# Copyright 2020 The Orbit 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.
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"""Provides a utility class for training in epochs."""

import tensorflow as tf


class EpochHelper:
  """A helper class handle bookkeeping of epochs in custom training loops."""

  def __init__(self, epoch_steps: int, global_step: tf.Variable):
    """Initializes the `EpochHelper` instance.

    Args:
      epoch_steps: An integer indicating how many steps are in an epoch.
      global_step: A `tf.Variable` providing the current global step.
    """
    self._epoch_steps = epoch_steps
    self._global_step = global_step
    self._current_epoch = None
    self._epoch_start_step = None
    self._in_epoch = False

  def epoch_begin(self):
    """Returns whether a new epoch should begin."""
    if self._in_epoch:
      return False
    current_step = self._global_step.numpy()
    self._epoch_start_step = current_step
    self._current_epoch = current_step // self._epoch_steps
    self._in_epoch = True
    return True

  def epoch_end(self):
    """Returns whether the current epoch should end."""
    if not self._in_epoch:
      raise ValueError("`epoch_end` can only be called inside an epoch.")
    current_step = self._global_step.numpy()
    epoch = current_step // self._epoch_steps

    if epoch > self._current_epoch:
      self._in_epoch = False
      return True
    return False

  @property
  def batch_index(self):
    """Index of the next batch within the current epoch."""
    return self._global_step.numpy() - self._epoch_start_step

  @property
  def current_epoch(self):
    return self._current_epoch
