighted, then the copy is really cheap:
        # __copy__ just returns self.
        Q = Q.copy(immutable=True, weighted=True)
        return super(PathSemigroup, cls).__classcall__(cls, Q)

    def __init__(self, Q):
        """
        Initialize ``self``.

        INPUT:

        - a :class:`~sage.graphs.digraph.DiGraph`.

        EXAMPLES:

        Note that usually a path semigroup is created using
        :meth:`sage.graphs.digraph.DiGraph.path_semigroup`. Here, we
        demonstrate the direct construction::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}}, immutable=True)
            sage: from sage.quivers.path_semigroup import PathSemigroup
            sage: P = PathSemigroup(Q)
            sage: P is DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}}).path_semigroup() # indirect doctest
            True
            sage: P
            Partial semigroup formed by the directed paths of Multi-digraph on 3 vertices

        While hardly of any use, it is possible to construct the path
        semigroup of an empty quiver (it is, of course, empty)::

            sage: D = DiGraph({})
            sage: A = D.path_semigroup(); A
            Partial semigroup formed by the directed paths of Digraph on 0 vertices
            sage: A.list()
            []

        .. TODO::

            When the graph has more than one edge, the proper category would be
            a "partial semigroup" or a "semigroupoid" but definitely not a
            semigroup!
        """
        #########
        # Verify that the graph labels are acceptable for this implementation
        # Check that edges are labelled with nonempty strings and do not begin
        # with 'e_' or contain '*'
        labels = Q.edge_labels()
        if len(set(labels)) != len(labels):
            raise ValueError("edge labels of the digraph must be unique")
        for x in labels:
            if not isinstance(x, str) or x == '':
                raise ValueError("edge labels of the digraph must be nonempty strings")
            if x[0:2] == 'e_':
                raise ValueError("edge labels of the digraph must not begin with 'e_'")
            if x.find('*') != -1:
                raise ValueError("edge labels of the digraph must not contain '*'")

        # Check validity of input: vertices have to be labelled 1,2,3,... and
        # edge labels must be unique
        for v in Q:
            if not isinstance(v, (Integer, int)):
                raise ValueError("vertices of the digraph must be labelled by integers")

        # Determine the category which this (partial) semigroup belongs to
        if Q.is_directed_acyclic():
            cat = FiniteEnumeratedSets()
        else:
            cat = InfiniteEnumeratedSets()
        self._sorted_edges = tuple(sorted(Q.edges(), key=lambda x: x[2]))
        self._labels = tuple([x[2] for x in self._sorted_edges])
        self._label_index = {s[2]: i for i, s in enumerate(self._sorted_edges)}
        self._nb_arrows = max(len(self._sorted_edges), 1)
        names = ['e_{0}'.format(v) for v in Q.vertex_iterator()]
        names += list(self._labels)
        self._quiver = Q
        if Q.num_verts() == 1:
            cat = cat.join([cat, Monoids()])
        else:
            cat = cat.join([cat, Semigroups()])
        Parent.__init__(self, names=names, category=cat)

    def _repr_(self):
        """
        String representation.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}})
            sage: Q.path_semigroup()
            Partial semigroup formed by the directed paths of Multi-digraph on 3 vertices
            sage: Q = DiGraph({1:{1:['a','b', 'c', 'd']}})
            sage: Q.path_semigroup()
            Monoid formed by the directed paths of Looped multi-digraph on 1 vertex
        """
        if self._quiver.num_verts() != 1:
            return "Partial semigroup formed by the directed paths of {}".format(self._quiver)
        return "Monoid formed by the directed paths of {}".format(self._quiver)

    def _coerce_map_from_(self, other):
        """
        A coercion from `A` to `B` exists if the underlying quiver
        of `A` is a sub-quiver of the underlying quiver of `B` (preserving
        names).

        EXAMPLES::

            sage: Q1 = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}})
            sage: Q2 = DiGraph({1:{2:['a'], 3:['c']}})
            sage: Q3 = DiGraph({1:{2:['a','x'], 3:['c']}, 3:{1:['d']}})
            sage: P1 = Q1.path_semigroup()
            sage: P2 = Q2.path_semigroup()
            sage: P3 = Q3.path_semigroup()
            sage: P1.has_coerce_map_from(P2)   # indirect doctest
            True
            sage: P1.has_coerce_map_from(P3)
            False
            sage: d = P1([(3,1,'d')]); d
            d
            sage: c = P2([(1,3,'c')]); c
            c
            sage: c.parent() is P1
            False
            sage: c in P1    # indirect doctest
            True
            sage: d*c        # indirect doctest
            d*c
            sage: (d*c).parent() is P1
            True
            sage: c3 = P3([(1,3,'c')]); c3
            c
            sage: c3 in P1   # indirect doctest
            False
            sage: d*c3
            Traceback (most recent call last):
            ...
            TypeError: unsupported operand parent(s) for *:
             'Partial semigroup formed by the directed paths of Multi-digraph on 3 vertices'
             and 'Partial semigroup formed by the directed paths of Multi-digraph on 3 vertices'
        """
        if not isinstance(other, PathSemigroup):
            return
        # This is what we would like to do:
        #     return other.quiver().is_subgraph(self._quiver, induced=False)
        # However, this is deprecated for non-induced subgraphs
        # of directed multi-graphs.
        #
        # We ignore the deprecation and do what the deprecated method is doing
        # internally, directly using the backend to make things faster.
        sQ = self._quiver._backend
        oQ = other.quiver()._backend
        if sQ.num_verts() < oQ.num_verts():
            return False
        if any(not sQ.has_vertex(v) for v in oQ.iterator_verts(None)):
            return False
        return all(sQ.has_edge(*e) for e in oQ.iterator_out_edges(oQ.iterator_verts(None), True))

    def _element_constructor_(self, data, check=True):
        """
        The accepted data are

        - the integer ``1``, which returns the first idempotent,
        - a list, whose only item is a tuple ``(v,v)`` for a vertex ``v``,
        - a list of edge labels,
        - a single edge label,
        - a list of triples ``(v, w, s)`` that are edges of the underlying quiver, or
        - an element of another path semigroup.

        EXAMPLES::

            sage: P = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}}).path_semigroup()
            sage: P(1)
            e_1
            sage: P([(3,3)])
            e_3
            sage: P(['c','d'])
            c*d
            sage: P('c')
            c
            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d'], 2:['e']}}).path_semigroup()
            sage: Q([(1,3,'c'), (3,1,'d'), (1,2,'b')])
            c*d*b
            sage: Q(P(['c','d']))
            c*d

        A ``TypeError`` or a ``ValueError`` is raised appropriately if the input
        is wrong::

            sage: G = DiGraph([(0,0,'a'), (0,1,'b'), (1,0,'c')], loops=True)
            sage: P = G.path_semigroup()

            sage: P([(0,0)])
            e_0
            sage: P([(2,2)])
            Traceback (most recent call last):
            ...
            ValueError: Startpoint 2 should belong to [0, 1]
            sage: P([(0,1,'a'),(1,0,'b')])
            Traceback (most recent call last):
            ...
            ValueError: (0, 1, 'a') is not an edge
            sage: P('d')
            Traceback (most recent call last):
            ...
            ValueError: data='d' is not the label of an edge
            sage: P([(0,0,'a'),(0,1,2,3)])
            Traceback (most recent call last):
            ...
            ValueError: each edge must be a triple, got (0, 1, 2, 3)
            sage: P(18)
            Traceback (most recent call last):
            ...
            TypeError: data=18 is not valid. A path must be initialized from
            either a tuple or a list
        """
        L = self._label_index
        E = self._sorted_edges
        if isinstance(data, QuiverPath):
            if data.parent() is self:
                return data
            start = data.initial_vertex()
            end = data.terminal_vertex()
            edge_index = {e: i for i, e in enumerate(E)}
            path = [edge_index.get(e) for e in data]
        elif not data:
            raise ValueError("No data given to define this path")
        elif data == 1:
            start = end = next(self._quiver.vertex_iterator())
            path = []
        elif isinstance(data, str):  # one edge
            i = L.get(data, None)
            if i is None:
                raise ValueError("data={!r} is not the label of an edge".format(data))
            start, end, _ = E[i]
            path = [i]
        elif not isinstance(data, (tuple, list)):
            raise TypeError("data={} is not valid. A path must be initialized from either a tuple or a list".format(data))
        elif isinstance(data[0], str):  # a list of labels
            start = L.get(data[0])
            if start is None:
                raise ValueError("data[0]={!r} is not the label of an edge".format(data[0]))
            start = E[start][0]
            end = L.get(data[-1])
            if end is None:
                raise ValueError("data[-1]={!r} is not the label of an edge".format(data[-1]))
            end = E[end][1]
            path = [L.get(e) for e in data]
        elif len(data) == 1 and len(data[0]) == 2:  # an idempotent
            start = data[0][0]
            end = data[0][1]
            path = []
        else:  # a list of edges
            if any(len(x) != 3 for x in data):
                x = next((x for x in data if len(x) != 3))
                raise ValueError("each edge must be a triple, got {}".format(x))
            start = data[0][0]
            end = data[-1][1]
            edge_index = {e: i for i, e in enumerate(E)}
            path = [edge_index.get(e) for e in data]

        if check:
            Q = self._quiver
            if start is None or start not in Q:
                raise ValueError("Startpoint {} should belong to {}".format(start, Q.vertices()))
            if end is None or end not in Q:
                raise ValueError("Endpoint {} should belong to {}".format(end, Q.vertices()))
            if not path:
                if start != end:
                    raise ValueError("Start and endpoint of a path of length 0 must coincide")
            else:
                if any(x is None for x in path):
                    i = next((i for i, x in enumerate(path) if x is None))
                    raise ValueError("{} is not an edge".format(data[i]))
                for n in range(1, len(path)):
                    e0 = E[path[n - 1]][1]
                    e1 = E[path[n]][0]
                    if e0 != e1:
                        raise ValueError("Edge {} ends at {}, but edge {} starts at {}".format(
                            E[path[n - 1]][2], e0, E[path[n]][2], e1))
                if E[path[0]][0] != start:
                    raise ValueError("First edge should start at vertex {}".format(start))
                if E[path[-1]][1] != end:
                    raise ValueError("Last edge should end at vertex {}".format(end))

        return self.element_class(self, start, end, path)

    @cached_method
    def arrows(self):
        """
        Return the elements corresponding to edges of the underlying quiver.

        EXAMPLES::

            sage: P = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}}).path_semigroup()
            sage: P.arrows()
            (a, b, c, d)
        """
        return tuple(self.element_class(self, e[0], e[1], [i])
                     for i, e in enumerate(self._sorted_edges))

    @cached_method
    def idempotents(self):
        """
        Return the idempotents corresponding to the vertices of the
        underlying quiver.

        EXAMPLES::

            sage: P = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}}).path_semigroup()
            sage: P.idempotents()
            (e_1, e_2, e_3)
        """
        return tuple(self.element_class(self, v, v, [])
                     for v in self._quiver.vertex_iterator())

    def ngens(self):
        """
        Return the number of generators (:meth:`arrows` and
        :meth:`idempotents`).

        EXAMPLES::

            sage: F = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}}).path_semigroup()
            sage: F.ngens()
            7
        """
        Q = self._quiver
        return Q.num_verts() + Q.num_edges()

    @cached_method
    def gen(self, i):
        """
        Return generator number `i`.

        INPUT:

        - ``i`` -- integer

        OUTPUT:

        An idempotent, if `i` is smaller than the number of vertices,
        or an arrow otherwise.

        EXAMPLES::

            sage: P = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}}).path_semigroup()
            sage: P.1         # indirect doctest
            e_2
            sage: P.idempotents()[1]
            e_2
            sage: P.5
            c
            sage: P.gens()[5]
            c
        """
        return self.gens()[i]

    @cached_method
    def gens(self):
        """
        Return the tuple of generators.

        .. NOTE::

            This coincides with the sum of the output of
            :meth:`idempotents` and :meth:`arrows`.

        EXAMPLES::

            sage: P = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}}).path_semigroup()
            sage: P.gens()
            (e_1, e_2, e_3, a, b, c, d)
            sage: P.gens() == P.idempotents() + P.arrows()
            True
        """
        return self.idempotents() + self.arrows()

    def is_finite(self):
        """
        This partial semigroup is finite if and only if the underlying
        quiver is acyclic.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}})
            sage: Q.path_semigroup().is_finite()
            True
            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}})
            sage: Q.path_semigroup().is_finite()
            False
        """
        return self._quiver.is_directed_acyclic() and not self._quiver.has_loops()

    def __len__(self):
        """
        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}})
            sage: F = Q.path_semigroup()
            sage: len(F)
            9
            sage: list(F)
            [e_1, e_2, e_3, a, b, c, d, a*d, b*d]
            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}})
            sage: F = Q.path_semigroup()
            sage: len(F)
            Traceback (most recent call last):
            ...
            ValueError: the underlying quiver has cycles, thus, there may be an infinity of directed paths
        """
        return len(self.all_paths())

    def cardinality(self):
        """
        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}})
            sage: F = Q.path_semigroup()
            sage: F.cardinality()
            9
            sage: A = F.algebra(QQ)
            sage: list(A.basis())
            [e_1, e_2, e_3, a, b, c, d, a*d, b*d]
            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}})
            sage: F = Q.path_semigroup()
            sage: F.cardinality()
            +Infinity
            sage: A = F.algebra(QQ)
            sage: list(A.basis())
            Traceback (most recent call last):
            ...
            ValueError: the underlying quiver has cycles, thus, there may be an infinity of directed paths
        """
        from sage.all import ZZ
        if self._quiver.is_directed_acyclic() and not self._quiver.has_loops():
            return ZZ(len(self))
        from sage.rings.infinity import Infinity
        return Infinity

    def __iter__(self):
        """
        Iterate over the elements of ``self``, i.e., over quiver paths.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}})
            sage: P = Q.path_semigroup()
            sage: list(P)
            [e_1, e_2, e_3, a, b, c, d, a*d, b*d]

        The elements are sorted by length. Of course, the list of elements
        is infinite for quivers with cycles. ::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['d']}, 3:{1:['c']}})
            sage: P = Q.path_semigroup()
            sage: P.is_finite()
            False
            sage: list(P)
            Traceback (most recent call last):
            ...
            ValueError: the underlying quiver has cycles, thus, there may be an infinity of directed paths

         However, one can iterate::

            sage: counter = 0
            sage: for p in P:
            ....:     counter += 1
            ....:     print(p)
            ....:     if counter==20:
            ....:         break
            e_1
            e_2
            e_3
            a
            b
            d
            c
            a*d
            b*d
            d*c
            c*a
            c*b
            a*d*c
            b*d*c
            d*c*a
            d*c*b
            c*a*d
            c*b*d
            a*d*c*a
            a*d*c*b

        """
        d = 0
        length_d_available = True
        while length_d_available:
            length_d_available = False
            for v in self._quiver.vertex_iterator():
                for w in self.iter_paths_by_length_and_startpoint(d, v):
                    length_d_available = True
                    yield w
            d += 1

    def iter_paths_by_length_and_startpoint(self, d, v):
        """
        An iterator over quiver paths with a fixed length and start point.

        INPUT:

        - ``d`` -- an integer, the path length
        - ``v`` -- a vertex, start point of the paths

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['d']}, 3:{1:['c']}})
            sage: P = Q.path_semigroup()
            sage: P.is_finite()
            False
            sage: list(P.iter_paths_by_length_and_startpoint(4,1))
            [a*d*c*a, a*d*c*b, b*d*c*a, b*d*c*b]
            sage: list(P.iter_paths_by_length_and_startpoint(5,1))
            [a*d*c*a*d, a*d*c*b*d, b*d*c*a*d, b*d*c*b*d]
            sage: list(P.iter_paths_by_length_and_startpoint(5,2))
            [d*c*a*d*c, d*c*b*d*c]

        TESTS::

             sage: Q = DiGraph({1:{1:['a','b', 'c', 'd']}})
             sage: P = Q.path_semigroup()
             sage: list(P.iter_paths_by_length_and_startpoint(2,1))
             [a*a,
              a*b,
              a*c,
              a*d,
              b*a,
              b*b,
              b*c,
              b*d,
              c*a,
              c*b,
              c*c,
              c*d,
              d*a,
              d*b,
              d*c,
              d*d]
             sage: len(list(P.iter_paths_by_length_and_startpoint(2,1)))
             16
        """
        # iterate over length d paths starting at vertex v
        if not d >= 0:
            raise ValueError("path length must be a non-negative integer")
        if v not in self._quiver:
            raise ValueError("the starting point {} is not a vertex of the underlying quiver".format(v))
        if not d:
            yield self.element_class(self, v, v, [])
        else:
            for w in self.iter_paths_by_length_and_startpoint(d - 1, v):
                for a in self._quiver._backend.iterator_out_edges([w.terminal_vertex()], True):
                    yield self(list(w) + [a], check=False)

    def iter_paths_by_length_and_endpoint(self, d, v):
        """
        An iterator over quiver paths with a fixed length and end point.

        INPUT:

        - ``d`` -- an integer, the path length
        - ``v`` -- a vertex, end point of the paths

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['d']}, 3:{1:['c']}})
            sage: F = Q.path_semigroup()
            sage: F.is_finite()
            False
            sage: list(F.iter_paths_by_length_and_endpoint(4,1))
            [c*a*d*c, c*b*d*c]
            sage: list(F.iter_paths_by_length_and_endpoint(5,1))
            [d*c*a*d*c, d*c*b*d*c]
            sage: list(F.iter_paths_by_length_and_endpoint(5,2))
            [c*a*d*c*a, c*b*d*c*a, c*a*d*c*b, c*b*d*c*b]
        """
        # iterate over length d paths ending at vertex v
        if not d >= 0:
            raise ValueError("path length must be a non-negative integer")
        if v not in self._quiver:
            raise ValueError("the starting point {} is not a vertex of the underlying quiver".format(v))
        if not d:
            yield self.element_class(self, v, v, [])
        else:
            for w in self.iter_paths_by_length_and_endpoint(d - 1, v):
                for a in self._quiver._backend.iterator_in_edges([w.initial_vertex()], True):
                    yield self([a] + list(w), check=False)

    def quiver(self):
        """
        Return the underlying quiver (i.e., digraph) of this path semigroup.

        .. NOTE::

            The returned digraph always is an immutable copy of the originally
            given digraph that is made weighted.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['d']}, 3:{1:['c']}},
            ....:             weighted=False)
            sage: F = Q.path_semigroup()
            sage: F.quiver() == Q
            False
            sage: Q.weighted(True)
            sage: F.quiver() == Q
            True
        """
        return self._quiver

    @cached_method
    def reverse(self):
        """
        The path semigroup of the reverse quiver.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['d']}, 3:{1:['c']}})
            sage: F = Q.path_semigroup()
            sage: F.reverse() is Q.reverse().path_semigroup()
            True
        """
        return self._quiver.reverse().path_semigroup()

    def algebra(self, k, order="negdegrevlex"):
        """
        Return the path algebra of the underlying quiver.

        INPUT:

        - ``k`` -- a commutative ring

        - ``order`` -- optional string, one of "negdegrevlex" (default),
          "degrevlex", "negdeglex" or "deglex", defining the monomial order to
          be used.

        .. NOTE::

            Monomial orders that are not degree orders are not supported.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['d']}, 3:{1:['c']}})
            sage: P = Q.path_semigroup()
            sage: P.algebra(GF(3))
            Path algebra of Multi-digraph on 3 vertices over Finite Field of size 3

        Now some example with different monomial orderings::

            sage: P1 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'))
            sage: P2 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'), order="degrevlex")
            sage: P3 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'), order="negdeglex")
            sage: P4 = DiGraph({1:{1:['x','y','z']}}).path_semigroup().algebra(GF(25,'t'), order="deglex")
            sage: P1.order_string()
            'negdegrevlex'
            sage: sage_eval('(x+2*z+1)^3', P1.gens_dict())
            e_1 + z + 3*x + 2*z*z + x*z + z*x + 3*x*x + 3*z*z*z + 4*x*z*z + 4*z*x*z + 2*x*x*z + 4*z*z*x + 2*x*z*x + 2*z*x*x + x*x*x
            sage: sage_eval('(x+2*z+1)^3', P2.gens_dict())
            3*z*z*z + 4*x*z*z + 4*z*x*z + 2*x*x*z + 4*z*z*x + 2*x*z*x + 2*z*x*x + x*x*x + 2*z*z + x*z + z*x + 3*x*x + z + 3*x + e_1
            sage: sage_eval('(x+2*z+1)^3', P3.gens_dict())
            e_1 + z + 3*x + 2*z*z + z*x + x*z + 3*x*x + 3*z*z*z + 4*z*z*x + 4*z*x*z + 2*z*x*x + 4*x*z*z + 2*x*z*x + 2*x*x*z + x*x*x
            sage: sage_eval('(x+2*z+1)^3', P4.gens_dict())
            3*z*z*z + 4*z*z*x + 4*z*x*z + 2*z*x*x + 4*x*z*z + 2*x*z*x + 2*x*x*z + x*x*x + 2*z*z + z*x + x*z + 3*x*x + z + 3*x + e_1

        """
        from sage.quivers.algebra import PathAlgebra
        return PathAlgebra(k, self, order)

    ###########################################################################
    #                                                                         #
    # REPRESENTATION THEORETIC FUNCTIONS                                      #
    #    These functions involve the representation theory of quivers.        #
    #                                                                         #
    ###########################################################################

    def representation(self, k, *args, **kwds):
        """
        Return a representation of the quiver.

        For more information see the
        :class:`~sage.quivers.representation.QuiverRep` documentation.

        TESTS::

            sage: Q = DiGraph({1:{3:['a']}, 2:{3:['b']}}).path_semigroup()
            sage: spaces = {1: QQ^2, 2: QQ^3, 3: QQ^2}
            sage: maps = {(1, 3, 'a'): (QQ^2).Hom(QQ^2).identity(), (2, 3, 'b'): [[1, 0], [0, 0], [0, 0]]}
            sage: M = Q.representation(QQ, spaces, maps)
            sage: M
            Representation with dimension vector (2, 3, 2)
        """
        return QuiverRep(k, self, *args, **kwds)

    def S(self, k, vertex):
        """
        Return the simple module over `k` at the given vertex
        ``vertex``.

        This module is literally simple only when `k` is a field.

        INPUT:

        - `k` -- ring, the base ring of the representation

        - ``vertex`` -- integer, a vertex of the quiver

        OUTPUT:

        - :class:`~sage.quivers.representation.QuiverRep`, the simple module
          at ``vertex`` with base ring `k`

        EXAMPLES::

            sage: P = DiGraph({1:{2:['a','b']}, 2:{3:['c','d']}}).path_semigroup()
            sage: S1 = P.S(GF(3), 1)
            sage: P.S(ZZ, 3).dimension_vector()
            (0, 0, 1)
            sage: P.S(ZZ, 1).dimension_vector()
            (1, 0, 0)

        The vertex given must be a vertex of the quiver::

            sage: P.S(QQ, 4)
            Traceback (most recent call last):
            ...
            ValueError: must specify a valid vertex of the quiver
        """
        if vertex not in self._quiver:
            raise ValueError("must specify a valid vertex of the quiver")

        # This is the module with k at the given vertex and zero elsewhere.  As
        # all maps are zero we only need to specify that the given vertex has
        # dimension 1 and the constructor will zero out everything else.
        return QuiverRep(k, self, {vertex: 1})

    simple = S

    def P(self, k, vertex):
        """
        Return the indecomposable projective module over `k` at the given
        vertex ``vertex``.

        This module is literally indecomposable only when `k` is a field.

        INPUT:

        - `k` -- ring, the base ring of the representation

        - ``vertex`` -- integer, a vertex of the quiver

        OUTPUT:

        - :class:`~sage.quivers.representation.QuiverRep`, the indecomposable
          projective module at ``vertex`` with base ring `k`

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['c','d']}}).path_semigroup()
            sage: P2 = Q.P(GF(3), 2)
            sage: Q.P(ZZ, 3).dimension_vector()
            (0, 0, 1)
            sage: Q.P(ZZ, 1).dimension_vector()
            (1, 2, 4)

        The vertex given must be a vertex of the quiver::

            sage: Q.P(QQ, 4)
            Traceback (most recent call last):
            ...
            ValueError: must specify a valid vertex of the quiver
        """
        if vertex not in self._quiver:
            raise ValueError("must specify a valid vertex of the quiver")
        return QuiverRep(k, self, [[(vertex, vertex)]], option='paths')

    projective = P

    def I(self, k, vertex):
        """
        Return the indecomposable injective module over `k` at the
        given vertex ``vertex``.

        This module is literally indecomposable only when `k` is a field.

        INPUT:

        - `k` -- ring, the base ring of the representation

        - ``vertex`` -- integer, a vertex of the quiver

        OUTPUT:

        - :class:`~sage.quivers.representation.QuiverRep`, the indecomposable
          injective module at vertex ``vertex`` with base ring `k`

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b']}, 2:{3:['c','d']}}).path_semigroup()
            sage: I2 = Q.I(GF(3), 2)
            sage: Q.I(ZZ, 3).dimension_vector()
            (4, 2, 1)
            sage: Q.I(ZZ, 1).dimension_vector()
            (1, 0, 0)

        The vertex given must be a vertex of the quiver::

            sage: Q.I(QQ, 4)
            Traceback (most recent call last):
            ...
            ValueError: must specify a valid vertex of the quiver
        """
        if vertex not in self._quiver:
            raise ValueError("must specify a valid vertex of the quiver")
        return QuiverRep(k, self, [[(vertex, vertex)]], option='dual paths')

    injective = I

    def free_module(self, k):
        """
        Return a free module of rank `1` over ``kP``, where `P` is
        ``self``. (In other words, the regular representation.)

        INPUT:

        - ``k`` -- ring, the base ring of the representation.

        OUTPUT:

        - :class:`~sage.quivers.representation.QuiverRep_with_path_basis`, the
          path algebra considered as a right module over itself.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a', 'b'], 3: ['c', 'd']}, 2:{3:['e']}}).path_semigroup()
            sage: Q.free_module(GF(3)).dimension_vector()
            (1, 3, 6)
        """
        return QuiverRep(k, self, [[(v, v)] for v in self._quiver], option='paths')

    @lazy_attribute
    def _poincare_series(self):
        r"""
        The Poincaré series matrix of this path semigroup.

        The coefficient `(i,j)` of the matrix is a generating function for the
        number of paths from vertex number `i` to vertex number `j`.

        EXAMPLES::

            sage: S = DiGraph({0:{1:['a'], 2:['b']}, 1:{0:['c'], 1:['d']}, 2:{0:['e'],2:['f']}}).path_semigroup()
            sage: S._poincare_series
            [             (-t + 1)/(-2*t^2 - t + 1)                     t/(-2*t^2 - t + 1)                     t/(-2*t^2 - t + 1)]
            [                    t/(-2*t^2 - t + 1) (t^2 + t - 1)/(-2*t^3 + t^2 + 2*t - 1)          -t^2/(-2*t^3 + t^2 + 2*t - 1)]
            [                    t/(-2*t^2 - t + 1)          -t^2/(-2*t^3 + t^2 + 2*t - 1) (-t^2 - t + 1)/(2*t^3 - t^2 - 2*t + 1)]

        Let us check that the result is plausible. We study the paths from
        vertex `1` to vertex `2`. The number of paths sorted by length are
        given by the power series expansion of the corresponding matrix
        entry::

            sage: P = PowerSeriesRing(ZZ, 't', default_prec=10)
            sage: P(S._poincare_series[1,2].numerator())/P(S._poincare_series[1,2].denominator())
            t^2 + 2*t^3 + 5*t^4 + 10*t^5 + 21*t^6 + 42*t^7 + 85*t^8 + 170*t^9 + 341*t^10 + 682*t^11 + O(t^12)

        For comparison, we list the paths that start at `1` and end at `2`, by
        length, and see that the Poincaré series matrix predicts the correct
        numbers::

            sage: [p for p in S.iter_paths_by_length_and_startpoint(1,1) if p.terminal_vertex()==2]
            []
            sage: [p for p in S.iter_paths_by_length_and_startpoint(2,1) if p.terminal_vertex()==2]
            [c*b]
            sage: [p for p in S.iter_paths_by_length_and_startpoint(3,1) if p.terminal_vertex()==2]
            [c*b*f, d*c*b]
            sage: [p for p in S.iter_paths_by_length_and_startpoint(4,1) if p.terminal_vertex()==2]
            [c*a*c*b, c*b*e*b, c*b*f*f, d*c*b*f, d*d*c*b]
            sage: len([p for p in S.iter_paths_by_length_and_startpoint(5,1) if p.terminal_vertex()==2])
            10
            sage: len([p for p in S.iter_paths_by_length_and_startpoint(10,1) if p.terminal_vertex()==2])
            341
        """
        P = ZZ['t']
        t = P.gen()
        M = self._quiver.adjacency_matrix()
        out = ~(1 - M * t)
        out.set_immutable()
        return out

    def all_paths(self, start=None, end=None):
        """
        List of all paths between a pair of vertices ``(start, end)``.

        INPUT:

        - ``start`` -- integer or ``None`` (default: ``None``); the initial
          vertex of the paths in the output; if ``None`` is given then
          the initial vertex is arbitrary.
        - ``end`` -- integer or ``None`` (default: ``None``); the terminal
          vertex of the paths in the output; if ``None`` is given then
          the terminal vertex is arbitrary

        OUTPUT:

        - list of paths, excluding the invalid path

        .. TODO::

            This currently does not work for quivers with cycles, even if
            there are only finitely many paths from ``start`` to ``end``.

        .. NOTE::

            If there are multiple edges between two vertices, the method
            :meth:`sage.graphs.digraph.all_paths` will not differentiate
            between them. But this method, which is not for digraphs but for
            their path semigroup associated with them, will.

        EXAMPLES::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 2:{3:['d']}})
            sage: F = Q.path_semigroup()
            sage: F.all_paths(1, 3)
            [a*d, b*d, c]

        If ``start=end`` then we expect only the trivial path at that vertex::

            sage: F.all_paths(1, 1)
            [e_1]

        The empty list is returned if there are no paths between the
        given vertices::

            sage: F.all_paths(3, 1)
            []

        If ``end=None`` then all edge paths beginning at ``start`` are
        returned, including the trivial path::

            sage: F.all_paths(2)
            [e_2, d]

        If ``start=None`` then all edge paths ending at ``end`` are
        returned, including the trivial path.  Note that the two edges
        from vertex 1 to vertex 2 count as two different edge paths::

            sage: F.all_paths(None, 2)
            [a, b, e_2]
            sage: F.all_paths(end=2)
            [a, b, e_2]

        If ``start=end=None`` then all edge paths are returned, including
        trivial paths::

            sage: F.all_paths()
            [e_1, a, b, a*d, b*d, c, e_2, d, e_3]

        The vertex given must be a vertex of the quiver::

            sage: F.all_paths(1, 4)
            Traceback (most recent call last):
            ...
            ValueError: the end vertex 4 is not a vertex of the quiver

        If the underlying quiver is cyclic, a ``ValueError``
        is raised::

            sage: Q = DiGraph({1:{2:['a','b'], 3:['c']}, 3:{1:['d']}})
            sage: F = Q.path_semigroup()
            sage: F.all_paths()
            Traceback (most recent call last):
            ...
            ValueError: the underlying quiver has cycles, thus, there may be an infinity of directed paths

        TESTS:

        We check a single edge with a multi-character label::

            sage: Q = DiGraph([[1,2,'abc']])
            sage: PQ = Q.path_semigroup()
            sage: PQ.all_paths(1,2)
            [abc]

        An example with multiple edges::

            sage: Q = DiGraph([[1,2,'abc'], [1,2,'def']], multiedges=True)
            sage: PQ = Q.path_semigroup()
            sage: PQ.all_paths(1,2)
            [abc, def]
        """
        # Check that given arguments are vertices
        if start is not None and start not in self._quiver:
            raise ValueError("the start vertex {} is not a vertex of the quiver".format(start))
        if end is not None and end not in self._quiver:
            raise ValueError("the end vertex {} is not a vertex of the quiver".format(end))

        # Handle quivers with cycles
        Q = self._quiver
        if not (Q.is_directed_acyclic()):
            raise ValueError("the underlying quiver has cycles, thus, there may be an infinity of directed paths")

        # Handle start=None
        if start is None:
            results = []
            for v in Q:
                results += self.all_paths(v, end)
            return results

        # Handle end=None
        if end is None:
            results = []
            for v in Q:
                results += self.all_paths(start, v)
            return results

        # Handle the trivial case
        if start == end:
            return [self.element_class(self, start, end, [])]

        # This function will recursively convert a path given in terms of
        # vertices to a list of QuiverPaths.
        def _v_to_e(path):
            if len(path) == 1:
                return [self.element_class(self, path[0], path[0], [])]
            paths = []
            l = Q.edge_label(path[0], path[1])
            if isinstance(l, str):
                for b in _v_to_e(path[1:]):
                    paths.append(self([(path[0], path[1], l)] + list(b), check=False))
            else:
                for a in l:
                    for b in _v_to_e(path[1:]):
                        paths.append(self([(path[0], path[1], a)] + list(b), check=False))
            return paths

        # For each vertex path we append the resulting edge paths
        result = []
        for path in Q.all_paths(start, end):
            result += _v_to_e(path)

        # The result is all paths from start to end
        return result
