#!/usr/bin/env python
# -*- coding: utf-8 -*-
# Copyright 1999-2021 Alibaba Group Holding Ltd.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
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from ..arithmetic.sqrt import sqrt
from .nanvar import nanvar


def nanstd(
    a, axis=None, dtype=None, out=None, ddof=0, keepdims=None, combine_size=None
):
    """
    Compute the standard deviation along the specified axis, while
    ignoring NaNs.

    Returns the standard deviation, a measure of the spread of a
    distribution, of the non-NaN tensor elements. The standard deviation is
    computed for the flattened tensor by default, otherwise over the
    specified axis.

    For all-NaN slices or slices with zero degrees of freedom, NaN is
    returned and a `RuntimeWarning` is raised.

    Parameters
    ----------
    a : array_like
        Calculate the standard deviation of the non-NaN values.
    axis : int, optional
        Axis along which the standard deviation is computed. The default is
        to compute the standard deviation of the flattened tensor.
    dtype : dtype, optional
        Type to use in computing the standard deviation. For tensors of
        integer type the default is float64, for tensors of float types it
        is the same as the tensor type.
    out : Tensor, optional
        Alternative output tensor in which to place the result. It must have
        the same shape as the expected output but the type (of the
        calculated values) will be cast if necessary.
    ddof : int, optional
        Means Delta Degrees of Freedom.  The divisor used in calculations
        is ``N - ddof``, where ``N`` represents the number of non-NaN
        elements.  By default `ddof` is zero.

    keepdims : bool, optional
        If this is set to True, the axes which are reduced are left
        in the result as dimensions with size one. With this option,
        the result will broadcast correctly against the original `a`.

        If this value is anything but the default it is passed through
        as-is to the relevant functions of the sub-classes.  If these
        functions do not have a `keepdims` kwarg, a RuntimeError will
        be raised.
    combine_size: int, optional
        The number of chunks to combine.

    Returns
    -------
    standard_deviation : ndarray, see dtype parameter above.
        If `out` is None, return a new array containing the standard
        deviation, otherwise return a reference to the output tensor. If
        ddof is >= the number of non-NaN elements in a slice or the slice
        contains only NaNs, then the result for that slice is NaN.

    See Also
    --------
    var, mean, std
    nanvar, nanmean

    Notes
    -----
    The standard deviation is the square root of the average of the squared
    deviations from the mean: ``std = sqrt(mean(abs(x - x.mean())**2))``.

    The average squared deviation is normally calculated as
    ``x.sum() / N``, where ``N = len(x)``.  If, however, `ddof` is
    specified, the divisor ``N - ddof`` is used instead. In standard
    statistical practice, ``ddof=1`` provides an unbiased estimator of the
    variance of the infinite population. ``ddof=0`` provides a maximum
    likelihood estimate of the variance for normally distributed variables.
    The standard deviation computed in this function is the square root of
    the estimated variance, so even with ``ddof=1``, it will not be an
    unbiased estimate of the standard deviation per se.

    Note that, for complex numbers, `std` takes the absolute value before
    squaring, so that the result is always real and nonnegative.

    For floating-point input, the *std* is computed using the same
    precision the input has. Depending on the input data, this can cause
    the results to be inaccurate, especially for float32 (see example
    below).  Specifying a higher-accuracy accumulator using the `dtype`
    keyword can alleviate this issue.

    Examples
    --------
    >>> import mars.tensor as mt

    >>> a = mt.array([[1, mt.nan], [3, 4]])
    >>> mt.nanstd(a).execute()
    1.247219128924647
    >>> mt.nanstd(a, axis=0).execute()
    array([ 1.,  0.])
    >>> mt.nanstd(a, axis=1).execute()
    array([ 0.,  0.5])

    """
    ret = sqrt(
        nanvar(
            a,
            axis=axis,
            dtype=dtype,
            out=out,
            ddof=ddof,
            keepdims=keepdims,
            combine_size=combine_size,
        )
    )
    if dtype is not None and ret.dtype != dtype:
        ret = ret.astype(dtype)
    return ret
