# Copyright 2018 The TensorFlow 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.
# ==============================================================================
"""Learning rate schedule."""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import tensorflow as tf


def step_learning_rate_with_linear_warmup(
    global_step,
    init_learning_rate,
    warmup_learning_rate,
    warmup_steps,
    learning_rate_levels,
    learning_rate_steps
):
    """Creates the step learning rate tensor with linear warmup."""

    def warmup_lr_fn():
        return warmup_learning_rate + \
            tf.cast(global_step,
                    dtype=tf.float32) / warmup_steps * (init_learning_rate - warmup_learning_rate)

    def learning_rate_fn():
        return tf.compat.v1.train.piecewise_constant(
            global_step,
            boundaries=learning_rate_steps,
            values=[init_learning_rate] + learning_rate_levels
        )

    learning_rate = tf.where(
        global_step < warmup_steps,
        warmup_lr_fn(),
        learning_rate_fn()
    )

    return learning_rate
