# Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
#
# 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 paddle
import paddle.nn as nn
from paddle.autograd import PyLayer


class GradientReversalFunction(PyLayer):
    """Gradient Reversal Layer from:
    Unsupervised Domain Adaptation by Backpropagation (Ganin & Lempitsky, 2015)

    Forward pass is the identity function. In the backward pass,
    the upstream gradients are multiplied by -lambda (i.e. gradient is reversed)
    """

    @staticmethod
    def forward(ctx, x, lambda_=1):
        """Forward in networks
        """
        ctx.save_for_backward(lambda_)
        return x.clone()

    @staticmethod
    def backward(ctx, grads):
        """Backward in networks
        """
        lambda_, = ctx.saved_tensor()
        dx = -lambda_ * grads
        return paddle.clip(dx, min=-0.5, max=0.5)


class GradientReversalLayer(nn.Layer):
    """Gradient Reversal Layer from:
    Unsupervised Domain Adaptation by Backpropagation (Ganin & Lempitsky, 2015)

    Forward pass is the identity function. In the backward pass,
    the upstream gradients are multiplied by -lambda (i.e. gradient is reversed)
    """

    def __init__(self, lambda_=1):
        super(GradientReversalLayer, self).__init__()
        self.lambda_ = lambda_

    def forward(self, x):
        """Forward in networks
        """
        return GradientReversalFunction.apply(x, self.lambda_)
