# Copyright The PyTorch Lightning 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 numbers
import warnings
from typing import Any, cast, Union

import torch

import pytorch_lightning as pl
from pytorch_lightning.overrides.base import _LightningModuleWrapperBase, _LightningPrecisionModuleWrapperBase
from pytorch_lightning.utilities.apply_func import apply_to_collection
from pytorch_lightning.utilities.rank_zero import rank_zero_warn


def _ignore_scalar_return_in_dp() -> None:
    # Users get confused by this warning so we silence it
    warnings.filterwarnings(
        "ignore",
        message=(
            "Was asked to gather along dimension 0, but all input tensors were scalars;"
            " will instead unsqueeze and return a vector."
        ),
    )


class LightningParallelModule(_LightningModuleWrapperBase):
    """Wraps the user's LightningModule and redirects the forward call to the appropriate method, either
    ``training_step``, ``validation_step``, ``test_step``, or ``predict_step``.

    This class is used in combination with :class:`~torch.nn.parallel.DataParallel` as shown in the example.
    It also takes care of converting Python scalars to Tensors and un-squeezes 0-dimensional Tensors as it is required
    by :class:`~torch.nn.parallel.DataParallel`.

    Example:

        dp_model = torch.nn.DataParallel(
            module=LightningParallelModule(lightning_module),
            device_ids=[3, 4],
            ...
        )

    Args:
        pl_module: the model to wrap
    """

    def __init__(self, pl_module: Union["pl.LightningModule", _LightningPrecisionModuleWrapperBase]) -> None:
        super().__init__(pl_module)
        _ignore_scalar_return_in_dp()

    def forward(self, *inputs: Any, **kwargs: Any) -> Any:
        self.update_replica_device_attributes(inputs)
        # forward call will redirect to training_step, validation_step, etc.
        output = super().forward(*inputs, **kwargs)

        def output_transform(data: Any) -> Any:
            device = cast(torch.device, self.module.device)
            data = python_scalar_to_tensor(data, device)
            data = unsqueeze_scalar_tensor(data)
            return data

        output = apply_to_collection(output, dtype=(numbers.Number, torch.Tensor), function=output_transform)
        return output

    def update_replica_device_attributes(self, inputs: Any) -> None:
        """Updates the device information of LightningModule by reading the device from the inputs. In
        :class:`~torch.nn.data_parallel.DataParallel` changes to the state during the `forward` pass are lost when
        the replicas get discarded. The only way to know the current device is from the inputs passed into the
        model.

        Args:
            inputs: A collection of inputs (typically a tuple). If the inputs don't contain tensors,
                a warning is shown that accessing ``self.device`` will not return the correct device.
        """
        replica_device = None

        def find_tensor_with_device(tensor: torch.Tensor) -> torch.Tensor:
            nonlocal replica_device
            if replica_device is None and tensor.device != torch.device("cpu"):
                replica_device = tensor.device
            return tensor

        apply_to_collection(inputs, dtype=torch.Tensor, function=find_tensor_with_device)

        if replica_device is not None:
            # by calling .to() we force the update to the self.device property
            self.module.to(device=replica_device)
        else:
            rank_zero_warn(
                "Could not determine on which device the inputs are."
                " When using DataParallel (strategy='dp'), be aware that in case you are using self.device"
                " in your code, it will reference only the root device."
            )


def python_scalar_to_tensor(data: Any, device: Union[str, torch.device] = torch.device("cpu")) -> Any:
    """Converts a Python scalar number to a torch tensor and places it on the given device."""
    if isinstance(data, numbers.Number):
        data = torch.tensor([data], device=device)
    return data


def unsqueeze_scalar_tensor(data: Any) -> Any:
    """Un-squeezes a 0-dim tensor."""
    if isinstance(data, torch.Tensor) and data.dim() == 0:
        data = data.unsqueeze(0)
    return data