# Copyright 2019,2020,2021 Sony Corporation.
# Copyright 2021 Sony Group Corporation.
#
# 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
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.


import nnabla.functions as F
from .utils import no_grad, get_output


def dropout_backward(inputs, p=0.5, seed=-1, output_mask=False):
    """
    Args:
      inputs (list of nn.Variable): Incomming grads/inputs to/of the forward function.
      kwargs (dict of arguments): Dictionary of the corresponding function arguments.

    Return:
      list of Variable: Return the gradients wrt inputs of the corresponding function.
    """

    if not output_mask:
        raise ValueError(
            "dropout_backward is supported for output_mask=True.")
    dy0 = inputs[0]
    dy1 = inputs[1]
    x0 = inputs[2]

    y1 = get_output(x0, "Dropout", nth_output=1)
    m0 = y1.get_unlinked_variable()  # mask
    dx0 = dy0 * m0 / (1 - p)
    return dx0
