# Copyright The Lightning AI team.
#
# 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 logging
from typing import Optional, Union

import torch

import lightning.pytorch as pl
from lightning.fabric.utilities.data import _set_sampler_epoch
from lightning.pytorch.loops import _Loop
from lightning.pytorch.loops.fetchers import _DataFetcher
from lightning.pytorch.loops.progress import _Progress
from lightning.pytorch.loops.training_epoch_loop import _TrainingEpochLoop
from lightning.pytorch.loops.utilities import _is_max_limit_reached, _select_data_fetcher
from lightning.pytorch.trainer import call
from lightning.pytorch.trainer.connectors.data_connector import (
    _check_dataloader_iterable,
    _DataLoaderSource,
    _parse_num_batches,
    _process_dataloader,
    _request_dataloader,
    _resolve_overfit_batches,
)
from lightning.pytorch.trainer.connectors.logger_connector.result import _ResultCollection
from lightning.pytorch.trainer.states import RunningStage, TrainerFn
from lightning.pytorch.utilities.combined_loader import _SUPPORTED_MODES, CombinedLoader
from lightning.pytorch.utilities.data import has_len_all_ranks
from lightning.pytorch.utilities.exceptions import MisconfigurationException, SIGTERMException
from lightning.pytorch.utilities.model_helpers import is_overridden
from lightning.pytorch.utilities.rank_zero import rank_zero_debug, rank_zero_info, rank_zero_warn
from lightning.pytorch.utilities.warnings import PossibleUserWarning

log = logging.getLogger(__name__)


class _FitLoop(_Loop):
    """This loop is the top-level loop where training starts.

    It simply counts the epochs and iterates from one to the next by calling ``TrainingEpochLoop.run()`` in its
    ``advance()`` method.

    Example::

        # FitLoop
        for epoch in range(max_epochs):
            # TrainingEpochLoop
            for batch_idx, batch in enumerate(train_dataloader):
                loss = lightning_module.training_step(batch, batch_idx)
                ...

                # ValidationEpochLoop
                for batch_idx, batch in enumerate(val_dataloader):
                    lightning_module.validation_step(batch, batch_idx)
                    ...
                ...
            ...

    Args:
        min_epochs: The minimum number of epochs
        max_epochs: The maximum number of epochs, can be set -1 to turn this limit off
    """

    def __init__(
        self,
        trainer: "pl.Trainer",
        min_epochs: Optional[int] = 0,
        max_epochs: Optional[int] = None,
    ) -> None:
        super().__init__(trainer)
        if isinstance(max_epochs, int) and max_epochs < -1:
            # Allow max_epochs to be zero, since this will be handled by fit_loop.done
            raise MisconfigurationException(
                f"`max_epochs` must be a non-negative integer or -1. You passed in {max_epochs}."
            )

        self.max_epochs = max_epochs
        self.min_epochs = min_epochs
        self.epoch_loop = _TrainingEpochLoop(trainer)
        self.epoch_progress = _Progress()
        self.max_batches: Union[int, float] = float("inf")

        self._data_source = _DataLoaderSource(None, "train_dataloader")
        self._combined_loader: Optional[CombinedLoader] = None
        self._data_fetcher: Optional[_DataFetcher] = None
        self._last_train_dl_reload_epoch = float("-inf")

    @property
    def total_batch_idx(self) -> int:
        """Returns the current batch index (across epochs)"""
        return self.epoch_loop.total_batch_idx

    @property
    def batch_idx(self) -> int:
        """Returns the current batch index (within this epoch)"""
        return self.epoch_loop.batch_idx

    @property
    def min_steps(self) -> Optional[int]:
        """Returns the minimum number of steps to run."""
        return self.epoch_loop.min_steps

    @property
    def max_steps(self) -> int:
        """Returns the maximum number of steps to run."""
        return self.epoch_loop.max_steps

    @_Loop.restarting.setter
    def restarting(self, restarting: bool) -> None:
        # if the last epoch completely finished, we are not actually restarting
        values = self.epoch_progress.current.ready, self.epoch_progress.current.started
        epoch_unfinished = any(v != self.epoch_progress.current.processed for v in values)
        restarting = restarting and epoch_unfinished or self._iteration_based_training()
        _Loop.restarting.fset(self, restarting)  # call the parent setter

    @property
    def _skip_backward(self) -> bool:
        """Determines whether the loop will skip backward during automatic optimization."""
        return self.epoch_loop.automatic_optimization._skip_backward

    @_skip_backward.setter
    def _skip_backward(self, value: bool) -> None:
        """Determines whether the loop will skip backward during automatic optimization."""
        self.epoch_loop.automatic_optimization._skip_backward = value

    @property
    def _results(self) -> _ResultCollection:
        if self.trainer.training:
            return self.epoch_loop._results
        if self.trainer.validating:
            return self.epoch_loop.val_loop._results
        raise RuntimeError("`FitLoop._results` property isn't defined. Accessed outside of scope")

    @property
    def _can_stop_early(self) -> bool:
        met_min_epochs = self.epoch_progress.current.processed >= self.min_epochs if self.min_epochs else True
        met_min_steps = self.epoch_loop.global_step >= self.min_steps if self.min_steps else True
        return met_min_epochs and met_min_steps

    @property
    def _should_reload_train_dl(self) -> bool:
        """Check if train dataloader should be reloaded."""
        n_epochs = self.trainer.reload_dataloaders_every_n_epochs
        return n_epochs and self.trainer.current_epoch - self._last_train_dl_reload_epoch >= n_epochs

    @property
    def done(self) -> bool:
        """Evaluates when to leave the loop."""
        if self.max_batches == 0:
            rank_zero_info("`Trainer.fit` stopped: No training batches.")
            return True

        # TODO: Move track steps inside training loop and move part of these condition inside training loop
        stop_steps = _is_max_limit_reached(self.epoch_loop.global_step, self.max_steps)
        if stop_steps:
            rank_zero_info(f"`Trainer.fit` stopped: `max_steps={self.max_steps!r}` reached.")
            return True

        # `processed` is increased before `on_train_epoch_end`, the hook where checkpoints are typically saved.
        # we use it here because the checkpoint data won't have `completed` increased yet
        assert isinstance(self.max_epochs, int)
        stop_epochs = _is_max_limit_reached(self.epoch_progress.current.processed, self.max_epochs)
        if stop_epochs:
            # in case they are not equal, override so `trainer.current_epoch` has the expected value
            self.epoch_progress.current.completed = self.epoch_progress.current.processed
            rank_zero_info(f"`Trainer.fit` stopped: `max_epochs={self.max_epochs!r}` reached.")
            return True

        if self.trainer.should_stop and self._can_stop_early:
            rank_zero_debug("`Trainer.fit` stopped: `trainer.should_stop` was set.")
            return True

        return False

    @property
    def skip(self) -> bool:
        """Whether we should skip the training and immediately return from the call to :meth:`run`."""
        # if `limit_train_batches == 0` then `setup_data` won't set the `self.max_batches` attribute (checked in `done`)
        # so we cannot use it solely
        return self.done or self.trainer.limit_train_batches == 0

    def run(self) -> None:
        self.setup_data()
        if self.skip:
            return
        self.reset()
        self.on_run_start()
        while not self.done:
            try:
                self.on_advance_start()
                self.advance()
                self.on_advance_end()
                self._restarting = False
            except StopIteration:
                break
        self._restarting = False
        self.on_run_end()

    def setup_data(self) -> None:
        if self._combined_loader is not None and not self._should_reload_train_dl:
            return

        trainer = self.trainer
        pl_module = trainer.lightning_module
        if trainer.limit_train_batches == 0 or not is_overridden("training_step", pl_module):
            return

        log.debug(f"{self.__class__.__name__}: resetting train dataloader")

        source = self._data_source
        train_dataloader = _request_dataloader(source)
        trainer.strategy.barrier("train_dataloader()")

        if not isinstance(train_dataloader, CombinedLoader):
            combined_loader = CombinedLoader(train_dataloader, "max_size_cycle")
        else:
            combined_loader = train_dataloader

        if trainer.overfit_batches > 0:
            _resolve_overfit_batches(combined_loader, mode=RunningStage.TRAINING)

        trainer_fn = TrainerFn.FITTING
        stage = RunningStage.TRAINING
        dataloaders = []
        for dl in combined_loader.flattened:
            _check_dataloader_iterable(dl, source, trainer_fn)
            dl = _process_dataloader(trainer, trainer_fn, stage, dl)
            dataloaders.append(dl)
        combined_loader.flattened = dataloaders
        self._combined_loader = combined_loader

        allow_zero_length = pl_module.allow_zero_length_dataloader_with_multiple_devices
        if trainer.datamodule is not None:
            allow_zero_length |= trainer.datamodule.allow_zero_length_dataloader_with_multiple_devices

        has_len_all_ranks_ = has_len_all_ranks(combined_loader, trainer.strategy, allow_zero_length)
        self.max_batches = len(combined_loader) if has_len_all_ranks_ else float("inf")
        if self.max_batches == 0:
            return

        self.max_batches = _parse_num_batches(stage, self.max_batches, trainer.limit_train_batches)

        # store epoch of dataloader reset for reload_dataloaders_every_n_epochs
        self._last_train_dl_reload_epoch = trainer.current_epoch

        if isinstance(trainer.val_check_interval, int):
            trainer.val_check_batch = trainer.val_check_interval
            if trainer.val_check_batch > self.max_batches and trainer.check_val_every_n_epoch is not None:
                raise ValueError(
                    f" `val_check_interval` ({trainer.val_check_interval}) must be less than or equal"
                    f" to the number of the training batches ({self.max_batches})."
                    " If you want to disable validation set `limit_val_batches` to 0.0 instead."
                    " If you want to validate based on the total training batches, set `check_val_every_n_epoch=None`."
                )
        else:
            if not has_len_all_ranks_:
                if trainer.val_check_interval == 1.0:
                    trainer.val_check_batch = float("inf")
                else:
                    raise MisconfigurationException(
                        "When using an IterableDataset for `train_dataloader`,"
                        " `Trainer(val_check_interval)` must be `1.0` or an int. An int k specifies"
                        " checking validation every k training batches."
                    )
            else:
                trainer.val_check_batch = int(self.max_batches * trainer.val_check_interval)
                trainer.val_check_batch = max(1, trainer.val_check_batch)

        if trainer.loggers and self.max_batches < trainer.log_every_n_steps:
            rank_zero_warn(
                f"The number of training batches ({self.max_batches}) is smaller than the logging interval"
                f" Trainer(log_every_n_steps={trainer.log_every_n_steps}). Set a lower value for log_every_n_steps if"
                " you want to see logs for the training epoch.",
                category=PossibleUserWarning,
            )

    def reset(self) -> None:
        """Resets the internal state of this loop."""
        assert self.trainer.model is not None
        self.trainer.model.train()
        torch.set_grad_enabled(True)

        if self.restarting:
            self.epoch_progress.reset_on_restart()

    def on_run_start(self) -> None:
        """Calls the ``on_train_start`` hook."""
        # update the current_epoch in-case of checkpoint reload
        if not self._iteration_based_training():
            self.epoch_progress.current.completed = self.epoch_progress.current.processed

        trainer = self.trainer

        # reload the evaluation dataloaders too for proper display in the progress bar
        if self.epoch_loop._should_check_val_epoch() and trainer.val_dataloaders is None:
            trainer.validating = True
            self.epoch_loop.val_loop.setup_data()
            trainer.training = True

        self._data_fetcher = _select_data_fetcher(trainer)

        call._call_callback_hooks(trainer, "on_train_start")
        call._call_lightning_module_hook(trainer, "on_train_start")
        call._call_strategy_hook(trainer, "on_train_start")

    def on_advance_start(self) -> None:
        """Prepares the dataloader for training and calls the hook ``on_train_epoch_start``"""
        trainer = self.trainer

        # might need to setup data again depending on `trainer.reload_dataloaders_every_n_epochs`
        self.setup_data()

        # update the epoch value for all samplers
        assert self._combined_loader is not None
        for i, dl in enumerate(self._combined_loader.flattened):
            _set_sampler_epoch(dl, self.epoch_progress.current.processed)

        self.epoch_progress.increment_ready()

        trainer._logger_connector.on_epoch_start()

        call._call_callback_hooks(trainer, "on_train_epoch_start")
        call._call_lightning_module_hook(trainer, "on_train_epoch_start")

        self.epoch_progress.increment_started()

    def advance(self) -> None:
        """Runs one whole epoch."""
        log.debug(f"{type(self).__name__}: advancing loop")

        combined_loader = self._combined_loader
        assert combined_loader is not None
        if combined_loader._mode == "sequential":
            raise ValueError(
                f'`{type(self).__name__}` does not support the `CombinedLoader(mode="sequential")` mode.'
                f" The available modes are: {[m for m in _SUPPORTED_MODES if m != 'sequential']}"
            )
        assert self._data_fetcher is not None
        self._data_fetcher.setup(combined_loader)
        with self.trainer.profiler.profile("run_training_epoch"):
            self.epoch_loop.run(self._data_fetcher)

    def on_advance_end(self) -> None:
        trainer = self.trainer
        # inform logger the batch loop has finished
        trainer._logger_connector.epoch_end_reached()

        self.epoch_progress.increment_processed()

        # call train epoch end hooks
        # we always call callback hooks first, but here we need to make an exception for the callbacks that
        # monitor a metric, otherwise they wouldn't be able to monitor a key logged in
        # `LightningModule.on_train_epoch_end`
        call._call_callback_hooks(trainer, "on_train_epoch_end", monitoring_callbacks=False)
        call._call_lightning_module_hook(trainer, "on_train_epoch_end")
        call._call_callback_hooks(trainer, "on_train_epoch_end", monitoring_callbacks=True)

        trainer._logger_connector.on_epoch_end()

        if self.epoch_loop._num_ready_batches_reached():
            # if we are restarting and the above condition holds, it's because we are reloading an epoch-end checkpoint.
            # since metric-based schedulers require access to metrics and those are not currently saved in the
            # checkpoint, the plateau schedulers shouldn't be updated
            self.epoch_loop.update_lr_schedulers("epoch", update_plateau_schedulers=not self.restarting)

        # we manually decrease here because loggers expect that the same step is used when logging epoch-end metrics
        # even when the batch loop has finished
        self.epoch_loop._batches_that_stepped -= 1
        # log epoch metrics
        trainer._logger_connector.update_train_epoch_metrics()
        self.epoch_loop._batches_that_stepped += 1

        self.epoch_progress.increment_completed()

        if trainer.received_sigterm:
            raise SIGTERMException

    def on_run_end(self) -> None:
        """Calls the ``on_train_end`` hook."""
        log.debug(f"{self.__class__.__name__}: train run ended")

        trainer = self.trainer
        call._call_callback_hooks(trainer, "on_train_end")
        call._call_lightning_module_hook(trainer, "on_train_end")
        call._call_strategy_hook(trainer, "on_train_end")

    def teardown(self) -> None:
        if self._data_fetcher is not None:
            self._data_fetcher.teardown()
            self._data_fetcher = None
        self.epoch_loop.teardown()

    def _should_accumulate(self) -> bool:
        """Whether the gradients should be accumulated."""
        return self.epoch_loop._should_accumulate()

    def _iteration_based_training(self) -> bool:
        return self.trainer.max_steps != -1
