# Copyright (c) Meta Platforms, Inc. and affiliates

import copy

import torch.distributed as dist
from torch.distributed._shard.sharded_tensor import Shard, ShardedTensor, ShardMetadata
from torch.distributed._shard.sharded_tensor.metadata import ShardedTensorMetadata
from torch.distributed.checkpoint.metadata import STATE_DICT_TYPE
from torch.distributed.remote_device import _remote_device

from ._traverse import OBJ_PATH, set_element, STATE_DICT_ITEM, traverse_state_dict
from .utils import _element_wise_add, _normalize_device_info


# TODO: We need to refactor this code.
def _flatten_sharded_tensors(state_dict: STATE_DICT_TYPE) -> STATE_DICT_TYPE:
    r"""
    Transform ``state_dict`` by flattening all nested ShardedTensor instances found.

    The resulting ShardedTensor instances are only correct regarding the local shard and
    MUST not be used for any other purpose but checkpointing, as no operator will work with them.

    This function should be used in conjunction with a state_dict produced by FSDP's
    StateDictType.SHARDED_STATE_DICT methods.
    """
    new_state_dict: STATE_DICT_TYPE = {}

    def rewrite_dict(path: OBJ_PATH, value: STATE_DICT_ITEM) -> None:
        if not isinstance(value, ShardedTensor):
            set_element(new_state_dict, path, value)
            return
        shards = value.local_shards()

        if len(shards) == 0:
            return
        if len(shards) != 1:
            set_element(new_state_dict, path, value)
            return

        outer_shard = shards[0]

        inner_st = outer_shard.tensor
        if not isinstance(inner_st, ShardedTensor):
            set_element(new_state_dict, path, value)
            return

        if len(inner_st.local_shards()) != 1:
            raise ValueError("Cannot handle inner tensor with more than 1 shard")
        inner_shard = inner_st.local_shards()[0]

        local_shards = [
            Shard(
                tensor=inner_shard.tensor,
                metadata=ShardMetadata(
                    shard_offsets=_element_wise_add(
                        outer_shard.metadata.shard_offsets,
                        inner_shard.metadata.shard_offsets,
                    ),
                    shard_sizes=inner_shard.metadata.shard_sizes,
                    placement=f"rank:{dist.get_rank()}/{inner_shard.tensor.device}",
                ),
            )
        ]

        st_meta: ShardedTensorMetadata = copy.deepcopy(value.metadata())
        other_rank = 0 if dist.get_rank() > 0 else 1
        device_info = _normalize_device_info(inner_shard.tensor.device.type, 0)

        # Remove the outer ST shard the inner ST covers
        for i, shard_md in enumerate(st_meta.shards_metadata):
            if shard_md.shard_offsets == outer_shard.metadata.shard_offsets:
                st_meta.shards_metadata.pop(i)
                break

        # Attribute other rank for the other shards
        for shard_md in st_meta.shards_metadata:
            shard_md.placement = _remote_device(f"rank:{other_rank}/{device_info}")

        # Add other inner shards from the inner tensor
        for inner_md in inner_st.metadata().shards_metadata:
            if inner_md.shard_offsets != inner_shard.metadata.shard_offsets:
                st_meta.shards_metadata.append(
                    ShardMetadata(
                        shard_offsets=_element_wise_add(
                            outer_shard.metadata.shard_offsets,
                            inner_md.shard_offsets,
                        ),
                        shard_sizes=inner_md.shard_sizes,
                        placement=f"rank:{other_rank}/{device_info}",
                    )
                )

        # Finally add this shard
        st_meta.shards_metadata.append(local_shards[0].metadata)

        st = ShardedTensor._init_from_local_shards_and_global_metadata(
            local_shards=local_shards,
            sharded_tensor_metadata=st_meta,
        )
        set_element(new_state_dict, path, st)

    traverse_state_dict(state_dict, rewrite_dict)
    return new_state_dict
