"""
Support and standalone functions for Robust Linear Models

References
----------
PJ Huber.  'Robust Statistics' John Wiley and Sons, Inc., New York, 1981.

R Venables, B Ripley. 'Modern Applied Statistics in S'
    Springer, New York, 2002.
"""
import numpy as np
from scipy.stats import norm as Gaussian
from . import norms
from statsmodels.tools import tools
from statsmodels.tools.validation import array_like, float_like


def mad(a, c=Gaussian.ppf(3/4.), axis=0, center=np.median):
    # c \approx .6745
    """
    The Median Absolute Deviation along given axis of an array

    Parameters
    ----------
    a : array_like
        Input array.
    c : float, optional
        The normalization constant.  Defined as scipy.stats.norm.ppf(3/4.),
        which is approximately .6745.
    axis : int, optional
        The default is 0. Can also be None.
    center : callable or float
        If a callable is provided, such as the default `np.median` then it
        is expected to be called center(a). The axis argument will be applied
        via np.apply_over_axes. Otherwise, provide a float.

    Returns
    -------
    mad : float
        `mad` = median(abs(`a` - center))/`c`
    """
    a = array_like(a, 'a', ndim=None)
    c = float_like(c, 'c')
    if callable(center) and a.size:
        center = np.apply_over_axes(center, a, axis)
    else:
        center = 0.0

    return np.median((np.abs(a-center)) / c, axis=axis)


class Huber(object):
    """
    Huber's proposal 2 for estimating location and scale jointly.

    Parameters
    ----------
    c : float, optional
        Threshold used in threshold for chi=psi**2.  Default value is 1.5.
    tol : float, optional
        Tolerance for convergence.  Default value is 1e-08.
    maxiter : int, optional0
        Maximum number of iterations.  Default value is 30.
    norm : statsmodels.robust.norms.RobustNorm, optional
        A robust norm used in M estimator of location. If None,
        the location estimator defaults to a one-step
        fixed point version of the M-estimator using Huber's T.

    call
        Return joint estimates of Huber's scale and location.

    Examples
    --------
    >>> import numpy as np
    >>> import statsmodels.api as sm
    >>> chem_data = np.array([2.20, 2.20, 2.4, 2.4, 2.5, 2.7, 2.8, 2.9, 3.03,
    ...        3.03, 3.10, 3.37, 3.4, 3.4, 3.4, 3.5, 3.6, 3.7, 3.7, 3.7, 3.7,
    ...        3.77, 5.28, 28.95])
    >>> sm.robust.scale.huber(chem_data)
    (array(3.2054980819923693), array(0.67365260010478967))
    """

    def __init__(self, c=1.5, tol=1.0e-08, maxiter=30, norm=None):
        self.c = c
        self.maxiter = maxiter
        self.tol = tol
        self.norm = norm
        tmp = 2 * Gaussian.cdf(c) - 1
        self.gamma = tmp + c**2 * (1 - tmp) - 2 * c * Gaussian.pdf(c)

    def __call__(self, a, mu=None, initscale=None, axis=0):
        """
        Compute Huber's proposal 2 estimate of scale, using an optional
        initial value of scale and an optional estimate of mu. If mu
        is supplied, it is not reestimated.

        Parameters
        ----------
        a : ndarray
            1d array
        mu : float or None, optional
            If the location mu is supplied then it is not reestimated.
            Default is None, which means that it is estimated.
        initscale : float or None, optional
            A first guess on scale.  If initscale is None then the standardized
            median absolute deviation of a is used.

        Notes
        -----
        `Huber` minimizes the function

        sum(psi((a[i]-mu)/scale)**2)

        as a function of (mu, scale), where

        psi(x) = np.clip(x, -self.c, self.c)
        """
        a = np.asarray(a)
        if mu is None:
            n = a.shape[0] - 1
            mu = np.median(a, axis=axis)
            est_mu = True
        else:
            n = a.shape[0]
            mu = mu
            est_mu = False

        if initscale is None:
            scale = mad(a, axis=axis)
        else:
            scale = initscale
        scale = tools.unsqueeze(scale, axis, a.shape)
        mu = tools.unsqueeze(mu, axis, a.shape)
        return self._estimate_both(a, scale, mu, axis, est_mu, n)

    def _estimate_both(self, a, scale, mu, axis, est_mu, n):
        """
        Estimate scale and location simultaneously with the following
        pseudo_loop:

        while not_converged:
            mu, scale = estimate_location(a, scale, mu), estimate_scale(a, scale, mu)

        where estimate_location is an M-estimator and estimate_scale implements
        the check used in Section 5.5 of Venables & Ripley
        """  # noqa:E501
        for _ in range(self.maxiter):
            # Estimate the mean along a given axis
            if est_mu:
                if self.norm is None:
                    # This is a one-step fixed-point estimator
                    # if self.norm == norms.HuberT
                    # It should be faster than using norms.HuberT
                    nmu = np.clip(a, mu-self.c*scale,
                                  mu+self.c*scale).sum(axis) / a.shape[axis]
                else:
                    nmu = norms.estimate_location(a, scale, self.norm, axis,
                                                  mu, self.maxiter, self.tol)
            else:
                # Effectively, do nothing
                nmu = mu.squeeze()
            nmu = tools.unsqueeze(nmu, axis, a.shape)

            subset = np.less_equal(np.abs((a - mu)/scale), self.c)
            card = subset.sum(axis)

            scale_num = np.sum(subset * (a - nmu)**2, axis)
            scale_denom = (n * self.gamma - (a.shape[axis] - card) * self.c**2)
            nscale = np.sqrt(scale_num / scale_denom)
            nscale = tools.unsqueeze(nscale, axis, a.shape)

            test1 = np.alltrue(np.less_equal(np.abs(scale - nscale),
                                             nscale * self.tol))
            test2 = np.alltrue(
                np.less_equal(np.abs(mu - nmu), nscale * self.tol))
            if not (test1 and test2):
                mu = nmu
                scale = nscale
            else:
                return nmu.squeeze(), nscale.squeeze()
        raise ValueError('joint estimation of location and scale failed '
                         'to converge in %d iterations' % self.maxiter)


huber = Huber()


class HuberScale(object):
    r"""
    Huber's scaling for fitting robust linear models.

    Huber's scale is intended to be used as the scale estimate in the
    IRLS algorithm and is slightly different than the `Huber` class.

    Parameters
    ----------
    d : float, optional
        d is the tuning constant for Huber's scale.  Default is 2.5
    tol : float, optional
        The convergence tolerance
    maxiter : int, optiona
        The maximum number of iterations.  The default is 30.

    Methods
    -------
    call
        Return's Huber's scale computed as below

    Notes
    --------
    Huber's scale is the iterative solution to

    scale_(i+1)**2 = 1/(n*h)*sum(chi(r/sigma_i)*sigma_i**2

    where the Huber function is

    chi(x) = (x**2)/2       for \|x\| < d
    chi(x) = (d**2)/2       for \|x\| >= d

    and the Huber constant h = (n-p)/n*(d**2 + (1-d**2)*\
            scipy.stats.norm.cdf(d) - .5 - d*sqrt(2*pi)*exp(-0.5*d**2)
    """
    def __init__(self, d=2.5, tol=1e-08, maxiter=30):
        self.d = d
        self.tol = tol
        self.maxiter = maxiter

    def __call__(self, df_resid, nobs, resid):
        h = df_resid / nobs * (
            self.d ** 2
            + (1 - self.d ** 2) * Gaussian.cdf(self.d)
            - .5 - self.d / (np.sqrt(2 * np.pi)) * np.exp(-.5 * self.d ** 2)
        )
        s = mad(resid)

        def subset(x):
            return np.less(np.abs(resid / x), self.d)

        def chi(s):
            return subset(s) * (resid / s) ** 2 / 2 + (1 - subset(s)) * \
                   (self.d ** 2 / 2)

        scalehist = [np.inf, s]
        niter = 1
        while (np.abs(scalehist[niter - 1] - scalehist[niter]) > self.tol
               and niter < self.maxiter):
            nscale = np.sqrt(1 / (nobs * h) * np.sum(chi(scalehist[-1])) *
                             scalehist[-1] ** 2)
            scalehist.append(nscale)
            niter += 1
            # TODO: raise on convergence failure?
        return scalehist[-1]


hubers_scale = HuberScale()