from copy import deepcopy

import torch.nn as nn
from torch.utils import checkpoint as cp

from ..builder import BACKBONES
from ..common import TAM
from .resnet import Bottleneck, ResNet


class TABlock(nn.Module):
    """Temporal Adaptive Block (TA-Block) for TANet.

    This block is proposed in `TAM: TEMPORAL ADAPTIVE MODULE FOR VIDEO
    RECOGNITION <https://arxiv.org/pdf/2005.06803>`_

    The temporal adaptive module (TAM) is embedded into ResNet-Block
    after the first Conv2D, which turns the vanilla ResNet-Block
    into TA-Block.

    Args:
        block (nn.Module): Residual blocks to be substituted.
        num_segments (int): Number of frame segments.
        tam_cfg (dict): Config for temporal adaptive module (TAM).
            Default: dict().
    """

    def __init__(self, block, num_segments, tam_cfg=dict()):
        super().__init__()
        self.tam_cfg = deepcopy(tam_cfg)
        self.block = block
        self.num_segments = num_segments
        self.tam = TAM(
            in_channels=block.conv1.out_channels,
            num_segments=num_segments,
            **self.tam_cfg)

        if not isinstance(self.block, Bottleneck):
            raise NotImplementedError('TA-Blocks have not been fully '
                                      'implemented except the pattern based '
                                      'on Bottleneck block.')

    def forward(self, x):
        assert isinstance(self.block, Bottleneck)

        def _inner_forward(x):
            """Forward wrapper for utilizing checkpoint."""
            identity = x

            out = self.block.conv1(x)
            out = self.tam(out)
            out = self.block.conv2(out)
            out = self.block.conv3(out)

            if self.block.downsample is not None:
                identity = self.block.downsample(x)

            out = out + identity

            return out

        if self.block.with_cp and x.requires_grad:
            out = cp.checkpoint(_inner_forward, x)
        else:
            out = _inner_forward(x)

        out = self.block.relu(out)

        return out


@BACKBONES.register_module()
class TANet(ResNet):
    """Temporal Adaptive Network (TANet) backbone.

    This backbone is proposed in `TAM: TEMPORAL ADAPTIVE MODULE FOR VIDEO
    RECOGNITION <https://arxiv.org/pdf/2005.06803>`_

    Embedding the temporal adaptive module (TAM) into ResNet to
    instantiate TANet.

    Args:
        depth (int): Depth of resnet, from {18, 34, 50, 101, 152}.
        num_segments (int): Number of frame segments.
        tam_cfg (dict | None): Config for temporal adaptive module (TAM).
            Default: dict().
        **kwargs (keyword arguments, optional): Arguments for ResNet except
            ```depth```.
    """

    def __init__(self, depth, num_segments, tam_cfg=dict(), **kwargs):
        super().__init__(depth, **kwargs)
        assert num_segments >= 3
        self.num_segments = num_segments
        self.tam_cfg = deepcopy(tam_cfg)

    def init_weights(self):
        super().init_weights()
        self.make_tam_modeling()

    def make_tam_modeling(self):
        """Replace ResNet-Block with TA-Block."""

        def make_tam_block(stage, num_segments, tam_cfg=dict()):
            blocks = list(stage.children())
            for i, block in enumerate(blocks):
                blocks[i] = TABlock(block, num_segments, deepcopy(tam_cfg))
            return nn.Sequential(*blocks)

        for i in range(self.num_stages):
            layer_name = f'layer{i + 1}'
            res_layer = getattr(self, layer_name)
            setattr(self, layer_name,
                    make_tam_block(res_layer, self.num_segments, self.tam_cfg))
