"""Support for aggregation-based AMG."""


from warnings import warn
import numpy as np
from scipy.sparse import csr_array, issparse, SparseEfficiencyWarning

from ..multilevel import MultilevelSolver
from ..relaxation.smoothing import change_smoothers
from ..relaxation.utils import relaxation_as_linear_operator
from ..util.utils import scale_T, get_Cpt_params, \
    eliminate_diag_dom_nodes, get_blocksize, \
    levelize_strength_or_aggregation, asfptype, \
    levelize_smooth_or_improve_candidates
from ..strength import classical_strength_of_connection, \
    symmetric_strength_of_connection, evolution_strength_of_connection, \
    energy_based_strength_of_connection, distance_strength_of_connection, \
    algebraic_distance, affinity_distance
from .aggregate import standard_aggregation, naive_aggregation, \
    lloyd_aggregation, pairwise_aggregation
from .tentative import fit_candidates
from .smooth import energy_prolongation_smoother


def rootnode_solver(A, B=None, BH=None,
                    symmetry='hermitian', strength='symmetric',
                    aggregate='standard', smooth='energy',
                    presmoother=('block_gauss_seidel',
                                 {'sweep': 'symmetric'}),
                    postsmoother=('block_gauss_seidel',
                                  {'sweep': 'symmetric'}),
                    improve_candidates=('block_gauss_seidel',
                                        {'sweep': 'symmetric',
                                         'iterations': 4}),
                    max_levels=10, max_coarse=10,
                    diagonal_dominance=False, keep=False, **kwargs):
    """Create a multilevel solver using root-node based Smoothed Aggregation (SA).

    See the notes below, for the major differences with the classical-style
    smoothed aggregation solver in aggregation.smoothed_aggregation_solver.

    Parameters
    ----------
    A : csr_array, bsr_array
        Sparse NxN matrix in CSR or BSR format.

    B : None, array
        Right near-nullspace candidates stored in the columns of an NxK array.
        K must be >= the blocksize of A (see reference [2]_). The default value
        ``B=None`` is equivalent to choosing the constant over each block-variable,
        ``B=np.kron(np.ones((A.shape[0]/get_blocksize(A), 1)), np.eye(get_blocksize(A)))``.

    BH : None, array
        Left near-nullspace candidates stored in the columns of an NxK array.
        BH is only used if symmetry='nonsymmetric'.  K must be >= the
        blocksize of A (see reference [2]_). The default value B=None is
        equivalent to choosing the constant over each block-variable,
        ``B=np.kron(np.ones((A.shape[0]/get_blocksize(A), 1)), np.eye(get_blocksize(A)))``.

    symmetry : str
        - 'symmetric' refers to both real and complex symmetric
        - 'hermitian' refers to both complex Hermitian and real Hermitian
        - 'nonsymmetric' i.e. nonsymmetric in a hermitian sense

        Note that for the strictly real case, symmetric and hermitian are the same.
        Note that this flag does not denote definiteness of the operator.

    strength : list
        Method used to determine the strength of connection between unknowns of
        the linear system.  Method-specific parameters may be passed in using a
        tuple, e.g. strength=('symmetric',{'theta' : 0.25 }). If strength=None,
        all nonzero entries of the matrix are considered strong.

    aggregate : list
        Method used to aggregate nodes.

    smooth : list
        Method used to smooth the tentative prolongator.  Method-specific
        parameters may be passed in using a tuple, e.g.  smooth=
        ('energy',{'krylov' : 'gmres'}).  Only 'energy' and None are valid
        prolongation smoothing options.

    presmoother : tuple, str, list
        Defines the presmoother for the multilevel cycling.  The default block
        Gauss-Seidel option defaults to point-wise Gauss-Seidel, if the matrix
        is CSR or is a BSR matrix with blocksize of 1.  See notes below for
        varying this parameter on a per level basis.

    postsmoother : tuple, str, list
        Same as presmoother, except defines the postsmoother.

    improve_candidates : tuple, str, list
        The ith entry defines the method used to improve the candidates B on
        level i.  If the list is shorter than max_levels, then the last entry
        will define the method for all levels lower.  If tuple or string, then
        this single relaxation descriptor defines improve_candidates on all
        levels.
        The list elements are relaxation descriptors of the form used for
        presmoother and postsmoother.  A value of None implies no action on B.

    max_levels : int
        Maximum number of levels to be used in the multilevel solver.

    max_coarse : int
        Maximum number of variables permitted on the coarse grid.

    diagonal_dominance : bool, tuple
        If True (or the first tuple entry is True), then avoid coarsening
        diagonally dominant rows.  The second tuple entry requires a
        dictionary, where the key value 'theta' is used to tune the diagonal
        dominance threshold.

    keep : bool
        Flag to indicate keeping extra operators in the hierarchy for
        diagnostics.  For example, if True, then strength of connection (C),
        tentative prolongation (T), aggregation (AggOp), and arrays
        storing the C-points (Cpts) and F-points (Fpts) are kept at
        each level.
    **kwargs : dict
        Extra keywords passed to the Multilevel class

        =============   =======================================================
        cycle_type      ['V','W','F'], Structrure of multigrid cycle
        coarse_solver   ['splu', 'lu', 'cholesky, 'pinv', 'gauss_seidel', ... ]
                        Solver used at the coarsest level of the MG hierarchy.
                        Optionally, may be a tuple (fn, args), where fn is a
                        string such as ['splu', 'lu', ...] or a callable
                        function, and args is a dictionary of arguments to be
                        passed to fn.
        =============   =======================================================

        See MultiLevel class for more details.

    Returns
    -------
    MultilevelSolver
        Multigrid hierarchy of matrices and prolongation operators.

    See Also
    --------
    MultilevelSolver
    aggregation.smoothed_aggregation_solver
    classical.ruge_stuben_solver

    Notes
    -----
         - Root-node style SA differs from classical SA primarily by preserving
           and identity block in the interpolation operator, P.  Each aggregate
           has a 'root-node' or 'center-node' associated with it, and this
           root-node is injected from the coarse grid to the fine grid.  The
           injection corresponds to the identity block.

         - Only smooth={'energy', None} is supported for prolongation
           smoothing.  See reference [2]_ below for more details on why the
           'energy' prolongation smoother is the natural counterpart to
           root-node style SA.

         - The additional parameters are passed through as arguments to
           MultilevelSolver.  Refer to pyamg.MultilevelSolver for additional
           documentation.

         - At each level, four steps are executed in order to define the coarser
           level operator.

           1. Matrix A is given and used to derive a strength matrix, C.

           2. Based on the strength matrix, indices are grouped or aggregated.

           3. The aggregates define coarse nodes and a tentative prolongation
              operator T is defined by injection

           4. The tentative prolongation operator is smoothed by a relaxation
              scheme to improve the quality and extent of interpolation from the
              aggregates to fine nodes.

         - The parameters smooth, strength, aggregate, presmoother, postsmoother
           can be varied on a per level basis.  For different methods on
           different levels, use a list as input so that the i-th entry defines
           the method at the i-th level.  If there are more levels in the
           hierarchy than list entries, the last entry will define the method
           for all levels lower.

           Examples are:
           smooth=[('jacobi', {'omega':1.0}), None, 'jacobi']
           presmoother=[('block_gauss_seidel', {'sweep':symmetric}), 'sor']
           aggregate=['standard', 'naive', 'lloyd', 'pairwise']
           strength=[('symmetric', {'theta':0.25}), ('symmetric', {'theta':0.08})]

         - Predefined strength of connection and aggregation schemes can be
           specified.  These options are best used together, but aggregation can
           be predefined while strength of connection is not.

           For predefined strength of connection, use a list consisting of
           tuples of the form ('predefined', {'C' : C0}), where C0 is a
           csr_array and each degree-of-freedom in C0 represents a supernode.
           For instance to predefine a three-level hierarchy, use
           [('predefined', {'C' : C0}), ('predefined', {'C' : C1}) ].

           Similarly for predefined aggregation, use a list of tuples.  For
           instance to predefine a three-level hierarchy, use [('predefined',
           {'AggOp' : Agg0}), ('predefined', {'AggOp' : Agg1}) ], where the
           dimensions of A, Agg0 and Agg1 are compatible, i.e.  Agg0.shape[1] ==
           A.shape[0] and Agg1.shape[1] == Agg0.shape[0].  Each AggOp is a
           csr_array.

           Because this is a root-nodes solver, if a member of the predefined
           aggregation list is predefined, it must be of the form
           ('predefined', {'AggOp' : Agg, 'Cnodes' : Cnodes}).

    References
    ----------
    .. [1996VaMa] Vanek, P. and Mandel, J. and Brezina, M.,
       "Algebraic Multigrid by Smoothed Aggregation for
       Second and Fourth Order Elliptic Problems",
       Computing, vol. 56, no. 3, pp. 179--196, 1996.
       http://citeseer.ist.psu.edu/vanek96algebraic.html
    .. [2] Olson, L. and Schroder, J. and Tuminaro, R.,
       "A general interpolation strategy for algebraic
       multigrid using energy minimization", SIAM Journal
       on Scientific Computing (SISC), vol. 33, pp.
       966--991, 2011.

    Examples
    --------
    >>> from pyamg import rootnode_solver
    >>> from pyamg.gallery import poisson
    >>> from scipy.sparse.linalg import cg
    >>> import numpy as np
    >>> A = poisson((100, 100), format='csr')           # matrix
    >>> b = np.ones((A.shape[0]))                   # RHS
    >>> ml = rootnode_solver(A)                     # AMG solver
    >>> M = ml.aspreconditioner(cycle='V')             # preconditioner
    >>> x, info = cg(A, b, rtol=1e-8, maxiter=30, M=M)   # solve with CG

    """
    if not issparse(A) or A.format not in ('bsr', 'csr'):
        try:
            A = csr_array(A)
            warn('Implicit conversion of A to CSR',
                 SparseEfficiencyWarning)
        except Exception as e:
            raise TypeError('Argument A must have type csr_array, '
                            'bsr_array, or be convertible to csr_array') from e

    A = asfptype(A)

    if symmetry not in ('symmetric', 'hermitian', 'nonsymmetric'):
        raise ValueError('Expected "symmetric", "nonsymmetric" '
                         'or "hermitian" for the symmetry parameter.')
    A.symmetry = symmetry

    if A.shape[0] != A.shape[1]:
        raise ValueError('expected square matrix')
    # Right near nullspace candidates use constant for each variable as default
    if B is None:
        B = np.kron(np.ones((int(A.shape[0]/get_blocksize(A)), 1), dtype=A.dtype),
                    np.eye(get_blocksize(A)))
    else:
        B = np.asarray(B, dtype=A.dtype)
        if len(B.shape) == 1:
            B = B.reshape(-1, 1)
        if B.shape[0] != A.shape[0]:
            raise ValueError('The near null-space modes B have incorrect \
                              dimensions for matrix A')
        if B.shape[1] < get_blocksize(A):
            raise ValueError('B.shape[1] must be >= the blocksize of A')

    # Left near nullspace candidates
    if A.symmetry == 'nonsymmetric':
        if BH is None:
            BH = B.copy()
        else:
            BH = np.asarray(BH, dtype=A.dtype)
            if len(BH.shape) == 1:
                BH = BH.reshape(-1, 1)
            if BH.shape[1] != B.shape[1]:
                raise ValueError('The number of left and right near \
                                  null-space modes B and BH, must be equal')
            if BH.shape[0] != A.shape[0]:
                raise ValueError('The near null-space modes BH have \
                                  incorrect dimensions for matrix A')

    # Levelize the user parameters, so that they become lists describing the
    # desired user option on each level.
    max_levels, max_coarse, strength =\
        levelize_strength_or_aggregation(strength, max_levels, max_coarse)
    max_levels, max_coarse, aggregate =\
        levelize_strength_or_aggregation(aggregate, max_levels, max_coarse)
    improve_candidates =\
        levelize_smooth_or_improve_candidates(improve_candidates, max_levels)
    smooth = levelize_smooth_or_improve_candidates(smooth, max_levels)

    # Construct multilevel structure
    levels = []
    levels.append(MultilevelSolver.Level())
    levels[-1].A = A          # matrix

    # Append near nullspace candidates
    levels[-1].B = B          # right candidates
    if A.symmetry == 'nonsymmetric':
        levels[-1].BH = BH    # left candidates

    while len(levels) < max_levels and \
            int(levels[-1].A.shape[0]/get_blocksize(levels[-1].A)) > max_coarse:
        _extend_hierarchy(levels, strength, aggregate, smooth,
                          improve_candidates, diagonal_dominance, keep)

    ml = MultilevelSolver(levels, **kwargs)
    change_smoothers(ml, presmoother, postsmoother)
    return ml


def _extend_hierarchy(levels, strength, aggregate, smooth, improve_candidates,
                      diagonal_dominance=False, keep=True):
    """Extend the multigrid hierarchy.

    Service routine to implement the strength of connection, aggregation,
    tentative prolongation construction, and prolongation smoothing.  Called by
    smoothed_aggregation_solver.

    """
    def unpack_arg(v):
        if isinstance(v, tuple):
            return v[0], v[1]
        return v, {}

    A = levels[-1].A
    B = levels[-1].B
    if A.symmetry == 'nonsymmetric':
        AH = A.T.conjugate().asformat(A.format)
        BH = levels[-1].BH

    # Compute the strength-of-connection matrix C, where larger
    # C[i, j] denote stronger couplings between i and j.
    fn, kwargs = unpack_arg(strength[len(levels)-1])
    if fn == 'symmetric':
        C = symmetric_strength_of_connection(A, **kwargs)
    elif fn == 'classical':
        C = classical_strength_of_connection(A, **kwargs)
    elif fn == 'distance':
        C = distance_strength_of_connection(A, **kwargs)
    elif fn in ('ode', 'evolution'):
        if 'B' in kwargs:
            C = evolution_strength_of_connection(A, **kwargs)
        else:
            C = evolution_strength_of_connection(A, B, **kwargs)
    elif fn == 'energy_based':
        C = energy_based_strength_of_connection(A, **kwargs)
    elif fn == 'predefined':
        C = kwargs['C'].tocsr()
    elif fn == 'algebraic_distance':
        C = algebraic_distance(A, **kwargs)
    elif fn == 'affinity':
        C = affinity_distance(A, **kwargs)
    elif fn is None:
        C = A.tocsr()
    else:
        raise ValueError(f'Unrecognized strength of connection method: {fn!s}')

    # Avoid coarsening diagonally dominant rows
    flag, kwargs = unpack_arg(diagonal_dominance)
    if flag:
        C = eliminate_diag_dom_nodes(A, C, **kwargs)

    # Compute the aggregation matrix AggOp (i.e., the nodal coarsening of A).
    # AggOp is a boolean matrix, where the sparsity pattern for the k-th column
    # denotes the fine-grid nodes agglomerated into k-th coarse-grid node.
    fn, kwargs = unpack_arg(aggregate[len(levels)-1])
    if fn == 'standard':
        AggOp, Cnodes = standard_aggregation(C, **kwargs)
    elif fn == 'naive':
        AggOp, Cnodes = naive_aggregation(C, **kwargs)
    elif fn == 'lloyd':
        AggOp, Cnodes = lloyd_aggregation(C, **kwargs)
    elif fn == 'pairwise':
        AggOp, Cnodes = pairwise_aggregation(A, **kwargs)
    elif fn == 'predefined':
        AggOp = kwargs['AggOp'].tocsr()
        Cnodes = kwargs['Cnodes']
    else:
        raise ValueError(f'Unrecognized aggregation method: {fn!s}')

    # Improve near nullspace candidates by relaxing on A B = 0
    fn, kwargs = unpack_arg(improve_candidates[len(levels)-1])
    if fn is not None:
        b = np.zeros((A.shape[0], 1), dtype=A.dtype)
        B = relaxation_as_linear_operator((fn, kwargs), A, b) @ B
        levels[-1].B = B
        if A.symmetry == 'nonsymmetric':
            BH = relaxation_as_linear_operator((fn, kwargs), AH, b) @ BH
            levels[-1].BH = BH

    # Compute the tentative prolongator, T, which is a tentative interpolation
    # matrix from the coarse-grid to the fine-grid.  T exactly interpolates
    # B_fine[:, 0:get_blocksize(A)] = T B_coarse[:, 0:get_blocksize(A)].
    T, dummy = fit_candidates(AggOp, B[:, 0:get_blocksize(A)])
    del dummy
    if A.symmetry == 'nonsymmetric':
        TH, dummyH = fit_candidates(AggOp, BH[:, 0:get_blocksize(A)])
        del dummyH

    # Create necessary root node matrices
    Cpt_params = (True, get_Cpt_params(A, Cnodes, AggOp, T))
    T = scale_T(T, Cpt_params[1]['P_I'], Cpt_params[1]['I_F'])
    if A.symmetry == 'nonsymmetric':
        TH = scale_T(TH, Cpt_params[1]['P_I'], Cpt_params[1]['I_F'])

    # Set coarse grid near nullspace modes as injected fine grid near
    # null-space modes
    B = Cpt_params[1]['P_I'].T@levels[-1].B
    if A.symmetry == 'nonsymmetric':
        BH = Cpt_params[1]['P_I'].T@levels[-1].BH

    # Smooth the tentative prolongator, so that it's accuracy is greatly
    # improved for algebraically smooth error.
    fn, kwargs = unpack_arg(smooth[len(levels)-1])
    if fn == 'energy':
        P = energy_prolongation_smoother(A, T, C, B, levels[-1].B,
                                         Cpt_params=Cpt_params, **kwargs)
    elif fn is None:
        P = T
    else:
        raise ValueError(f'Unrecognized prolongation smoother method: {fn!s}')

    # Compute the restriction matrix R, which interpolates from the fine-grid
    # to the coarse-grid.  If A is nonsymmetric, then R must be constructed
    # based on A.H.  Otherwise R = P.H or P.T.
    symmetry = A.symmetry
    if symmetry == 'hermitian':
        R = P.T.conjugate()
    elif symmetry == 'symmetric':
        R = P.T
    elif symmetry == 'nonsymmetric':
        fn, kwargs = unpack_arg(smooth[len(levels)-1])
        if fn == 'energy':
            R = energy_prolongation_smoother(AH, TH, C, BH, levels[-1].BH,
                                             Cpt_params=Cpt_params, **kwargs)
            R = R.T.conjugate()
        elif fn is None:
            R = T.T.conjugate()
        else:
            raise ValueError(f'Unrecognized prolongation smoother method: {fn!s}')

    if keep:
        levels[-1].C = C                         # strength of connection matrix
        levels[-1].AggOp = AggOp                 # aggregation operator
        levels[-1].T = T                         # tentative prolongator
        levels[-1].Fpts = Cpt_params[1]['Fpts']  # Fpts
        levels[-1].P_I = Cpt_params[1]['P_I']    # Injection operator
        levels[-1].I_F = Cpt_params[1]['I_F']    # Identity on F-pts
        levels[-1].I_C = Cpt_params[1]['I_C']    # Identity on C-pts

    levels[-1].P = P                             # smoothed prolongator
    levels[-1].R = R                             # restriction operator
    levels[-1].Cpts = Cpt_params[1]['Cpts']      # Cpts (i.e., rootnodes)

    levels.append(MultilevelSolver.Level())
    A = R @ A @ P                                # Galerkin operator
    A.symmetry = symmetry
    levels[-1].A = A
    levels[-1].B = B                             # right near nullspace candidates

    if A.symmetry == 'nonsymmetric':
        levels[-1].BH = BH                       # left near nullspace candidates
