import numpy as np

from ._rolling_ball_cy import apply_kernel, apply_kernel_nan


def rolling_ball(image, *, radius=100, kernel=None,
                 nansafe=False, num_threads=None):
    """Estimate background intensity by rolling/translating a kernel.

    This rolling ball algorithm estimates background intensity for a
    ndimage in case of uneven exposure. It is a generalization of the
    frequently used rolling ball algorithm [1]_.

    Parameters
    ----------
    image : ndarray
        The image to be filtered.
    radius : int, optional
        Radius of a ball shaped kernel to be rolled/translated in the image.
        Used if ``kernel = None``.
    kernel : ndarray, optional
        The kernel to be rolled/translated in the image. It must have the
        same number of dimensions as ``image``. Kernel is filled with the
        intensity of the kernel at that position.
    nansafe: bool, optional
        If ``False`` (default) assumes that none of the values in ``image``
        are ``np.nan``, and uses a faster implementation.
    num_threads: int, optional
        The maximum number of threads to use. If ``None`` use the OpenMP
        default value; typically equal to the maximum number of virtual cores.
        Note: This is an upper limit to the number of threads. The exact number
        is determined by the system's OpenMP library.

    Returns
    -------
    background : ndarray
        The estimated background of the image.

    Notes
    -----
    For the pixel that has its background intensity estimated (without loss
    of generality at ``center``) the rolling ball method centers ``kernel``
    under it and raises the kernel until the surface touches the image umbra
    at some ``pos=(y,x)``. The background intensity is then estimated
    using the image intensity at that position (``image[pos]``) plus the
    difference of ``kernel[center] - kernel[pos]``.

    This algorithm assumes that dark pixels correspond to the background. If
    you have a bright background, invert the image before passing it to the
    function, e.g., using `utils.invert`. See the gallery example for details.

    This algorithm is sensitive to noise (in particular salt-and-pepper
    noise). If this is a problem in your image, you can apply mild
    gaussian smoothing before passing the image to this function.

    References
    ----------
    .. [1] Sternberg, Stanley R. "Biomedical image processing." Computer 1
           (1983): 22-34. :DOI:`10.1109/MC.1983.1654163`

    Examples
    --------
    >>> import numpy as np
    >>> from skimage import data
    >>> from skimage.restoration import rolling_ball
    >>> image = data.coins()
    >>> background = rolling_ball(data.coins())
    >>> filtered_image = image - background


    >>> import numpy as np
    >>> from skimage import data
    >>> from skimage.restoration import rolling_ball, ellipsoid_kernel
    >>> image = data.coins()
    >>> kernel = ellipsoid_kernel((101, 101), 75)
    >>> background = rolling_ball(data.coins(), kernel=kernel)
    >>> filtered_image = image - background
    """

    image = np.asarray(image)
    img = image.astype(float)

    if num_threads is None:
        num_threads = 0

    if kernel is None:
        kernel = ball_kernel(radius, image.ndim)

    kernel_shape = np.asarray(kernel.shape)
    kernel_center = (kernel_shape // 2)
    center_intensity = kernel[tuple(kernel_center)]

    intensity_difference = center_intensity - kernel
    intensity_difference[kernel == np.Inf] = np.Inf
    intensity_difference = intensity_difference.astype(img.dtype)
    intensity_difference = intensity_difference.reshape(-1)

    img = np.pad(img, kernel_center[:, np.newaxis],
                 constant_values=np.Inf, mode="constant")

    func = apply_kernel_nan if nansafe else apply_kernel
    background = func(
        img.reshape(-1),
        intensity_difference,
        np.zeros_like(image, dtype=img.dtype).reshape(-1),
        np.array(image.shape, dtype=np.intp),
        np.array(img.shape, dtype=np.intp),
        kernel_shape.astype(np.intp),
        num_threads
    )

    background = background.astype(image.dtype, copy=False)

    return background


def ball_kernel(radius, ndim):
    """Create a ball kernel for restoration.rolling_ball.

    Parameters
    ----------
    radius : int
        Radius of the ball.
    ndim : int
        Number of dimensions of the ball. ``ndim`` should match the
        dimensionality of the image the kernel will be applied to.

    Returns
    -------
    kernel : ndarray
        The kernel containing the surface intensity of the top half
        of the ellipsoid.

    See Also
    --------
    rolling_ball
    """

    kernel_coords = np.stack(
        np.meshgrid(
            *[np.arange(-x, x + 1) for x in [np.ceil(radius)] * ndim],
            indexing='ij'
        ),
        axis=-1
    )

    sum_of_squares = np.sum(kernel_coords ** 2, axis=-1)
    distance_from_center = np.sqrt(sum_of_squares)
    kernel = np.sqrt(np.clip(radius ** 2 - sum_of_squares, 0, None))
    kernel[distance_from_center > radius] = np.Inf

    return kernel


def ellipsoid_kernel(shape, intensity):
    """Create an ellipoid kernel for restoration.rolling_ball.

    Parameters
    ----------
    shape : arraylike
        Length of the principal axis of the ellipsoid (excluding
        the intensity axis). The kernel needs to have the same
        dimensionality as the image it will be applied to.
    intensity : int
        Length of the intensity axis of the ellipsoid.

    Returns
    -------
    kernel : ndarray
        The kernel containing the surface intensity of the top half
        of the ellipsoid.

    See Also
    --------
    rolling_ball
    """

    shape = np.asarray(shape)
    semi_axis = np.clip(shape // 2, 1, None)

    kernel_coords = np.stack(
        np.meshgrid(
            *[np.arange(-x, x + 1) for x in semi_axis],
            indexing='ij'
        ),
        axis=-1)

    intensity_scaling = 1 - np.sum((kernel_coords / semi_axis) ** 2, axis=-1)
    kernel = intensity * np.sqrt(np.clip(intensity_scaling, 0, None))
    kernel[intensity_scaling < 0] = np.Inf

    return kernel
