import dataclasses
import itertools
from collections.abc import Iterable
from typing import Any, Union

import torch
from torch.utils._python_dispatch import is_traceable_wrapper_subclass


# This is technically very similar to SubclassCreatingMeta
# in aot_autograd, but we don't need all the stuff in there
# so just recreated a new dataclass.
@dataclasses.dataclass
class SubclassCreationMeta:
    start_idx: int
    num_tensors: int
    class_type: Any
    attrs: dict[str, "SubclassCreationMeta"]
    metadata: Any
    outer_size: Iterable[Union[None, int, torch.SymInt]]
    outer_stride: Iterable[Union[None, int, torch.SymInt]]


class UnwrapTensorSubclass(torch.nn.Module):
    def forward(self, *tensors) -> torch.Tensor:  # type: ignore[no-untyped-def]
        todo: list[torch.Tensor] = list(tensors)

        def _unwrap_tensor_subclasses(subclass_meta, tensors, offset):  # type: ignore[no-untyped-def]
            if subclass_meta is None:
                return tensors[offset], offset + 1
            inner_tensors = {}
            for attr, meta in subclass_meta.attrs.items():
                built_tensor, offset = _unwrap_tensor_subclasses(meta, tensors, offset)
                inner_tensors[attr] = built_tensor
            rebuilt = subclass_meta.class_type.__tensor_unflatten__(
                inner_tensors,
                subclass_meta.metadata,
                subclass_meta.outer_size,
                subclass_meta.outer_stride,
            )
            return rebuilt, offset

        return _unwrap_tensor_subclasses(self.subclass_meta, todo, 0)[0]

    def right_inverse(self, tensor: torch.Tensor) -> list[torch.Tensor]:
        if type(tensor) is torch.Tensor:
            raise AssertionError("tensor must be a subclass, not torch.Tensor")
        plain_tensors: list[torch.Tensor] = []

        def _create_subclass_meta(tensor, idx, plain_tensor_container):  # type: ignore[no-untyped-def]
            if type(tensor) is torch.Tensor:
                plain_tensor_container.append(tensor)
                return None, idx + 1
            inner_tensors_attrnames, metadata = tensor.__tensor_flatten__()  # type: ignore[attr-defined]
            new_idx = idx
            attr_to_meta = {}
            for attr in inner_tensors_attrnames:
                val = getattr(tensor, attr)
                subclass_meta, new_idx = _create_subclass_meta(
                    val, new_idx, plain_tensor_container
                )
                attr_to_meta[attr] = subclass_meta
            return (
                SubclassCreationMeta(
                    start_idx=idx,
                    num_tensors=new_idx - idx,
                    class_type=type(tensor),
                    attrs=attr_to_meta,
                    metadata=metadata,
                    outer_size=tensor.size(),
                    outer_stride=tensor.stride(),
                ),
                new_idx,
            )

        self.subclass_meta = _create_subclass_meta(tensor, 0, plain_tensors)[0]
        return plain_tensors


def unwrap_tensor_subclass_parameters(module: torch.nn.Module) -> torch.nn.Module:
    """
    Model transformation that replaces all the parameters that are subclasses to plain tensors.
    This reduces runtime overhead of flattening/unflattening the parameters.

    This transformation adds parametrization with `torch.nn.utils.parametrize`.
    The FQNs of the subclass parameters will be changed and state_dict will become incompatible with the original model.
    E.g.
    Original model state_dict: {"p1": torch.testing._internal.TwoTensor}
    becomes: {"parametrizations.p2.original0": torch.Tensor, "parametrizations.p2.original1": torch.Tensor}

    """
    for name, tensor in itertools.chain(
        list(module.named_parameters(recurse=False)),
        # pyrefly: ignore [no-matching-overload]
        list(module.named_buffers(recurse=False)),
    ):
        if is_traceable_wrapper_subclass(tensor):
            torch.nn.utils.parametrize.register_parametrization(
                module, name, UnwrapTensorSubclass()
            )

    for child in module.children():
        unwrap_tensor_subclass_parameters(child)

    return module
