# cython: cpow=True

import re
import warnings
import numpy as np
from scipy._lib._util import check_random_state
from ._rotation_groups import create_group

cimport numpy as np
cimport cython
from cython.view cimport array
from libc.math cimport sqrt, sin, cos, atan2, acos, hypot, isnan, NAN, pi

np.import_array()

# utilities for empty array initialization
cdef inline double[:] _empty1(int n) noexcept:
    return array(shape=(n,), itemsize=sizeof(double), format=b"d")
cdef inline double[:, :] _empty2(int n1, int n2) noexcept :
    return array(shape=(n1, n2), itemsize=sizeof(double), format=b"d")
cdef inline double[:, :, :] _empty3(int n1, int n2, int n3) noexcept:
    return array(shape=(n1, n2, n3), itemsize=sizeof(double), format=b"d")
cdef inline double[:, :] _zeros2(int n1, int n2) noexcept:
    cdef double[:, :] arr = array(shape=(n1, n2),
        itemsize=sizeof(double), format=b"d")
    arr[:, :] = 0
    return arr

# flat implementations of numpy functions
@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline double[:] _cross3(const double[:] a, const double[:] b) noexcept:
    cdef double[:] result = _empty1(3)
    result[0] = a[1]*b[2] - a[2]*b[1]
    result[1] = a[2]*b[0] - a[0]*b[2]
    result[2] = a[0]*b[1] - a[1]*b[0]
    return result

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline double _dot3(const double[:] a, const double[:] b) noexcept nogil:
    return a[0]*b[0] + a[1]*b[1] + a[2]*b[2]

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline double _norm3(const double[:] elems) noexcept nogil:
    return sqrt(_dot3(elems, elems))

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline double _normalize4(double[:] elems) noexcept nogil:
    cdef double norm = sqrt(_dot3(elems, elems) + elems[3]*elems[3])

    if norm == 0:
        return NAN

    elems[0] /= norm
    elems[1] /= norm
    elems[2] /= norm
    elems[3] /= norm

    return norm

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline int _argmax4(double[:] a) noexcept nogil:
    cdef int imax = 0
    cdef double vmax = a[0]

    for i in range(1, 4):
        if a[i] > vmax:
            imax = i
            vmax = a[i]

    return imax

ctypedef unsigned char uchar

cdef double[3] _ex = [1, 0, 0]
cdef double[3] _ey = [0, 1, 0]
cdef double[3] _ez = [0, 0, 1]

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline const double[:] _elementary_basis_vector(uchar axis) noexcept:
    if axis == b'x': return _ex
    elif axis == b'y': return _ey
    elif axis == b'z': return _ez
    
@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline int _elementary_basis_index(uchar axis) noexcept:
    if axis == b'x': return 0
    elif axis == b'y': return 1
    elif axis == b'z': return 2

# Reduce the quaternion double coverage of the rotation group to a unique
# canonical "positive" single cover
@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline void _quat_canonical_single(double[:] q) noexcept nogil:
    if ((q[3] < 0)
        or (q[3] == 0 and q[0] < 0)
        or (q[3] == 0 and q[0] == 0 and q[1] < 0)
        or (q[3] == 0 and q[0] == 0 and q[1] == 0 and q[2] < 0)):
        q[0] *= -1.0
        q[1] *= -1.0
        q[2] *= -1.0
        q[3] *= -1.0

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline void _quat_canonical(double[:, :] q) noexcept:
    cdef Py_ssize_t n = q.shape[0]
    for ind in range(n):
        _quat_canonical_single(q[ind])

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline void _get_angles(
    double[:] angles, bint extrinsic, bint symmetric, bint sign, 
    double lamb, double a, double b, double c, double d):
    
    # intrinsic/extrinsic conversion helpers
    cdef int angle_first, angle_third
    if extrinsic:
        angle_first = 0
        angle_third = 2
    else:
        angle_first = 2
        angle_third = 0

    cdef double half_sum, half_diff
    cdef int case

    # Step 2
    # Compute second angle...
    angles[1] = 2 * atan2(hypot(c, d), hypot(a, b))

    # ... and check if equal to is 0 or pi, causing a singularity
    if abs(angles[1]) <= 1e-7:
        case = 1
    elif abs(angles[1] - <double>pi) <= 1e-7:
        case = 2
    else:
        case = 0 # normal case

    # Step 3
    # compute first and third angles, according to case
    half_sum = atan2(b, a)
    half_diff = atan2(d, c)
    
    if case == 0:  # no singularities
        angles[angle_first] = half_sum - half_diff
        angles[angle_third] = half_sum + half_diff
    
    else:  # any degenerate case
        angles[2] = 0
        if case == 1:
            angles[0] = 2 * half_sum
        else:
            angles[0] = 2 * half_diff * (-1 if extrinsic else 1)
            
    # for Tait-Bryan/asymmetric sequences
    if not symmetric:
        angles[angle_third] *= sign
        angles[1] -= lamb

    for idx in range(3):
        if angles[idx] < -pi:
            angles[idx] += 2 * pi
        elif angles[idx] > pi:
            angles[idx] -= 2 * pi

    if case != 0:
        warnings.warn("Gimbal lock detected. Setting third angle to zero "
                      "since it is not possible to uniquely determine "
                      "all angles.", stacklevel=3)

@cython.boundscheck(False)
@cython.wraparound(False)
cdef double[:, :] _compute_euler_from_matrix(
    np.ndarray[double, ndim=3] matrix, const uchar[:] seq, bint extrinsic
) noexcept:
    # This is being replaced by the newer: _compute_euler_from_quat
    #
    # The algorithm assumes intrinsic frame transformations. The algorithm
    # in the paper is formulated for rotation matrices which are transposition
    # rotation matrices used within Rotation.
    # Adapt the algorithm for our case by
    # 1. Instead of transposing our representation, use the transpose of the
    #    O matrix as defined in the paper, and be careful to swap indices
    # 2. Reversing both axis sequence and angles for extrinsic rotations
    #
    # Based on Malcolm D. Shuster, F. Landis Markley, "General formula for
    # extraction the Euler angles", Journal of guidance, control, and
    # dynamics, vol. 29.1, pp. 215-221. 2006

    if extrinsic:
        seq = seq[::-1]

    cdef Py_ssize_t num_rotations = matrix.shape[0]

    # Step 0
    # Algorithm assumes axes as column vectors, here we use 1D vectors
    cdef const double[:] n1 = _elementary_basis_vector(seq[0])
    cdef const double[:] n2 = _elementary_basis_vector(seq[1])
    cdef const double[:] n3 = _elementary_basis_vector(seq[2])

    # Step 2
    cdef double sl = _dot3(_cross3(n1, n2), n3)
    cdef double cl = _dot3(n1, n3)

    # angle offset is lambda from the paper referenced in [2] from docstring of
    # `as_euler` function
    cdef double offset = atan2(sl, cl)
    cdef double[:, :] c_ = _empty2(3, 3)
    c_[0, :] = n2
    c_[1, :] = _cross3(n1, n2)
    c_[2, :] = n1
    cdef np.ndarray[double, ndim=2] c = np.asarray(c_)

    rot = np.array([
        [1, 0, 0],
        [0, cl, sl],
        [0, -sl, cl],
    ])

    # some forward definitions
    cdef double[:, :] angles = _empty2(num_rotations, 3)
    cdef double[:, :] matrix_trans # transformed matrix
    cdef double[:] _angles # accessor for each rotation
    cdef np.ndarray[double, ndim=2] res
    cdef double eps = 1e-7
    cdef bint safe1, safe2, safe, adjust

    for ind in range(num_rotations):
        _angles = angles[ind, :]

        # Step 3
        res = np.dot(c, matrix[ind, :, :])
        matrix_trans = np.dot(res, c.T.dot(rot))

        # Step 4
        # Ensure less than unit norm
        matrix_trans[2, 2] = min(matrix_trans[2, 2], 1)
        matrix_trans[2, 2] = max(matrix_trans[2, 2], -1)
        _angles[1] = acos(matrix_trans[2, 2])

        # Steps 5, 6
        safe1 = abs(_angles[1]) >= eps
        safe2 = abs(_angles[1] - <double>pi) >= eps
        safe = safe1 and safe2

        # Step 4 (Completion)
        _angles[1] += offset

        # 5b
        if safe:
            _angles[0] = atan2(matrix_trans[0, 2], -matrix_trans[1, 2])
            _angles[2] = atan2(matrix_trans[2, 0], matrix_trans[2, 1])

        if extrinsic:
            # For extrinsic, set first angle to zero so that after reversal we
            # ensure that third angle is zero
            # 6a
            if not safe:
                _angles[0] = 0
            # 6b
            if not safe1:
                _angles[2] = atan2(matrix_trans[1, 0] - matrix_trans[0, 1],
                                   matrix_trans[0, 0] + matrix_trans[1, 1])
            # 6c
            if not safe2:
                _angles[2] = -atan2(matrix_trans[1, 0] + matrix_trans[0, 1],
                                    matrix_trans[0, 0] - matrix_trans[1, 1])
        else:
            # For intrinsic, set third angle to zero
            # 6a
            if not safe:
                _angles[2] = 0
            # 6b
            if not safe1:
                _angles[0] = atan2(matrix_trans[1, 0] - matrix_trans[0, 1],
                                   matrix_trans[0, 0] + matrix_trans[1, 1])
            # 6c
            if not safe2:
                _angles[0] = atan2(matrix_trans[1, 0] + matrix_trans[0, 1],
                                   matrix_trans[0, 0] - matrix_trans[1, 1])

        # Step 7
        if seq[0] == seq[2]:
            # lambda = 0, so we can only ensure angle2 -> [0, pi]
            adjust = _angles[1] < 0 or _angles[1] > pi
        else:
            # lambda = + or - pi/2, so we can ensure angle2 -> [-pi/2, pi/2]
            adjust = _angles[1] < -pi / 2 or _angles[1] > pi / 2

        # Dont adjust gimbal locked angle sequences
        if adjust and safe:
            _angles[0] += pi
            _angles[1] = 2 * offset - _angles[1]
            _angles[2] -= pi

        for i in range(3):
            if _angles[i] < -pi:
                _angles[i] += 2 * pi
            elif _angles[i] > pi:
                _angles[i] -= 2 * pi

        if extrinsic:
            # reversal
            _angles[0], _angles[2] = _angles[2], _angles[0]

        # Step 8
        if not safe:
            warnings.warn("Gimbal lock detected. Setting third angle to zero "
                          "since it is not possible to uniquely determine "
                          "all angles.")

    return angles

@cython.boundscheck(False)
@cython.wraparound(False)
cdef double[:, :] _compute_euler_from_quat(
    np.ndarray[double, ndim=2] quat, const uchar[:] seq, bint extrinsic
) noexcept:
    # The algorithm assumes extrinsic frame transformations. The algorithm
    # in the paper is formulated for rotation quaternions, which are stored
    # directly by Rotation.
    # Adapt the algorithm for our case by reversing both axis sequence and 
    # angles for intrinsic rotations when needed
    
    if not extrinsic:
        seq = seq[::-1]
        
    cdef int i = _elementary_basis_index(seq[0])
    cdef int j = _elementary_basis_index(seq[1])
    cdef int k = _elementary_basis_index(seq[2])

    cdef bint symmetric = i == k
    if symmetric:
        k = 3 - i - j # get third axis
        
    # Step 0
    # Check if permutation is even (+1) or odd (-1)     
    cdef int sign = (i - j) * (j - k) * (k - i) // 2

    cdef Py_ssize_t num_rotations = quat.shape[0]

    # some forward definitions
    cdef double a, b, c, d
    cdef double[:, :] angles = _empty2(num_rotations, 3)

    for ind in range(num_rotations):

        # Step 1
        # Permutate quaternion elements            
        if symmetric:
            a = quat[ind, 3]
            b = quat[ind, i]
            c = quat[ind, j]
            d = quat[ind, k] * sign
        else:
            a = quat[ind, 3] - quat[ind, j]
            b = quat[ind, i] + quat[ind, k] * sign
            c = quat[ind, j] + quat[ind, 3]
            d = quat[ind, k] * sign - quat[ind, i]

        _get_angles(angles[ind], extrinsic, symmetric, sign, pi / 2, a, b, c, d)

    return angles

@cython.boundscheck(False)
@cython.wraparound(False)
cdef double[:, :] _compute_davenport_from_quat(
    np.ndarray[double, ndim=2] quat, np.ndarray[double, ndim=1] n1,
    np.ndarray[double, ndim=1] n2, np.ndarray[double, ndim=1] n3,
    bint extrinsic
):
    # The algorithm assumes extrinsic frame transformations. The algorithm
    # in the paper is formulated for rotation quaternions, which are stored
    # directly by Rotation.
    # Adapt the algorithm for our case by reversing both axis sequence and
    # angles for intrinsic rotations when needed

    if not extrinsic:
        n1, n3 = n3, n1

    cdef double[:] n_cross = _cross3(n1, n2)
    cdef double lamb = atan2(_dot3(n3, n_cross), _dot3(n3, n1))

    cdef int correct_set = False
    if lamb < 0:
        # alternative set of angles compatible with as_euler implementation
        n2 = -n2
        lamb = -lamb
        n_cross[0] = -n_cross[0]
        n_cross[1] = -n_cross[1]
        n_cross[2] = -n_cross[2]
        correct_set = True

    cdef double[:] quat_lamb = np.array([
            sin(lamb / 2) * n2[0],
            sin(lamb / 2) * n2[1],
            sin(lamb / 2) * n2[2],
            cos(lamb / 2)]
    )

    cdef Py_ssize_t num_rotations = quat.shape[0]

    # some forward definitions
    cdef double[:, :] angles = _empty2(num_rotations, 3)
    cdef double[:] quat_transformed = _empty1(4)
    cdef double a, b, c, d

    for ind in range(num_rotations):
        _compose_quat_single(quat_lamb, quat[ind], quat_transformed)

        # Step 1
        # Permutate quaternion elements
        a = quat_transformed[3]
        b = _dot3(quat_transformed[:3], n1)
        c = _dot3(quat_transformed[:3], n2)
        d = _dot3(quat_transformed[:3], n_cross)

        _get_angles(angles[ind], extrinsic, False, 1, lamb, a, b, c, d)

        if correct_set:
            angles[ind, 1] = -angles[ind, 1]

    return angles

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline void _compose_quat_single( # calculate p * q into r
    const double[:] p, const double[:] q, double[:] r
) noexcept:
    cdef double[:] cross = _cross3(p[:3], q[:3])

    r[0] = p[3]*q[0] + q[3]*p[0] + cross[0]
    r[1] = p[3]*q[1] + q[3]*p[1] + cross[1]
    r[2] = p[3]*q[2] + q[3]*p[2] + cross[2]
    r[3] = p[3]*q[3] - p[0]*q[0] - p[1]*q[1] - p[2]*q[2]

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline double[:, :] _compose_quat(
    const double[:, :] p, const double[:, :] q
) noexcept:
    cdef Py_ssize_t n = max(p.shape[0], q.shape[0])
    cdef double[:, :] product = _empty2(n, 4)

    # dealing with broadcasting
    if p.shape[0] == 1:
        for ind in range(n):
            _compose_quat_single(p[0], q[ind], product[ind])
    elif q.shape[0] == 1:
        for ind in range(n):
            _compose_quat_single(p[ind], q[0], product[ind])
    else:
        for ind in range(n):
            _compose_quat_single(p[ind], q[ind], product[ind])

    return product

@cython.boundscheck(False)
@cython.wraparound(False)
cdef inline double[:, :] _make_elementary_quat(
    uchar axis, const double[:] angles
) noexcept:
    cdef Py_ssize_t n = angles.shape[0]
    cdef double[:, :] quat = _zeros2(n, 4)

    cdef int axis_ind
    if axis == b'x':   axis_ind = 0
    elif axis == b'y': axis_ind = 1
    elif axis == b'z': axis_ind = 2

    for ind in range(n):
        quat[ind, 3] = cos(angles[ind] / 2)
        quat[ind, axis_ind] = sin(angles[ind] / 2)
    return quat

@cython.boundscheck(False)
@cython.wraparound(False)
cdef double[:, :] _elementary_quat_compose(
    const uchar[:] seq, const double[:, :] angles, bint intrinsic=False
) noexcept:
    cdef double[:, :] result = _make_elementary_quat(seq[0], angles[:, 0])
    cdef Py_ssize_t seq_len = seq.shape[0]

    for idx in range(1, seq_len):
        if intrinsic:
            result = _compose_quat(
                result,
                _make_elementary_quat(seq[idx], angles[:, idx]))
        else:
            result = _compose_quat(
                _make_elementary_quat(seq[idx], angles[:, idx]),
                result)
    return result

def _format_angles(angles, degrees, num_axes):
    angles = np.asarray(angles, dtype=float)
    if degrees:
        angles = np.deg2rad(angles)

    is_single = False
    # Prepare angles to have shape (num_rot, num_axes)
    if num_axes == 1:
        if angles.ndim == 0:
            # (1, 1)
            angles = angles.reshape((1, 1))
            is_single = True
        elif angles.ndim == 1:
            # (N, 1)
            angles = angles[:, None]
        elif angles.ndim == 2 and angles.shape[-1] != 1:
            raise ValueError("Expected `angles` parameter to have shape "
                             "(N, 1), got {}.".format(angles.shape))
        elif angles.ndim > 2:
            raise ValueError("Expected float, 1D array, or 2D array for "
                             "parameter `angles` corresponding to `seq`, "
                             "got shape {}.".format(angles.shape))
    else:  # 2 or 3 axes
        if angles.ndim not in [1, 2] or angles.shape[-1] != num_axes:
            raise ValueError("Expected `angles` to be at most "
                             "2-dimensional with width equal to number "
                             "of axes specified, got "
                             "{} for shape".format(angles.shape))

        if angles.ndim == 1:
            # (1, num_axes)
            angles = angles[None, :]
            is_single = True

    # By now angles should have shape (num_rot, num_axes)
    # sanity check
    if angles.ndim != 2 or angles.shape[-1] != num_axes:
        raise ValueError("Expected angles to have shape (num_rotations, "
                         "num_axes), got {}.".format(angles.shape))

    return angles, is_single

cdef class Rotation:
    """Rotation in 3 dimensions.

    This class provides an interface to initialize from and represent rotations
    with:

    - Quaternions
    - Rotation Matrices
    - Rotation Vectors
    - Modified Rodrigues Parameters
    - Euler Angles

    The following operations on rotations are supported:

    - Application on vectors
    - Rotation Composition
    - Rotation Inversion
    - Rotation Indexing

    Indexing within a rotation is supported since multiple rotation transforms
    can be stored within a single `Rotation` instance.

    To create `Rotation` objects use ``from_...`` methods (see examples below).
    ``Rotation(...)`` is not supposed to be instantiated directly.

    Attributes
    ----------
    single

    Methods
    -------
    __len__
    from_quat
    from_matrix
    from_rotvec
    from_mrp
    from_euler
    from_davenport
    as_quat
    as_matrix
    as_rotvec
    as_mrp
    as_euler
    as_davenport
    concatenate
    apply
    __mul__
    __pow__
    inv
    magnitude
    approx_equal
    mean
    reduce
    create_group
    __getitem__
    identity
    random
    align_vectors

    See Also
    --------
    Slerp

    Notes
    -----
    .. versionadded:: 1.2.0

    Examples
    --------
    >>> from scipy.spatial.transform import Rotation as R
    >>> import numpy as np

    A `Rotation` instance can be initialized in any of the above formats and
    converted to any of the others. The underlying object is independent of the
    representation used for initialization.

    Consider a counter-clockwise rotation of 90 degrees about the z-axis. This
    corresponds to the following quaternion (in scalar-last format):

    >>> r = R.from_quat([0, 0, np.sin(np.pi/4), np.cos(np.pi/4)])

    The rotation can be expressed in any of the other formats:

    >>> r.as_matrix()
    array([[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
    [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
    [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]])
    >>> r.as_rotvec()
    array([0.        , 0.        , 1.57079633])
    >>> r.as_euler('zyx', degrees=True)
    array([90.,  0.,  0.])

    The same rotation can be initialized using a rotation matrix:

    >>> r = R.from_matrix([[0, -1, 0],
    ...                    [1, 0, 0],
    ...                    [0, 0, 1]])

    Representation in other formats:

    >>> r.as_quat()
    array([0.        , 0.        , 0.70710678, 0.70710678])
    >>> r.as_rotvec()
    array([0.        , 0.        , 1.57079633])
    >>> r.as_euler('zyx', degrees=True)
    array([90.,  0.,  0.])

    The rotation vector corresponding to this rotation is given by:

    >>> r = R.from_rotvec(np.pi/2 * np.array([0, 0, 1]))

    Representation in other formats:

    >>> r.as_quat()
    array([0.        , 0.        , 0.70710678, 0.70710678])
    >>> r.as_matrix()
    array([[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
           [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
           [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]])
    >>> r.as_euler('zyx', degrees=True)
    array([90.,  0.,  0.])

    The ``from_euler`` method is quite flexible in the range of input formats
    it supports. Here we initialize a single rotation about a single axis:

    >>> r = R.from_euler('z', 90, degrees=True)

    Again, the object is representation independent and can be converted to any
    other format:

    >>> r.as_quat()
    array([0.        , 0.        , 0.70710678, 0.70710678])
    >>> r.as_matrix()
    array([[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
           [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
           [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]])
    >>> r.as_rotvec()
    array([0.        , 0.        , 1.57079633])

    It is also possible to initialize multiple rotations in a single instance
    using any of the ``from_...`` functions. Here we initialize a stack of 3
    rotations using the ``from_euler`` method:

    >>> r = R.from_euler('zyx', [
    ... [90, 0, 0],
    ... [0, 45, 0],
    ... [45, 60, 30]], degrees=True)

    The other representations also now return a stack of 3 rotations. For
    example:

    >>> r.as_quat()
    array([[0.        , 0.        , 0.70710678, 0.70710678],
           [0.        , 0.38268343, 0.        , 0.92387953],
           [0.39190384, 0.36042341, 0.43967974, 0.72331741]])

    Applying the above rotations onto a vector:

    >>> v = [1, 2, 3]
    >>> r.apply(v)
    array([[-2.        ,  1.        ,  3.        ],
           [ 2.82842712,  2.        ,  1.41421356],
           [ 2.24452282,  0.78093109,  2.89002836]])

    A `Rotation` instance can be indexed and sliced as if it were a single
    1D array or list:

    >>> r.as_quat()
    array([[0.        , 0.        , 0.70710678, 0.70710678],
           [0.        , 0.38268343, 0.        , 0.92387953],
           [0.39190384, 0.36042341, 0.43967974, 0.72331741]])
    >>> p = r[0]
    >>> p.as_matrix()
    array([[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
           [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
           [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]])
    >>> q = r[1:3]
    >>> q.as_quat()
    array([[0.        , 0.38268343, 0.        , 0.92387953],
           [0.39190384, 0.36042341, 0.43967974, 0.72331741]])

    In fact it can be converted to numpy.array:

    >>> r_array = np.asarray(r)
    >>> r_array.shape
    (3,)
    >>> r_array[0].as_matrix()
    array([[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
           [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
           [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]])

    Multiple rotations can be composed using the ``*`` operator:

    >>> r1 = R.from_euler('z', 90, degrees=True)
    >>> r2 = R.from_rotvec([np.pi/4, 0, 0])
    >>> v = [1, 2, 3]
    >>> r2.apply(r1.apply(v))
    array([-2.        , -1.41421356,  2.82842712])
    >>> r3 = r2 * r1 # Note the order
    >>> r3.apply(v)
    array([-2.        , -1.41421356,  2.82842712])

    A rotation can be composed with itself using the ``**`` operator:

    >>> p = R.from_rotvec([1, 0, 0])
    >>> q = p ** 2
    >>> q.as_rotvec()
    array([2., 0., 0.])

    Finally, it is also possible to invert rotations:

    >>> r1 = R.from_euler('z', [90, 45], degrees=True)
    >>> r2 = r1.inv()
    >>> r2.as_euler('zyx', degrees=True)
    array([[-90.,   0.,   0.],
           [-45.,   0.,   0.]])

    The following function can be used to plot rotations with Matplotlib by
    showing how they transform the standard x, y, z coordinate axes:

    >>> import matplotlib.pyplot as plt

    >>> def plot_rotated_axes(ax, r, name=None, offset=(0, 0, 0), scale=1):
    ...     colors = ("#FF6666", "#005533", "#1199EE")  # Colorblind-safe RGB
    ...     loc = np.array([offset, offset])
    ...     for i, (axis, c) in enumerate(zip((ax.xaxis, ax.yaxis, ax.zaxis),
    ...                                       colors)):
    ...         axlabel = axis.axis_name
    ...         axis.set_label_text(axlabel)
    ...         axis.label.set_color(c)
    ...         axis.line.set_color(c)
    ...         axis.set_tick_params(colors=c)
    ...         line = np.zeros((2, 3))
    ...         line[1, i] = scale
    ...         line_rot = r.apply(line)
    ...         line_plot = line_rot + loc
    ...         ax.plot(line_plot[:, 0], line_plot[:, 1], line_plot[:, 2], c)
    ...         text_loc = line[1]*1.2
    ...         text_loc_rot = r.apply(text_loc)
    ...         text_plot = text_loc_rot + loc[0]
    ...         ax.text(*text_plot, axlabel.upper(), color=c,
    ...                 va="center", ha="center")
    ...     ax.text(*offset, name, color="k", va="center", ha="center",
    ...             bbox={"fc": "w", "alpha": 0.8, "boxstyle": "circle"})

    Create three rotations - the identity and two Euler rotations using
    intrinsic and extrinsic conventions:

    >>> r0 = R.identity()
    >>> r1 = R.from_euler("ZYX", [90, -30, 0], degrees=True)  # intrinsic
    >>> r2 = R.from_euler("zyx", [90, -30, 0], degrees=True)  # extrinsic

    Add all three rotations to a single plot:

    >>> ax = plt.figure().add_subplot(projection="3d", proj_type="ortho")
    >>> plot_rotated_axes(ax, r0, name="r0", offset=(0, 0, 0))
    >>> plot_rotated_axes(ax, r1, name="r1", offset=(3, 0, 0))
    >>> plot_rotated_axes(ax, r2, name="r2", offset=(6, 0, 0))
    >>> _ = ax.annotate(
    ...     "r0: Identity Rotation\\n"
    ...     "r1: Intrinsic Euler Rotation (ZYX)\\n"
    ...     "r2: Extrinsic Euler Rotation (zyx)",
    ...     xy=(0.6, 0.7), xycoords="axes fraction", ha="left"
    ... )
    >>> ax.set(xlim=(-1.25, 7.25), ylim=(-1.25, 1.25), zlim=(-1.25, 1.25))
    >>> ax.set(xticks=range(-1, 8), yticks=[-1, 0, 1], zticks=[-1, 0, 1])
    >>> ax.set_aspect("equal", adjustable="box")
    >>> ax.figure.set_size_inches(6, 5)
    >>> plt.tight_layout()

    Show the plot:

    >>> plt.show()

    These examples serve as an overview into the `Rotation` class and highlight
    major functionalities. For more thorough examples of the range of input and
    output formats supported, consult the individual method's examples.

    """
    cdef double[:, :] _quat
    cdef bint _single

    @cython.boundscheck(False)
    @cython.wraparound(False)
    def __init__(self, quat, normalize=True, copy=True, scalar_first=False):
        self._single = False
        quat = np.asarray(quat, dtype=float)

        if quat.ndim not in [1, 2] or quat.shape[len(quat.shape) - 1] != 4:
            raise ValueError("Expected `quat` to have shape (4,) or (N, 4), "
                             f"got {quat.shape}.")

        # If a single quaternion is given, convert it to a 2D 1 x 4 matrix but
        # set self._single to True so that we can return appropriate objects
        # in the `to_...` methods
        if quat.shape == (4,):
            quat = quat[None, :]
            self._single = True

        cdef Py_ssize_t num_rotations = quat.shape[0]

        if scalar_first:
            quat = np.roll(quat, -1, axis=1)
        elif normalize or copy:
            quat = quat.copy()

        if normalize:
            for ind in range(num_rotations):
                if isnan(_normalize4(quat[ind, :])):
                    raise ValueError("Found zero norm quaternions in `quat`.")

        self._quat = quat

    def __getstate__(self):
        return np.asarray(self._quat, dtype=float), self._single

    def __setstate__(self, state):
        quat, single = state
        self._quat = quat.copy()
        self._single = single

    @property
    def single(self):
        """Whether this instance represents a single rotation."""
        return self._single

    def __bool__(self):
        """Comply with Python convention for objects to be True.

        Required because `Rotation.__len__()` is defined and not always truthy.
        """
        return True

    @cython.embedsignature(True)
    def __len__(self):
        """Number of rotations contained in this object.

        Multiple rotations can be stored in a single instance.

        Returns
        -------
        length : int
            Number of rotations stored in object.

        Raises
        ------
        TypeError if the instance was created as a single rotation.
        """
        if self._single:
            raise TypeError("Single rotation has no len().")

        return self._quat.shape[0]

    @cython.embedsignature(True)
    @classmethod
    def from_quat(cls, quat, *, scalar_first=False):
        """Initialize from quaternions.

        Rotations in 3 dimensions can be represented using unit norm
        quaternions [1]_.

        The 4 components of a quaternion are divided into a scalar part ``w``
        and a vector part ``(x, y, z)`` and can be expressed from the angle
        ``theta`` and the axis ``n`` of a rotation as follows::

            w = cos(theta / 2)
            x = sin(theta / 2) * n_x
            y = sin(theta / 2) * n_y
            z = sin(theta / 2) * n_z

        There are 2 conventions to order the components in a quaternion:

        - scalar-first order -- ``(w, x, y, z)``
        - scalar-last order -- ``(x, y, z, w)``

        The choice is controlled by `scalar_first` argument.
        By default, it is False and the scalar-last order is assumed.

        Advanced users may be interested in the "double cover" of 3D space by
        the quaternion representation [2]_. As of version 1.11.0, the
        following subset (and only this subset) of operations on a `Rotation`
        ``r`` corresponding to a quaternion ``q`` are guaranteed to preserve
        the double cover property: ``r = Rotation.from_quat(q)``,
        ``r.as_quat(canonical=False)``, ``r.inv()``, and composition using the
        ``*`` operator such as ``r*r``.

        Parameters
        ----------
        quat : array_like, shape (N, 4) or (4,)
            Each row is a (possibly non-unit norm) quaternion representing an
            active rotation. Each quaternion will be normalized to unit norm.
        scalar_first : bool, optional
            Whether the scalar component goes first or last.
            Default is False, i.e. the scalar-last order is assumed.

        Returns
        -------
        rotation : `Rotation` instance
            Object containing the rotations represented by input quaternions.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation
        .. [2] Hanson, Andrew J. "Visualizing quaternions."
            Morgan Kaufmann Publishers Inc., San Francisco, CA. 2006.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R

        A rotation can be initialzied from a quaternion with the scalar-last
        (default) or scalar-first component order as shown below:

        >>> r = R.from_quat([0, 0, 0, 1])
        >>> r.as_matrix()
        array([[1., 0., 0.],
               [0., 1., 0.],
               [0., 0., 1.]])
        >>> r = R.from_quat([1, 0, 0, 0], scalar_first=True)
        >>> r.as_matrix()
        array([[1., 0., 0.],
               [0., 1., 0.],
               [0., 0., 1.]])

        It is possible to initialize multiple rotations in a single object by
        passing a 2-dimensional array:

        >>> r = R.from_quat([
        ... [1, 0, 0, 0],
        ... [0, 0, 0, 1]
        ... ])
        >>> r.as_quat()
        array([[1., 0., 0., 0.],
               [0., 0., 0., 1.]])
        >>> r.as_quat().shape
        (2, 4)

        It is also possible to have a stack of a single rotation:

        >>> r = R.from_quat([[0, 0, 0, 1]])
        >>> r.as_quat()
        array([[0., 0., 0., 1.]])
        >>> r.as_quat().shape
        (1, 4)

        Quaternions are normalized before initialization.

        >>> r = R.from_quat([0, 0, 1, 1])
        >>> r.as_quat()
        array([0.        , 0.        , 0.70710678, 0.70710678])
        """
        return cls(quat, normalize=True, scalar_first=scalar_first)

    @cython.embedsignature(True)
    @classmethod
    @cython.boundscheck(False)
    @cython.wraparound(False)
    def from_matrix(cls, matrix):
        """Initialize from rotation matrix.

        Rotations in 3 dimensions can be represented with 3 x 3 proper
        orthogonal matrices [1]_. If the input is not proper orthogonal,
        an approximation is created using the method described in [2]_.

        Parameters
        ----------
        matrix : array_like, shape (N, 3, 3) or (3, 3)
            A single matrix or a stack of matrices, where ``matrix[i]`` is
            the i-th matrix.

        Returns
        -------
        rotation : `Rotation` instance
            Object containing the rotations represented by the rotation
            matrices.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Rotation_matrix#In_three_dimensions
        .. [2] F. Landis Markley, "Unit Quaternion from Rotation Matrix",
               Journal of guidance, control, and dynamics vol. 31.2, pp.
               440-442, 2008.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Initialize a single rotation:

        >>> r = R.from_matrix([
        ... [0, -1, 0],
        ... [1, 0, 0],
        ... [0, 0, 1]])
        >>> r.as_matrix().shape
        (3, 3)

        Initialize multiple rotations in a single object:

        >>> r = R.from_matrix([
        ... [
        ...     [0, -1, 0],
        ...     [1, 0, 0],
        ...     [0, 0, 1],
        ... ],
        ... [
        ...     [1, 0, 0],
        ...     [0, 0, -1],
        ...     [0, 1, 0],
        ... ]])
        >>> r.as_matrix().shape
        (2, 3, 3)

        If input matrices are not special orthogonal (orthogonal with
        determinant equal to +1), then a special orthogonal estimate is stored:

        >>> a = np.array([
        ... [0, -0.5, 0],
        ... [0.5, 0, 0],
        ... [0, 0, 0.5]])
        >>> np.linalg.det(a)
        0.12500000000000003
        >>> r = R.from_matrix(a)
        >>> matrix = r.as_matrix()
        >>> matrix
        array([[-0.38461538, -0.92307692,  0.        ],
               [ 0.92307692, -0.38461538,  0.        ],
               [ 0.        ,  0.        ,  1.        ]])
        >>> np.linalg.det(matrix)
        1.0000000000000002

        It is also possible to have a stack containing a single rotation:

        >>> r = R.from_matrix([[
        ... [0, -1, 0],
        ... [1, 0, 0],
        ... [0, 0, 1]]])
        >>> r.as_matrix()
        array([[[ 0., -1.,  0.],
                [ 1.,  0.,  0.],
                [ 0.,  0.,  1.]]])
        >>> r.as_matrix().shape
        (1, 3, 3)

        Notes
        -----
        This function was called from_dcm before.

        .. versionadded:: 1.4.0
        """
        is_single = False
        matrix = np.asarray(matrix, dtype=float)

        if (matrix.ndim not in [2, 3] or
            matrix.shape[len(matrix.shape)-2:] != (3, 3)):
            raise ValueError("Expected `matrix` to have shape (3, 3) or "
                             "(N, 3, 3), got {}".format(matrix.shape))

        # If a single matrix is given, convert it to 3D 1 x 3 x 3 matrix but
        # set self._single to True so that we can return appropriate objects in
        # the `to_...` methods
        cdef double[:, :, :] cmatrix
        if matrix.shape == (3, 3):
            cmatrix = matrix[None, :, :]
            is_single = True
        else:
            cmatrix = matrix

        cdef Py_ssize_t num_rotations = cmatrix.shape[0]
        cdef Py_ssize_t i, j, k
        cdef double[:] decision = _empty1(4)
        cdef int choice

        cdef double[:, :] quat = _empty2(num_rotations, 4)

        for ind in range(num_rotations):
            decision[0] = cmatrix[ind, 0, 0]
            decision[1] = cmatrix[ind, 1, 1]
            decision[2] = cmatrix[ind, 2, 2]
            decision[3] = cmatrix[ind, 0, 0] + cmatrix[ind, 1, 1] \
                        + cmatrix[ind, 2, 2]
            choice = _argmax4(decision)

            if choice != 3:
                i = choice
                j = (i + 1) % 3
                k = (j + 1) % 3

                quat[ind, i] = 1 - decision[3] + 2 * cmatrix[ind, i, i]
                quat[ind, j] = cmatrix[ind, j, i] + cmatrix[ind, i, j]
                quat[ind, k] = cmatrix[ind, k, i] + cmatrix[ind, i, k]
                quat[ind, 3] = cmatrix[ind, k, j] - cmatrix[ind, j, k]
            else:
                quat[ind, 0] = cmatrix[ind, 2, 1] - cmatrix[ind, 1, 2]
                quat[ind, 1] = cmatrix[ind, 0, 2] - cmatrix[ind, 2, 0]
                quat[ind, 2] = cmatrix[ind, 1, 0] - cmatrix[ind, 0, 1]
                quat[ind, 3] = 1 + decision[3]

            # normalize
            _normalize4(quat[ind])

        if is_single:
            return cls(quat[0], normalize=False, copy=False)
        else:
            return cls(quat, normalize=False, copy=False)

    @cython.embedsignature(True)
    @classmethod
    @cython.boundscheck(False)
    @cython.wraparound(False)
    def from_rotvec(cls, rotvec, degrees=False):
        """Initialize from rotation vectors.

        A rotation vector is a 3 dimensional vector which is co-directional to
        the axis of rotation and whose norm gives the angle of rotation [1]_.

        Parameters
        ----------
        rotvec : array_like, shape (N, 3) or (3,)
            A single vector or a stack of vectors, where `rot_vec[i]` gives
            the ith rotation vector.
        degrees : bool, optional
            If True, then the given magnitudes are assumed to be in degrees.
            Default is False.

            .. versionadded:: 1.7.0

        Returns
        -------
        rotation : `Rotation` instance
            Object containing the rotations represented by input rotation
            vectors.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Axis%E2%80%93angle_representation#Rotation_vector

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Initialize a single rotation:

        >>> r = R.from_rotvec(np.pi/2 * np.array([0, 0, 1]))
        >>> r.as_rotvec()
        array([0.        , 0.        , 1.57079633])
        >>> r.as_rotvec().shape
        (3,)

        Initialize a rotation in degrees, and view it in degrees:

        >>> r = R.from_rotvec(45 * np.array([0, 1, 0]), degrees=True)
        >>> r.as_rotvec(degrees=True)
        array([ 0., 45.,  0.])

        Initialize multiple rotations in one object:

        >>> r = R.from_rotvec([
        ... [0, 0, np.pi/2],
        ... [np.pi/2, 0, 0]])
        >>> r.as_rotvec()
        array([[0.        , 0.        , 1.57079633],
               [1.57079633, 0.        , 0.        ]])
        >>> r.as_rotvec().shape
        (2, 3)

        It is also possible to have a stack of a single rotaton:

        >>> r = R.from_rotvec([[0, 0, np.pi/2]])
        >>> r.as_rotvec().shape
        (1, 3)

        """
        is_single = False
        rotvec = np.asarray(rotvec, dtype=float)
        if degrees:
            rotvec = np.deg2rad(rotvec)

        if rotvec.ndim not in [1, 2] or rotvec.shape[len(rotvec.shape)-1] != 3:
            raise ValueError("Expected `rot_vec` to have shape (3,) "
                             "or (N, 3), got {}".format(rotvec.shape))

        # If a single vector is given, convert it to a 2D 1 x 3 matrix but
        # set self._single to True so that we can return appropriate objects
        # in the `as_...` methods
        cdef double[:, :] crotvec
        if rotvec.shape == (3,):
            crotvec = rotvec[None, :]
            is_single = True
        else:
            crotvec = rotvec

        cdef Py_ssize_t num_rotations = crotvec.shape[0]
        cdef double angle, scale, angle2
        cdef double[:, :] quat = _empty2(num_rotations, 4)

        for ind in range(num_rotations):
            angle = _norm3(crotvec[ind, :])

            if angle <= 1e-3:  # small angle Taylor series expansion
                angle2 = angle * angle
                scale = 0.5 - angle2 / 48 + angle2 * angle2 / 3840
            else:  # large angle
                scale = sin(angle / 2) / angle

            quat[ind, 0] = scale * crotvec[ind, 0]
            quat[ind, 1] = scale * crotvec[ind, 1]
            quat[ind, 2] = scale * crotvec[ind, 2]
            quat[ind, 3] = cos(angle / 2)

        if is_single:
            return cls(quat[0], normalize=False, copy=False)
        else:
            return cls(quat, normalize=False, copy=False)

    @cython.embedsignature(True)
    @classmethod
    def from_euler(cls, seq, angles, degrees=False):
        """Initialize from Euler angles.

        Rotations in 3-D can be represented by a sequence of 3
        rotations around a sequence of axes. In theory, any three axes spanning
        the 3-D Euclidean space are enough. In practice, the axes of rotation are
        chosen to be the basis vectors.

        The three rotations can either be in a global frame of reference
        (extrinsic) or in a body centred frame of reference (intrinsic), which
        is attached to, and moves with, the object under rotation [1]_.

        Parameters
        ----------
        seq : string
            Specifies sequence of axes for rotations. Up to 3 characters
            belonging to the set {'X', 'Y', 'Z'} for intrinsic rotations, or
            {'x', 'y', 'z'} for extrinsic rotations. Extrinsic and intrinsic
            rotations cannot be mixed in one function call.
        angles : float or array_like, shape (N,) or (N, [1 or 2 or 3])
            Euler angles specified in radians (`degrees` is False) or degrees
            (`degrees` is True).
            For a single character `seq`, `angles` can be:

            - a single value
            - array_like with shape (N,), where each `angle[i]`
              corresponds to a single rotation
            - array_like with shape (N, 1), where each `angle[i, 0]`
              corresponds to a single rotation

            For 2- and 3-character wide `seq`, `angles` can be:

            - array_like with shape (W,) where `W` is the width of
              `seq`, which corresponds to a single rotation with `W` axes
            - array_like with shape (N, W) where each `angle[i]`
              corresponds to a sequence of Euler angles describing a single
              rotation

        degrees : bool, optional
            If True, then the given angles are assumed to be in degrees.
            Default is False.

        Returns
        -------
        rotation : `Rotation` instance
            Object containing the rotation represented by the sequence of
            rotations around given axes with given angles.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Euler_angles#Definition_by_intrinsic_rotations

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R

        Initialize a single rotation along a single axis:

        >>> r = R.from_euler('x', 90, degrees=True)
        >>> r.as_quat().shape
        (4,)

        Initialize a single rotation with a given axis sequence:

        >>> r = R.from_euler('zyx', [90, 45, 30], degrees=True)
        >>> r.as_quat().shape
        (4,)

        Initialize a stack with a single rotation around a single axis:

        >>> r = R.from_euler('x', [90], degrees=True)
        >>> r.as_quat().shape
        (1, 4)

        Initialize a stack with a single rotation with an axis sequence:

        >>> r = R.from_euler('zyx', [[90, 45, 30]], degrees=True)
        >>> r.as_quat().shape
        (1, 4)

        Initialize multiple elementary rotations in one object:

        >>> r = R.from_euler('x', [90, 45, 30], degrees=True)
        >>> r.as_quat().shape
        (3, 4)

        Initialize multiple rotations in one object:

        >>> r = R.from_euler('zyx', [[90, 45, 30], [35, 45, 90]], degrees=True)
        >>> r.as_quat().shape
        (2, 4)

        """
        num_axes = len(seq)
        if num_axes < 1 or num_axes > 3:
            raise ValueError("Expected axis specification to be a non-empty "
                             "string of upto 3 characters, got {}".format(seq))

        intrinsic = (re.match(r'^[XYZ]{1,3}$', seq) is not None)
        extrinsic = (re.match(r'^[xyz]{1,3}$', seq) is not None)
        if not (intrinsic or extrinsic):
            raise ValueError("Expected axes from `seq` to be from ['x', 'y', "
                             "'z'] or ['X', 'Y', 'Z'], got {}".format(seq))

        if any(seq[i] == seq[i+1] for i in range(num_axes - 1)):
            raise ValueError("Expected consecutive axes to be different, "
                             "got {}".format(seq))

        seq = seq.lower()

        angles, is_single = _format_angles(angles, degrees, num_axes)

        quat = _elementary_quat_compose(seq.encode(), angles, intrinsic)

        if is_single:
            return cls(quat[0], normalize=False, copy=False)
        else:
            return cls(quat, normalize=False, copy=False)

    @cython.embedsignature(True)
    @classmethod
    def from_davenport(cls, axes, order, angles, degrees=False):
        """Initialize from Davenport angles.

        Rotations in 3-D can be represented by a sequence of 3
        rotations around a sequence of axes.

        The three rotations can either be in a global frame of reference
        (extrinsic) or in a body centred frame of reference (intrinsic), which
        is attached to, and moves with, the object under rotation [1]_.

        For both Euler angles and Davenport angles, consecutive axes must 
        be are orthogonal (``axis2`` is orthogonal to both ``axis1`` and 
        ``axis3``). For Euler angles, there is an additional relationship 
        between ``axis1`` or ``axis3``, with two possibilities:

            - ``axis1`` and ``axis3`` are also orthogonal (asymmetric sequence)
            - ``axis1 == axis3`` (symmetric sequence)

        For Davenport angles, this last relationship is relaxed [2]_, and only
        the consecutive orthogonal axes requirement is maintained.

        Parameters
        ----------
        axes : array_like, shape (3,) or ([1 or 2 or 3], 3)
            Axis of rotation, if one dimensional. If two dimensional, describes the 
            sequence of axes for rotations, where each axes[i, :] is the ith
            axis. If more than one axis is given, then the second axis must be
            orthogonal to both the first and third axes.
        order : string
            If it is equal to 'e' or 'extrinsic', the sequence will be 
            extrinsic. If it is equal to 'i' or 'intrinsic', sequence 
            will be treated as intrinsic.
        angles : float or array_like, shape (N,) or (N, [1 or 2 or 3])
            Euler angles specified in radians (`degrees` is False) or degrees
            (`degrees` is True).
            For a single axis, `angles` can be:

            - a single value
            - array_like with shape (N,), where each `angle[i]`
              corresponds to a single rotation
            - array_like with shape (N, 1), where each `angle[i, 0]`
              corresponds to a single rotation

            For 2 and 3 axes, `angles` can be:

            - array_like with shape (W,) where `W` is the number of rows of
              `axes`, which corresponds to a single rotation with `W` axes
            - array_like with shape (N, W) where each `angle[i]`
              corresponds to a sequence of Davenport angles describing a 
              single rotation

        degrees : bool, optional
            If True, then the given angles are assumed to be in degrees. 
            Default is False.

        Returns
        -------
        rotation : `Rotation` instance
            Object containing the rotation represented by the sequence of
            rotations around given axes with given angles.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Euler_angles#Definition_by_intrinsic_rotations
        .. [2] Shuster, Malcolm & Markley, Landis. (2003). Generalization of
               the Euler Angles. Journal of the Astronautical Sciences. 51. 123-132. 10.1007/BF03546304.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R

        Davenport angles are a generalization of Euler angles, when we use the
        canonical basis axes:

        >>> ex = [1, 0, 0]
        >>> ey = [0, 1, 0]
        >>> ez = [0, 0, 1]

        Initialize a single rotation with a given axis sequence:

        >>> axes = [ez, ey, ex]
        >>> r = R.from_davenport(axes, 'extrinsic', [90, 0, 0], degrees=True)
        >>> r.as_quat().shape
        (4,)

        It is equivalent to Euler angles in this case:

        >>> r.as_euler('zyx', degrees=True)
        array([90.,  0., -0.])

        Initialize multiple rotations in one object:

        >>> r = R.from_davenport(axes, 'extrinsic', [[90, 45, 30], [35, 45, 90]], degrees=True)
        >>> r.as_quat().shape
        (2, 4)

        Using only one or two axes is also possible:

        >>> r = R.from_davenport([ez, ex], 'extrinsic', [[90, 45], [35, 45]], degrees=True)
        >>> r.as_quat().shape
        (2, 4)

        Non-canonical axes are possible, and they do not need to be normalized,
        as long as consecutive axes are orthogonal:

        >>> e1 = [2, 0, 0]
        >>> e2 = [0, 1, 0]
        >>> e3 = [1, 0, 1]
        >>> axes = [e1, e2, e3]
        >>> r = R.from_davenport(axes, 'extrinsic', [90, 45, 30], degrees=True)
        >>> r.as_quat()
        [ 0.701057,  0.430459, -0.092296,  0.560986]
        """
        if order in ['e', 'extrinsic']:
            extrinsic = True
        elif order in ['i', 'intrinsic']:
            extrinsic = False
        else:
            raise ValueError("order should be 'e'/'extrinsic' for extrinsic "
                             "sequences or 'i'/'intrinsic' for intrinsic "
                             "sequences, got {}".format(order))

        axes = np.asarray(axes)
        if axes.ndim == 1:
            axes = axes.reshape([1, 3])

        num_axes = len(axes)

        if axes.shape[1] != 3:
            raise ValueError("Axes must be vectors of length 3.")

        if num_axes < 1 or num_axes > 3:
            raise ValueError("Expected up to 3 axes, got {}".format(num_axes))

        # normalize axes
        norm = np.repeat(np.linalg.norm(axes, axis=1), 3)
        axes = axes / norm.reshape(num_axes, 3)

        if (num_axes > 1 and abs(np.dot(axes[0], axes[1])) >= 1e-7 or 
            num_axes > 2 and abs(np.dot(axes[1], axes[2])) >= 1e-7):
            raise ValueError("Consecutive axes must be orthogonal.")
            
        angles, is_single = _format_angles(angles, degrees, num_axes)

        q = Rotation.identity(len(angles))
        for i in range(num_axes):
            qi = Rotation.from_rotvec(angles[:, i, np.newaxis] * axes[i])
            if extrinsic:
                q = qi * q
            else:
                q = q * qi

        return q[0] if is_single else q

    @cython.embedsignature(True)
    @classmethod
    @cython.boundscheck(False)
    @cython.wraparound(False)
    def from_mrp(cls, mrp):
        """Initialize from Modified Rodrigues Parameters (MRPs).

        MRPs are a 3 dimensional vector co-directional to the axis of rotation and whose
        magnitude is equal to ``tan(theta / 4)``, where ``theta`` is the angle of rotation
        (in radians) [1]_.

        MRPs have a singularity at 360 degrees which can be avoided by ensuring the angle of
        rotation does not exceed 180 degrees, i.e. switching the direction of the rotation when
        it is past 180 degrees.

        Parameters
        ----------
        mrp : array_like, shape (N, 3) or (3,)
            A single vector or a stack of vectors, where `mrp[i]` gives
            the ith set of MRPs.

        Returns
        -------
        rotation : `Rotation` instance
            Object containing the rotations represented by input MRPs.

        References
        ----------
        .. [1] Shuster, M. D. "A Survey of Attitude Representations",
               The Journal of Astronautical Sciences, Vol. 41, No.4, 1993,
               pp. 475-476

        Notes
        -----

        .. versionadded:: 1.6.0

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Initialize a single rotation:

        >>> r = R.from_mrp([0, 0, 1])
        >>> r.as_euler('xyz', degrees=True)
        array([0.        , 0.        , 180.      ])
        >>> r.as_euler('xyz').shape
        (3,)

        Initialize multiple rotations in one object:

        >>> r = R.from_mrp([
        ... [0, 0, 1],
        ... [1, 0, 0]])
        >>> r.as_euler('xyz', degrees=True)
        array([[0.        , 0.        , 180.      ],
               [180.0     , 0.        , 0.        ]])
        >>> r.as_euler('xyz').shape
        (2, 3)

        It is also possible to have a stack of a single rotation:

        >>> r = R.from_mrp([[0, 0, np.pi/2]])
        >>> r.as_euler('xyz').shape
        (1, 3)

        """
        is_single = False
        mrp = np.asarray(mrp, dtype=float)

        if mrp.ndim not in [1, 2] or mrp.shape[len(mrp.shape) - 1] != 3:
            raise ValueError("Expected `mrp` to have shape (3,) "
                             "or (N, 3), got {}".format(mrp.shape))

        # If a single vector is given, convert it to a 2D 1 x 3 matrix but
        # set self._single to True so that we can return appropriate objects
        # in the `as_...` methods
        cdef double[:, :] cmrp
        if mrp.shape == (3,):
            cmrp = mrp[None, :]
            is_single = True
        else:
            cmrp = mrp

        cdef Py_ssize_t num_rotations = cmrp.shape[0]
        cdef double[:, :] quat = _empty2(num_rotations, 4)
        cdef double mrp_squared_plus_1

        for ind in range(num_rotations):
            mrp_squared_plus_1 = 1 + _dot3(cmrp[ind, :], cmrp[ind, :])

            quat[ind, 0] = 2 * cmrp[ind, 0] / mrp_squared_plus_1
            quat[ind, 1] = 2 * cmrp[ind, 1] / mrp_squared_plus_1
            quat[ind, 2] = 2 * cmrp[ind, 2] / mrp_squared_plus_1
            quat[ind, 3] = (2 - mrp_squared_plus_1) / mrp_squared_plus_1

        if is_single:
            return cls(quat[0], normalize=False, copy=False)
        else:
            return cls(quat, normalize=False, copy=False)

    @cython.embedsignature(True)
    def as_quat(self, canonical=False, *, scalar_first=False):
        """Represent as quaternions.

        Rotations in 3 dimensions can be represented using unit norm
        quaternions [1]_.

        The 4 components of a quaternion are divided into a scalar part ``w``
        and a vector part ``(x, y, z)`` and can be expressed from the angle
        ``theta`` and the axis ``n`` of a rotation as follows::

            w = cos(theta / 2)
            x = sin(theta / 2) * n_x
            y = sin(theta / 2) * n_y
            z = sin(theta / 2) * n_z

        There are 2 conventions to order the components in a quaternion:

        - scalar-first order -- ``(w, x, y, z)``
        - scalar-last order -- ``(x, y, z, w)``

        The choice is controlled by `scalar_first` argument.
        By default, it is False and the scalar-last order is used.

        The mapping from quaternions to rotations is
        two-to-one, i.e. quaternions ``q`` and ``-q``, where ``-q`` simply
        reverses the sign of each component, represent the same spatial
        rotation. 

        Parameters
        ----------
        canonical : `bool`, default False
            Whether to map the redundant double cover of rotation space to a
            unique "canonical" single cover. If True, then the quaternion is
            chosen from {q, -q} such that the w term is positive. If the w term
            is 0, then the quaternion is chosen such that the first nonzero
            term of the x, y, and z terms is positive.
        scalar_first : bool, optional
            Whether the scalar component goes first or last.
            Default is False, i.e. the scalar-last order is used.

        Returns
        -------
        quat : `numpy.ndarray`, shape (4,) or (N, 4)
            Shape depends on shape of inputs used for initialization.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        A rotation can be represented as a quaternion with either scalar-last
        (default) or scalar-first component order.
        This is shown for a single rotation:

        >>> r = R.from_matrix(np.eye(3))
        >>> r.as_quat()
        array([0., 0., 0., 1.])
        >>> r.as_quat(scalar_first=True)
        array([1., 0., 0., 0.])

        When multiple rotations are stored in a single Rotation object, the
        result will be a 2-dimensional array:

        >>> r = R.from_rotvec([[np.pi, 0, 0], [0, 0, np.pi/2]])
        >>> r.as_quat().shape
        (2, 4)

        Quaternions can be mapped from a redundant double cover of the
        rotation space to a canonical representation with a positive w term.

        >>> r = R.from_quat([0, 0, 0, -1])
        >>> r.as_quat()
        array([0. , 0. , 0. , -1.])
        >>> r.as_quat(canonical=True)
        array([0. , 0. , 0. , 1.])
        """
        if self._single:
            q = np.array(self._quat[0], copy=True)
            if canonical:
                _quat_canonical_single(q)
        else:
            q = np.array(self._quat, copy=True)
            if canonical:
                _quat_canonical(q)

        if scalar_first:
            q = np.roll(q, 1, axis=None if self._single else 1)

        return q

    @cython.embedsignature(True)
    @cython.boundscheck(False)
    @cython.wraparound(False)
    def as_matrix(self):
        """Represent as rotation matrix.

        3D rotations can be represented using rotation matrices, which
        are 3 x 3 real orthogonal matrices with determinant equal to +1 [1]_.

        Returns
        -------
        matrix : ndarray, shape (3, 3) or (N, 3, 3)
            Shape depends on shape of inputs used for initialization.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Rotation_matrix#In_three_dimensions

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Represent a single rotation:

        >>> r = R.from_rotvec([0, 0, np.pi/2])
        >>> r.as_matrix()
        array([[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
               [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
               [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]])
        >>> r.as_matrix().shape
        (3, 3)

        Represent a stack with a single rotation:

        >>> r = R.from_quat([[1, 1, 0, 0]])
        >>> r.as_matrix()
        array([[[ 0.,  1.,  0.],
                [ 1.,  0.,  0.],
                [ 0.,  0., -1.]]])
        >>> r.as_matrix().shape
        (1, 3, 3)

        Represent multiple rotations:

        >>> r = R.from_rotvec([[np.pi/2, 0, 0], [0, 0, np.pi/2]])
        >>> r.as_matrix()
        array([[[ 1.00000000e+00,  0.00000000e+00,  0.00000000e+00],
                [ 0.00000000e+00,  2.22044605e-16, -1.00000000e+00],
                [ 0.00000000e+00,  1.00000000e+00,  2.22044605e-16]],
               [[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
                [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
                [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]]])
        >>> r.as_matrix().shape
        (2, 3, 3)

        Notes
        -----
        This function was called as_dcm before.

        .. versionadded:: 1.4.0
        """
        cdef double[:, :] quat = self._quat
        cdef Py_ssize_t num_rotations = quat.shape[0]
        cdef double[:, :, :] matrix = _empty3(num_rotations, 3, 3)

        cdef double x, y, z, w, x2, y2, z2, w2
        cdef double xy, zw, xz, yw, yz, xw

        for ind in range(num_rotations):
            x = quat[ind, 0]
            y = quat[ind, 1]
            z = quat[ind, 2]
            w = quat[ind, 3]

            x2 = x * x
            y2 = y * y
            z2 = z * z
            w2 = w * w

            xy = x * y
            zw = z * w
            xz = x * z
            yw = y * w
            yz = y * z
            xw = x * w

            matrix[ind, 0, 0] = x2 - y2 - z2 + w2
            matrix[ind, 1, 0] = 2 * (xy + zw)
            matrix[ind, 2, 0] = 2 * (xz - yw)

            matrix[ind, 0, 1] = 2 * (xy - zw)
            matrix[ind, 1, 1] = - x2 + y2 - z2 + w2
            matrix[ind, 2, 1] = 2 * (yz + xw)

            matrix[ind, 0, 2] = 2 * (xz + yw)
            matrix[ind, 1, 2] = 2 * (yz - xw)
            matrix[ind, 2, 2] = - x2 - y2 + z2 + w2

        ret = np.asarray(matrix)
        if self._single:
            return ret[0]
        else:
            return ret

    @cython.embedsignature(True)
    @cython.boundscheck(False)
    @cython.wraparound(False)
    def as_rotvec(self, degrees=False):
        """Represent as rotation vectors.

        A rotation vector is a 3 dimensional vector which is co-directional to
        the axis of rotation and whose norm gives the angle of rotation [1]_.

        Parameters
        ----------
        degrees : boolean, optional
            Returned magnitudes are in degrees if this flag is True, else they are
            in radians. Default is False.

            .. versionadded:: 1.7.0

        Returns
        -------
        rotvec : ndarray, shape (3,) or (N, 3)
            Shape depends on shape of inputs used for initialization.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Axis%E2%80%93angle_representation#Rotation_vector

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Represent a single rotation:

        >>> r = R.from_euler('z', 90, degrees=True)
        >>> r.as_rotvec()
        array([0.        , 0.        , 1.57079633])
        >>> r.as_rotvec().shape
        (3,)

        Represent a rotation in degrees:

        >>> r = R.from_euler('YX', (-90, -90), degrees=True)
        >>> s = r.as_rotvec(degrees=True)
        >>> s
        array([-69.2820323, -69.2820323, -69.2820323])
        >>> np.linalg.norm(s)
        120.00000000000001

        Represent a stack with a single rotation:

        >>> r = R.from_quat([[0, 0, 1, 1]])
        >>> r.as_rotvec()
        array([[0.        , 0.        , 1.57079633]])
        >>> r.as_rotvec().shape
        (1, 3)

        Represent multiple rotations in a single object:

        >>> r = R.from_quat([[0, 0, 1, 1], [1, 1, 0, 1]])
        >>> r.as_rotvec()
        array([[0.        , 0.        , 1.57079633],
               [1.35102172, 1.35102172, 0.        ]])
        >>> r.as_rotvec().shape
        (2, 3)

        """

        cdef Py_ssize_t num_rotations = len(self._quat)
        cdef double angle, scale, angle2
        cdef double[:, :] rotvec = _empty2(num_rotations, 3)
        cdef double[:] quat

        for ind in range(num_rotations):
            quat = self._quat[ind, :].copy()
            _quat_canonical_single(quat)  # w > 0 ensures that 0 <= angle <= pi

            angle = 2 * atan2(_norm3(quat), quat[3])

            if angle <= 1e-3:  # small angle Taylor series expansion
                angle2 = angle * angle
                scale = 2 + angle2 / 12 + 7 * angle2 * angle2 / 2880
            else:  # large angle
                scale = angle / sin(angle / 2)

            rotvec[ind, 0] = scale * quat[0]
            rotvec[ind, 1] = scale * quat[1]
            rotvec[ind, 2] = scale * quat[2]

        if degrees:
            rotvec = np.rad2deg(rotvec)

        if self._single:
            return np.asarray(rotvec[0])
        else:
            return np.asarray(rotvec)

    @cython.embedsignature(True)
    def _compute_euler(self, seq, degrees, algorithm):
        # Prepare axis sequence to call Euler angles conversion algorithm.
        
        if len(seq) != 3:
            raise ValueError("Expected 3 axes, got {}.".format(seq))

        intrinsic = (re.match(r'^[XYZ]{1,3}$', seq) is not None)
        extrinsic = (re.match(r'^[xyz]{1,3}$', seq) is not None)
        if not (intrinsic or extrinsic):
            raise ValueError("Expected axes from `seq` to be from "
                             "['x', 'y', 'z'] or ['X', 'Y', 'Z'], "
                             "got {}".format(seq))

        if any(seq[i] == seq[i+1] for i in range(2)):
            raise ValueError("Expected consecutive axes to be different, "
                             "got {}".format(seq))

        seq = seq.lower()
            
        if algorithm == 'from_matrix':
            matrix = self.as_matrix()
            if matrix.ndim == 2:
                matrix = matrix[None, :, :]
            angles = np.asarray(_compute_euler_from_matrix(
                matrix, seq.encode(), extrinsic))
        elif algorithm == 'from_quat':
            quat = self.as_quat()
            if quat.ndim == 1:
                quat = quat[None, :]
            angles = np.asarray(_compute_euler_from_quat(
                    quat, seq.encode(), extrinsic))
        else:
            # algorithm can only be 'from_quat' or 'from_matrix'
            assert False
            
        if degrees:
            angles = np.rad2deg(angles)

        return angles[0] if self._single else angles

    @cython.embedsignature(True)
    def _as_euler_from_matrix(self, seq, degrees=False):
        """Represent as Euler angles.

        Any orientation can be expressed as a composition of 3 elementary
        rotations. Once the axis sequence has been chosen, Euler angles define
        the angle of rotation around each respective axis [1]_.

        The algorithm from [2]_ has been used to calculate Euler angles for the
        rotation about a given sequence of axes.

        Euler angles suffer from the problem of gimbal lock [3]_, where the
        representation loses a degree of freedom and it is not possible to
        determine the first and third angles uniquely. In this case,
        a warning is raised, and the third angle is set to zero. Note however
        that the returned angles still represent the correct rotation.

        Parameters
        ----------
        seq : string, length 3
            3 characters belonging to the set {'X', 'Y', 'Z'} for intrinsic
            rotations, or {'x', 'y', 'z'} for extrinsic rotations [1]_.
            Adjacent axes cannot be the same.
            Extrinsic and intrinsic rotations cannot be mixed in one function
            call.
        degrees : boolean, optional
            Returned angles are in degrees if this flag is True, else they are
            in radians. Default is False.

        Returns
        -------
        angles : ndarray, shape (3,) or (N, 3)
            Shape depends on shape of inputs used to initialize object.
            The returned angles are in the range:

            - First angle belongs to [-180, 180] degrees (both inclusive)
            - Third angle belongs to [-180, 180] degrees (both inclusive)
            - Second angle belongs to:

                - [-90, 90] degrees if all axes are different (like xyz)
                - [0, 180] degrees if first and third axes are the same
                  (like zxz)

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Euler_angles#Definition_by_intrinsic_rotations
        .. [2] Malcolm D. Shuster, F. Landis Markley, "General formula for
               extraction the Euler angles", Journal of guidance, control, and
               dynamics, vol. 29.1, pp. 215-221. 2006
        .. [3] https://en.wikipedia.org/wiki/Gimbal_lock#In_applied_mathematics

        """
        return self._compute_euler(seq, degrees, 'from_matrix')

    @cython.embedsignature(True)
    def as_euler(self, seq, degrees=False):
        """Represent as Euler angles.

        Any orientation can be expressed as a composition of 3 elementary
        rotations. Once the axis sequence has been chosen, Euler angles define
        the angle of rotation around each respective axis [1]_.

        The algorithm from [2]_ has been used to calculate Euler angles for the 
        rotation about a given sequence of axes.

        Euler angles suffer from the problem of gimbal lock [3]_, where the
        representation loses a degree of freedom and it is not possible to
        determine the first and third angles uniquely. In this case,
        a warning is raised, and the third angle is set to zero. Note however
        that the returned angles still represent the correct rotation.

        Parameters
        ----------
        seq : string, length 3
            3 characters belonging to the set {'X', 'Y', 'Z'} for intrinsic
            rotations, or {'x', 'y', 'z'} for extrinsic rotations [1]_.
            Adjacent axes cannot be the same.
            Extrinsic and intrinsic rotations cannot be mixed in one function
            call.
        degrees : boolean, optional
            Returned angles are in degrees if this flag is True, else they are
            in radians. Default is False.

        Returns
        -------
        angles : ndarray, shape (3,) or (N, 3)
            Shape depends on shape of inputs used to initialize object.
            The returned angles are in the range:

            - First angle belongs to [-180, 180] degrees (both inclusive)
            - Third angle belongs to [-180, 180] degrees (both inclusive)
            - Second angle belongs to:

                - [-90, 90] degrees if all axes are different (like xyz)
                - [0, 180] degrees if first and third axes are the same
                  (like zxz)

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Euler_angles#Definition_by_intrinsic_rotations
        .. [2] Bernardes E, Viollet S (2022) Quaternion to Euler angles 
               conversion: A direct, general and computationally efficient 
               method. PLoS ONE 17(11): e0276302. 
               https://doi.org/10.1371/journal.pone.0276302
        .. [3] https://en.wikipedia.org/wiki/Gimbal_lock#In_applied_mathematics

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Represent a single rotation:

        >>> r = R.from_rotvec([0, 0, np.pi/2])
        >>> r.as_euler('zxy', degrees=True)
        array([90.,  0.,  0.])
        >>> r.as_euler('zxy', degrees=True).shape
        (3,)

        Represent a stack of single rotation:

        >>> r = R.from_rotvec([[0, 0, np.pi/2]])
        >>> r.as_euler('zxy', degrees=True)
        array([[90.,  0.,  0.]])
        >>> r.as_euler('zxy', degrees=True).shape
        (1, 3)

        Represent multiple rotations in a single object:

        >>> r = R.from_rotvec([
        ... [0, 0, np.pi/2],
        ... [0, -np.pi/3, 0],
        ... [np.pi/4, 0, 0]])
        >>> r.as_euler('zxy', degrees=True)
        array([[ 90.,   0.,   0.],
               [  0.,   0., -60.],
               [  0.,  45.,   0.]])
        >>> r.as_euler('zxy', degrees=True).shape
        (3, 3)

        """
        return self._compute_euler(seq, degrees, 'from_quat')

    @cython.embedsignature(True)
    def as_davenport(self, axes, order, degrees=False):
        """Represent as Davenport angles.

        Any orientation can be expressed as a composition of 3 elementary
        rotations.

        For both Euler angles and Davenport angles, consecutive axes must 
        be are orthogonal (``axis2`` is orthogonal to both ``axis1`` and 
        ``axis3``). For Euler angles, there is an additional relationship 
        between ``axis1`` or ``axis3``, with two possibilities:

            - ``axis1`` and ``axis3`` are also orthogonal (asymmetric sequence)
            - ``axis1 == axis3`` (symmetric sequence)

        For Davenport angles, this last relationship is relaxed [1]_, and only
        the consecutive orthogonal axes requirement is maintained.

        A slightly modified version of the algorithm from [2]_ has been used to
        calculate Davenport angles for the rotation about a given sequence of
        axes.

        Davenport angles, just like Euler angles, suffer from the problem of
        gimbal lock [3]_, where the representation loses a degree of freedom
        and it is not possible to determine the first and third angles
        uniquely. In this case, a warning is raised, and the third angle is set
        to zero. Note however that the returned angles still represent the
        correct rotation.

        Parameters
        ----------
        axes : array_like, shape (3,) or ([1 or 2 or 3], 3)
            Axis of rotation, if one dimensional. If two dimensional, describes the 
            sequence of axes for rotations, where each axes[i, :] is the ith
            axis. If more than one axis is given, then the second axis must be
            orthogonal to both the first and third axes.
        order : string
            If it belongs to the set {'e', 'extrinsic'}, the sequence will be 
            extrinsic. If if belongs to the set {'i', 'intrinsic'}, sequence 
            will be treated as intrinsic.
        degrees : boolean, optional
            Returned angles are in degrees if this flag is True, else they are
            in radians. Default is False.

        Returns
        -------
        angles : ndarray, shape (3,) or (N, 3)
            Shape depends on shape of inputs used to initialize object.
            The returned angles are in the range:

            - First angle belongs to [-180, 180] degrees (both inclusive)
            - Third angle belongs to [-180, 180] degrees (both inclusive)
            - Second angle belongs to a set of size 180 degrees, 
              given by: ``[-abs(lambda), 180 - abs(lambda)]``, where ``lambda``
              is the angle between the first and third axes.

        References
        ----------
        .. [1] Shuster, Malcolm & Markley, Landis. (2003). Generalization of
               the Euler Angles. Journal of the Astronautical Sciences. 51. 123-132. 10.1007/BF03546304.
        .. [2] Bernardes E, Viollet S (2022) Quaternion to Euler angles
               conversion: A direct, general and computationally efficient method.
               PLoS ONE 17(11): e0276302. 10.1371/journal.pone.0276302
        .. [3] https://en.wikipedia.org/wiki/Gimbal_lock#In_applied_mathematics

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Davenport angles are a generalization of Euler angles, when we use the
        canonical basis axes:

        >>> ex = [1, 0, 0]
        >>> ey = [0, 1, 0]
        >>> ez = [0, 0, 1]

        Represent a single rotation:

        >>> r = R.from_rotvec([0, 0, np.pi/2])
        >>> r.as_davenport([ez, ex, ey], 'extrinsic', degrees=True)
        array([90.,  0.,  0.])
        >>> r.as_euler('zxy', degrees=True)
        array([90.,  0.,  0.])
        >>> r.as_davenport([ez, ex, ey], 'extrinsic', degrees=True).shape
        (3,)

        Represent a stack of single rotation:

        >>> r = R.from_rotvec([[0, 0, np.pi/2]])
        >>> r.as_davenport([ez, ex, ey], 'extrinsic', degrees=True)
        array([[90.,  0.,  0.]])
        >>> r.as_davenport([ez, ex, ey], 'extrinsic', degrees=True).shape
        (1, 3)

        Represent multiple rotations in a single object:

        >>> r = R.from_rotvec([
        ... [0, 0, 90],
        ... [45, 0, 0]], degrees=True)
        >>> r.as_davenport([ez, ex, ey], 'extrinsic', degrees=True)
        array([[90.,  0.,  0.],
               [ 0., 45.,  0.]])
        >>> r.as_davenport([ez, ex, ey], 'extrinsic', degrees=True).shape
        (2, 3)
        """
        if order in ['e', 'extrinsic']:
            extrinsic = True
        elif order in ['i', 'intrinsic']:
            extrinsic = False
        else:
            raise ValueError("order should be 'e'/'extrinsic' for extrinsic "
                             "sequences or 'i'/'intrinsic' for intrinsic "
                             "sequences, got {}".format(order))

        if len(axes) != 3:
            raise ValueError("Expected 3 axes, got {}.".format(len(axes)))

        axes = np.asarray(axes)

        if axes.shape[1] != 3:
            raise ValueError("Axes must be vectors of length 3.")

        n1, n2, n3 = axes

        # normalize axes
        n1 = n1 / np.linalg.norm(n1)
        n2 = n2 / np.linalg.norm(n2)
        n3 = n3 / np.linalg.norm(n3)

        if np.dot(n1, n2) >= 1e-7:
            raise ValueError("Consecutive axes must be orthogonal.")
        if np.dot(n2, n3) >= 1e-7:
            raise ValueError("Consecutive axes must be orthogonal.")

        quat = self.as_quat()
        if quat.ndim == 1:
            quat = quat[None, :]
        angles = np.asarray(_compute_davenport_from_quat(
                quat, n1, n2, n3, extrinsic))

        if degrees:
            angles = np.rad2deg(angles)

        return angles[0] if self._single else angles

    @cython.embedsignature(True)
    def as_mrp(self):
        """Represent as Modified Rodrigues Parameters (MRPs).

        MRPs are a 3 dimensional vector co-directional to the axis of rotation and whose
        magnitude is equal to ``tan(theta / 4)``, where ``theta`` is the angle of rotation
        (in radians) [1]_.

        MRPs have a singularity at 360 degrees which can be avoided by ensuring the angle of
        rotation does not exceed 180 degrees, i.e. switching the direction of the rotation when
        it is past 180 degrees. This function will always return MRPs corresponding to a rotation
        of less than or equal to 180 degrees.

        Returns
        -------
        mrps : ndarray, shape (3,) or (N, 3)
            Shape depends on shape of inputs used for initialization.

        References
        ----------
        .. [1] Shuster, M. D. "A Survey of Attitude Representations",
               The Journal of Astronautical Sciences, Vol. 41, No.4, 1993,
               pp. 475-476

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Represent a single rotation:

        >>> r = R.from_rotvec([0, 0, np.pi])
        >>> r.as_mrp()
        array([0.        , 0.        , 1.         ])
        >>> r.as_mrp().shape
        (3,)

        Represent a stack with a single rotation:

        >>> r = R.from_euler('xyz', [[180, 0, 0]], degrees=True)
        >>> r.as_mrp()
        array([[1.       , 0.        , 0.         ]])
        >>> r.as_mrp().shape
        (1, 3)

        Represent multiple rotations:

        >>> r = R.from_rotvec([[np.pi/2, 0, 0], [0, 0, np.pi/2]])
        >>> r.as_mrp()
        array([[0.41421356, 0.        , 0.        ],
               [0.        , 0.        , 0.41421356]])
        >>> r.as_mrp().shape
        (2, 3)

        Notes
        -----

        .. versionadded:: 1.6.0
        """
        cdef Py_ssize_t num_rotations = len(self._quat)
        cdef double[:, :] mrps = _empty2(num_rotations, 3)
        cdef int sign
        cdef double denominator

        for ind in range(num_rotations):

            # Ensure we are calculating the set of MRPs that correspond
            # to a rotation of <= 180
            sign = -1 if self._quat[ind, 3] < 0 else 1

            denominator = 1 + sign * self._quat[ind, 3]
            for i in range(3):
                mrps[ind, i] = sign * self._quat[ind, i] / denominator

        if self._single:
            return np.asarray(mrps[0])
        else:
            return np.asarray(mrps)

    @cython.embedsignature(True)
    @classmethod
    def concatenate(cls, rotations):
        """Concatenate a sequence of `Rotation` objects into a single object.

        This is useful if you want to, for example, take the mean of a set of
        rotations and need to pack them into a single object to do so.

        Parameters
        ----------
        rotations : sequence of `Rotation` objects
            The rotations to concatenate.

        Returns
        -------
        concatenated : `Rotation` instance
            The concatenated rotations.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> r1 = R.from_rotvec([0, 0, 1])
        >>> r2 = R.from_rotvec([0, 0, 2])
        >>> rc = R.concatenate([r1, r2])
        >>> rc.as_rotvec()
        array([[0., 0., 1.],
               [0., 0., 2.]])
        >>> rc.mean().as_rotvec()
        array([0., 0., 1.5])

        Note that it may be simpler to create the desired rotations by passing
        in a single list of the data during initialization, rather then by
        concatenating:

        >>> R.from_rotvec([[0, 0, 1], [0, 0, 2]]).as_rotvec()
        array([[0., 0., 1.],
               [0., 0., 2.]])

        Notes
        -----
        .. versionadded:: 1.8.0
        """
        if not all(isinstance(x, Rotation) for x in rotations):
            raise TypeError("input must contain Rotation objects only")

        quats = np.concatenate([np.atleast_2d(x.as_quat()) for x in rotations])
        return cls(quats, normalize=False)

    @cython.embedsignature(True)
    def apply(self, vectors, inverse=False):
        """Apply this rotation to a set of vectors.

        If the original frame rotates to the final frame by this rotation, then
        its application to a vector can be seen in two ways:

            - As a projection of vector components expressed in the final frame
              to the original frame.
            - As the physical rotation of a vector being glued to the original
              frame as it rotates. In this case the vector components are
              expressed in the original frame before and after the rotation.

        In terms of rotation matrices, this application is the same as
        ``self.as_matrix() @ vectors``.

        Parameters
        ----------
        vectors : array_like, shape (3,) or (N, 3)
            Each `vectors[i]` represents a vector in 3D space. A single vector
            can either be specified with shape `(3, )` or `(1, 3)`. The number
            of rotations and number of vectors given must follow standard numpy
            broadcasting rules: either one of them equals unity or they both
            equal each other.
        inverse : boolean, optional
            If True then the inverse of the rotation(s) is applied to the input
            vectors. Default is False.

        Returns
        -------
        rotated_vectors : ndarray, shape (3,) or (N, 3)
            Result of applying rotation on input vectors.
            Shape depends on the following cases:

                - If object contains a single rotation (as opposed to a stack
                  with a single rotation) and a single vector is specified with
                  shape ``(3,)``, then `rotated_vectors` has shape ``(3,)``.
                - In all other cases, `rotated_vectors` has shape ``(N, 3)``,
                  where ``N`` is either the number of rotations or vectors.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Single rotation applied on a single vector:

        >>> vector = np.array([1, 0, 0])
        >>> r = R.from_rotvec([0, 0, np.pi/2])
        >>> r.as_matrix()
        array([[ 2.22044605e-16, -1.00000000e+00,  0.00000000e+00],
               [ 1.00000000e+00,  2.22044605e-16,  0.00000000e+00],
               [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]])
        >>> r.apply(vector)
        array([2.22044605e-16, 1.00000000e+00, 0.00000000e+00])
        >>> r.apply(vector).shape
        (3,)

        Single rotation applied on multiple vectors:

        >>> vectors = np.array([
        ... [1, 0, 0],
        ... [1, 2, 3]])
        >>> r = R.from_rotvec([0, 0, np.pi/4])
        >>> r.as_matrix()
        array([[ 0.70710678, -0.70710678,  0.        ],
               [ 0.70710678,  0.70710678,  0.        ],
               [ 0.        ,  0.        ,  1.        ]])
        >>> r.apply(vectors)
        array([[ 0.70710678,  0.70710678,  0.        ],
               [-0.70710678,  2.12132034,  3.        ]])
        >>> r.apply(vectors).shape
        (2, 3)

        Multiple rotations on a single vector:

        >>> r = R.from_rotvec([[0, 0, np.pi/4], [np.pi/2, 0, 0]])
        >>> vector = np.array([1,2,3])
        >>> r.as_matrix()
        array([[[ 7.07106781e-01, -7.07106781e-01,  0.00000000e+00],
                [ 7.07106781e-01,  7.07106781e-01,  0.00000000e+00],
                [ 0.00000000e+00,  0.00000000e+00,  1.00000000e+00]],
               [[ 1.00000000e+00,  0.00000000e+00,  0.00000000e+00],
                [ 0.00000000e+00,  2.22044605e-16, -1.00000000e+00],
                [ 0.00000000e+00,  1.00000000e+00,  2.22044605e-16]]])
        >>> r.apply(vector)
        array([[-0.70710678,  2.12132034,  3.        ],
               [ 1.        , -3.        ,  2.        ]])
        >>> r.apply(vector).shape
        (2, 3)

        Multiple rotations on multiple vectors. Each rotation is applied on the
        corresponding vector:

        >>> r = R.from_euler('zxy', [
        ... [0, 0, 90],
        ... [45, 30, 60]], degrees=True)
        >>> vectors = [
        ... [1, 2, 3],
        ... [1, 0, -1]]
        >>> r.apply(vectors)
        array([[ 3.        ,  2.        , -1.        ],
               [-0.09026039,  1.11237244, -0.86860844]])
        >>> r.apply(vectors).shape
        (2, 3)

        It is also possible to apply the inverse rotation:

        >>> r = R.from_euler('zxy', [
        ... [0, 0, 90],
        ... [45, 30, 60]], degrees=True)
        >>> vectors = [
        ... [1, 2, 3],
        ... [1, 0, -1]]
        >>> r.apply(vectors, inverse=True)
        array([[-3.        ,  2.        ,  1.        ],
               [ 1.09533535, -0.8365163 ,  0.3169873 ]])

        """
        vectors = np.asarray(vectors)
        if vectors.ndim > 2 or vectors.shape[-1] != 3:
            raise ValueError("Expected input of shape (3,) or (P, 3), "
                             "got {}.".format(vectors.shape))

        single_vector = False
        if vectors.shape == (3,):
            single_vector = True
            vectors = vectors[None, :]

        matrix = self.as_matrix()
        if self._single:
            matrix = matrix[None, :, :]

        n_vectors = vectors.shape[0]
        n_rotations = len(self._quat)

        if n_vectors != 1 and n_rotations != 1 and n_vectors != n_rotations:
            raise ValueError("Expected equal numbers of rotations and vectors "
                             ", or a single rotation, or a single vector, got "
                             "{} rotations and {} vectors.".format(
                                n_rotations, n_vectors))

        if inverse:
            result = np.einsum('ikj,ik->ij', matrix, vectors)
        else:
            result = np.einsum('ijk,ik->ij', matrix, vectors)

        if self._single and single_vector:
            return result[0]
        else:
            return result

    @cython.embedsignature(True)
    def __mul__(Rotation self, Rotation other):
        """Compose this rotation with the other.

        If `p` and `q` are two rotations, then the composition of 'q followed
        by p' is equivalent to `p * q`. In terms of rotation matrices,
        the composition can be expressed as
        ``p.as_matrix() @ q.as_matrix()``.

        Parameters
        ----------
        other : `Rotation` instance
            Object containing the rotations to be composed with this one. Note
            that rotation compositions are not commutative, so ``p * q`` is
            generally different from ``q * p``.

        Returns
        -------
        composition : `Rotation` instance
            This function supports composition of multiple rotations at a time.
            The following cases are possible:

            - Either ``p`` or ``q`` contains a single rotation. In this case
              `composition` contains the result of composing each rotation in
              the other object with the single rotation.
            - Both ``p`` and ``q`` contain ``N`` rotations. In this case each
              rotation ``p[i]`` is composed with the corresponding rotation
              ``q[i]`` and `output` contains ``N`` rotations.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Composition of two single rotations:

        >>> p = R.from_quat([0, 0, 1, 1])
        >>> q = R.from_quat([1, 0, 0, 1])
        >>> p.as_matrix()
        array([[ 0., -1.,  0.],
               [ 1.,  0.,  0.],
               [ 0.,  0.,  1.]])
        >>> q.as_matrix()
        array([[ 1.,  0.,  0.],
               [ 0.,  0., -1.],
               [ 0.,  1.,  0.]])
        >>> r = p * q
        >>> r.as_matrix()
        array([[0., 0., 1.],
               [1., 0., 0.],
               [0., 1., 0.]])

        Composition of two objects containing equal number of rotations:

        >>> p = R.from_quat([[0, 0, 1, 1], [1, 0, 0, 1]])
        >>> q = R.from_rotvec([[np.pi/4, 0, 0], [-np.pi/4, 0, np.pi/4]])
        >>> p.as_quat()
        array([[0.        , 0.        , 0.70710678, 0.70710678],
               [0.70710678, 0.        , 0.        , 0.70710678]])
        >>> q.as_quat()
        array([[ 0.38268343,  0.        ,  0.        ,  0.92387953],
               [-0.37282173,  0.        ,  0.37282173,  0.84971049]])
        >>> r = p * q
        >>> r.as_quat()
        array([[ 0.27059805,  0.27059805,  0.65328148,  0.65328148],
               [ 0.33721128, -0.26362477,  0.26362477,  0.86446082]])

        """
        len_self = len(self._quat)
        len_other = len(other._quat)
        if not(len_self == 1 or len_other == 1 or len_self == len_other):
            raise ValueError("Expected equal number of rotations in both "
                             "or a single rotation in either object, "
                             "got {} rotations in first and {} rotations in "
                             "second object.".format(
                                len(self), len(other)))
        result = _compose_quat(self._quat, other._quat)
        if self._single and other._single:
            result = result[0]
        return self.__class__(result, normalize=True, copy=False)

    @cython.embedsignature(True)
    def __pow__(Rotation self, float n, modulus):
        """Compose this rotation with itself `n` times.

        Composition of a rotation ``p`` with itself can be extended to
        non-integer ``n`` by considering the power ``n`` to be a scale factor
        applied to the angle of rotation about the rotation's fixed axis. The
        expression ``q = p ** n`` can also be expressed as
        ``q = Rotation.from_rotvec(n * p.as_rotvec())``.

        If ``n`` is negative, then the rotation is inverted before the power
        is applied. In other words, ``p ** -abs(n) == p.inv() ** abs(n)``.

        Parameters
        ----------
        n : float
            The number of times to compose the rotation with itself.
        modulus : None
            This overridden argument is not applicable to Rotations and must be
            ``None``.

        Returns
        -------
        power : `Rotation` instance
            If the input Rotation ``p`` contains ``N`` multiple rotations, then
            the output will contain ``N`` rotations where the ``i`` th rotation
            is equal to ``p[i] ** n``

        Notes
        -----
        For example, a power of 2 will double the angle of rotation, and a
        power of 0.5 will halve the angle. There are three notable cases: if
        ``n == 1`` then the original rotation is returned, if ``n == 0``
        then the identity rotation is returned, and if ``n == -1`` then
        ``p.inv()`` is returned.

        Note that fractional powers ``n`` which effectively take a root of
        rotation, do so using the shortest path smallest representation of that
        angle (the principal root). This means that powers of ``n`` and ``1/n``
        are not necessarily inverses of each other. For example, a 0.5 power of
        a +240 degree rotation will be calculated as the 0.5 power of a -120
        degree rotation, with the result being a rotation of -60 rather than
        +120 degrees.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R

        Raising a rotation to a power:

        >>> p = R.from_rotvec([1, 0, 0])
        >>> q = p ** 2
        >>> q.as_rotvec()
        array([2., 0., 0.])
        >>> r = p ** 0.5
        >>> r.as_rotvec()
        array([0.5, 0., 0.])

        Inverse powers do not necessarily cancel out:

        >>> p = R.from_rotvec([0, 0, 120], degrees=True)
        >>> ((p ** 2) ** 0.5).as_rotvec(degrees=True)
        array([  -0.,   -0., -60.])

        """
        if modulus is not None:
            raise NotImplementedError("modulus not supported")

        # Exact short-cuts
        if n == 0:
            return Rotation.identity(None if self._single else len(self._quat))
        elif n == -1:
            return self.inv()
        elif n == 1:
            if self._single:
                return self.__class__(self._quat[0], normalize=False, copy=True)
            else:
                return self.__class__(self._quat, normalize=False, copy=True)
        else:  # general scaling of rotation angle
            return Rotation.from_rotvec(n * self.as_rotvec())

    @cython.embedsignature(True)
    def inv(self):
        """Invert this rotation.

        Composition of a rotation with its inverse results in an identity
        transformation.

        Returns
        -------
        inverse : `Rotation` instance
            Object containing inverse of the rotations in the current instance.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np

        Inverting a single rotation:

        >>> p = R.from_euler('z', 45, degrees=True)
        >>> q = p.inv()
        >>> q.as_euler('zyx', degrees=True)
        array([-45.,   0.,   0.])

        Inverting multiple rotations:

        >>> p = R.from_rotvec([[0, 0, np.pi/3], [-np.pi/4, 0, 0]])
        >>> q = p.inv()
        >>> q.as_rotvec()
        array([[-0.        , -0.        , -1.04719755],
               [ 0.78539816, -0.        , -0.        ]])

        """
        cdef np.ndarray quat = np.array(self._quat, copy=True)
        quat[:, 0] *= -1
        quat[:, 1] *= -1
        quat[:, 2] *= -1
        if self._single:
            quat = quat[0]
        return self.__class__(quat, normalize=False, copy=False)

    @cython.embedsignature(True)
    @cython.boundscheck(False)
    @cython.wraparound(False)
    def magnitude(self):
        """Get the magnitude(s) of the rotation(s).

        Returns
        -------
        magnitude : ndarray or float
            Angle(s) in radians, float if object contains a single rotation
            and ndarray if object contains multiple rotations. The magnitude
            will always be in the range [0, pi].

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np
        >>> r = R.from_quat(np.eye(4))
        >>> r.as_quat()
        array([[ 1., 0., 0., 0.],
               [ 0., 1., 0., 0.],
               [ 0., 0., 1., 0.],
               [ 0., 0., 0., 1.]])
        >>> r.magnitude()
        array([3.14159265, 3.14159265, 3.14159265, 0.        ])

        Magnitude of a single rotation:

        >>> r[0].magnitude()
        3.141592653589793
        """
        cdef double[:, :] quat = self._quat
        cdef Py_ssize_t num_rotations = quat.shape[0]
        cdef double[:] angles = _empty1(num_rotations)

        for ind in range(num_rotations):
            angles[ind] = 2 * atan2(_norm3(quat[ind, :3]), abs(quat[ind, 3]))

        if self._single:
            return angles[0]
        else:
            return np.asarray(angles)


    @cython.embedsignature(True)
    @cython.boundscheck(False)
    @cython.wraparound(False)
    def approx_equal(Rotation self, Rotation other, atol=None, degrees=False):
        """Determine if another rotation is approximately equal to this one.

        Equality is measured by calculating the smallest angle between the
        rotations, and checking to see if it is smaller than `atol`.

        Parameters
        ----------
        other : `Rotation` instance
            Object containing the rotations to measure against this one.
        atol : float, optional
            The absolute angular tolerance, below which the rotations are
            considered equal. If not given, then set to 1e-8 radians by
            default.
        degrees : bool, optional
            If True and `atol` is given, then `atol` is measured in degrees. If
            False (default), then atol is measured in radians.

        Returns
        -------
        approx_equal : ndarray or bool
            Whether the rotations are approximately equal, bool if object
            contains a single rotation and ndarray if object contains multiple
            rotations.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> import numpy as np
        >>> p = R.from_quat([0, 0, 0, 1])
        >>> q = R.from_quat(np.eye(4))
        >>> p.approx_equal(q)
        array([False, False, False, True])

        Approximate equality for a single rotation:

        >>> p.approx_equal(q[0])
        False
        """
        if atol is None:
            if degrees:
                warnings.warn("atol must be set to use the degrees flag, "
                              "defaulting to 1e-8 radians.")
            atol = 1e-8  # radians
        elif degrees:
            atol = np.deg2rad(atol)

        angles = (other * self.inv()).magnitude()
        return angles < atol

    @cython.embedsignature(True)
    def mean(self, weights=None):
        """Get the mean of the rotations.

        The mean used is the chordal L2 mean (also called the projected or
        induced arithmetic mean) [1]_. If ``A`` is a set of rotation matrices,
        then the mean ``M`` is the rotation matrix that minimizes the
        following loss function:

        .. math::

            L(M) = \\sum_{i = 1}^{n} w_i \\lVert \\mathbf{A}_i -
            \\mathbf{M} \\rVert^2 ,

        where :math:`w_i`'s are the `weights` corresponding to each matrix.

        Parameters
        ----------
        weights : array_like shape (N,), optional
            Weights describing the relative importance of the rotations. If
            None (default), then all values in `weights` are assumed to be
            equal.

        Returns
        -------
        mean : `Rotation` instance
            Object containing the mean of the rotations in the current
            instance.

        References
        ----------
        .. [1] Hartley, Richard, et al.,
                "Rotation Averaging", International Journal of Computer Vision
                103, 2013, pp. 267-305.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> r = R.from_euler('zyx', [[0, 0, 0],
        ...                          [1, 0, 0],
        ...                          [0, 1, 0],
        ...                          [0, 0, 1]], degrees=True)
        >>> r.mean().as_euler('zyx', degrees=True)
        array([0.24945696, 0.25054542, 0.24945696])
        """
        if weights is None:
            weights = np.ones(len(self))
        else:
            weights = np.asarray(weights)
            if weights.ndim != 1:
                raise ValueError("Expected `weights` to be 1 dimensional, got "
                                 "shape {}.".format(weights.shape))
            if weights.shape[0] != len(self):
                raise ValueError("Expected `weights` to have number of values "
                                 "equal to number of rotations, got "
                                 "{} values and {} rotations.".format(
                                    weights.shape[0], len(self)))
            if np.any(weights < 0):
                raise ValueError("`weights` must be non-negative.")

        quat = np.asarray(self._quat)
        K = np.dot(weights * quat.T, quat)
        _, v = np.linalg.eigh(K)
        return self.__class__(v[:, -1], normalize=False)

    @cython.embedsignature(True)
    def reduce(self, left=None, right=None, return_indices=False):
        """Reduce this rotation with the provided rotation groups.

        Reduction of a rotation ``p`` is a transformation of the form
        ``q = l * p * r``, where ``l`` and ``r`` are chosen from `left` and
        `right` respectively, such that rotation ``q`` has the smallest
        magnitude.

        If `left` and `right` are rotation groups representing symmetries of
        two objects rotated by ``p``, then ``q`` is the rotation of the
        smallest magnitude to align these objects considering their symmetries.

        Parameters
        ----------
        left : `Rotation` instance, optional
            Object containing the left rotation(s). Default value (None)
            corresponds to the identity rotation.
        right : `Rotation` instance, optional
            Object containing the right rotation(s). Default value (None)
            corresponds to the identity rotation.
        return_indices : bool, optional
            Whether to return the indices of the rotations from `left` and
            `right` used for reduction.

        Returns
        -------
        reduced : `Rotation` instance
            Object containing reduced rotations.
        left_best, right_best: integer ndarray
            Indices of elements from `left` and `right` used for reduction.
        """
        if left is None and right is None:
            reduced = self.__class__(self._quat, normalize=False, copy=True)
            if return_indices:
                return reduced, None, None
            else:
                return reduced
        elif right is None:
            right = Rotation.identity()
        elif left is None:
            left = Rotation.identity()

        # Levi-Civita tensor for triple product computations
        e = np.zeros((3, 3, 3))
        e[0, 1, 2] = e[1, 2, 0] = e[2, 0, 1] = 1
        e[0, 2, 1] = e[2, 1, 0] = e[1, 0, 2] = -1

        # We want to calculate the real components of q = l * p * r. It can
        # be shown that:
        #     qs = ls * ps * rs - ls * dot(pv, rv) - ps * dot(lv, rv)
        #          - rs * dot(lv, pv) - dot(cross(lv, pv), rv)
        # where ls and lv denote the scalar and vector components of l.

        def split_rotation(R):
            q = np.atleast_2d(R.as_quat())
            return q[:, -1], q[:, :-1]

        p = self
        ps, pv = split_rotation(p)
        ls, lv = split_rotation(left)
        rs, rv = split_rotation(right)

        qs = np.abs(np.einsum('i,j,k', ls, ps, rs) -
                    np.einsum('i,jx,kx', ls, pv, rv) -
                    np.einsum('ix,j,kx', lv, ps, rv) -
                    np.einsum('ix,jx,k', lv, pv, rs) -
                    np.einsum('xyz,ix,jy,kz', e, lv, pv, rv))
        qs = np.reshape(np.moveaxis(qs, 1, 0), (qs.shape[1], -1))

        # Find best indices from scalar components
        max_ind = np.argmax(np.reshape(qs, (len(qs), -1)), axis=1)
        left_best = max_ind // len(rv)
        right_best = max_ind % len(rv)

        if not left.single:
            left = left[left_best]
        if not right.single:
            right = right[right_best]

        # Reduce the rotation using the best indices
        reduced = left * p * right
        if self._single:
            # Reduce the rotation using the best indices
            reduced = self.__class__(reduced.as_quat()[0], normalize=False)
            left_best = left_best[0]
            right_best = right_best[0]

        if return_indices:
            if left is None:
                left_best = None
            if right is None:
                right_best = None
            return reduced, left_best, right_best
        else:
            return reduced

    @cython.embedsignature(True)
    @classmethod
    def create_group(cls, group, axis='Z'):
        """Create a 3D rotation group.

        Parameters
        ----------
        group : string
            The name of the group. Must be one of 'I', 'O', 'T', 'Dn', 'Cn',
            where `n` is a positive integer. The groups are:

                * I: Icosahedral group
                * O: Octahedral group
                * T: Tetrahedral group
                * D: Dicyclic group
                * C: Cyclic group

        axis : integer
            The cyclic rotation axis. Must be one of ['X', 'Y', 'Z'] (or
            lowercase). Default is 'Z'. Ignored for groups 'I', 'O', and 'T'.

        Returns
        -------
        rotation : `Rotation` instance
            Object containing the elements of the rotation group.

        Notes
        -----
        This method generates rotation groups only. The full 3-dimensional
        point groups [PointGroups]_ also contain reflections.

        References
        ----------
        .. [PointGroups] `Point groups
           <https://en.wikipedia.org/wiki/Point_groups_in_three_dimensions>`_
           on Wikipedia.
        """
        return create_group(cls, group, axis=axis)

    @cython.embedsignature(True)
    def __getitem__(self, indexer):
        """Extract rotation(s) at given index(es) from object.

        Create a new `Rotation` instance containing a subset of rotations
        stored in this object.

        Parameters
        ----------
        indexer : index, slice, or index array
            Specifies which rotation(s) to extract. A single indexer must be
            specified, i.e. as if indexing a 1 dimensional array or list.

        Returns
        -------
        rotation : `Rotation` instance
            Contains
                - a single rotation, if `indexer` is a single index
                - a stack of rotation(s), if `indexer` is a slice, or and index
                  array.

        Raises
        ------
        TypeError if the instance was created as a single rotation.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R
        >>> r = R.from_quat([
        ... [1, 1, 0, 0],
        ... [0, 1, 0, 1],
        ... [1, 1, -1, 0]])
        >>> r.as_quat()
        array([[ 0.70710678,  0.70710678,  0.        ,  0.        ],
               [ 0.        ,  0.70710678,  0.        ,  0.70710678],
               [ 0.57735027,  0.57735027, -0.57735027,  0.        ]])

        Indexing using a single index:

        >>> p = r[0]
        >>> p.as_quat()
        array([0.70710678, 0.70710678, 0.        , 0.        ])

        Array slicing:

        >>> q = r[1:3]
        >>> q.as_quat()
        array([[ 0.        ,  0.70710678,  0.        ,  0.70710678],
               [ 0.57735027,  0.57735027, -0.57735027,  0.        ]])

        """
        if self._single:
            raise TypeError("Single rotation is not subscriptable.")

        return self.__class__(np.asarray(self._quat)[indexer], normalize=False)

    def __setitem__(self, indexer, value):
        """Set rotation(s) at given index(es) from object.

        Parameters
        ----------
        indexer : index, slice, or index array
            Specifies which rotation(s) to replace. A single indexer must be
            specified, i.e. as if indexing a 1 dimensional array or list.

        value : `Rotation` instance
            The rotations to set.

        Raises
        ------
        TypeError if the instance was created as a single rotation.

        Notes
        -----

        .. versionadded:: 1.8.0
        """
        if self._single:
            raise TypeError("Single rotation is not subscriptable.")

        if not isinstance(value, Rotation):
            raise TypeError("value must be a Rotation object")

        quat = np.asarray(self._quat)
        quat[indexer] = value.as_quat()
        self._quat = quat

    @cython.embedsignature(True)
    @classmethod
    def identity(cls, num=None):
        """Get identity rotation(s).

        Composition with the identity rotation has no effect.

        Parameters
        ----------
        num : int or None, optional
            Number of identity rotations to generate. If None (default), then a
            single rotation is generated.

        Returns
        -------
        identity : Rotation object
            The identity rotation.
        """
        if num is None:
            q = [0, 0, 0, 1]
        else:
            q = np.zeros((num, 4))
            q[:, 3] = 1
        return cls(q, normalize=False)

    @cython.embedsignature(True)
    @classmethod
    def random(cls, num=None, random_state=None):
        """Generate uniformly distributed rotations.

        Parameters
        ----------
        num : int or None, optional
            Number of random rotations to generate. If None (default), then a
            single rotation is generated.
        random_state : {None, int, `numpy.random.Generator`,
                        `numpy.random.RandomState`}, optional

            If `seed` is None (or `np.random`), the `numpy.random.RandomState`
            singleton is used.
            If `seed` is an int, a new ``RandomState`` instance is used,
            seeded with `seed`.
            If `seed` is already a ``Generator`` or ``RandomState`` instance
            then that instance is used.

        Returns
        -------
        random_rotation : `Rotation` instance
            Contains a single rotation if `num` is None. Otherwise contains a
            stack of `num` rotations.

        Notes
        -----
        This function is optimized for efficiently sampling random rotation
        matrices in three dimensions. For generating random rotation matrices
        in higher dimensions, see `scipy.stats.special_ortho_group`.

        Examples
        --------
        >>> from scipy.spatial.transform import Rotation as R

        Sample a single rotation:

        >>> R.random().as_euler('zxy', degrees=True)
        array([-110.5976185 ,   55.32758512,   76.3289269 ])  # random

        Sample a stack of rotations:

        >>> R.random(5).as_euler('zxy', degrees=True)
        array([[-110.5976185 ,   55.32758512,   76.3289269 ],  # random
               [ -91.59132005,  -14.3629884 ,  -93.91933182],
               [  25.23835501,   45.02035145, -121.67867086],
               [ -51.51414184,  -15.29022692, -172.46870023],
               [ -81.63376847,  -27.39521579,    2.60408416]])

        See Also
        --------
        scipy.stats.special_ortho_group

       """
        random_state = check_random_state(random_state)

        if num is None:
            sample = random_state.normal(size=4)
        else:
            sample = random_state.normal(size=(num, 4))

        return cls(sample)

    @cython.embedsignature(True)
    @classmethod
    def align_vectors(cls, a, b, weights=None, return_sensitivity=False):
        """Estimate a rotation to optimally align two sets of vectors.

        Find a rotation between frames A and B which best aligns a set of
        vectors `a` and `b` observed in these frames. The following loss
        function is minimized to solve for the rotation matrix
        :math:`C`:

        .. math::

            L(C) = \\frac{1}{2} \\sum_{i = 1}^{n} w_i \\lVert \\mathbf{a}_i -
            C \\mathbf{b}_i \\rVert^2 ,

        where :math:`w_i`'s are the `weights` corresponding to each vector.

        The rotation is estimated with Kabsch algorithm [1]_, and solves what
        is known as the "pointing problem", or "Wahba's problem" [2]_.

        There are two special cases. The first is if a single vector is given
        for `a` and `b`, in which the shortest distance rotation that aligns
        `b` to `a` is returned.

        The second is when one of the weights is infinity. In this case, the
        shortest distance rotation between the primary infinite weight vectors
        is calculated as above. Then, the rotation about the aligned primary
        vectors is calculated such that the secondary vectors are optimally
        aligned per the above loss function. The result is the composition
        of these two rotations. The result via this process is the same as the
        Kabsch algorithm as the corresponding weight approaches infinity in
        the limit. For a single secondary vector this is known as the
        "align-constrain" algorithm [3]_.

        For both special cases (single vectors or an infinite weight), the
        sensitivity matrix does not have physical meaning and an error will be
        raised if it is requested. For an infinite weight, the primary vectors
        act as a constraint with perfect alignment, so their contribution to
        `rssd` will be forced to 0 even if they are of different lengths.

        Parameters
        ----------
        a : array_like, shape (3,) or (N, 3)
            Vector components observed in initial frame A. Each row of `a`
            denotes a vector.
        b : array_like, shape (3,) or (N, 3)
            Vector components observed in another frame B. Each row of `b`
            denotes a vector.
        weights : array_like shape (N,), optional
            Weights describing the relative importance of the vector
            observations. If None (default), then all values in `weights` are
            assumed to be 1. One and only one weight may be infinity, and
            weights must be positive.
        return_sensitivity : bool, optional
            Whether to return the sensitivity matrix. See Notes for details.
            Default is False.

        Returns
        -------
        rotation : `Rotation` instance
            Best estimate of the rotation that transforms `b` to `a`.
        rssd : float
            Stands for "root sum squared distance". Square root of the weighted
            sum of the squared distances between the given sets of vectors
            after alignment. It is equal to ``sqrt(2 * minimum_loss)``, where
            ``minimum_loss`` is the loss function evaluated for the found
            optimal rotation.
            Note that the result will also be weighted by the vectors'
            magnitudes, so perfectly aligned vector pairs will have nonzero
            `rssd` if they are not of the same length. So, depending on the
            use case it may be desirable to normalize the input vectors to unit
            length before calling this method.
        sensitivity_matrix : ndarray, shape (3, 3)
            Sensitivity matrix of the estimated rotation estimate as explained
            in Notes. Returned only when `return_sensitivity` is True. Not
            valid if aligning a single pair of vectors or if there is an
            infinite weight, in which cases an error will be raised.

        Notes
        -----
        The sensitivity matrix gives the sensitivity of the estimated rotation
        to small perturbations of the vector measurements. Specifically we
        consider the rotation estimate error as a small rotation vector of
        frame A. The sensitivity matrix is proportional to the covariance of
        this rotation vector assuming that the vectors in `a` was measured with
        errors significantly less than their lengths. To get the true
        covariance matrix, the returned sensitivity matrix must be multiplied
        by harmonic mean [4]_ of variance in each observation. Note that
        `weights` are supposed to be inversely proportional to the observation
        variances to get consistent results. For example, if all vectors are
        measured with the same accuracy of 0.01 (`weights` must be all equal),
        then you should multiple the sensitivity matrix by 0.01**2 to get the
        covariance.

        Refer to [5]_ for more rigorous discussion of the covariance
        estimation. See [6]_ for more discussion of the pointing problem and
        minimal proper pointing.

        References
        ----------
        .. [1] https://en.wikipedia.org/wiki/Kabsch_algorithm
        .. [2] https://en.wikipedia.org/wiki/Wahba%27s_problem
        .. [3] Magner, Robert,
                "Extending target tracking capabilities through trajectory and
                momentum setpoint optimization." Small Satellite Conference,
                2018.
        .. [4] https://en.wikipedia.org/wiki/Harmonic_mean
        .. [5] F. Landis Markley,
                "Attitude determination using vector observations: a fast
                optimal matrix algorithm", Journal of Astronautical Sciences,
                Vol. 41, No.2, 1993, pp. 261-280.
        .. [6] Bar-Itzhack, Itzhack Y., Daniel Hershkowitz, and Leiba Rodman,
                "Pointing in Real Euclidean Space", Journal of Guidance,
                Control, and Dynamics, Vol. 20, No. 5, 1997, pp. 916-922.

        Examples
        --------
        >>> import numpy as np
        >>> from scipy.spatial.transform import Rotation as R

        Here we run the baseline Kabsch algorithm to best align two sets of
        vectors, where there is noise on the last two vector measurements of
        the ``b`` set:

        >>> a = [[0, 1, 0], [0, 1, 1], [0, 1, 1]]
        >>> b = [[1, 0, 0], [1, 1.1, 0], [1, 0.9, 0]]
        >>> rot, rssd, sens = R.align_vectors(a, b, return_sensitivity=True)
        >>> rot.as_matrix()
        array([[0., 0., 1.],
               [1., 0., 0.],
               [0., 1., 0.]])

        When we apply the rotation to ``b``, we get vectors close to ``a``:

        >>> rot.apply(b)
        array([[0. , 1. , 0. ],
               [0. , 1. , 1.1],
               [0. , 1. , 0.9]])

        The error for the first vector is 0, and for the last two the error is
        magnitude 0.1. The `rssd` is the square root of the sum of the
        weighted squared errors, and the default weights are all 1, so in this
        case the `rssd` is calculated as
        ``sqrt(1 * 0**2 + 1 * 0.1**2 + 1 * (-0.1)**2) = 0.141421356237308``

        >>> a - rot.apply(b)
        array([[ 0., 0.,  0. ],
               [ 0., 0., -0.1],
               [ 0., 0.,  0.1]])
        >>> np.sqrt(np.sum(np.ones(3) @ (a - rot.apply(b))**2))
        0.141421356237308
        >>> rssd
        0.141421356237308

        The sensitivity matrix for this example is as follows:

        >>> sens
        array([[0.2, 0. , 0.],
               [0. , 1.5, 1.],
               [0. , 1. , 1.]])

        Special case 1: Find a minimum rotation between single vectors:

        >>> a = [1, 0, 0]
        >>> b = [0, 1, 0]
        >>> rot, _ = R.align_vectors(a, b)
        >>> rot.as_matrix()
        array([[0., 1., 0.],
               [-1., 0., 0.],
               [0., 0., 1.]])
        >>> rot.apply(b)
        array([1., 0., 0.])

        Special case 2: One infinite weight. Here we find a rotation between
        primary and secondary vectors that can align exactly:

        >>> a = [[0, 1, 0], [0, 1, 1]]
        >>> b = [[1, 0, 0], [1, 1, 0]]
        >>> rot, _ = R.align_vectors(a, b, weights=[np.inf, 1])
        >>> rot.as_matrix()
        array([[0., 0., 1.],
               [1., 0., 0.],
               [0., 1., 0.]])
        >>> rot.apply(b)
        array([[0., 1., 0.],
               [0., 1., 1.]])

        Here the secondary vectors must be best-fit:

        >>> a = [[0, 1, 0], [0, 1, 1]]
        >>> b = [[1, 0, 0], [1, 2, 0]]
        >>> rot, _ = R.align_vectors(a, b, weights=[np.inf, 1])
        >>> rot.as_matrix()
        array([[0., 0., 1.],
               [1., 0., 0.],
               [0., 1., 0.]])
        >>> rot.apply(b)
        array([[0., 1., 0.],
               [0., 1., 2.]])
        """
        # Check input vectors
        a_original = np.array(a, dtype=float)
        b_original = np.array(b, dtype=float)
        a = np.atleast_2d(a_original)
        b = np.atleast_2d(b_original)
        if a.shape[-1] != 3:
            raise ValueError("Expected input `a` to have shape (3,) or "
                             "(N, 3), got {}".format(a_original.shape))
        if b.shape[-1] != 3:
            raise ValueError("Expected input `b` to have shape (3,) or "
                             "(N, 3), got {}".format(b_original.shape))
        if a.shape != b.shape:
            raise ValueError("Expected inputs `a` and `b` to have same shapes"
                             ", got {} and {} respectively.".format(
                                 a_original.shape, b_original.shape))
        N = len(a)

        # Check weights
        if weights is None:
            weights = np.ones(N)
        else:
            weights = np.array(weights, dtype=float)
            if weights.ndim != 1:
                raise ValueError("Expected `weights` to be 1 dimensional, got "
                                 "shape {}.".format(weights.shape))
            if N > 1 and (weights.shape[0] != N):
                raise ValueError("Expected `weights` to have number of values "
                                 "equal to number of input vectors, got "
                                 "{} values and {} vectors.".format(
                                    weights.shape[0], N))
            if (weights < 0).any():
                raise ValueError("`weights` may not contain negative values")

        # For the special case of a single vector pair, we use the infinite
        # weight code path
        if N == 1:
            weight_is_inf = np.array([True])
        else:
            weight_is_inf = np.isposinf(weights)
        n_inf = np.sum(weight_is_inf)

        # Check for an infinite weight, which indicates that the corresponding
        # vector pair is the primary unmoving reference to which we align the
        # other secondary vectors
        if n_inf > 1:
            raise ValueError("Only one infinite weight is allowed")

        elif n_inf == 1:
            if return_sensitivity:
                raise ValueError("Cannot return sensitivity matrix with an "
                                 "infinite weight or one vector pair")
            a_pri = a[weight_is_inf]
            b_pri = b[weight_is_inf]
            a_pri_norm = _norm3(a_pri[0])
            b_pri_norm = _norm3(b_pri[0])
            if a_pri_norm == 0 or b_pri_norm == 0:
                raise ValueError("Cannot align zero length primary vectors")
            a_pri /= a_pri_norm
            b_pri /= b_pri_norm

            # We first find the minimum angle rotation between the primary
            # vectors.
            cross = np.cross(b_pri[0], a_pri[0])
            cross_norm = _norm3(cross)
            theta = atan2(cross_norm, _dot3(a_pri[0], b_pri[0]))
            tolerance = 1e-3  # tolerance for small angle approximation (rad)
            R_flip = cls.identity()
            if (np.pi - theta) < tolerance:
                # Near pi radians, the Taylor series appoximation of x/sin(x)
                # diverges, so for numerical stability we flip pi and then
                # rotate back by the small angle pi - theta
                if cross_norm == 0:
                    # For antiparallel vectors, cross = [0, 0, 0] so we need to
                    # manually set an arbitrary orthogonal axis of rotation
                    i = np.argmin(np.abs(a_pri[0]))
                    r = np.zeros(3)
                    r[i - 1], r[i - 2] = a_pri[0][i - 2], -a_pri[0][i - 1]
                else:
                    r = cross  # Shortest angle orthogonal axis of rotation
                R_flip = Rotation.from_rotvec(r / np.linalg.norm(r) * np.pi)
                theta = np.pi - theta
                cross = -cross
            if abs(theta) < tolerance:
                # Small angle Taylor series approximation for numerical stability
                theta2 = theta * theta
                r = cross * (1 + theta2 / 6 + theta2 * theta2 * 7 / 360)
            else:
                r = cross * theta / np.sin(theta)
            R_pri = cls.from_rotvec(r) * R_flip

            if N == 1:
                # No secondary vectors, so we are done
                R_opt = R_pri
            else:
                a_sec = a[~weight_is_inf]
                b_sec = b[~weight_is_inf]
                weights_sec = weights[~weight_is_inf]

                # We apply the first rotation to the b vectors to align the
                # primary vectors, resulting in vectors c. The secondary
                # vectors must now be rotated about that primary vector to best
                # align c to a.
                c_sec = R_pri.apply(b_sec)

                # Calculate vector components for the angle calculation. The
                # angle phi to rotate a single 2D vector C to align to 2D
                # vector A in the xy plane can be found with the equation
                # phi = atan2(cross(C, A), dot(C, A))
                #     = atan2(|C|*|A|*sin(phi), |C|*|A|*cos(phi))
                # The below equations perform the same operation, but with the
                # 3D rotation restricted to the 2D plane normal to a_pri, where
                # the secondary vectors are projected into that plane. We then
                # find the composite angle from the weighted sum of the
                # axial components in that plane, minimizing the 2D alignment
                # problem.
                # Note that einsum('ij,ij->i', X, Y) is the row-wise dot
                # product of X and Y.
                sin_term = np.einsum('ij,ij->i', np.cross(c_sec, a_sec), a_pri)
                cos_term = (np.einsum('ij,ij->i', c_sec, a_sec)
                            - (np.einsum('ij,ij->i', c_sec, a_pri)
                               * np.einsum('ij,ij->i', a_sec, a_pri)))
                phi = atan2(np.sum(weights_sec * sin_term),
                            np.sum(weights_sec * cos_term))
                R_sec = cls.from_rotvec(phi * a_pri[0])

                # Compose these to get the optimal rotation
                R_opt = R_sec * R_pri

            # Calculated the root sum squared distance. We force the error to
            # be zero for the infinite weight vectors since they will align
            # exactly.
            weights_inf_zero = weights.copy()
            if N > 1 or np.isposinf(weights[0]):
                # Skip non-infinite weight single vectors pairs, we used the
                # infinite weight code path but don't want to zero that weight
                weights_inf_zero[weight_is_inf] = 0
            a_est = R_opt.apply(b)
            rssd = np.sqrt(np.sum(weights_inf_zero @ (a - a_est)**2))

            return R_opt, rssd

        # If no infinite weights and multiple vectors, proceed with normal
        # algorithm
        # Note that einsum('ji,jk->ik', X, Y) is equivalent to np.dot(X.T, Y)
        B = np.einsum('ji,jk->ik', weights[:, None] * a, b)
        u, s, vh = np.linalg.svd(B)

        # Correct improper rotation if necessary (as in Kabsch algorithm)
        if np.linalg.det(u @ vh) < 0:
            s[-1] = -s[-1]
            u[:, -1] = -u[:, -1]

        C = np.dot(u, vh)

        if s[1] + s[2] < 1e-16 * s[0]:
            warnings.warn("Optimal rotation is not uniquely or poorly defined "
                          "for the given sets of vectors.")

        rssd = np.sqrt(max(
            np.sum(weights * np.sum(b ** 2 + a ** 2, axis=1)) - 2 * np.sum(s),
            0))

        if return_sensitivity:
            zeta = (s[0] + s[1]) * (s[1] + s[2]) * (s[2] + s[0])
            kappa = s[0] * s[1] + s[1] * s[2] + s[2] * s[0]
            with np.errstate(divide='ignore', invalid='ignore'):
                sensitivity = np.mean(weights) / zeta * (
                        kappa * np.eye(3) + np.dot(B, B.T))
            return cls.from_matrix(C), rssd, sensitivity
        else:
            return cls.from_matrix(C), rssd

class Slerp:
    """Spherical Linear Interpolation of Rotations.

    The interpolation between consecutive rotations is performed as a rotation
    around a fixed axis with a constant angular velocity [1]_. This ensures
    that the interpolated rotations follow the shortest path between initial
    and final orientations.

    Parameters
    ----------
    times : array_like, shape (N,)
        Times of the known rotations. At least 2 times must be specified.
    rotations : `Rotation` instance
        Rotations to perform the interpolation between. Must contain N
        rotations.

    Methods
    -------
    __call__

    See Also
    --------
    Rotation

    Notes
    -----
    .. versionadded:: 1.2.0

    References
    ----------
    .. [1] https://en.wikipedia.org/wiki/Slerp#Quaternion_Slerp

    Examples
    --------
    >>> from scipy.spatial.transform import Rotation as R
    >>> from scipy.spatial.transform import Slerp

    Setup the fixed keyframe rotations and times:

    >>> key_rots = R.random(5, random_state=2342345)
    >>> key_times = [0, 1, 2, 3, 4]

    Create the interpolator object:

    >>> slerp = Slerp(key_times, key_rots)

    Interpolate the rotations at the given times:

    >>> times = [0, 0.5, 0.25, 1, 1.5, 2, 2.75, 3, 3.25, 3.60, 4]
    >>> interp_rots = slerp(times)

    The keyframe rotations expressed as Euler angles:

    >>> key_rots.as_euler('xyz', degrees=True)
    array([[ 14.31443779, -27.50095894,  -3.7275787 ],
           [ -1.79924227, -24.69421529, 164.57701743],
           [146.15020772,  43.22849451, -31.34891088],
           [ 46.39959442,  11.62126073, -45.99719267],
           [-88.94647804, -49.64400082, -65.80546984]])

    The interpolated rotations expressed as Euler angles. These agree with the
    keyframe rotations at both endpoints of the range of keyframe times.

    >>> interp_rots.as_euler('xyz', degrees=True)
    array([[  14.31443779,  -27.50095894,   -3.7275787 ],
           [   4.74588574,  -32.44683966,   81.25139984],
           [  10.71094749,  -31.56690154,   38.06896408],
           [  -1.79924227,  -24.69421529,  164.57701743],
           [  11.72796022,   51.64207311, -171.7374683 ],
           [ 146.15020772,   43.22849451,  -31.34891088],
           [  68.10921869,   20.67625074,  -48.74886034],
           [  46.39959442,   11.62126073,  -45.99719267],
           [  12.35552615,    4.21525086,  -64.89288124],
           [ -30.08117143,  -19.90769513,  -78.98121326],
           [ -88.94647804,  -49.64400082,  -65.80546984]])

    """
    def __init__(self, times, rotations):
        if not isinstance(rotations, Rotation):
            raise TypeError("`rotations` must be a `Rotation` instance.")

        if rotations.single or len(rotations) == 1:
            raise ValueError("`rotations` must be a sequence of at least 2 rotations.")

        times = np.asarray(times)
        if times.ndim != 1:
            raise ValueError("Expected times to be specified in a 1 "
                             "dimensional array, got {} "
                             "dimensions.".format(times.ndim))

        if times.shape[0] != len(rotations):
            raise ValueError("Expected number of rotations to be equal to "
                             "number of timestamps given, got {} rotations "
                             "and {} timestamps.".format(
                                len(rotations), times.shape[0]))
        self.times = times
        self.timedelta = np.diff(times)

        if np.any(self.timedelta <= 0):
            raise ValueError("Times must be in strictly increasing order.")

        self.rotations = rotations[:-1]
        self.rotvecs = (self.rotations.inv() * rotations[1:]).as_rotvec()

    def __call__(self, times):
        """Interpolate rotations.

        Compute the interpolated rotations at the given `times`.

        Parameters
        ----------
        times : array_like
            Times to compute the interpolations at. Can be a scalar or
            1-dimensional.

        Returns
        -------
        interpolated_rotation : `Rotation` instance
            Object containing the rotations computed at given `times`.

        """
        # Clearly differentiate from self.times property
        compute_times = np.asarray(times)
        if compute_times.ndim > 1:
            raise ValueError("`times` must be at most 1-dimensional.")

        single_time = compute_times.ndim == 0
        compute_times = np.atleast_1d(compute_times)

        # side = 'left' (default) excludes t_min.
        ind = np.searchsorted(self.times, compute_times) - 1
        # Include t_min. Without this step, index for t_min equals -1
        ind[compute_times == self.times[0]] = 0
        if np.any(np.logical_or(ind < 0, ind > len(self.rotations) - 1)):
            raise ValueError("Interpolation times must be within the range "
                             "[{}, {}], both inclusive.".format(
                                self.times[0], self.times[-1]))

        alpha = (compute_times - self.times[ind]) / self.timedelta[ind]

        result = (self.rotations[ind] *
                  Rotation.from_rotvec(self.rotvecs[ind] * alpha[:, None]))

        if single_time:
            result = result[0]

        return result
