# Lint as: python3
# Copyright 2020 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
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# ==============================================================================
"""Optimizer factory class."""
from typing import Union

import tensorflow as tf

import tensorflow_addons.optimizers as tfa_optimizers

from official.modeling.optimization import lr_schedule
from official.modeling.optimization.configs import optimization_config as opt_cfg
from official.nlp import optimization as nlp_optimization

OPTIMIZERS_CLS = {
    'sgd': tf.keras.optimizers.SGD,
    'adam': tf.keras.optimizers.Adam,
    'adamw': nlp_optimization.AdamWeightDecay,
    'lamb': tfa_optimizers.LAMB,
    'rmsprop': tf.keras.optimizers.RMSprop
}

LR_CLS = {
    'stepwise': tf.keras.optimizers.schedules.PiecewiseConstantDecay,
    'polynomial': tf.keras.optimizers.schedules.PolynomialDecay,
    'exponential': tf.keras.optimizers.schedules.ExponentialDecay,
    'cosine': tf.keras.experimental.CosineDecay,
    'power': lr_schedule.DirectPowerDecay,
}

WARMUP_CLS = {
    'linear': lr_schedule.LinearWarmup,
    'polynomial': lr_schedule.PolynomialWarmUp
}


class OptimizerFactory(object):
  """Optimizer factory class.

  This class builds learning rate and optimizer based on an optimization config.
  To use this class, you need to do the following:
  (1) Define optimization config, this includes optimizer, and learning rate
      schedule.
  (2) Initialize the class using the optimization config.
  (3) Build learning rate.
  (4) Build optimizer.

  This is a typical example for using this class:
  params = {
        'optimizer': {
            'type': 'sgd',
            'sgd': {'momentum': 0.9}
        },
        'learning_rate': {
            'type': 'stepwise',
            'stepwise': {'boundaries': [10000, 20000],
                         'values': [0.1, 0.01, 0.001]}
        },
        'warmup': {
            'type': 'linear',
            'linear': {'warmup_steps': 500, 'warmup_learning_rate': 0.01}
        }
    }
  opt_config = OptimizationConfig(params)
  opt_factory = OptimizerFactory(opt_config)
  lr = opt_factory.build_learning_rate()
  optimizer = opt_factory.build_optimizer(lr)
  """

  def __init__(self, config: opt_cfg.OptimizationConfig):
    """Initializing OptimizerFactory.

    Args:
      config: OptimizationConfig instance contain optimization config.
    """
    self._config = config
    self._optimizer_config = config.optimizer.get()
    self._optimizer_type = config.optimizer.type

    if self._optimizer_type is None:
      raise ValueError('Optimizer type must be specified')

    self._lr_config = config.learning_rate.get()
    self._lr_type = config.learning_rate.type

    if self._lr_type is None:
      raise ValueError('Learning rate type must be specified')

    self._warmup_config = config.warmup.get()
    self._warmup_type = config.warmup.type

  def build_learning_rate(self):
    """Build learning rate.

    Builds learning rate from config. Learning rate schedule is built according
    to the learning rate config. If learning rate type is consant,
    lr_config.learning_rate is returned.

    Returns:
      tf.keras.optimizers.schedules.LearningRateSchedule instance. If
      learning rate type is consant, lr_config.learning_rate is returned.
    """
    if self._lr_type == 'constant':
      lr = self._lr_config.learning_rate
    else:
      lr = LR_CLS[self._lr_type](**self._lr_config.as_dict())

    if self._warmup_config:
      lr = WARMUP_CLS[self._warmup_type](lr, **self._warmup_config.as_dict())

    return lr

  def build_optimizer(
      self, lr: Union[tf.keras.optimizers.schedules.LearningRateSchedule,
                      float]):
    """Build optimizer.

    Builds optimizer from config. It takes learning rate as input, and builds
    the optimizer according to the optimizer config. Typically, the learning
    rate built using self.build_lr() is passed as an argument to this method.

    Args:
      lr: A floating point value, or a
        tf.keras.optimizers.schedules.LearningRateSchedule instance.

    Returns:
      tf.keras.optimizers.Optimizer instance.
    """

    optimizer_dict = self._optimizer_config.as_dict()
    optimizer_dict['learning_rate'] = lr

    optimizer = OPTIMIZERS_CLS[self._optimizer_type](**optimizer_dict)
    return optimizer
