import warnings

import numpy as np
from scipy.spatial import cKDTree


def hausdorff_distance(image0, image1):
    """Calculate the Hausdorff distance between nonzero elements of given images.

    The Hausdorff distance [1]_ is the maximum distance between any point on
    ``image0`` and its nearest point on ``image1``, and vice-versa.

    Parameters
    ----------
    image0, image1 : ndarray
        Arrays where ``True`` represents a point that is included in a
        set of points. Both arrays must have the same shape.

    Returns
    -------
    distance : float
        The Hausdorff distance between coordinates of nonzero pixels in
        ``image0`` and ``image1``, using the Euclidian distance.

    References
    ----------
    .. [1] http://en.wikipedia.org/wiki/Hausdorff_distance

    Examples
    --------
    >>> points_a = (3, 0)
    >>> points_b = (6, 0)
    >>> shape = (7, 1)
    >>> image_a = np.zeros(shape, dtype=bool)
    >>> image_b = np.zeros(shape, dtype=bool)
    >>> image_a[points_a] = True
    >>> image_b[points_b] = True
    >>> hausdorff_distance(image_a, image_b)
    3.0

    """
    a_points = np.transpose(np.nonzero(image0))
    b_points = np.transpose(np.nonzero(image1))

    # Handle empty sets properly:
    # - if both sets are empty, return zero
    # - if only one set is empty, return infinity
    if len(a_points) == 0:
        return 0 if len(b_points) == 0 else np.inf
    elif len(b_points) == 0:
        return np.inf

    return max(max(cKDTree(a_points).query(b_points, k=1)[0]),
               max(cKDTree(b_points).query(a_points, k=1)[0]))


def hausdorff_pair(image0, image1):
    """Returns pair of points that are Hausdorff distance apart between nonzero
    elements of given images.

    The Hausdorff distance [1]_ is the maximum distance between any point on
    ``image0`` and its nearest point on ``image1``, and vice-versa.

    Parameters
    ----------
    image0, image1 : ndarray
        Arrays where ``True`` represents a point that is included in a
        set of points. Both arrays must have the same shape.

    Returns
    -------
    point_a, point_b : array
        A pair of points that have Hausdorff distance between them.

    References
    ----------
    .. [1] http://en.wikipedia.org/wiki/Hausdorff_distance

    Examples
    --------
    >>> points_a = (3, 0)
    >>> points_b = (6, 0)
    >>> shape = (7, 1)
    >>> image_a = np.zeros(shape, dtype=bool)
    >>> image_b = np.zeros(shape, dtype=bool)
    >>> image_a[points_a] = True
    >>> image_b[points_b] = True
    >>> hausdorff_pair(image_a, image_b)
    (array([3, 0]), array([6, 0]))

    """
    a_points = np.transpose(np.nonzero(image0))
    b_points = np.transpose(np.nonzero(image1))

    # If either of the sets are empty, there is no corresponding pair of points
    if len(a_points) == 0 or len(b_points) == 0:
        warnings.warn("One or both of the images is empty.", stacklevel=2)
        return (), ()

    nearest_dists_from_b, nearest_a_point_indices_from_b = cKDTree(a_points) \
        .query(b_points)
    nearest_dists_from_a, nearest_b_point_indices_from_a = cKDTree(b_points) \
        .query(a_points)

    max_index_from_a = nearest_dists_from_b.argmax()
    max_index_from_b = nearest_dists_from_a.argmax()

    max_dist_from_a = nearest_dists_from_b[max_index_from_a]
    max_dist_from_b = nearest_dists_from_a[max_index_from_b]

    if max_dist_from_b > max_dist_from_a:
        return a_points[max_index_from_b], \
            b_points[nearest_b_point_indices_from_a[max_index_from_b]]
    else:
        return a_points[nearest_a_point_indices_from_b[max_index_from_a]], \
            b_points[max_index_from_a]