"""
Homomorphisms of Lie Algebras

AUTHORS:

- Travis Scrimshaw (07-15-2013): Initial implementation
- Eero Hakavuori (08-09-2018): Morphisms defined by a generating subset
"""
# ****************************************************************************
#  Copyright (C) 2013 Travis Scrimshaw <tscrim at ucdavis.edu>
#                2018 Eero Hakavuori <eero.hakavuori at gmail.com>
#
#  Distributed under the terms of the GNU General Public License (GPL)
#                  https://www.gnu.org/licenses/
# *****************************************************************************

from sage.misc.cachefunc import cached_method
from sage.categories.morphism import Morphism
from sage.categories.morphism import SetMorphism
from sage.categories.homset import Homset, Hom
from sage.structure.sequence import Sequence, Sequence_generic
from sage.structure.element import get_coercion_model
from sage.structure.richcmp import richcmp
from sage.matrix.constructor import matrix
from itertools import combinations

# TODO: Refactor out common functionality with RingHomomorphism_im_gens
class LieAlgebraHomomorphism_im_gens(Morphism):
    r"""
    A homomorphism of Lie algebras.

    Let `\mathfrak{g}` and `\mathfrak{g}^{\prime}` be Lie algebras.
    A linear map `f \colon \mathfrak{g} \to \mathfrak{g}^{\prime}` is a
    homomorphism (of Lie algebras) if `f([x, y]) = [f(x), f(y)]` for all
    `x, y \in \mathfrak{g}`. Thus homomorphisms are completely determined
    by the image of the generators of `\mathfrak{g}`.

    EXAMPLES::

        sage: L = LieAlgebra(QQ, 'x,y,z')
        sage: Lyn = L.Lyndon()
        sage: H = L.Hall()
        doctest:warning...:
        FutureWarning: The Hall basis has not been fully proven correct, but currently no bugs are known
        See http://trac.sagemath.org/16823 for details.
        sage: phi = Lyn.coerce_map_from(H); phi
        Lie algebra morphism:
          From: Free Lie algebra generated by (x, y, z) over Rational Field in the Hall basis
          To:   Free Lie algebra generated by (x, y, z) over Rational Field in the Lyndon basis
          Defn: x |--> x
                y |--> y
                z |--> z
    """
    def __init__(self, parent, im_gens, check=True):
        """
        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: phi = Lyn.coerce_map_from(H)

        We skip the category test because the Homset's element class
        does not match this class::

            sage: TestSuite(phi).run(skip=['_test_category'])
        """
        Morphism.__init__(self, parent)
        if not isinstance(im_gens, Sequence_generic):
            if not isinstance(im_gens, (tuple, list)):
                im_gens = [im_gens]
            im_gens = Sequence(im_gens, parent.codomain(), immutable=True)
        if check:
            if len(im_gens) != len(parent.domain().lie_algebra_generators()):
                raise ValueError("number of images must equal number of generators")
            # TODO: Implement a (meaningful) _is_valid_homomorphism_()
            #if not parent.domain()._is_valid_homomorphism_(parent.codomain(), im_gens):
            #    raise ValueError("relations do not all (canonically) map to 0 under map determined by images of generators.")
        if not im_gens.is_immutable():
            import copy
            im_gens = copy.copy(im_gens)
            im_gens.set_immutable()
        self._im_gens = im_gens

    def _repr_type(self):
        """
        TESTS::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: f = Lyn.coerce_map_from(H)
            sage: type(f)
            <class 'sage.algebras.lie_algebras.morphism.LieAlgebraHomomorphism_im_gens'>
            sage: f._repr_type()
            'Lie algebra'
        """
        return "Lie algebra"

    def im_gens(self):
        """
        Return the images of the generators of the domain.

        OUTPUT:

        - ``list`` -- a copy of the list of gens (it is safe to change this)

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: f = Lyn.coerce_map_from(H)
            sage: f.im_gens()
            [x, y, z]
        """
        return list(self._im_gens)

    def _richcmp_(self, other, op):
        """
        Rich comparisons.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: x,y,z = H.gens()
            sage: f = Hom(Lyn, H)([x,y,z])
            sage: g = Hom(Lyn, H)([x,y,z])
            sage: h = Hom(Lyn, H)([y,x,z])
            sage: f == g
            True
            sage: f is g
            False
            sage: f != h
            True
        """
        return richcmp(self._im_gens, other._im_gens, op)

    def __hash__(self):
        """
        Return the hash of this morphism.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: phi = Lyn.coerce_map_from(H)
            sage: hash(phi) == hash(phi)
            True
        """
        return hash(self._im_gens)

    def _repr_defn(self):
        """
        Used in constructing string representation of ``self``.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: phi = Lyn.coerce_map_from(H)
            sage: print(phi._repr_defn())
            x |--> x
            y |--> y
            z |--> z
        """
        D = self.domain()
        ig = self._im_gens
        return '\n'.join(['%s |--> %s'%(x, ig[i]) for i, x in enumerate(D.gens())])

    def _call_(self, x):
        """
        Evaluate this homomorphism at ``x``.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: phi = Lyn.coerce_map_from(H)
            sage: a = H.graded_basis(5)[12]; a
            [z, [y, [x, [x, z]]]]
            sage: phi(a)
            [x, [[x, z], [y, z]]] + [x, [[[x, z], z], y]]
             + [[x, y], [[x, z], z]] + [[x, [y, z]], [x, z]]
        """
        return x._im_gens_(self.codomain(), self.im_gens())

class LieAlgebraHomset(Homset):
    """
    Homset between two Lie algebras.

    .. TODO::

        This is a very minimal implementation which does not
        have coercions of the morphisms.
    """
    def __init__(self, X, Y, category=None, base=None, check=True):
        """
        Initalize ``self``.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: HS = Hom(Lyn, H)

        We skip the elements test since homsets are not proper parents::

            sage: TestSuite(HS).run(skip=["_test_elements"])
        """
        if base is None:
            base = X.base_ring()
        Homset.__init__(self, X, Y, category, base, check)

    def _repr_(self):
        """
        Return a string representation of ``self``.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: Hom(Lyn, H)
            Set of Lie algebra morphisms
             from Free Lie algebra generated by (x, y, z) over Rational Field in the Lyndon basis
             to Free Lie algebra generated by (x, y, z) over Rational Field in the Hall basis
        """
        return "Set of Lie algebra morphisms from {} to {}".format(
                                self.domain(), self.codomain())

    def __call__(self, im_gens, check=True):
        """
        Construct a morphism from ``im_gens``.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: HS = Hom(Lyn, H)
            sage: x,y,z = Lyn.gens()
            sage: phi = HS([z,x,y])
            sage: phi(x + Lyn[z,y])
            z - [x, y]

            sage: HS = Hom(Lyn, Lyn)
            sage: phi = HS([z,x,y])
            sage: a = Lyn([z, [x, [[y, z], x]]]); a
            [x, [x, [[y, z], z]]] + [x, [[x, z], [y, z]]] - [[x, [y, z]], [x, z]]
            sage: phi(a)
            [[x, [[y, z], z]], y] + 2*[[[x, z], [y, z]], y] + [[[[x, z], z], y], y]
            sage: phi(phi(phi(a))) == a
            True
        """
        if isinstance(im_gens, Morphism):
            return im_gens
        from sage.categories.lie_algebras import LieAlgebras
        if (self.domain() in LieAlgebras.FiniteDimensional.WithBasis
            and self.codomain() in LieAlgebras.FiniteDimensional.WithBasis):
            try:
                return LieAlgebraMorphism_from_generators(self.domain(), im_gens,
                                                          codomain=self.codomain(),
                                                          check=check)
            except (TypeError, ValueError):
                pass
        return LieAlgebraHomomorphism_im_gens(self, im_gens)

    @cached_method
    def zero(self):
        """
        Return the zero morphism.

        EXAMPLES::

            sage: L = LieAlgebra(QQ, 'x,y,z')
            sage: Lyn = L.Lyndon()
            sage: H = L.Hall()
            sage: HS = Hom(Lyn, H)
            sage: HS.zero()
            Generic morphism:
              From: Free Lie algebra generated by (x, y, z) over Rational Field in the Lyndon basis
              To:   Free Lie algebra generated by (x, y, z) over Rational Field in the Hall basis
        """
        return SetMorphism(self, lambda x: self.codomain().zero())

    _an_element_ = zero

class LieAlgebraMorphism_from_generators(LieAlgebraHomomorphism_im_gens):
    r"""
    A morphism between two Lie algebras defined by images of a
    generating set as a Lie algebra.

    This is the Lie algebra morphism `\phi \colon L \to K` defined on
    the chosen basis of `L` to that of `K` be using the image of some
    generating set (as a Lie algebra) of `L`.

    INPUT:

    - ``on_generators`` -- dictionary ``{X: Y}`` of the images `Y`
      in ``codomain`` of elements `X` of ``domain``
    - ``codomain`` -- a Lie algebra (optional); this is inferred
      from the values of ``on_generators`` if not given
    - ``check`` -- (default: ``True``) boolean; if ``False`` the
      values  on the Lie brackets implied by ``on_generators`` will
      not be checked for contradictory values

    EXAMPLES:

    A reflection of one horizontal vector in the Heisenberg algebra::

        sage: L.<X,Y,Z> = LieAlgebra(QQ, {('X','Y'): {'Z':1}})
        sage: phi = L.morphism({X:-X, Y:Y}); phi
        Lie algebra endomorphism of Lie algebra on 3 generators (X, Y, Z) over Rational Field
          Defn: X |--> -X
                Y |--> Y
                Z |--> -Z

    There is no Lie algebra morphism that reflects one horizontal vector,
    but not the vertical one::

        sage: L.morphism({X:-X, Y:Y, Z:Z})
        Traceback (most recent call last):
        ...
        ValueError: this does not define a Lie algebra morphism;
         contradictory values for brackets of length 2

    Checking for mistakes can be disabled, which can produce
    invalid results::

        sage: phi = L.morphism({X:-X, Y:Y, Z:Z}, check=False); phi
        Lie algebra endomorphism of Lie algebra on 3 generators (X, Y, Z) over Rational Field
          Defn: X |--> -X
                Y |--> Y
                Z |--> Z
        sage: L[phi(X), phi(Y)] == phi(L[X,Y])
        False

    The set of keys must generate the Lie algebra::

        sage: L.morphism({X: X})
        Traceback (most recent call last):
        ...
        ValueError: [X] is not a generating set of Lie algebra on 3 generators
        (X, Y, Z) over Rational Field

    Over non-fields, generating subsets are more restricted::

        sage: L.<X,Y,Z> = LieAlgebra(ZZ, {('X','Y'): {'Z':2}})
        sage: L.morphism({X: X, Y: Y})
        Traceback (most recent call last):
        ...
        ValueError: [X, Y] is not a generating set of Lie algebra on 3
        generators (X, Y, Z) over Integer Ring

    The generators do not have to correspond to the defined generating
    set of the domain::

        sage: L.<X,Y,Z,W> = LieAlgebra(QQ, {('X','Y'): {'Z':1}, ('X','Z'): {'W':1}})
        sage: K.<A,B,C> = LieAlgebra(QQ, {('A','B'): {'C':2}})
        sage: phi = L.morphism({X+2*Y: A, X-Y: B}); phi
        Lie algebra morphism:
          From: Lie algebra on 4 generators (X, Y, Z, W) over Rational Field
          To:   Lie algebra on 3 generators (A, B, C) over Rational Field
          Defn: X |--> 1/3*A + 2/3*B
                Y |--> 1/3*A - 1/3*B
                Z |--> -2/3*C
                W |--> 0
        sage: phi(X+2*Y)
        A
        sage: phi(X)
        1/3*A + 2/3*B
        sage: phi(W)
        0
        sage: phi(Z)
        -2/3*C
        sage: all(K[phi(p), phi(q)] == phi(L[p,q])
        ....:     for p in L.basis() for q in L.basis())
        True

    A quotient type Lie algebra morphism::

        sage: K.<A,B> = LieAlgebra(SR, abelian=True)
        sage: L.morphism({X: A, Y: B})
        Lie algebra morphism:
          From: Lie algebra on 4 generators (X, Y, Z, W) over Rational Field
          To:   Abelian Lie algebra on 2 generators (A, B) over Symbolic Ring
          Defn: X |--> A
                Y |--> B
                Z |--> 0
                W |--> 0
    """
    def __init__(self, on_generators, domain=None, codomain=None, check=True):
        r"""
        Initialize ``self``.

        The keys of ``on_generators`` need to generate ``domain``
        as a Lie algebra.

        .. TODO::

            It might be possible to extract an explicit bracket relation that
            fails whenever some linear system fails to be solved. This would
            allow outputting an even more explicit error.

        TESTS:

        Test suite for a morphism::

            sage: L.<X,Y,Z,W> = LieAlgebra(QQ, {('X','Y'): {'Z':1}, ('X','Z'): {'W':1}})
            sage: K.<A,B,C> = LieAlgebra(QQ, {('A','B'): {'C':2}})
            sage: phi = L.morphism({X+2*Y: A, X-Y: B})
            sage: TestSuite(phi).run(skip=['_test_category'])

        Failure of inferring codomain::

            sage: L.<X> = LieAlgebra(QQ, abelian=True)
            sage: L.morphism({X: int(1)})
            Traceback (most recent call last):
            ...
            TypeError: codomain <type 'int'> is not a Lie algebra

            sage: from sage.algebras.lie_algebras.morphism import LieAlgebraMorphism_from_generators
            sage: LieAlgebraMorphism_from_generators({ZZ(1): X})
            Traceback (most recent call last):
            ...
            TypeError: domain Integer Ring is not a Lie algebra
            sage: LieAlgebraMorphism_from_generators({})
            Traceback (most recent call last):
            ...
            ValueError: no elements to infer domain from
            sage: LieAlgebraMorphism_from_generators({}, domain=L)
            Traceback (most recent call last):
            ...
            ValueError: no elements to infer codomain from
        """
        from sage.categories.lie_algebras import LieAlgebras

        cm = get_coercion_model()
        if domain is None:
            if not on_generators:
                raise ValueError("no elements to infer domain from")
            domain = cm.common_parent(*on_generators)
            if domain not in LieAlgebras:
                raise TypeError("domain %s is not a Lie algebra" % domain)
        if codomain is None:
            if not on_generators:
                raise ValueError("no elements to infer codomain from")
            codomain = cm.common_parent(*list(on_generators.values()))
            if codomain not in LieAlgebras:
                raise TypeError("codomain %s is not a Lie algebra" % codomain)

        parent = Hom(domain, codomain)
        m = domain.module()
        cm = codomain.module()

        spanning_set = [X.to_vector() for X in on_generators]
        im_gens = [Y.to_vector() for Y in on_generators.values()]

        if not im_gens:
            LieAlgebraHomomorphism_im_gens.__init__(self, parent, [], check=check)
            return

        # helper function to solve linear systems Ax = b, where both x and b
        # are vectors of vectors
        def solve_linear_system(A, b, check):
            R = cm.base_ring()
            A_inv = A.solve_left(matrix.identity(A.ncols()))

            if check:
                # Verify validity of solution x = A_inv*b. Since b is a vector of
                # vectors, need to expand the matrix product by hand.
                M = A * A_inv
                for Mi, bk in zip(M.rows(), b):
                    test_bk = sum((R(Mij) * bj for Mij,bj in zip(Mi,b)), cm.zero())
                    if test_bk != bk:
                        raise ValueError("contradictory linear system")

            return [sum((R(Aij) * bk for Aij,bk in zip(Ai,b)), cm.zero())
                    for Ai in A_inv.rows()]

        bracketlength = 1
        n = 0
        while True:
            sm = m.submodule(spanning_set)
            A = matrix(sm.base_ring(), [sm.coordinate_vector(X) for X in spanning_set])
            try:
                im_gens = solve_linear_system(A, im_gens, check)
            except ValueError:
                raise ValueError("this does not define a Lie algebra morphism; "
                                 "contradictory values for brackets of length %d"
                                 % bracketlength)

            spanning_set = list(sm.basis())
            if n == len(spanning_set):
                # no increase in dimension => no further values will be computed
                break

            # compute brackets and repeat
            bracketlength += 1
            n = len(spanning_set)
            for i,j in combinations(range(n), 2):
                # add the value of the bracket to known images
                Z = domain.bracket(spanning_set[i], spanning_set[j])
                imZ = codomain.bracket(im_gens[i], im_gens[j])
                spanning_set.append(Z.to_vector())
                im_gens.append(imZ.to_vector())

        # verify that sm is the full module m
        if not sm.has_coerce_map_from(m):
            raise ValueError("%s is not a generating set of %s"
                             % (list(on_generators), domain))

        A = matrix(m.base_ring(), spanning_set)
        im_gens = solve_linear_system(A, im_gens, check)

        LieAlgebraHomomorphism_im_gens.__init__(self, parent, im_gens, check=check)

    def _call_(self, x):
        """
        Evaluate this homomorphism at ``x``.

        EXAMPLES::

            sage: L.<X,Y,Z,W> = LieAlgebra(QQ, {('X','Y'): {'Z':1}, ('X','Z'): {'W':1}})
            sage: K.<A,B> = LieAlgebra(SR, abelian=True)
            sage: phi = L.morphism({X: A, Y: B})
            sage: phi(X)
            A
            sage: phi(Y)
            B
            sage: phi(Z)
            0
            sage: phi(W)
            0
            sage: phi(-X + 3*Y)
            -A + 3*B

            sage: K.<A,B,C> = LieAlgebra(QQ, {('A','B'): {'C':2}})
            sage: phi = L.morphism({X+2*Y: A, X-Y: B})
            sage: phi(X)
            1/3*A + 2/3*B
            sage: phi(Y)
            1/3*A - 1/3*B
            sage: phi(3*X+Y)
            4/3*A + 5/3*B
            sage: phi(3/2*W-Z+Y)
            1/3*A - 1/3*B + 2/3*C
        """
        C = self.codomain()
        return C.sum(c * self._im_gens[i] for i, c in x.to_vector().iteritems())
