from .utils import gen_output


def scatter_add_(output, index, input, dim=0):
    """ -> Tensor

    Sums up all values from the tensor :attr:`input` into :attr:`output` at
    the indices specified in the :attr:`index` tensor along an given axis
    :attr:`dim`. For each value in :attr:`input`, its output index is specified
    by its index in :attr:`input` for dimension != :attr:`dim` and by the
    corresponding value in :attr:`index` for dimension = :attr:`dim`. If
    multiple indices reference the same location, their contributions add.

    If :attr:`input` and :attr:`index` are n-dimensional tensors with size
    :math:`(x_0, ..., x_{i-1}, x_i, x_{i+1}, ..., x_{n-1})` and
    :attr:`dim` = i, then :attr:`output` must be an n-dimensional tensor with
    size :math:`(x_0, ..., x_{i-1}, y, x_{i+1}, ..., x_{n-1})`. Moreover, the
    values of :attr:`index` must be between `0` and `output.size(dim) - 1`.

    For one-dimensional tensors, the operation computes
    :math:`output_i = output_i + \sum_j input_j`, where sum is over
    :math:`j` such that :math:`index_j = i`.

    Args:
        output (Tensor): The destination tensor
        index (LongTensor): The indices of elements to scatter
        input (Tensor): The source tensor
        dim (int, optional): The axis along which to index

    Example::
        >> input = torch.Tensor([[2, 0, 1, 4, 3], [0,2, 1, 3, 4]])
        >> index = torch.LongTensor([[4, 5, 2, 3], [0, 0, 2, 2, 1]])
        >> output = torch.zeros(2, 6)
        >> scatter_add_(output, index, input, dim=1)
        0  0  4  3  3  0
        2  4  4  0  0  0
        [torch.FloatTensor of size 2x6]
    """
    return output.scatter_add_(dim, index, input)


def scatter_add(index, input, dim=0, size=None, fill_value=0):
    """ -> Tensor

    Sums ap all values from the tensor :attr:`input` at the indices
    specified in the :attr:`index` tensor along an given axis :attr:`dim`.
    The output size at dimension :attr:`dim` is given by :attr:`size` and must
    be at least size `index.max(dim) - 1`. If :attr:`size` is not given, a
    minimal sized output tensor is returned. The output tensor is initially
    filled with the specified value at :attr:`fill_value`.

    For one-dimensional tensors, the operation computes
    :math:`output_i = fill_value + \sum_j input_j`, where sum is over
    :math:`j` such that :math:`index_j = i`.

    A more detailed explanation is described in :meth:`~scatter_add_`.

    Args:
        index (LongTensor): The indices of elements to scatter
        input (Tensor): The source tensor
        dim (int, optional): The axis along which to index
        size (int, optional): Output size at dimension :attr:`dim`
        fill_value (int, optional): Initial filling of output tensor
    """
    output = gen_output(index, input, dim, size, fill_value)
    return scatter_add_(output, index, input, dim)