from math import dist
from colossalai.tensor.dist_spec import _DistSpec
from colossalai.nn.layer.utils import divide
from numpy import prod
from contextlib import contextmanager
import torch
import torch.distributed as dist


class TransformDistSpec(torch.autograd.Function):

    @staticmethod
    def forward(ctx, tensor, old_dist_spec, dist_spec, forward_trans_func, backward_trans_func):
        ctx.old_dist_spec = old_dist_spec
        ctx.dist_spec = dist_spec
        ctx.backward_trans_func = backward_trans_func
        return forward_trans_func(tensor, old_dist_spec, dist_spec)

    @staticmethod
    def backward(ctx, grad_outputs):
        return ctx.backward_trans_func(grad_outputs, ctx.dist_spec, ctx.old_dist_spec), None, None, None, None


class DistSpecManager:

    _use_autograd_function: bool = True

    @staticmethod
    def _shard_as(tensor: torch.Tensor, old_dist_spec: _DistSpec, dist_spec: _DistSpec) -> torch.Tensor:
        chunk = tensor
        idx = dist_spec.process_group.rank()
        num_parts = prod(dist_spec.num_partitions)
        for i, dim in enumerate(dist_spec.dims):
            num_parts //= dist_spec.num_partitions[i]
            chunk_size = divide(tensor.size(dim), dist_spec.num_partitions[i])
            chunk = chunk.narrow(dim, idx // num_parts * chunk_size, chunk_size)
            idx %= num_parts
        return chunk.detach().contiguous()

    @staticmethod
    def _gather(tensor: torch.Tensor, old_dist_spec: _DistSpec) -> torch.Tensor:
        buffer = [torch.empty_like(tensor) for _ in range(old_dist_spec.process_group.size())]
        dist.all_gather(buffer, tensor, group=old_dist_spec.process_group)
        for i in range(len(old_dist_spec.dims) - 1, -1, -1):
            new_buffer = []
            dim = old_dist_spec.dims[i]
            num_parts = old_dist_spec.num_partitions[i]
            for start in range(0, len(buffer), num_parts):
                new_buffer.append(torch.cat(buffer[start:start + num_parts], dim))
            buffer = new_buffer
        assert len(buffer) == 1
        return buffer[0]

    @staticmethod
    def _r2r(tensor: torch.Tensor, old_dist_spec: _DistSpec, dist_spec: _DistSpec) -> torch.Tensor:
        if old_dist_spec.process_group is not None and old_dist_spec.process_group != dist_spec.process_group \
            and dist_spec.process_group is not None:
            raise NotImplementedError
        return tensor

    @staticmethod
    def _r2s(tensor: torch.Tensor, old_dist_spec: _DistSpec, dist_spec: _DistSpec) -> torch.Tensor:
        if old_dist_spec.process_group is not None and old_dist_spec.process_group != dist_spec.process_group:
            raise NotImplementedError
        return DistSpecManager._shard_as(tensor, old_dist_spec, dist_spec)

    @staticmethod
    def _s2r(tensor: torch.Tensor, old_dist_spec: _DistSpec, dist_spec: _DistSpec) -> torch.Tensor:
        if old_dist_spec.process_group != dist_spec.process_group \
            and dist_spec.process_group is not None:
            raise NotImplementedError
        return DistSpecManager._gather(tensor, old_dist_spec)

    @staticmethod
    def _s2s(tensor: torch.Tensor, old_dist_spec: _DistSpec, dist_spec: _DistSpec) -> torch.Tensor:
        if old_dist_spec.process_group != dist_spec.process_group:
            raise NotImplementedError
        if old_dist_spec == dist_spec:
            return tensor
        tensor = DistSpecManager._gather(tensor, old_dist_spec)
        return DistSpecManager._shard_as(tensor, old_dist_spec, dist_spec)

    @staticmethod
    def handle_trans_spec(tensor: torch.Tensor, old_dist_spec: _DistSpec, dist_spec: _DistSpec) -> torch.Tensor:
        forward_trans_handle = getattr(DistSpecManager, f'_{old_dist_spec.placement.value}2{dist_spec.placement.value}')
        if not DistSpecManager._use_autograd_function:
            return forward_trans_handle(tensor, old_dist_spec, dist_spec)
        backward_trans_handle = getattr(DistSpecManager,
                                        f'_{dist_spec.placement.value}2{old_dist_spec.placement.value}')
        return TransformDistSpec.apply(tensor, old_dist_spec, dist_spec, forward_trans_handle, backward_trans_handle)

    @staticmethod
    @contextmanager
    def no_grad():
        try:
            DistSpecManager._use_autograd_function = False
            yield
        finally:
            DistSpecManager._use_autograd_function = True