from __future__ import division, print_function, absolute_import

import numpy as np
from scipy._lib._util import _asarray_validated

__all__ = ["logsumexp"]


def logsumexp(a, axis=None, b=None, keepdims=False, return_sign=False):
    """Compute the log of the sum of exponentials of input elements.

    Parameters
    ----------
    a : array_like
        Input array.
    axis : None or int or tuple of ints, optional
        Axis or axes over which the sum is taken. By default `axis` is None,
        and all elements are summed.

        .. versionadded:: 0.11.0
    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 array.

        .. versionadded:: 0.15.0
    b : array-like, optional
        Scaling factor for exp(`a`) must be of the same shape as `a` or
        broadcastable to `a`. These values may be negative in order to
        implement subtraction.

        .. versionadded:: 0.12.0
    return_sign : bool, optional
        If this is set to True, the result will be a pair containing sign
        information; if False, results that are negative will be returned
        as NaN. Default is False (no sign information).

        .. versionadded:: 0.16.0

    Returns
    -------
    res : ndarray
        The result, ``np.log(np.sum(np.exp(a)))`` calculated in a numerically
        more stable way. If `b` is given then ``np.log(np.sum(b*np.exp(a)))``
        is returned.
    sgn : ndarray
        If return_sign is True, this will be an array of floating-point
        numbers matching res and +1, 0, or -1 depending on the sign
        of the result. If False, only one result is returned.

    See Also
    --------
    numpy.logaddexp, numpy.logaddexp2

    Notes
    -----
    Numpy has a logaddexp function which is very similar to `logsumexp`, but
    only handles two arguments. `logaddexp.reduce` is similar to this
    function, but may be less stable.

    Examples
    --------
    >>> from scipy.special import logsumexp
    >>> a = np.arange(10)
    >>> np.log(np.sum(np.exp(a)))
    9.4586297444267107
    >>> logsumexp(a)
    9.4586297444267107

    With weights

    >>> a = np.arange(10)
    >>> b = np.arange(10, 0, -1)
    >>> logsumexp(a, b=b)
    9.9170178533034665
    >>> np.log(np.sum(b*np.exp(a)))
    9.9170178533034647

    Returning a sign flag

    >>> logsumexp([1,2],b=[1,-1],return_sign=True)
    (1.5413248546129181, -1.0)

    Notice that `logsumexp` does not directly support masked arrays. To use it
    on a masked array, convert the mask into zero weights:

    >>> a = np.ma.array([np.log(2), 2, np.log(3)],
    ...                  mask=[False, True, False])
    >>> b = (~a.mask).astype(int)
    >>> logsumexp(a.data, b=b), np.log(5)
    1.6094379124341005, 1.6094379124341005

    """
    a = _asarray_validated(a, check_finite=False)
    if b is not None:
        a, b = np.broadcast_arrays(a, b)
        if np.any(b == 0):
            a = a + 0.  # promote to at least float
            a[b == 0] = -np.inf

    a_max = np.amax(a, axis=axis, keepdims=True)

    if a_max.ndim > 0:
        a_max[~np.isfinite(a_max)] = 0
    elif not np.isfinite(a_max):
        a_max = 0

    if b is not None:
        b = np.asarray(b)
        tmp = b * np.exp(a - a_max)
    else:
        tmp = np.exp(a - a_max)

    # suppress warnings about log of zero
    with np.errstate(divide='ignore'):
        s = np.sum(tmp, axis=axis, keepdims=keepdims)
        if return_sign:
            sgn = np.sign(s)
            s *= sgn  # /= makes more sense but we need zero -> zero
        out = np.log(s)

    if not keepdims:
        a_max = np.squeeze(a_max, axis=axis)
    out += a_max

    if return_sign:
        return out, sgn
    else:
        return out