# Copyright 2022 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.
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#     http://www.apache.org/licenses/LICENSE-2.0
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# ==============================================================================
"""AdamW optimizer implementation."""


import tensorflow.compat.v2 as tf

from keras.optimizers import optimizer
from keras.saving.object_registration import register_keras_serializable

# isort: off
from tensorflow.python.util.tf_export import keras_export


@register_keras_serializable()
@keras_export(
    "keras.optimizers.AdamW", "keras.optimizers.experimental.AdamW", v1=[]
)
class AdamW(optimizer.Optimizer):
    r"""Optimizer that implements the AdamW algorithm.

    AdamW optimization is a stochastic gradient descent method that is based on
    adaptive estimation of first-order and second-order moments with an added
    method to decay weights per the techniques discussed in the paper,
    'Decoupled Weight Decay Regularization' by
    [Loshchilov, Hutter et al., 2019](https://arxiv.org/abs/1711.05101).

    According to
    [Kingma et al., 2014](http://arxiv.org/abs/1412.6980),
    the underying Adam method is "*computationally
    efficient, has little memory requirement, invariant to diagonal rescaling of
    gradients, and is well suited for problems that are large in terms of
    data/parameters*".

    Args:
      learning_rate: A `tf.Tensor`, floating point value, a schedule that is a
        `tf.keras.optimizers.schedules.LearningRateSchedule`, or a callable
        that takes no arguments and returns the actual value to use. The
        learning rate. Defaults to 0.001.
      weight_decay: A `tf.Tensor`, floating point value. The weight decay.
        Defaults to 0.004.
      beta_1: A float value or a constant float tensor, or a callable
        that takes no arguments and returns the actual value to use. The
        exponential decay rate for the 1st moment estimates. Defaults to 0.9.
      beta_2: A float value or a constant float tensor, or a callable
        that takes no arguments and returns the actual value to use. The
        exponential decay rate for the 2nd moment estimates. Defaults to 0.999.
      epsilon: A small constant for numerical stability. This epsilon is
        "epsilon hat" in the Kingma and Ba paper (in the formula just before
        Section 2.1), not the epsilon in Algorithm 1 of the paper. Defaults to
        1e-7.
      amsgrad: Boolean. Whether to apply AMSGrad variant of this algorithm from
        the paper "On the Convergence of Adam and beyond". Defaults to `False`.
      {{base_optimizer_keyword_args}}

    Reference:
      - [Loshchilov et al., 2019](https://arxiv.org/abs/1711.05101)
      - [Kingma et al., 2014](http://arxiv.org/abs/1412.6980) for `adam`
      - [Reddi et al., 2018](
          https://openreview.net/pdf?id=ryQu7f-RZ) for `amsgrad`.

    Notes:

    The sparse implementation of this algorithm (used when the gradient is an
    IndexedSlices object, typically because of `tf.gather` or an embedding
    lookup in the forward pass) does apply momentum to variable slices even if
    they were not used in the forward pass (meaning they have a gradient equal
    to zero). Momentum decay (beta1) is also applied to the entire momentum
    accumulator. This means that the sparse behavior is equivalent to the dense
    behavior (in contrast to some momentum implementations which ignore momentum
    unless a variable slice was actually used).
    """

    def __init__(
        self,
        learning_rate=0.001,
        weight_decay=0.004,
        beta_1=0.9,
        beta_2=0.999,
        epsilon=1e-7,
        amsgrad=False,
        clipnorm=None,
        clipvalue=None,
        global_clipnorm=None,
        use_ema=False,
        ema_momentum=0.99,
        ema_overwrite_frequency=None,
        jit_compile=True,
        name="AdamW",
        **kwargs
    ):
        super().__init__(
            name=name,
            clipnorm=clipnorm,
            clipvalue=clipvalue,
            global_clipnorm=global_clipnorm,
            use_ema=use_ema,
            ema_momentum=ema_momentum,
            ema_overwrite_frequency=ema_overwrite_frequency,
            jit_compile=jit_compile,
            **kwargs
        )
        self._learning_rate = self._build_learning_rate(learning_rate)
        self.weight_decay = weight_decay
        self.beta_1 = beta_1
        self.beta_2 = beta_2
        self.epsilon = epsilon
        self.amsgrad = amsgrad

        if self.weight_decay is None:
            raise ValueError(
                "Missing value of `weight_decay` which is required and"
                " must be a float value."
            )

    def build(self, var_list):
        """Initialize optimizer variables.

        AdamW optimizer has 3 types of variables: momentums, velocities and
        velocity_hat (only set when amsgrad is applied),

        Args:
          var_list: list of model variables to build AdamW variables on.
        """
        super().build(var_list)
        if hasattr(self, "_built") and self._built:
            return
        self._built = True
        self._momentums = []
        self._velocities = []
        for var in var_list:
            self._momentums.append(
                self.add_variable_from_reference(
                    model_variable=var, variable_name="m"
                )
            )
            self._velocities.append(
                self.add_variable_from_reference(
                    model_variable=var, variable_name="v"
                )
            )
        if self.amsgrad:
            self._velocity_hats = []
            for var in var_list:
                self._velocity_hats.append(
                    self.add_variable_from_reference(
                        model_variable=var, variable_name="vhat"
                    )
                )

    def update_step(self, gradient, variable):
        """Update step given gradient and the associated model variable."""
        beta_1_power = None
        beta_2_power = None
        lr = tf.cast(self.learning_rate, variable.dtype)
        local_step = tf.cast(self.iterations + 1, variable.dtype)
        beta_1_power = tf.pow(tf.cast(self.beta_1, variable.dtype), local_step)
        beta_2_power = tf.pow(tf.cast(self.beta_2, variable.dtype), local_step)

        var_key = self._var_key(variable)
        m = self._momentums[self._index_dict[var_key]]
        v = self._velocities[self._index_dict[var_key]]

        alpha = lr * tf.sqrt(1 - beta_2_power) / (1 - beta_1_power)

        if isinstance(gradient, tf.IndexedSlices):
            # Sparse gradients.
            m.assign_add(-m * (1 - self.beta_1))
            m.scatter_add(
                tf.IndexedSlices(
                    gradient.values * (1 - self.beta_1), gradient.indices
                )
            )
            v.assign_add(-v * (1 - self.beta_2))
            v.scatter_add(
                tf.IndexedSlices(
                    tf.square(gradient.values) * (1 - self.beta_2),
                    gradient.indices,
                )
            )
            if self.amsgrad:
                v_hat = self._velocity_hats[self._index_dict[var_key]]
                v_hat.assign(tf.maximum(v_hat, v))
                v = v_hat
            variable.assign_sub((m * alpha) / (tf.sqrt(v) + self.epsilon))
        else:
            # Dense gradients.
            m.assign_add((gradient - m) * (1 - self.beta_1))
            v.assign_add((tf.square(gradient) - v) * (1 - self.beta_2))
            if self.amsgrad:
                v_hat = self._velocity_hats[self._index_dict[var_key]]
                v_hat.assign(tf.maximum(v_hat, v))
                v = v_hat
            variable.assign_sub((m * alpha) / (tf.sqrt(v) + self.epsilon))

    def get_config(self):
        config = super().get_config()

        config.update(
            {
                "learning_rate": self._serialize_hyperparameter(
                    self._learning_rate
                ),
                "weight_decay": self.weight_decay,
                "beta_1": self.beta_1,
                "beta_2": self.beta_2,
                "epsilon": self.epsilon,
                "amsgrad": self.amsgrad,
            }
        )
        return config


AdamW.__doc__ = AdamW.__doc__.replace(
    "{{base_optimizer_keyword_args}}", optimizer.base_optimizer_keyword_args
)