"""Abstract linear algebra library.

This module defines a class hierarchy that implements a kind of "lazy"
matrix representation, called the ``LinearOperator``. It can be used to do
linear algebra with extremely large sparse or structured matrices, without
representing those explicitly in memory. Such matrices can be added,
multiplied, transposed, etc.

As a motivating example, suppose you want have a matrix where almost all of
the elements have the value one. The standard sparse matrix representation
skips the storage of zeros, but not ones. By contrast, a LinearOperator is
able to represent such matrices efficiently. First, we need a compact way to
represent an all-ones matrix::

    >>> import numpy as np
    >>> from scipy.sparse.linalg._interface import LinearOperator
    >>> class Ones(LinearOperator):
    ...     def __init__(self, shape):
    ...         super().__init__(dtype=None, shape=shape)
    ...     def _matvec(self, x):
    ...         return np.repeat(x.sum(), self.shape[0])

Instances of this class emulate ``np.ones(shape)``, but using a constant
amount of storage, independent of ``shape``. The ``_matvec`` method specifies
how this linear operator multiplies with (operates on) a vector. We can now
add this operator to a sparse matrix that stores only offsets from one::

    >>> from scipy.sparse.linalg._interface import aslinearoperator
    >>> from scipy.sparse import csr_array
    >>> offsets = csr_array([[1, 0, 2], [0, -1, 0], [0, 0, 3]])
    >>> A = aslinearoperator(offsets) + Ones(offsets.shape)
    >>> A.dot([1, 2, 3])
    array([13,  4, 15])

The result is the same as that given by its dense, explicitly-stored
counterpart::

    >>> (np.ones(A.shape, A.dtype) + offsets.toarray()).dot([1, 2, 3])
    array([13,  4, 15])

Several algorithms in the ``scipy.sparse`` library are able to operate on
``LinearOperator`` instances.
"""

import warnings

import numpy as np

from scipy.sparse import issparse
from scipy.sparse._sputils import isshape, isintlike, asmatrix, is_pydata_spmatrix

__all__ = ['LinearOperator', 'aslinearoperator']


class LinearOperator:
    """Common interface for performing matrix vector products

    Many iterative methods (e.g. cg, gmres) do not need to know the
    individual entries of a matrix to solve a linear system A@x=b.
    Such solvers only require the computation of matrix vector
    products, A@v where v is a dense vector.  This class serves as
    an abstract interface between iterative solvers and matrix-like
    objects.

    To construct a concrete LinearOperator, either pass appropriate
    callables to the constructor of this class, or subclass it.

    A subclass must implement either one of the methods ``_matvec``
    and ``_matmat``, and the attributes/properties ``shape`` (pair of
    integers) and ``dtype`` (may be None). It may call the ``__init__``
    on this class to have these attributes validated. Implementing
    ``_matvec`` automatically implements ``_matmat`` (using a naive
    algorithm) and vice-versa.

    Optionally, a subclass may implement ``_rmatvec`` or ``_adjoint``
    to implement the Hermitian adjoint (conjugate transpose). As with
    ``_matvec`` and ``_matmat``, implementing either ``_rmatvec`` or
    ``_adjoint`` implements the other automatically. Implementing
    ``_adjoint`` is preferable; ``_rmatvec`` is mostly there for
    backwards compatibility.

    Parameters
    ----------
    shape : tuple
        Matrix dimensions (M, N).
    matvec : callable f(v)
        Returns returns A @ v.
    rmatvec : callable f(v)
        Returns A^H @ v, where A^H is the conjugate transpose of A.
    matmat : callable f(V)
        Returns A @ V, where V is a dense matrix with dimensions (N, K).
    dtype : dtype
        Data type of the matrix.
    rmatmat : callable f(V)
        Returns A^H @ V, where V is a dense matrix with dimensions (M, K).

    Attributes
    ----------
    args : tuple
        For linear operators describing products etc. of other linear
        operators, the operands of the binary operation.
    ndim : int
        Number of dimensions (this is always 2)

    See Also
    --------
    aslinearoperator : Construct LinearOperators

    Notes
    -----
    The user-defined matvec() function must properly handle the case
    where v has shape (N,) as well as the (N,1) case.  The shape of
    the return type is handled internally by LinearOperator.

    It is highly recommended to explicitly specify the `dtype`, otherwise
    it is determined automatically at the cost of a single matvec application
    on `int8` zero vector using the promoted `dtype` of the output.
    Python `int` could be difficult to automatically cast to numpy integers
    in the definition of the `matvec` so the determination may be inaccurate.
    It is assumed that `matmat`, `rmatvec`, and `rmatmat` would result in
    the same dtype of the output given an `int8` input as `matvec`.

    LinearOperator instances can also be multiplied, added with each
    other and exponentiated, all lazily: the result of these operations
    is always a new, composite LinearOperator, that defers linear
    operations to the original operators and combines the results.

    More details regarding how to subclass a LinearOperator and several
    examples of concrete LinearOperator instances can be found in the
    external project `PyLops <https://pylops.readthedocs.io>`_.


    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse.linalg import LinearOperator
    >>> def mv(v):
    ...     return np.array([2*v[0], 3*v[1]])
    ...
    >>> A = LinearOperator((2,2), matvec=mv)
    >>> A
    <2x2 _CustomLinearOperator with dtype=int8>
    >>> A.matvec(np.ones(2))
    array([ 2.,  3.])
    >>> A @ np.ones(2)
    array([ 2.,  3.])

    """

    ndim = 2
    # Necessary for right matmul with numpy arrays.
    __array_ufunc__ = None

    def __new__(cls, *args, **kwargs):
        if cls is LinearOperator:
            # Operate as _CustomLinearOperator factory.
            return super().__new__(_CustomLinearOperator)
        else:
            obj = super().__new__(cls)

            if (type(obj)._matvec == LinearOperator._matvec
                    and type(obj)._matmat == LinearOperator._matmat):
                warnings.warn("LinearOperator subclass should implement"
                              " at least one of _matvec and _matmat.",
                              category=RuntimeWarning, stacklevel=2)

            return obj

    def __init__(self, dtype, shape):
        """Initialize this LinearOperator.

        To be called by subclasses. ``dtype`` may be None; ``shape`` should
        be convertible to a length-2 tuple.
        """
        if dtype is not None:
            dtype = np.dtype(dtype)

        shape = tuple(shape)
        if not isshape(shape):
            raise ValueError(f"invalid shape {shape!r} (must be 2-d)")

        self.dtype = dtype
        self.shape = shape

    def _init_dtype(self):
        """Determine the dtype by executing `matvec` on an `int8` test vector.

        In `np.promote_types` hierarchy, the type `int8` is the smallest,
        so we call `matvec` on `int8` and use the promoted dtype of the output
        to set the default `dtype` of the `LinearOperator`.
        We assume that `matmat`, `rmatvec`, and `rmatmat` would result in
        the same dtype of the output given an `int8` input as `matvec`.

        Called from subclasses at the end of the __init__ routine.
        """
        if self.dtype is None:
            v = np.zeros(self.shape[-1], dtype=np.int8)
            try:
                matvec_v = np.asarray(self.matvec(v))
            except OverflowError:
                # Python large `int` promoted to `np.int64`or `np.int32`
                self.dtype = np.dtype(int)
            else:
                self.dtype = matvec_v.dtype

    def _matmat(self, X):
        """Default matrix-matrix multiplication handler.

        Falls back on the user-defined _matvec method, so defining that will
        define matrix multiplication (though in a very suboptimal way).
        """

        return np.hstack([self.matvec(col.reshape(-1,1)) for col in X.T])

    def _matvec(self, x):
        """Default matrix-vector multiplication handler.

        If self is a linear operator of shape (M, N), then this method will
        be called on a shape (N,) or (N, 1) ndarray, and should return a
        shape (M,) or (M, 1) ndarray.

        This default implementation falls back on _matmat, so defining that
        will define matrix-vector multiplication as well.
        """
        return self.matmat(x.reshape(-1, 1))

    def matvec(self, x):
        """Matrix-vector multiplication.

        Performs the operation y=A@x where A is an MxN linear
        operator and x is a column vector or 1-d array.

        Parameters
        ----------
        x : {matrix, ndarray}
            An array with shape (N,) or (N,1).

        Returns
        -------
        y : {matrix, ndarray}
            A matrix or ndarray with shape (M,) or (M,1) depending
            on the type and shape of the x argument.

        Notes
        -----
        This matvec wraps the user-specified matvec routine or overridden
        _matvec method to ensure that y has the correct shape and type.

        """

        x = np.asanyarray(x)

        M,N = self.shape

        if x.shape != (N,) and x.shape != (N,1):
            raise ValueError('dimension mismatch')

        y = self._matvec(x)

        if isinstance(x, np.matrix):
            y = asmatrix(y)
        else:
            y = np.asarray(y)

        if x.ndim == 1:
            y = y.reshape(M)
        elif x.ndim == 2:
            y = y.reshape(M,1)
        else:
            raise ValueError('invalid shape returned by user-defined matvec()')

        return y

    def rmatvec(self, x):
        """Adjoint matrix-vector multiplication.

        Performs the operation y = A^H @ x where A is an MxN linear
        operator and x is a column vector or 1-d array.

        Parameters
        ----------
        x : {matrix, ndarray}
            An array with shape (M,) or (M,1).

        Returns
        -------
        y : {matrix, ndarray}
            A matrix or ndarray with shape (N,) or (N,1) depending
            on the type and shape of the x argument.

        Notes
        -----
        This rmatvec wraps the user-specified rmatvec routine or overridden
        _rmatvec method to ensure that y has the correct shape and type.

        """

        x = np.asanyarray(x)

        M,N = self.shape

        if x.shape != (M,) and x.shape != (M,1):
            raise ValueError('dimension mismatch')

        y = self._rmatvec(x)

        if isinstance(x, np.matrix):
            y = asmatrix(y)
        else:
            y = np.asarray(y)

        if x.ndim == 1:
            y = y.reshape(N)
        elif x.ndim == 2:
            y = y.reshape(N,1)
        else:
            raise ValueError('invalid shape returned by user-defined rmatvec()')

        return y

    def _rmatvec(self, x):
        """Default implementation of _rmatvec; defers to adjoint."""
        if type(self)._adjoint == LinearOperator._adjoint:
            # _adjoint not overridden, prevent infinite recursion
            if (hasattr(self, "_rmatmat")
                    and type(self)._rmatmat != LinearOperator._rmatmat):
                # Try to use _rmatmat as a fallback
                return self._rmatmat(x.reshape(-1, 1)).reshape(-1)
            raise NotImplementedError
        else:
            return self.H.matvec(x)

    def matmat(self, X):
        """Matrix-matrix multiplication.

        Performs the operation y=A@X where A is an MxN linear
        operator and X dense N*K matrix or ndarray.

        Parameters
        ----------
        X : {matrix, ndarray}
            An array with shape (N,K).

        Returns
        -------
        Y : {matrix, ndarray}
            A matrix or ndarray with shape (M,K) depending on
            the type of the X argument.

        Notes
        -----
        This matmat wraps any user-specified matmat routine or overridden
        _matmat method to ensure that y has the correct type.

        """
        if not (issparse(X) or is_pydata_spmatrix(X)):
            X = np.asanyarray(X)

        if X.ndim != 2:
            raise ValueError(f'expected 2-d ndarray or matrix, not {X.ndim}-d')

        if X.shape[0] != self.shape[1]:
            raise ValueError(f'dimension mismatch: {self.shape}, {X.shape}')

        try:
            Y = self._matmat(X)
        except Exception as e:
            if issparse(X) or is_pydata_spmatrix(X):
                raise TypeError(
                    "Unable to multiply a LinearOperator with a sparse matrix."
                    " Wrap the matrix in aslinearoperator first."
                ) from e
            raise

        if isinstance(Y, np.matrix):
            Y = asmatrix(Y)

        return Y

    def rmatmat(self, X):
        """Adjoint matrix-matrix multiplication.

        Performs the operation y = A^H @ x where A is an MxN linear
        operator and x is a column vector or 1-d array, or 2-d array.
        The default implementation defers to the adjoint.

        Parameters
        ----------
        X : {matrix, ndarray}
            A matrix or 2D array.

        Returns
        -------
        Y : {matrix, ndarray}
            A matrix or 2D array depending on the type of the input.

        Notes
        -----
        This rmatmat wraps the user-specified rmatmat routine.

        """
        if not (issparse(X) or is_pydata_spmatrix(X)):
            X = np.asanyarray(X)

        if X.ndim != 2:
            raise ValueError('expected 2-d ndarray or matrix, not %d-d'
                             % X.ndim)

        if X.shape[0] != self.shape[0]:
            raise ValueError(f'dimension mismatch: {self.shape}, {X.shape}')

        try:
            Y = self._rmatmat(X)
        except Exception as e:
            if issparse(X) or is_pydata_spmatrix(X):
                raise TypeError(
                    "Unable to multiply a LinearOperator with a sparse matrix."
                    " Wrap the matrix in aslinearoperator() first."
                ) from e
            raise

        if isinstance(Y, np.matrix):
            Y = asmatrix(Y)
        return Y

    def _rmatmat(self, X):
        """Default implementation of _rmatmat defers to rmatvec or adjoint."""
        if type(self)._adjoint == LinearOperator._adjoint:
            return np.hstack([self.rmatvec(col.reshape(-1, 1)) for col in X.T])
        else:
            return self.H.matmat(X)

    def __call__(self, x):
        return self@x

    def __mul__(self, x):
        return self.dot(x)

    def __truediv__(self, other):
        if not np.isscalar(other):
            raise ValueError("Can only divide a linear operator by a scalar.")

        return _ScaledLinearOperator(self, 1.0/other)

    def dot(self, x):
        """Matrix-matrix or matrix-vector multiplication.

        Parameters
        ----------
        x : array_like
            1-d or 2-d array, representing a vector or matrix.

        Returns
        -------
        Ax : array
            1-d or 2-d array (depending on the shape of x) that represents
            the result of applying this linear operator on x.

        """
        if isinstance(x, LinearOperator):
            return _ProductLinearOperator(self, x)
        elif np.isscalar(x):
            return _ScaledLinearOperator(self, x)
        else:
            if not issparse(x) and not is_pydata_spmatrix(x):
                # Sparse matrices shouldn't be converted to numpy arrays.
                x = np.asarray(x)

            if x.ndim == 1 or x.ndim == 2 and x.shape[1] == 1:
                return self.matvec(x)
            elif x.ndim == 2:
                return self.matmat(x)
            else:
                raise ValueError(f'expected 1-d or 2-d array or matrix, got {x!r}')

    def __matmul__(self, other):
        if np.isscalar(other):
            raise ValueError("Scalar operands are not allowed, "
                             "use '*' instead")
        return self.__mul__(other)

    def __rmatmul__(self, other):
        if np.isscalar(other):
            raise ValueError("Scalar operands are not allowed, "
                             "use '*' instead")
        return self.__rmul__(other)

    def __rmul__(self, x):
        if np.isscalar(x):
            return _ScaledLinearOperator(self, x)
        else:
            return self._rdot(x)

    def _rdot(self, x):
        """Matrix-matrix or matrix-vector multiplication from the right.

        Parameters
        ----------
        x : array_like
            1-d or 2-d array, representing a vector or matrix.

        Returns
        -------
        xA : array
            1-d or 2-d array (depending on the shape of x) that represents
            the result of applying this linear operator on x from the right.

        Notes
        -----
        This is copied from dot to implement right multiplication.
        """
        if isinstance(x, LinearOperator):
            return _ProductLinearOperator(x, self)
        elif np.isscalar(x):
            return _ScaledLinearOperator(self, x)
        else:
            if not issparse(x) and not is_pydata_spmatrix(x):
                # Sparse matrices shouldn't be converted to numpy arrays.
                x = np.asarray(x)

            # We use transpose instead of rmatvec/rmatmat to avoid
            # unnecessary complex conjugation if possible.
            if x.ndim == 1 or x.ndim == 2 and x.shape[0] == 1:
                return self.T.matvec(x.T).T
            elif x.ndim == 2:
                return self.T.matmat(x.T).T
            else:
                raise ValueError(f'expected 1-d or 2-d array or matrix, got {x!r}')

    def __pow__(self, p):
        if np.isscalar(p):
            return _PowerLinearOperator(self, p)
        else:
            return NotImplemented

    def __add__(self, x):
        if isinstance(x, LinearOperator):
            return _SumLinearOperator(self, x)
        else:
            return NotImplemented

    def __neg__(self):
        return _ScaledLinearOperator(self, -1)

    def __sub__(self, x):
        return self.__add__(-x)

    def __repr__(self):
        M,N = self.shape
        if self.dtype is None:
            dt = 'unspecified dtype'
        else:
            dt = 'dtype=' + str(self.dtype)

        return '<%dx%d %s with %s>' % (M, N, self.__class__.__name__, dt)

    def adjoint(self):
        """Hermitian adjoint.

        Returns the Hermitian adjoint of self, aka the Hermitian
        conjugate or Hermitian transpose. For a complex matrix, the
        Hermitian adjoint is equal to the conjugate transpose.

        Can be abbreviated self.H instead of self.adjoint().

        Returns
        -------
        A_H : LinearOperator
            Hermitian adjoint of self.
        """
        return self._adjoint()

    H = property(adjoint)

    def transpose(self):
        """Transpose this linear operator.

        Returns a LinearOperator that represents the transpose of this one.
        Can be abbreviated self.T instead of self.transpose().
        """
        return self._transpose()

    T = property(transpose)

    def _adjoint(self):
        """Default implementation of _adjoint; defers to rmatvec."""
        return _AdjointLinearOperator(self)

    def _transpose(self):
        """ Default implementation of _transpose; defers to rmatvec + conj"""
        return _TransposedLinearOperator(self)


class _CustomLinearOperator(LinearOperator):
    """Linear operator defined in terms of user-specified operations."""

    def __init__(self, shape, matvec, rmatvec=None, matmat=None,
                 dtype=None, rmatmat=None):
        super().__init__(dtype, shape)

        self.args = ()

        self.__matvec_impl = matvec
        self.__rmatvec_impl = rmatvec
        self.__rmatmat_impl = rmatmat
        self.__matmat_impl = matmat

        self._init_dtype()

    def _matmat(self, X):
        if self.__matmat_impl is not None:
            return self.__matmat_impl(X)
        else:
            return super()._matmat(X)

    def _matvec(self, x):
        return self.__matvec_impl(x)

    def _rmatvec(self, x):
        func = self.__rmatvec_impl
        if func is None:
            raise NotImplementedError("rmatvec is not defined")
        return self.__rmatvec_impl(x)

    def _rmatmat(self, X):
        if self.__rmatmat_impl is not None:
            return self.__rmatmat_impl(X)
        else:
            return super()._rmatmat(X)

    def _adjoint(self):
        return _CustomLinearOperator(shape=(self.shape[1], self.shape[0]),
                                     matvec=self.__rmatvec_impl,
                                     rmatvec=self.__matvec_impl,
                                     matmat=self.__rmatmat_impl,
                                     rmatmat=self.__matmat_impl,
                                     dtype=self.dtype)


class _AdjointLinearOperator(LinearOperator):
    """Adjoint of arbitrary Linear Operator"""

    def __init__(self, A):
        shape = (A.shape[1], A.shape[0])
        super().__init__(dtype=A.dtype, shape=shape)
        self.A = A
        self.args = (A,)

    def _matvec(self, x):
        return self.A._rmatvec(x)

    def _rmatvec(self, x):
        return self.A._matvec(x)

    def _matmat(self, x):
        return self.A._rmatmat(x)

    def _rmatmat(self, x):
        return self.A._matmat(x)

class _TransposedLinearOperator(LinearOperator):
    """Transposition of arbitrary Linear Operator"""

    def __init__(self, A):
        shape = (A.shape[1], A.shape[0])
        super().__init__(dtype=A.dtype, shape=shape)
        self.A = A
        self.args = (A,)

    def _matvec(self, x):
        # NB. np.conj works also on sparse matrices
        return np.conj(self.A._rmatvec(np.conj(x)))

    def _rmatvec(self, x):
        return np.conj(self.A._matvec(np.conj(x)))

    def _matmat(self, x):
        # NB. np.conj works also on sparse matrices
        return np.conj(self.A._rmatmat(np.conj(x)))

    def _rmatmat(self, x):
        return np.conj(self.A._matmat(np.conj(x)))

def _get_dtype(operators, dtypes=None):
    if dtypes is None:
        dtypes = []
    for obj in operators:
        if obj is not None and hasattr(obj, 'dtype'):
            dtypes.append(obj.dtype)
    return np.result_type(*dtypes)


class _SumLinearOperator(LinearOperator):
    def __init__(self, A, B):
        if not isinstance(A, LinearOperator) or \
                not isinstance(B, LinearOperator):
            raise ValueError('both operands have to be a LinearOperator')
        if A.shape != B.shape:
            raise ValueError(f'cannot add {A} and {B}: shape mismatch')
        self.args = (A, B)
        super().__init__(_get_dtype([A, B]), A.shape)

    def _matvec(self, x):
        return self.args[0].matvec(x) + self.args[1].matvec(x)

    def _rmatvec(self, x):
        return self.args[0].rmatvec(x) + self.args[1].rmatvec(x)

    def _rmatmat(self, x):
        return self.args[0].rmatmat(x) + self.args[1].rmatmat(x)

    def _matmat(self, x):
        return self.args[0].matmat(x) + self.args[1].matmat(x)

    def _adjoint(self):
        A, B = self.args
        return A.H + B.H


class _ProductLinearOperator(LinearOperator):
    def __init__(self, A, B):
        if not isinstance(A, LinearOperator) or \
                not isinstance(B, LinearOperator):
            raise ValueError('both operands have to be a LinearOperator')
        if A.shape[1] != B.shape[0]:
            raise ValueError(f'cannot multiply {A} and {B}: shape mismatch')
        super().__init__(_get_dtype([A, B]),
                                                     (A.shape[0], B.shape[1]))
        self.args = (A, B)

    def _matvec(self, x):
        return self.args[0].matvec(self.args[1].matvec(x))

    def _rmatvec(self, x):
        return self.args[1].rmatvec(self.args[0].rmatvec(x))

    def _rmatmat(self, x):
        return self.args[1].rmatmat(self.args[0].rmatmat(x))

    def _matmat(self, x):
        return self.args[0].matmat(self.args[1].matmat(x))

    def _adjoint(self):
        A, B = self.args
        return B.H @ A.H


class _ScaledLinearOperator(LinearOperator):
    def __init__(self, A, alpha):
        if not isinstance(A, LinearOperator):
            raise ValueError('LinearOperator expected as A')
        if not np.isscalar(alpha):
            raise ValueError('scalar expected as alpha')
        if isinstance(A, _ScaledLinearOperator):
            A, alpha_original = A.args
            # Avoid in-place multiplication so that we don't accidentally mutate
            # the original prefactor.
            alpha = alpha * alpha_original

        dtype = _get_dtype([A], [type(alpha)])
        super().__init__(dtype, A.shape)
        self.args = (A, alpha)
        # Note: args[1] is alpha (a scalar), so use `*` below, not `@`

    def _matvec(self, x):
        return self.args[1] * self.args[0].matvec(x)

    def _rmatvec(self, x):
        return np.conj(self.args[1]) * self.args[0].rmatvec(x)

    def _rmatmat(self, x):
        return np.conj(self.args[1]) * self.args[0].rmatmat(x)

    def _matmat(self, x):
        return self.args[1] * self.args[0].matmat(x)

    def _adjoint(self):
        A, alpha = self.args
        return A.H * np.conj(alpha)


class _PowerLinearOperator(LinearOperator):
    def __init__(self, A, p):
        if not isinstance(A, LinearOperator):
            raise ValueError('LinearOperator expected as A')
        if A.shape[0] != A.shape[1]:
            raise ValueError(f'square LinearOperator expected, got {A!r}')
        if not isintlike(p) or p < 0:
            raise ValueError('non-negative integer expected as p')

        super().__init__(_get_dtype([A]), A.shape)
        self.args = (A, p)

    def _power(self, fun, x):
        res = np.array(x, copy=True)
        for i in range(self.args[1]):
            res = fun(res)
        return res

    def _matvec(self, x):
        return self._power(self.args[0].matvec, x)

    def _rmatvec(self, x):
        return self._power(self.args[0].rmatvec, x)

    def _rmatmat(self, x):
        return self._power(self.args[0].rmatmat, x)

    def _matmat(self, x):
        return self._power(self.args[0].matmat, x)

    def _adjoint(self):
        A, p = self.args
        return A.H ** p


class MatrixLinearOperator(LinearOperator):
    def __init__(self, A):
        super().__init__(A.dtype, A.shape)
        self.A = A
        self.__adj = None
        self.args = (A,)

    def _matmat(self, X):
        return self.A.dot(X)

    def _adjoint(self):
        if self.__adj is None:
            self.__adj = _AdjointMatrixOperator(self.A)
        return self.__adj


class _AdjointMatrixOperator(MatrixLinearOperator):
    def __init__(self, adjoint_array):
        self.A = adjoint_array.T.conj()
        self.args = (adjoint_array,)
        self.shape = adjoint_array.shape[1], adjoint_array.shape[0]

    @property
    def dtype(self):
        return self.args[0].dtype

    def _adjoint(self):
        return MatrixLinearOperator(self.args[0])


class IdentityOperator(LinearOperator):
    def __init__(self, shape, dtype=None):
        super().__init__(dtype, shape)

    def _matvec(self, x):
        return x

    def _rmatvec(self, x):
        return x

    def _rmatmat(self, x):
        return x

    def _matmat(self, x):
        return x

    def _adjoint(self):
        return self


def aslinearoperator(A):
    """Return A as a LinearOperator.

    'A' may be any of the following types:
     - ndarray
     - matrix
     - sparse array (e.g. csr_array, lil_array, etc.)
     - LinearOperator
     - An object with .shape and .matvec attributes

    See the LinearOperator documentation for additional information.

    Notes
    -----
    If 'A' has no .dtype attribute, the data type is determined by calling
    :func:`LinearOperator.matvec()` - set the .dtype attribute to prevent this
    call upon the linear operator creation.

    Examples
    --------
    >>> import numpy as np
    >>> from scipy.sparse.linalg import aslinearoperator
    >>> M = np.array([[1,2,3],[4,5,6]], dtype=np.int32)
    >>> aslinearoperator(M)
    <2x3 MatrixLinearOperator with dtype=int32>
    """
    if isinstance(A, LinearOperator):
        return A

    elif isinstance(A, np.ndarray) or isinstance(A, np.matrix):
        if A.ndim > 2:
            raise ValueError('array must have ndim <= 2')
        A = np.atleast_2d(np.asarray(A))
        return MatrixLinearOperator(A)

    elif issparse(A) or is_pydata_spmatrix(A):
        return MatrixLinearOperator(A)

    else:
        if hasattr(A, 'shape') and hasattr(A, 'matvec'):
            rmatvec = None
            rmatmat = None
            dtype = None

            if hasattr(A, 'rmatvec'):
                rmatvec = A.rmatvec
            if hasattr(A, 'rmatmat'):
                rmatmat = A.rmatmat
            if hasattr(A, 'dtype'):
                dtype = A.dtype
            return LinearOperator(A.shape, A.matvec, rmatvec=rmatvec,
                                  rmatmat=rmatmat, dtype=dtype)

        else:
            raise TypeError('type not understood')
