# 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.
from contextlib import contextmanager
from typing import Any, Callable, cast, Dict, Generator, Literal, Optional, Union

import torch
from torch import Tensor
from torch.optim import LBFGS, Optimizer

import lightning.pytorch as pl
from lightning.fabric.accelerators.cuda import _patch_cuda_is_available
from lightning.fabric.plugins.precision.amp import _optimizer_handles_unscaling
from lightning.fabric.utilities.types import Optimizable
from lightning.pytorch.plugins.precision.precision_plugin import PrecisionPlugin
from lightning.pytorch.utilities import GradClipAlgorithmType
from lightning.pytorch.utilities.exceptions import MisconfigurationException


class MixedPrecisionPlugin(PrecisionPlugin):
    """Plugin for Automatic Mixed Precision (AMP) training with ``torch.autocast``.

    Args:
        precision: Whether to use ``torch.float16`` (``'16-mixed'``) or ``torch.bfloat16`` (``'bf16-mixed'``).
        device: The device for ``torch.autocast``.
        scaler: An optional :class:`torch.cuda.amp.GradScaler` to use.
    """

    def __init__(
        self,
        precision: Literal["16-mixed", "bf16-mixed"],
        device: str,
        scaler: Optional[torch.cuda.amp.GradScaler] = None,
    ) -> None:
        if precision not in ("16-mixed", "bf16-mixed"):
            raise ValueError(
                f"`Passed `{type(self).__name__}(precision={precision!r})`."
                f" Precision must be '16-mixed' or 'bf16-mixed'."
            )

        self.precision = cast(Literal["16-mixed", "bf16-mixed"], str(precision))
        if scaler is None and self.precision == "16-mixed":
            with _patch_cuda_is_available():
                # if possible, we defer CUDA initialization to support strategies that will attempt forks
                scaler = torch.cuda.amp.GradScaler()
        if scaler is not None and self.precision == "bf16-mixed":
            raise MisconfigurationException(f"`precision='bf16-mixed'` does not use a scaler, found {scaler}.")
        self.device = device
        self.scaler = scaler

    def pre_backward(self, tensor: Tensor, module: "pl.LightningModule") -> Tensor:  # type: ignore[override]
        if self.scaler is not None:
            tensor = self.scaler.scale(tensor)
        return super().pre_backward(tensor, module)

    def optimizer_step(  # type: ignore[override]
        self,
        optimizer: Optimizable,
        model: "pl.LightningModule",
        closure: Callable[[], Any],
        **kwargs: Any,
    ) -> Any:
        if self.scaler is None:
            # skip scaler logic, as bfloat16 does not require scaler
            return super().optimizer_step(optimizer, model=model, closure=closure, **kwargs)
        if isinstance(optimizer, LBFGS):
            raise MisconfigurationException("AMP and the LBFGS optimizer are not compatible.")
        closure_result = closure()

        # If backward was skipped in automatic optimization (return None), unscaling is not needed
        skip_unscaling = closure_result is None and model.automatic_optimization

        if not _optimizer_handles_unscaling(optimizer) and not skip_unscaling:
            # Unscaling needs to be performed here in case we are going to apply gradient clipping.
            # Optimizers that perform unscaling in their `.step()` method are not supported (e.g., fused Adam).
            # Note: `unscale` happens after the closure is executed, but before the `on_before_optimizer_step` hook.
            self.scaler.unscale_(optimizer)

        self._after_closure(model, optimizer)

        # in manual optimization, the closure does not return a value
        if not skip_unscaling:
            # note: the scaler will skip the `optimizer.step` if nonfinite gradients are found
            step_output = self.scaler.step(optimizer, **kwargs)
            self.scaler.update()
            return step_output
        return closure_result

    def clip_gradients(
        self,
        optimizer: Optimizer,
        clip_val: Union[int, float] = 0.0,
        gradient_clip_algorithm: GradClipAlgorithmType = GradClipAlgorithmType.NORM,
    ) -> None:
        if clip_val > 0 and _optimizer_handles_unscaling(optimizer):
            raise RuntimeError(
                f"The current optimizer, {type(optimizer).__qualname__}, does not allow for gradient clipping"
                " because it performs unscaling of gradients internally. HINT: Are you using a 'fused' optimizer?"
            )
        super().clip_gradients(optimizer=optimizer, clip_val=clip_val, gradient_clip_algorithm=gradient_clip_algorithm)

    def autocast_context_manager(self) -> torch.autocast:
        # the dtype could be automatically inferred but we need to manually set it due to a bug upstream
        # https://github.com/pytorch/pytorch/issues/67233
        return torch.autocast(self.device, dtype=torch.bfloat16 if self.precision == "bf16-mixed" else torch.half)

    @contextmanager
    def forward_context(self) -> Generator[None, None, None]:
        """Enable autocast context."""
        with self.autocast_context_manager():
            yield

    def state_dict(self) -> Dict[str, Any]:
        if self.scaler is not None:
            return self.scaler.state_dict()
        return {}

    def load_state_dict(self, state_dict: Dict[str, Any]) -> None:
        if self.scaler is not None:
            self.scaler.load_state_dict(state_dict)
