# Copyright (c) 2020, NVIDIA CORPORATION.  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 time

from lightning.pytorch.callbacks import Callback
from lightning.pytorch.utilities import rank_zero_only


class LogEpochTimeCallback(Callback):
    """Simple callback that logs how long each epoch takes, in seconds, to a pytorch lightning log"""

    @rank_zero_only
    def on_train_epoch_start(self, trainer, pl_module):
        """Record epoch start time."""
        self.epoch_start = time.time()

    @rank_zero_only
    def on_train_epoch_end(self, trainer, pl_module):
        """Log elapsed epoch time."""
        curr_time = time.time()
        duration = curr_time - self.epoch_start
        trainer.logger.log_metrics({"epoch_time": duration}, step=trainer.global_step)