from warnings import warn, catch_warnings, simplefilter

import numpy as np
from numpy import asarray
from scipy.sparse import (issparse, SparseEfficiencyWarning,
                          csr_array, csc_array, eye_array, diags_array)
from scipy.sparse._sputils import (is_pydata_spmatrix, convert_pydata_sparse_to_scipy,
                                   get_index_dtype, safely_cast_index_arrays)
from scipy.linalg import LinAlgError
import copy
import threading

from . import _superlu

noScikit = False
try:
    import scikits.umfpack as umfpack
except ImportError:
    noScikit = True

useUmfpack = threading.local()


__all__ = ['use_solver', 'spsolve', 'splu', 'spilu', 'factorized',
           'MatrixRankWarning', 'spsolve_triangular', 'is_sptriangular', 'spbandwidth']


class MatrixRankWarning(UserWarning):
    """Warning for exactly singular matrices."""
    pass


def use_solver(**kwargs):
    """
    Select default sparse direct solver to be used.

    Parameters
    ----------
    **kwargs
        The following options may be passed as keyword arguments.

        useUmfpack : bool, optional
            Use UMFPACK [1]_, [2]_, [3]_, [4]_. over SuperLU. Has effect only
            if ``scikits.umfpack`` is installed. Default: True
        assumeSortedIndices : bool, optional
            Allow UMFPACK to skip the step of sorting indices for a CSR/CSC matrix.
            Has effect only if useUmfpack is True and ``scikits.umfpack`` is
            installed. Default: False

    Notes
    -----
    The default sparse solver is UMFPACK when available
    (``scikits.umfpack`` is installed). This can be changed by passing
    useUmfpack = False, which then causes the always present SuperLU
    based solver to be used.

    UMFPACK requires a CSR/CSC matrix to have sorted column/row indices. If
    sure that the matrix fulfills this, pass ``assumeSortedIndices=True``
    to gain some speed.

    References
    ----------
    .. [1] T. A. Davis, Algorithm 832:  UMFPACK - an unsymmetric-pattern
           multifrontal method with a column pre-ordering strategy, ACM
           Trans. on Mathematical Software, 30(2), 2004, pp. 196--199.
           :doi:`10.1145/992200.992206`.

    .. [2] T. A. Davis, A column pre-ordering strategy for the
           unsymmetric-pattern multifrontal method, ACM Trans.
           on Mathematical Software, 30(2), 2004, pp. 165--195.
           :doi:`10.1145/992200.992205`.

    .. [3] T. A. Davis and I. S. Duff, A combined unifrontal/multifrontal
           method for unsymmetric sparse matrices, ACM Trans. on
           Mathematical Software, 25(1), 1999, pp. 1--19.
           :doi:`10.1145/305658.287640`.

    .. [4] T. A. Davis and I. S. Duff, An unsymmetric-pattern multifrontal
           method for sparse LU factorization, SIAM J. Matrix Analysis and
           Computations, 18(1), 1997, pp. 140--158.
           :doi:`10.1137/S0895479894246905T`.

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse.linalg import use_solver, spsolve
    >>> from scipy.sparse import csc_array
    >>> R = np.random.randn(5, 5)
    >>> A = csc_array(R)
    >>> b = np.random.randn(5)
    >>> use_solver(useUmfpack=False) # enforce superLU over UMFPACK
    >>> x = spsolve(A, b)
    >>> np.allclose(A.dot(x), b)
    True
    >>> use_solver(useUmfpack=True) # reset umfPack usage to default
    """
    global useUmfpack
    if 'useUmfpack' in kwargs:
        useUmfpack.u = kwargs['useUmfpack']
    if useUmfpack.u and 'assumeSortedIndices' in kwargs:
        umfpack.configure(assumeSortedIndices=kwargs['assumeSortedIndices'])

def _get_umf_family(A):
    """Get umfpack family string given the sparse matrix dtype."""
    _families = {
        (np.float64, np.int32): 'di',
        (np.complex128, np.int32): 'zi',
        (np.float64, np.int64): 'dl',
        (np.complex128, np.int64): 'zl'
    }

    # A.dtype.name can only be "float64" or
    # "complex128" in control flow
    f_type = getattr(np, A.dtype.name)
    # control flow may allow for more index
    # types to get through here
    i_type = getattr(np, A.indices.dtype.name)

    try:
        family = _families[(f_type, i_type)]

    except KeyError as e:
        msg = ('only float64 or complex128 matrices with int32 or int64 '
               f'indices are supported! (got: matrix: {f_type}, indices: {i_type})')
        raise ValueError(msg) from e

    # See gh-8278. Considered converting only if
    # A.shape[0]*A.shape[1] > np.iinfo(np.int32).max,
    # but that didn't always fix the issue.
    family = family[0] + "l"
    A_new = copy.copy(A)
    A_new.indptr = np.asarray(A.indptr, dtype=np.int64)
    A_new.indices = np.asarray(A.indices, dtype=np.int64)

    return family, A_new

def spsolve(A, b, permc_spec=None, use_umfpack=True):
    """Solve the sparse linear system Ax=b, where b may be a vector or a matrix.

    Parameters
    ----------
    A : ndarray or sparse array or matrix
        The square matrix A will be converted into CSC or CSR form
    b : ndarray or sparse array or matrix
        The matrix or vector representing the right hand side of the equation.
        If a vector, b.shape must be (n,) or (n, 1).
    permc_spec : str, optional
        How to permute the columns of the matrix for sparsity preservation.
        (default: 'COLAMD')

        - ``NATURAL``: natural ordering.
        - ``MMD_ATA``: minimum degree ordering on the structure of A^T A.
        - ``MMD_AT_PLUS_A``: minimum degree ordering on the structure of A^T+A.
        - ``COLAMD``: approximate minimum degree column ordering [1]_, [2]_.

    use_umfpack : bool, optional
        if True (default) then use UMFPACK for the solution [3]_, [4]_, [5]_,
        [6]_ . This is only referenced if b is a vector and
        ``scikits.umfpack`` is installed.

    Returns
    -------
    x : ndarray or sparse array or matrix
        the solution of the sparse linear equation.
        If b is a vector, then x is a vector of size A.shape[1]
        If b is a matrix, then x is a matrix of size (A.shape[1], b.shape[1])

    Notes
    -----
    For solving the matrix expression AX = B, this solver assumes the resulting
    matrix X is sparse, as is often the case for very sparse inputs.  If the
    resulting X is dense, the construction of this sparse result will be
    relatively expensive.  In that case, consider converting A to a dense
    matrix and using scipy.linalg.solve or its variants.

    References
    ----------
    .. [1] T. A. Davis, J. R. Gilbert, S. Larimore, E. Ng, Algorithm 836:
           COLAMD, an approximate column minimum degree ordering algorithm,
           ACM Trans. on Mathematical Software, 30(3), 2004, pp. 377--380.
           :doi:`10.1145/1024074.1024080`

    .. [2] T. A. Davis, J. R. Gilbert, S. Larimore, E. Ng, A column approximate
           minimum degree ordering algorithm, ACM Trans. on Mathematical
           Software, 30(3), 2004, pp. 353--376. :doi:`10.1145/1024074.1024079`

    .. [3] T. A. Davis, Algorithm 832:  UMFPACK - an unsymmetric-pattern
           multifrontal method with a column pre-ordering strategy, ACM
           Trans. on Mathematical Software, 30(2), 2004, pp. 196--199.
           :doi:`10.1145/992200.992206`.

    .. [4] T. A. Davis, A column pre-ordering strategy for the
           unsymmetric-pattern multifrontal method, ACM Trans.
           on Mathematical Software, 30(2), 2004, pp. 165--195.
           :doi:`10.1145/992200.992205`.

    .. [5] T. A. Davis and I. S. Duff, A combined unifrontal/multifrontal
           method for unsymmetric sparse matrices, ACM Trans. on
           Mathematical Software, 25(1), 1999, pp. 1--19.
           :doi:`10.1145/305658.287640`.

    .. [6] T. A. Davis and I. S. Duff, An unsymmetric-pattern multifrontal
           method for sparse LU factorization, SIAM J. Matrix Analysis and
           Computations, 18(1), 1997, pp. 140--158.
           :doi:`10.1137/S0895479894246905T`.


    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse import csc_array
    >>> from scipy.sparse.linalg import spsolve
    >>> A = csc_array([[3, 2, 0], [1, -1, 0], [0, 5, 1]], dtype=float)
    >>> B = csc_array([[2, 0], [-1, 0], [2, 0]], dtype=float)
    >>> x = spsolve(A, B)
    >>> np.allclose(A.dot(x).toarray(), B.toarray())
    True
    """
    is_pydata_sparse = is_pydata_spmatrix(b)
    pydata_sparse_cls = b.__class__ if is_pydata_sparse else None
    A = convert_pydata_sparse_to_scipy(A)
    b = convert_pydata_sparse_to_scipy(b)

    if not (issparse(A) and A.format in ("csc", "csr")):
        A = csc_array(A)
        warn('spsolve requires A be CSC or CSR matrix format',
             SparseEfficiencyWarning, stacklevel=2)

    # b is a vector only if b have shape (n,) or (n, 1)
    b_is_sparse = issparse(b)
    if not b_is_sparse:
        b = asarray(b)
    b_is_vector = ((b.ndim == 1) or (b.ndim == 2 and b.shape[1] == 1))

    # sum duplicates for non-canonical format
    A.sum_duplicates()
    A = A._asfptype()  # upcast to a floating point format
    result_dtype = np.promote_types(A.dtype, b.dtype)
    if A.dtype != result_dtype:
        A = A.astype(result_dtype)
    if b.dtype != result_dtype:
        b = b.astype(result_dtype)

    # validate input shapes
    M, N = A.shape
    if (M != N):
        raise ValueError(f"matrix must be square (has shape {(M, N)})")

    if M != b.shape[0]:
        raise ValueError(f"matrix - rhs dimension mismatch ({A.shape} - {b.shape[0]})")

    if not hasattr(useUmfpack, 'u'):
        useUmfpack.u = not noScikit

    use_umfpack = use_umfpack and useUmfpack.u

    if b_is_vector and use_umfpack:
        if b_is_sparse:
            b_vec = b.toarray()
        else:
            b_vec = b
        b_vec = asarray(b_vec, dtype=A.dtype).ravel()

        if noScikit:
            raise RuntimeError('Scikits.umfpack not installed.')

        if A.dtype.char not in 'dD':
            raise ValueError("convert matrix data to double, please, using"
                  " .astype(), or set linsolve.useUmfpack.u = False")

        umf_family, A = _get_umf_family(A)
        umf = umfpack.UmfpackContext(umf_family)
        x = umf.linsolve(umfpack.UMFPACK_A, A, b_vec,
                         autoTranspose=True)
    else:
        if b_is_vector and b_is_sparse:
            b = b.toarray()
            b_is_sparse = False

        if not b_is_sparse:
            if A.format == "csc":
                flag = 1  # CSC format
            else:
                flag = 0  # CSR format

            indices = A.indices.astype(np.intc, copy=False)
            indptr = A.indptr.astype(np.intc, copy=False)
            options = dict(ColPerm=permc_spec)
            x, info = _superlu.gssv(N, A.nnz, A.data, indices, indptr,
                                    b, flag, options=options)
            if info == 0:
                pass  # Success
            elif 0 < info <= N:
                warn("Matrix is exactly singular", MatrixRankWarning, stacklevel=2)
                x.fill(np.nan)
            elif info > N:
                raise MemoryError("Out of memory during gssv")
            else:
                # gssv is not supposed to return any info < 0
                # However, it calls gstrf, which can set info < 0
                raise Exception(f"gssv exited with unknown exit code {info}")
            if b_is_vector:
                x = x.ravel()
        else:
            # b is sparse
            Afactsolve = factorized(A)

            if not (b.format == "csc" or is_pydata_spmatrix(b)):
                warn('spsolve is more efficient when sparse b '
                     'is in the CSC matrix format',
                     SparseEfficiencyWarning, stacklevel=2)
                b = csc_array(b)

            # Create a sparse output matrix by repeatedly applying
            # the sparse factorization to solve columns of b.
            data_segs = []
            row_segs = []
            col_segs = []
            for j in range(b.shape[1]):
                bj = b[:, j].toarray().ravel()
                xj = Afactsolve(bj)
                w = np.flatnonzero(xj)
                segment_length = w.shape[0]
                row_segs.append(w)
                col_segs.append(np.full(segment_length, j, dtype=int))
                data_segs.append(np.asarray(xj[w], dtype=A.dtype))
            sparse_data = np.concatenate(data_segs)
            idx_dtype = get_index_dtype(maxval=max(b.shape))
            sparse_row = np.concatenate(row_segs, dtype=idx_dtype)
            sparse_col = np.concatenate(col_segs, dtype=idx_dtype)
            x = A.__class__((sparse_data, (sparse_row, sparse_col)),
                           shape=b.shape, dtype=A.dtype)

            if is_pydata_sparse:
                x = pydata_sparse_cls.from_scipy_sparse(x)

    return x


def splu(A, permc_spec=None, diag_pivot_thresh=None,
         relax=None, panel_size=None, options=None):
    """
    Compute the LU decomposition of a sparse, square matrix.

    Parameters
    ----------
    A : sparse array or matrix
        Sparse array to factorize. Most efficient when provided in CSC
        format. Other formats will be converted to CSC before factorization.
    permc_spec : str, optional
        How to permute the columns of the matrix for sparsity preservation.
        (default: 'COLAMD')

        - ``NATURAL``: natural ordering.
        - ``MMD_ATA``: minimum degree ordering on the structure of A^T A.
        - ``MMD_AT_PLUS_A``: minimum degree ordering on the structure of A^T+A.
        - ``COLAMD``: approximate minimum degree column ordering

    diag_pivot_thresh : float, optional
        Threshold used for a diagonal entry to be an acceptable pivot.
        See SuperLU user's guide for details [1]_
    relax : int, optional
        Expert option for customizing the degree of relaxing supernodes.
        See SuperLU user's guide for details [1]_
    panel_size : int, optional
        Expert option for customizing the panel size.
        See SuperLU user's guide for details [1]_
    options : dict, optional
        Dictionary containing additional expert options to SuperLU.
        See SuperLU user guide [1]_ (section 2.4 on the 'Options' argument)
        for more details. For example, you can specify
        ``options=dict(Equil=False, IterRefine='SINGLE'))``
        to turn equilibration off and perform a single iterative refinement.

    Returns
    -------
    invA : scipy.sparse.linalg.SuperLU
        Object, which has a ``solve`` method.

    See Also
    --------
    spilu : incomplete LU decomposition

    Notes
    -----
    When a real array is factorized and the returned SuperLU object's ``solve()``
    method is used with complex arguments an error is generated. Instead, cast the
    initial array to complex and then factorize.

    This function uses the SuperLU library.

    References
    ----------
    .. [1] SuperLU https://portal.nersc.gov/project/sparse/superlu/

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse import csc_array
    >>> from scipy.sparse.linalg import splu
    >>> A = csc_array([[1., 0., 0.], [5., 0., 2.], [0., -1., 0.]], dtype=float)
    >>> B = splu(A)
    >>> x = np.array([1., 2., 3.], dtype=float)
    >>> B.solve(x)
    array([ 1. , -3. , -1.5])
    >>> A.dot(B.solve(x))
    array([ 1.,  2.,  3.])
    >>> B.solve(A.dot(x))
    array([ 1.,  2.,  3.])
    """

    if is_pydata_spmatrix(A):
        A_cls = type(A)
        def csc_construct_func(*a, cls=A_cls):
            return cls.from_scipy_sparse(csc_array(*a))
        A = A.to_scipy_sparse().tocsc()
    else:
        csc_construct_func = csc_array

    if not (issparse(A) and A.format == "csc"):
        A = csc_array(A)
        warn('splu converted its input to CSC format',
             SparseEfficiencyWarning, stacklevel=2)

    # sum duplicates for non-canonical format
    A.sum_duplicates()
    A = A._asfptype()  # upcast to a floating point format

    M, N = A.shape
    if (M != N):
        raise ValueError("can only factor square matrices")  # is this true?

    indices, indptr = safely_cast_index_arrays(A, np.intc, "SuperLU")

    _options = dict(DiagPivotThresh=diag_pivot_thresh, ColPerm=permc_spec,
                    PanelSize=panel_size, Relax=relax)
    if options is not None:
        _options.update(options)

    # Ensure that no column permutations are applied
    if (_options["ColPerm"] == "NATURAL"):
        _options["SymmetricMode"] = True

    return _superlu.gstrf(N, A.nnz, A.data, indices, indptr,
                          csc_construct_func=csc_construct_func,
                          ilu=False, options=_options)


def spilu(A, drop_tol=None, fill_factor=None, drop_rule=None, permc_spec=None,
          diag_pivot_thresh=None, relax=None, panel_size=None, options=None):
    """
    Compute an incomplete LU decomposition for a sparse, square matrix.

    The resulting object is an approximation to the inverse of `A`.

    Parameters
    ----------
    A : (N, N) array_like
        Sparse array to factorize. Most efficient when provided in CSC format.
        Other formats will be converted to CSC before factorization.
    drop_tol : float, optional
        Drop tolerance (``0 <= tol <= 1``) for an incomplete LU decomposition.
        (default: 1e-4)

        Note that `drop_tol` primarily affects entries generated as fill-in
        during the ILU factorization; for matrices that produce little or no
        fill-in, changing this parameter may have no visible effect on the
        sparsity pattern of the factors.
    fill_factor : float, optional
        Specifies the fill ratio upper bound (``>= 1.0``) for ILU. (default: 10)
    drop_rule : str, optional
        Comma-separated string of drop rules to use.
        Available rules: ``basic``, ``prows``, ``column``, ``area``,
        ``secondary``, ``dynamic``, ``interp``. (Default: ``basic,area``)

        See SuperLU documentation for details.
    permc_spec : str, optional
        How to permute the columns of the matrix for sparsity preservation.
        (default: 'COLAMD')

        - ``NATURAL``: natural ordering.
        - ``MMD_ATA``: minimum degree ordering on the structure of A^T A.
        - ``MMD_AT_PLUS_A``: minimum degree ordering on the structure of A^T+A.
        - ``COLAMD``: approximate minimum degree column ordering

    diag_pivot_thresh : float, optional
        Threshold used for a diagonal entry to be an acceptable pivot.
        See SuperLU user's guide for details [1]_
    relax : int, optional
        Expert option for customizing the degree of relaxing supernodes.
        See SuperLU user's guide for details [1]_
    panel_size : int, optional
        Expert option for customizing the panel size.
        See SuperLU user's guide for details [1]_
    options : dict, optional
        Dictionary containing additional expert options to SuperLU.
        See SuperLU user guide [1]_ (section 2.4 on the 'Options' argument)
        for more details. For example, you can specify
        ``options=dict(Equil=False, IterRefine='SINGLE'))``
        to turn equilibration off and perform a single iterative refinement.

    Returns
    -------
    invA_approx : scipy.sparse.linalg.SuperLU
        Object, which has a ``solve`` method.

    See Also
    --------
    splu : complete LU decomposition

    Notes
    -----
    When a real array is factorized and the returned SuperLU object's ``solve()`` method
    is used with complex arguments an error is generated. Instead, cast the initial
    array to complex and then factorize.

    To improve the better approximation to the inverse, you may need to
    increase `fill_factor` AND decrease `drop_tol`.

    The effect of `drop_tol` is matrix-dependent. In particular, `drop_tol`
    does not guarantee that existing off-diagonal entries will be removed;
    it controls dropping of candidate entries during factorization (often
    fill-in). For some sparsity patterns, the ILU factors can be identical
    to the full LU factors even for large `drop_tol`.

    This function uses the SuperLU library.

    References
    ----------
    .. [1] SuperLU https://portal.nersc.gov/project/sparse/superlu/

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse import csc_array
    >>> from scipy.sparse.linalg import spilu
    >>> A = csc_array([[1., 0., 0.], [5., 0., 2.], [0., -1., 0.]], dtype=float)
    >>> B = spilu(A)
    >>> x = np.array([1., 2., 3.], dtype=float)
    >>> B.solve(x)
    array([ 1. , -3. , -1.5])
    >>> A.dot(B.solve(x))
    array([ 1.,  2.,  3.])
    >>> B.solve(A.dot(x))
    array([ 1.,  2.,  3.])
    """

    if is_pydata_spmatrix(A):
        A_cls = type(A)
        def csc_construct_func(*a, cls=A_cls):
            return cls.from_scipy_sparse(csc_array(*a))
        A = A.to_scipy_sparse().tocsc()
    else:
        csc_construct_func = csc_array

    if not (issparse(A) and A.format == "csc"):
        A = csc_array(A)
        warn('spilu converted its input to CSC format',
             SparseEfficiencyWarning, stacklevel=2)

    # sum duplicates for non-canonical format
    A.sum_duplicates()
    A = A._asfptype()  # upcast to a floating point format

    M, N = A.shape
    if (M != N):
        raise ValueError("can only factor square matrices")  # is this true?

    indices, indptr = safely_cast_index_arrays(A, np.intc, "SuperLU")

    _options = dict(ILU_DropRule=drop_rule, ILU_DropTol=drop_tol,
                    ILU_FillFactor=fill_factor,
                    DiagPivotThresh=diag_pivot_thresh, ColPerm=permc_spec,
                    PanelSize=panel_size, Relax=relax)
    if options is not None:
        _options.update(options)

    # Ensure that no column permutations are applied
    if (_options["ColPerm"] == "NATURAL"):
        _options["SymmetricMode"] = True

    return _superlu.gstrf(N, A.nnz, A.data, indices, indptr,
                          csc_construct_func=csc_construct_func,
                          ilu=True, options=_options)


def factorized(A):
    """
    Return a function for solving a sparse linear system, with A pre-factorized.

    Parameters
    ----------
    A : (N, N) array_like
        Input. A in CSC format is most efficient. A CSR format matrix will
        be converted to CSC before factorization.

    Returns
    -------
    solve : callable
        To solve the linear system of equations given in `A`, the `solve`
        callable should be passed an ndarray of shape (N,).

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse.linalg import factorized
    >>> from scipy.sparse import csc_array
    >>> A = np.array([[ 3. ,  2. , -1. ],
    ...               [ 2. , -2. ,  4. ],
    ...               [-1. ,  0.5, -1. ]])
    >>> solve = factorized(csc_array(A)) # Makes LU decomposition.
    >>> rhs1 = np.array([1, -2, 0])
    >>> solve(rhs1) # Uses the LU factors.
    array([ 1., -2., -2.])

    """
    if is_pydata_spmatrix(A):
        A = A.to_scipy_sparse().tocsc()

    if not hasattr(useUmfpack, 'u'):
        useUmfpack.u = not noScikit

    if useUmfpack.u:
        if noScikit:
            raise RuntimeError('Scikits.umfpack not installed.')

        if not (issparse(A) and A.format == "csc"):
            A = csc_array(A)
            warn('splu converted its input to CSC format',
                 SparseEfficiencyWarning, stacklevel=2)

        A = A._asfptype()  # upcast to a floating point format

        if A.dtype.char not in 'dD':
            raise ValueError("convert matrix data to double, please, using"
                  " .astype(), or set linsolve.useUmfpack.u = False")

        umf_family, A = _get_umf_family(A)
        umf = umfpack.UmfpackContext(umf_family)

        # Make LU decomposition.
        umf.numeric(A)

        def solve(b):
            with np.errstate(divide="ignore", invalid="ignore"):
                # Ignoring warnings with numpy >= 1.23.0, see gh-16523
                result = umf.solve(umfpack.UMFPACK_A, A, b, autoTranspose=True)

            return result

        return solve
    else:
        return splu(A).solve


def spsolve_triangular(A, b, lower=True, overwrite_A=False, overwrite_b=False,
                       unit_diagonal=False):
    """
    Solve the equation ``A x = b`` for `x`, assuming A is a triangular matrix.

    Parameters
    ----------
    A : (M, M) sparse array or matrix
        A sparse square triangular matrix. Should be in CSR or CSC format.
    b : (M,) or (M, N) array_like
        Right-hand side matrix in ``A x = b``
    lower : bool, optional
        Whether `A` is a lower or upper triangular matrix.
        Default is lower triangular matrix.
    overwrite_A : bool, optional
        Allow changing `A`.
        Enabling gives a performance gain. Default is False.
    overwrite_b : bool, optional
        Allow overwriting data in `b`.
        Enabling gives a performance gain. Default is False.
        If `overwrite_b` is True, it should be ensured that
        `b` has an appropriate dtype to be able to store the result.
    unit_diagonal : bool, optional
        If True, diagonal elements of `a` are assumed to be 1.

        .. versionadded:: 1.4.0

    Returns
    -------
    x : (M,) or (M, N) ndarray
        Solution to the system ``A x = b``. Shape of return matches shape
        of `b`.

    Raises
    ------
    LinAlgError
        If `A` is singular or not triangular.
    ValueError
        If shape of `A` or shape of `b` do not match the requirements.

    Notes
    -----
    .. versionadded:: 0.19.0

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse import csc_array
    >>> from scipy.sparse.linalg import spsolve_triangular
    >>> A = csc_array([[3, 0, 0], [1, -1, 0], [2, 0, 1]], dtype=float)
    >>> B = np.array([[2, 0], [-1, 0], [2, 0]], dtype=float)
    >>> x = spsolve_triangular(A, B)
    >>> np.allclose(A.dot(x), B)
    True
    """

    if is_pydata_spmatrix(A):
        A = A.to_scipy_sparse().tocsc()

    trans = "N"
    if issparse(A) and A.format == "csr":
        A = A.T
        trans = "T"
        lower = not lower

    if not (issparse(A) and A.format == "csc"):
        warn('CSC or CSR matrix format is required. Converting to CSC matrix.',
             SparseEfficiencyWarning, stacklevel=2)
        A = csc_array(A)
    elif not overwrite_A:
        A = A.copy()


    M, N = A.shape
    if M != N:
        raise ValueError(
            f'A must be a square matrix but its shape is {A.shape}.')

    if unit_diagonal:
        with catch_warnings():
            simplefilter('ignore', SparseEfficiencyWarning)
            A.setdiag(1)
    else:
        diag = A.diagonal()
        if np.any(diag == 0):
            raise LinAlgError(
                'A is singular: zero entry on diagonal.')
        invdiag = 1/diag
        if trans == "N":
            A = A @ diags_array(invdiag)
        else:
            A = (A.T @ diags_array(invdiag)).T

    # sum duplicates for non-canonical format
    A.sum_duplicates()

    b = np.asanyarray(b)

    if b.ndim not in [1, 2]:
        raise ValueError(
            f'b must have 1 or 2 dims but its shape is {b.shape}.')
    if M != b.shape[0]:
        raise ValueError(
            'The size of the dimensions of A must be equal to '
            'the size of the first dimension of b but the shape of A is '
            f'{A.shape} and the shape of b is {b.shape}.'
        )

    result_dtype = np.promote_types(np.promote_types(A.dtype, np.float32), b.dtype)
    if A.dtype != result_dtype:
        A = A.astype(result_dtype)
    if b.dtype != result_dtype:
        b = b.astype(result_dtype)
    elif not overwrite_b:
        b = b.copy()

    if lower:
        L = A
        U = csc_array((N, N), dtype=result_dtype)
    else:
        L = eye_array(N, dtype=result_dtype, format='csc')
        U = A
        U.setdiag(0)

    L_indices, L_indptr = safely_cast_index_arrays(L, np.intc, "SuperLU")
    U_indices, U_indptr = safely_cast_index_arrays(U, np.intc, "SuperLU")

    x, info = _superlu.gstrs(trans,
                             N, L.nnz, L.data, L_indices, L_indptr,
                             N, U.nnz, U.data, U_indices, U_indptr,
                             b)
    if info:
        raise LinAlgError('A is singular.')

    if not unit_diagonal:
        invdiag = invdiag.reshape(-1, *([1] * (len(x.shape) - 1)))
        x = x * invdiag

    return x


def is_sptriangular(A):
    """Returns 2-tuple indicating lower/upper triangular structure for sparse ``A``.

    Checks for triangular structure in ``A``. The result is summarized in
    two boolean values ``lower`` and ``upper`` to designate whether ``A`` is
    lower triangular or upper triangular respectively. Diagonal ``A`` will
    result in both being True. Non-triangular structure results in False for both.

    Only the sparse structure is used here. Values are not checked for zeros.

    This function will convert a copy of ``A`` to CSC format if it is not already
    CSR or CSC format. So it may be more efficient to convert it yourself if you
    have other uses for the CSR/CSC version.

    If ``A`` is not square, the portions outside the upper left square of the
    matrix do not affect its triangular structure. You probably want to work
    with the square portion of the matrix, though it is not requred here.

    Parameters
    ----------
    A : SciPy sparse array or matrix
        A sparse matrix preferrably in CSR or CSC format.

    Returns
    -------
    lower, upper : 2-tuple of bool
        Whether `A` is lower / upper triangular.

        .. versionadded:: 1.15.0

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse import csc_array, eye_array
    >>> from scipy.sparse.linalg import is_sptriangular
    >>> A = csc_array([[3, 0, 0], [1, -1, 0], [2, 0, 1]], dtype=float)
    >>> is_sptriangular(A)
    (True, False)
    >>> D = eye_array(3, format='csr')
    >>> is_sptriangular(D)
    (True, True)
    """
    if not (issparse(A) and A.format in ("csc", "csr", "coo", "dia", "dok", "lil")):
        warn('is_sptriangular needs sparse and not BSR format. Converting to CSR.',
             SparseEfficiencyWarning, stacklevel=2)
        A = csr_array(A)

    # bsr is better off converting to csr
    if A.format == "dia":
        return A.offsets.max() <= 0, A.offsets.min() >= 0
    elif A.format == "coo":
        rows, cols = A.coords
        return (cols <= rows).all(), (cols >= rows).all()
    elif A.format == "dok":
        return all(c <= r for r, c in A.keys()), all(c >= r for r, c in A.keys())
    elif A.format == "lil":
        lower = all(col <= row for row, cols in enumerate(A.rows) for col in cols)
        upper = all(col >= row for row, cols in enumerate(A.rows) for col in cols)
        return lower, upper
    # format in ("csc", "csr")
    indptr, indices = A.indptr, A.indices
    N = len(indptr) - 1

    lower, upper = True, True
    # check middle, 1st, last col (treat as CSC and switch at end if CSR)
    for col in [N // 2, 0, -1]:
        rows = indices[indptr[col]:indptr[col + 1]]
        upper = upper and (col >= rows).all()
        lower = lower and (col <= rows).all()
        if not upper and not lower:
            return False, False
    # check all cols
    cols = np.repeat(np.arange(N), np.diff(indptr))
    rows = indices
    upper = upper and (cols >= rows).all()
    lower = lower and (cols <= rows).all()
    if A.format == 'csr':
        return upper, lower
    return lower, upper


def spbandwidth(A):
    """Return the lower and upper bandwidth of a 2D numeric array.

    Computes the lower and upper limits on the bandwidth of the
    sparse 2D array ``A``. The result is summarized as a 2-tuple
    of positive integers ``(lo, hi)``. A zero denotes no sub/super
    diagonal entries on that side (triangular). The maximum value
    for ``lo`` (``hi``) is one less than the number of rows(cols).

    Only the sparse structure is used here. Values are not checked for zeros.

    Parameters
    ----------
    A : SciPy sparse array or matrix
        A sparse matrix preferrably in CSR or CSC format.

    Returns
    -------
    below, above : 2-tuple of int
        The distance to the farthest non-zero diagonal below/above the
        main diagonal.

        .. versionadded:: 1.15.0

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse.linalg import spbandwidth
    >>> from scipy.sparse import csc_array, eye_array
    >>> A = csc_array([[3, 0, 0], [1, -1, 0], [2, 0, 1]], dtype=float)
    >>> spbandwidth(A)
    (2, 0)
    >>> D = eye_array(3, format='csr')
    >>> spbandwidth(D)
    (0, 0)
    """
    if not (issparse(A) and A.format in ("csc", "csr", "coo", "dia", "dok")):
        warn('spbandwidth needs sparse format not LIL and BSR. Converting to CSR.',
             SparseEfficiencyWarning, stacklevel=2)
        A = csr_array(A)

    # bsr and lil are better off converting to csr
    if A.format == "dia":
        return max(0, -A.offsets.min().item()), max(0, A.offsets.max().item())
    if A.format in ("csc", "csr"):
        indptr, indices = A.indptr, A.indices
        N = len(indptr) - 1
        gap = np.repeat(np.arange(N), np.diff(indptr)) - indices
        if A.format == 'csr':
            gap = -gap
    elif A.format == "coo":
        gap = A.coords[1] - A.coords[0]
    elif A.format == "dok":
        gap = [(c - r) for r, c in A.keys()] + [0]
        return -min(gap), max(gap)
    return max(-np.min(gap).item(), 0), max(np.max(gap).item(), 0)
