# Copyright 2019 The FastEstimator 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 tensorflow as tf
import numpy as np

from tensorflow.keras import layers


class FadeIn(layers.Add):
    """ A layer that returns a linear combination of two tensors.

    Args:
        fade_in_alpha (float): value for weight of the linear combination between the two input tensors
    """
    def __init__(self, fade_in_alpha, **kwargs):
        super().__init__(**kwargs)
        self.fade_in_alpha = fade_in_alpha

    def get_config(self):
        return {'fade_in_alpha': self.fade_in_alpha}

    def _merge_function(self, inputs):
        assert len(inputs) == 2, "FadeIn only supports two layers"
        output = ((1.0 - self.fade_in_alpha) * inputs[0]) + (self.fade_in_alpha * inputs[1])
        return output
