from torch.autograd import Variable
import torch
from .module import Module
from .container import Sequential
from .activation import LogSoftmax
from .. import functional as F


def _assert_no_grad(variable):
    assert not variable.requires_grad, \
        "nn criterions don't compute the gradient w.r.t. targets - please " \
        "mark these variables as volatile or not requiring gradients"


class _Loss(Module):

    def __init__(self, size_average=True):
        super(_Loss, self).__init__()
        self.size_average = size_average

    def forward(self, input, target):
        _assert_no_grad(target)
        backend_fn = getattr(self._backend, type(self).__name__)
        return backend_fn(self.size_average)(input, target)


class _WeighedLoss(_Loss):

    def __init__(self, weight=None, size_average=True):
        super(_WeighedLoss, self).__init__(size_average)
        self.register_buffer('weight', weight)

    def forward(self, input, target):
        _assert_no_grad(target)
        backend_fn = getattr(self._backend, type(self).__name__)
        return backend_fn(self.size_average, weight=self.weight)(input, target)


class L1Loss(_Loss):
    r"""Creates a criterion that measures the mean absolute value of the
    element-wise difference between input `x` and target `y`:

    :math:`{loss}(x, y)  = 1/n \sum |x_i - y_i|`

    `x` and `y` arbitrary shapes with a total of `n` elements each.

    The sum operation still operates over all the elements, and divides by `n`.

    The division by `n` can be avoided if one sets the constructor argument `sizeAverage=False`
    """
    pass


class NLLLoss(_WeighedLoss):
    r"""The negative log likelihood loss. It is useful to train a classication problem with n classes

    If provided, the optional argument `weights` should be a 1D Tensor assigning
    weight to each of the classes.

    This is particularly useful when you have an unbalanced training set.

    The input given through a forward call is expected to contain log-probabilities
    of each class: input has to be a 2D Tensor of size `(minibatch, n)`

    Obtaining log-probabilities in a neural network is easily achieved by
    adding a  `LogSoftmax`  layer in the last layer of your network.

    You may use `CrossEntropyLoss`  instead, if you prefer not to add an extra layer.

    The target that this loss expects is a class index `(0 to N-1, where N = number of classes)`

    The loss can be described as::

        loss(x, class) = -x[class]

    or in the case of the weights argument it is specified as follows::

        loss(x, class) = -weights[class] * x[class]

    Args:
        weight (Tensor, optional): a manual rescaling weight given to each class.
                                   If given, has to be a Tensor of size "nclasses"
        size_average (bool, optional): By default, the losses are averaged over observations for each minibatch.
                                       However, if the field sizeAverage is set to False,
                                       the losses are instead summed for each minibatch.


    Shape:
        - Input: :math:`(N, C)` where `C = number of classes`
        - Target: :math:`(N)` where each value is `0 <= targets[i] <= C-1`

    Attributes:
        weight: the class-weights given as input to the constructor

    Examples::

        >>> m = nn.LogSoftmax()
        >>> loss = nn.NLLLoss()
        >>> # input is of size nBatch x nClasses = 3 x 5
        >>> input = autograd.Variable(torch.randn(3, 5))
        >>> # each element in target has to have 0 <= value < nclasses
        >>> target = autograd.Variable(torch.LongTensor([1, 0, 4]))
        >>> output = loss(m(input), target)
        >>> output.backward()
    """
    pass


class NLLLoss2d(_Loss):
    r"""This is negative log likehood loss, but for image inputs. It computes NLL loss per-pixel.

    This loss does not support per-class weights

    Args:
        size_average: By default, the losses are averaged over observations for each minibatch.
                      However, if the field sizeAverage is set to False, the losses
                      are instead summed for each minibatch. Default: True

    Shape:
        - Input: :math:`(N, C, H, W)` where `C = number of classes`
        - Target: :math:`(N, H, W)` where each value is `0 <= targets[i] <= C-1`

    Examples:
        >>> m = nn.Conv2d(16, 32, (3, 3)).float()
        >>> loss = nn.NLLLoss2d()
        >>> # input is of size nBatch x nClasses x height x width
        >>> input = autograd.Variable(torch.randn(3, 16, 10, 10))
        >>> # each element in target has to have 0 <= value < nclasses
        >>> target = autograd.Variable(torch.LongTensor(3, 8, 8).random_(0, 4))
        >>> output = loss(m(input), target)
        >>> output.backward()
    """
    pass


class KLDivLoss(_WeighedLoss):
    r"""The `Kullback-Leibler divergence`_ Loss

    KL divergence is a useful distance measure for continuous distributions
    and is often useful when performing direct regression over the space of
    (discretely sampled) continuous output distributions.

    As with `NLLLoss`, the `input` given is expected to contain
    *log-probabilities*, however unlike `ClassNLLLoss`, `input` is not
    restricted to a 2D Tensor, because the criterion is applied element-wise.

    This criterion expects a `target` `Tensor` of the same size as the
    `input` `Tensor`.

    The loss can be described as:
    :math:`loss(x, target) = 1/n \sum(target_i * (log(target_i) - x_i))`

    By default, the losses are averaged for each minibatch over observations
    **as well as** over dimensions. However, if the field
    `sizeAverage` is set to `False`, the losses are instead summed.

    .. _Kullback-Leibler divergence:
        https://en.wikipedia.org/wiki/Kullback-Leibler_divergence
    """
    pass


class MSELoss(_Loss):
    r"""Creates a criterion that measures the mean squared error between
    `n` elements in the input `x` and target `y`:

    :math:`loss(x, y) = 1/n \sum |x_i - y_i|^2`

    `x` and `y` arbitrary shapes with a total of `n` elements each
    the sum operation still operates over all the elements, and divides by `n`.

    The division by `n` can be avoided if one sets the internal variable
    `sizeAverage` to `False`

    """
    pass


class BCELoss(_WeighedLoss):
    r"""Creates a criterion that measures the Binary Cross Entropy
    between the target and the output:

    :math:`loss(o, t) = - 1/n \sum_i (t[i] * log(o[i]) + (1 - t[i]) * log(1 - o[i]))`

    or in the case of the weights argument being specified:

    :math:`loss(o, t) = - 1/n \sum_i weights[i] * (t[i] * log(o[i]) + (1 - t[i]) * log(1 - o[i]))`

    This is used for measuring the error of a reconstruction in for example
    an auto-encoder. Note that the targets `t[i]` should be numbers between 0 and 1,
    for instance, the output of an `nn.Sigmoid` layer.

    By default, the losses are averaged for each minibatch over observations
    *as well as* over dimensions. However, if the field `sizeAverage` is set
    to `False`, the losses are instead summed.

    """
    pass


class HingeEmbeddingLoss(_Loss):
    r"""Measures the loss given an input `x` which is a 2D mini-batch tensor
    and a labels `y`, a 1D tensor containg values (`1` or `-1`).
    This is usually used for measuring whether two inputs are similar or dissimilar,
    e.g. using the L1 pairwise distance, and is typically used for learning
    nonlinear embeddings or semi-supervised learning::

                         { x_i,                  if y_i ==  1
        loss(x, y) = 1/n {
                         { max(0, margin - x_i), if y_i == -1

    `x` and `y` arbitrary shapes with a total of `n` elements each
    the sum operation still operates over all the elements, and divides by `n`.

    The division by `n` can be avoided if one sets the internal variable `sizeAverage=False`.

    The `margin` has a default value of `1`, or can be set in the constructor.
    """
    pass


class MultiLabelMarginLoss(_Loss):
    r"""Creates a criterion that optimizes a multi-class multi-classification
    hinge loss (margin-based loss) between input `x`  (a 2D mini-batch `Tensor`) and
    output `y` (which is a 2D `Tensor` of target class indices).
    For each sample in the mini-batch::

        loss(x, y) = sum_ij(max(0, 1 - (x[y[j]] - x[i]))) / x.size(0)

    where `i == 0` to `x.size(0)`, `j == 0` to `y.size(0)`,
    `y[j] != 0`, and `i != y[j]` for all `i` and `j`.

    `y` and `x` must have the same size.

    The criterion only considers the first non zero `y[j]` targets.

    This allows for different samples to have variable amounts of target classes
    """
    pass


class SmoothL1Loss(_Loss):
    r"""Creates a criterion that uses a squared term if the absolute
    element-wise error falls below 1 and an L1 term otherwise.
    It is less sensitive to outliers than the `MSELoss` and in some cases
    prevents exploding gradients (e.g. see "Fast R-CNN" paper by Ross Girshick).
    Also known as the Huber loss::

                              { 0.5 * (x_i - y_i)^2, if |x_i - y_i| < 1
        loss(x, y) = 1/n \sum {
                              { |x_i - y_i| - 0.5,   otherwise

    `x` and `y` arbitrary shapes with a total of `n` elements each
    the sum operation still operates over all the elements, and divides by `n`.

    The division by `n` can be avoided if one sets the internal variable
    `sizeAverage` to `False`
    """
    pass


class SoftMarginLoss(_Loss):
    r"""Creates a criterion that optimizes a two-class classification
    logistic loss between input `x` (a 2D mini-batch Tensor) and
    target `y` (which is a tensor containing either `1` or `-1`).

    ::

        loss(x, y) = sum_i (log(1 + exp(-y[i]*x[i]))) / x.nelement()

    The normalization by the number of elements in the input can be disabled by
    setting `self.sizeAverage` to `False`.
    """
    pass


class CrossEntropyLoss(_WeighedLoss):
    r"""This criterion combines `LogSoftMax` and `NLLLoss` in one single class.

    It is useful when training a classification problem with `n` classes.
    If provided, the optional argument `weights` should be a 1D `Tensor`
    assigning weight to each of the classes.
    This is particularly useful when you have an unbalanced training set.

    The `input` is expected to contain scores for each class.

    `input` has to be a 2D `Tensor` of size `batch x n`.

    This criterion expects a class index (0 to nClasses-1) as the
    `target` for each value of a 1D tensor of size `n`

    The loss can be described as::

        loss(x, class) = -log(exp(x[class]) / (\sum_j exp(x[j])))
                       = -x[class] + log(\sum_j exp(x[j]))

    or in the case of the `weights` argument being specified::

        loss(x, class) = weights[class] * (-x[class] + log(\sum_j exp(x[j])))

    The losses are averaged across observations for each minibatch.

    Shape:
        - Input: :math:`(N, C)` where `C = number of classes`
        - Target: :math:`(N)` where each value is `0 <= targets[i] <= C-1`

    """

    def forward(self, input, target):
        _assert_no_grad(target)
        return F.cross_entropy(input, target,
                               self.weight, self.size_average)


class MultiLabelSoftMarginLoss(_WeighedLoss):
    r"""Creates a criterion that optimizes a multi-label one-versus-all
    loss based on max-entropy, between input `x`  (a 2D mini-batch `Tensor`) and
    target `y` (a binary 2D `Tensor`). For each sample in the minibatch::

       loss(x, y) = - sum_i (y[i] log( exp(x[i]) / (1 + exp(x[i])))
                             + (1-y[i]) log(1/(1+exp(x[i])))) / x:nElement()

    where `i == 0` to `x.nElement()-1`, `y[i]  in {0,1}`.
    `y` and `x` must have the same size.
    """

    def forward(self, input, target):
        return F.binary_cross_entropy(torch.sigmoid(input), target,
                                      self.weight, self.size_average)


class CosineEmbeddingLoss(Module):
    r"""Creates a criterion that measures the loss given  an input tensors x1, x2
    and a `Tensor` label `y` with values 1 or -1.
    This is used for measuring whether two inputs are similar or dissimilar,
    using the cosine distance, and is typically used for learning nonlinear
    embeddings or semi-supervised learning.

    `margin` should be a number from `-1` to `1`, `0` to `0.5` is suggested.
    If `margin` is missing, the default value is `0`.

    The loss function for each sample is::

                     { 1 - cos(x1, x2),              if y ==  1
        loss(x, y) = {
                     { max(0, cos(x1, x2) - margin), if y == -1

    If the internal variable `sizeAverage` is equal to `True`,
    the loss function averages the loss over the batch samples;
    if `sizeAverage` is `False`, then the loss function sums over the
    batch samples. By default, `sizeAverage = True`.
    """

    def __init__(self, margin=0, size_average=True):
        super(CosineEmbeddingLoss, self).__init__()
        self.margin = margin
        self.size_average = size_average

    def forward(self, input1, input2, target):
        return self._backend.CosineEmbeddingLoss(self.margin,
                                                 self.size_average)(input1, input2, target)


class MarginRankingLoss(Module):
    r"""Creates a criterion that measures the loss given
    inputs `x1`, `x2`, two 1D min-batch `Tensor`s,
    and a label 1D mini-batch tensor `y` with values (`1` or `-1`).

    If `y == 1` then it assumed the first input should be ranked higher
    (have a larger value) than the second input, and vice-versa for `y == -1`.

    The loss function for each sample in the mini-batch is::

        loss(x, y) = max(0, -y * (x1 - x2) + margin)

    if the internal variable `sizeAverage = True`,
    the loss function averages the loss over the batch samples;
    if `sizeAverage = False`, then the loss function sums over the batch samples.
    By default, `sizeAverage` equals to `True`.
    """

    def __init__(self, margin=0, size_average=True):
        super(MarginRankingLoss, self).__init__()
        self.margin = margin
        self.size_average = size_average

    def forward(self, input1, input2, target):
        return self._backend.MarginRankingLoss(self.margin,
                                               self.size_average)(input1, input2, target)


class MultiMarginLoss(Module):
    r"""Creates a criterion that optimizes a multi-class classification hinge loss
    (margin-based loss) between input `x` (a 2D mini-batch `Tensor`) and
    output `y` (which is a 1D tensor of target class indices, `0` <= `y` <= `x.size(1)`):

    For each mini-batch sample::

        loss(x, y) = sum_i(max(0, (margin - x[y] + x[i]))^p) / x.size(0)
                     where `i == 0` to `x.size(0)` and `i != y`.

    Optionally, you can give non-equal weighting on the classes by passing
    a 1D `weights` tensor into the constructor.

    The loss function then becomes:
        loss(x, y) = sum_i(max(0, w[y] * (margin - x[y] - x[i]))^p) / x.size(0)

    By default, the losses are averaged over observations for each minibatch.
    However, if the field `sizeAverage` is set to `False`,
    the losses are instead summed.
    """

    def __init__(self, p=1, margin=1, weight=None, size_average=True):
        super(MultiMarginLoss, self).__init__()
        if p != 1 and p != 2:
            raise ValueError("only p == 1 and p == 2 supported")
        assert weight is None or weight.dim() == 1
        self.p = p
        self.margin = margin
        self.size_average = size_average
        self.weight = weight

    def forward(self, input, target):
        return self._backend.MultiMarginLoss(self.size_average, self.p,
                                             self.margin, weight=self.weight)(input, target)


# TODO: L1HingeEmbeddingCriterion
# TODO: MSECriterion weight
# TODO: ClassSimplexCriterion
