from typing import Sequence

import numpy as np
import torch
import torch.nn as nn
from mmcv.cnn import Linear, build_activation_layer, build_norm_layer
from mmcv.cnn.bricks.drop import build_dropout
from mmcv.cnn.bricks.transformer import PatchEmbed
from mmcv.cnn.utils.weight_init import constant_init, trunc_normal_init
from mmcv.runner import BaseModule, ModuleList, Sequential
from mmcv.utils import deprecated_api_warning

from ..utils import LayerScale, MultiheadAttention, resize_pos_embed, to_2tuple
from ..builder import BACKBONES
from .vision_transformer import VisionTransformer


class DeiT3FFN(BaseModule):
    """FFN for DeiT3.

    The differences between DeiT3FFN & FFN:
        1. Use LayerScale.

    Args:
        embed_dims (int): The feature dimension. Same as
            `MultiheadAttention`. Defaults: 256.
        feedforward_channels (int): The hidden dimension of FFNs.
            Defaults: 1024.
        num_fcs (int, optional): The number of fully-connected layers in
            FFNs. Default: 2.
        act_cfg (dict, optional): The activation config for FFNs.
            Default: dict(type='ReLU')
        ffn_drop (float, optional): Probability of an element to be
            zeroed in FFN. Default 0.0.
        add_identity (bool, optional): Whether to add the
            identity connection. Default: `True`.
        dropout_layer (obj:`ConfigDict`): The dropout_layer used
            when adding the shortcut.
        use_layer_scale (bool): Whether to use layer_scale in
            DeiT3FFN. Defaults to True.
        init_cfg (obj:`mmcv.ConfigDict`): The Config for initialization.
            Default: None.
    """

    @deprecated_api_warning(
        {
            'dropout': 'ffn_drop',
            'add_residual': 'add_identity'
        },
        cls_name='FFN')
    def __init__(self,
                 embed_dims=256,
                 feedforward_channels=1024,
                 num_fcs=2,
                 act_cfg=dict(type='ReLU', inplace=True),
                 ffn_drop=0.,
                 dropout_layer=None,
                 add_identity=True,
                 use_layer_scale=True,
                 init_cfg=None,
                 **kwargs):
        super().__init__(init_cfg)
        assert num_fcs >= 2, 'num_fcs should be no less ' \
            f'than 2. got {num_fcs}.'
        self.embed_dims = embed_dims
        self.feedforward_channels = feedforward_channels
        self.num_fcs = num_fcs
        self.act_cfg = act_cfg
        self.activate = build_activation_layer(act_cfg)

        layers = []
        in_channels = embed_dims
        for _ in range(num_fcs - 1):
            layers.append(
                Sequential(
                    Linear(in_channels, feedforward_channels), self.activate,
                    nn.Dropout(ffn_drop)))
            in_channels = feedforward_channels
        layers.append(Linear(feedforward_channels, embed_dims))
        layers.append(nn.Dropout(ffn_drop))
        self.layers = Sequential(*layers)
        self.dropout_layer = build_dropout(
            dropout_layer) if dropout_layer else torch.nn.Identity()
        self.add_identity = add_identity

        if use_layer_scale:
            self.gamma2 = LayerScale(embed_dims, data_format='channels_last')
        else:
            self.gamma2 = nn.Identity()

    @deprecated_api_warning({'residual': 'identity'}, cls_name='FFN')
    def forward(self, x, identity=None):
        """Forward function for `FFN`.

        The function would add x to the output tensor if residue is None.
        """
        out = self.gamma2(self.layers(x))
        if not self.add_identity:
            return self.dropout_layer(out)
        if identity is None:
            identity = x
        return identity + self.dropout_layer(out)


class DeiT3TransformerEncoderLayer(BaseModule):
    """Implements one encoder layer in DeiT3.

    The differences between DeiT3TransformerEncoderLayer and the vanilla
    TransformerEncoderLayer: Using LayerScale.

    Args:
        embed_dims (int): The feature dimension
        num_heads (int): Parallel attention heads
        feedforward_channels (int): The hidden dimension for FFNs
        drop_rate (float): Probability of an element to be zeroed
            after the feed forward layer. Defaults to 0.
        attn_drop_rate (float): The drop out rate for attention output weights.
            Defaults to 0.
        drop_path_rate (float): Stochastic depth rate. Defaults to 0.
        num_fcs (int): The number of fully-connected layers for FFNs.
            Defaults to 2.
        qkv_bias (bool): enable bias for qkv if True. Defaults to True.
        use_layer_scale (bool): Whether to use layer_scale in
            DeiT3TransformerEncoderLayer. Defaults to True.
        act_cfg (dict): The activation config for FFNs.
            Defaluts to ``dict(type='GELU')``.
        norm_cfg (dict): Config dict for normalization layer.
            Defaults to ``dict(type='LN')``.
        init_cfg (dict, optional): Initialization config dict.
            Defaults to None.
    """

    def __init__(self,
                 embed_dims,
                 num_heads,
                 feedforward_channels,
                 drop_rate=0.,
                 attn_drop_rate=0.,
                 drop_path_rate=0.,
                 num_fcs=2,
                 qkv_bias=True,
                 use_layer_scale=True,
                 act_cfg=dict(type='GELU'),
                 norm_cfg=dict(type='LN'),
                 init_cfg=None):
        super(DeiT3TransformerEncoderLayer, self).__init__(init_cfg=init_cfg)

        self.embed_dims = embed_dims

        self.norm1_name, norm1 = build_norm_layer(
            norm_cfg, self.embed_dims, postfix=1)
        self.add_module(self.norm1_name, norm1)

        self.attn = MultiheadAttention(
            embed_dims=embed_dims,
            num_heads=num_heads,
            attn_drop=attn_drop_rate,
            proj_drop=drop_rate,
            dropout_layer=dict(type='DropPath', drop_prob=drop_path_rate),
            qkv_bias=qkv_bias,
            use_layer_scale=use_layer_scale)

        self.norm2_name, norm2 = build_norm_layer(
            norm_cfg, self.embed_dims, postfix=2)
        self.add_module(self.norm2_name, norm2)

        self.ffn = DeiT3FFN(
            embed_dims=embed_dims,
            feedforward_channels=feedforward_channels,
            num_fcs=num_fcs,
            ffn_drop=drop_rate,
            dropout_layer=dict(type='DropPath', drop_prob=drop_path_rate),
            act_cfg=act_cfg,
            use_layer_scale=use_layer_scale)

    @property
    def norm1(self):
        return getattr(self, self.norm1_name)

    @property
    def norm2(self):
        return getattr(self, self.norm2_name)

    def init_weights(self):
        super(DeiT3TransformerEncoderLayer, self).init_weights()
        for m in self.ffn.modules():
            if isinstance(m, nn.Linear):
                nn.init.xavier_uniform_(m.weight)
                nn.init.normal_(m.bias, std=1e-6)

    def forward(self, x):
        x = x + self.attn(self.norm1(x))
        x = self.ffn(self.norm2(x), identity=x)
        return x


@BACKBONES.register_module()
class DeiT3(VisionTransformer):
    """DeiT3 backbone.

    A PyTorch implement of : `DeiT III: Revenge of the ViT
    <https://arxiv.org/pdf/2204.07118.pdf>`_

    The differences between DeiT3 & VisionTransformer:

    1. Use LayerScale.
    2. Concat cls token after adding pos_embed.

    Args:
        arch (str | dict): DeiT3 architecture. If use string,
            choose from 'small', 'base', 'medium', 'large' and 'huge'.
            If use dict, it should have below keys:

            - **embed_dims** (int): The dimensions of embedding.
            - **num_layers** (int): The number of transformer encoder layers.
            - **num_heads** (int): The number of heads in attention modules.
            - **feedforward_channels** (int): The hidden dimensions in
              feedforward modules.

            Defaults to 'base'.
        img_size (int | tuple): The expected input image shape. Because we
            support dynamic input shape, just set the argument to the most
            common input image shape. Defaults to 224.
        patch_size (int | tuple): The patch size in patch embedding.
            Defaults to 16.
        in_channels (int): The num of input channels. Defaults to 3.
        out_indices (Sequence | int): Output from which stages.
            Defaults to -1, means the last stage.
        drop_rate (float): Probability of an element to be zeroed.
            Defaults to 0.
        drop_path_rate (float): stochastic depth rate. Defaults to 0.
        qkv_bias (bool): Whether to add bias for qkv in attention modules.
            Defaults to True.
        norm_cfg (dict): Config dict for normalization layer.
            Defaults to ``dict(type='LN')``.
        final_norm (bool): Whether to add a additional layer to normalize
            final feature map. Defaults to True.
        with_cls_token (bool): Whether concatenating class token into image
            tokens as transformer input. Defaults to True.
        output_cls_token (bool): Whether output the cls_token. If set True,
            ``with_cls_token`` must be True. Defaults to True.
        use_layer_scale (bool): Whether to use layer_scale in  DeiT3.
            Defaults to True.
        interpolate_mode (str): Select the interpolate mode for position
            embeding vector resize. Defaults to "bicubic".
        patch_cfg (dict): Configs of patch embeding. Defaults to an empty dict.
        layer_cfgs (Sequence | dict): Configs of each transformer layer in
            encoder. Defaults to an empty dict.
        init_cfg (dict, optional): Initialization config dict.
            Defaults to None.
    """
    arch_zoo = {
        **dict.fromkeys(
            ['s', 'small'], {
                'embed_dims': 384,
                'num_layers': 12,
                'num_heads': 6,
                'feedforward_channels': 1536,
            }),
        **dict.fromkeys(
            ['m', 'medium'], {
                'embed_dims': 512,
                'num_layers': 12,
                'num_heads': 8,
                'feedforward_channels': 2048,
            }),
        **dict.fromkeys(
            ['b', 'base'], {
                'embed_dims': 768,
                'num_layers': 12,
                'num_heads': 12,
                'feedforward_channels': 3072
            }),
        **dict.fromkeys(
            ['l', 'large'], {
                'embed_dims': 1024,
                'num_layers': 24,
                'num_heads': 16,
                'feedforward_channels': 4096
            }),
        **dict.fromkeys(
            ['h', 'huge'], {
                'embed_dims': 1280,
                'num_layers': 32,
                'num_heads': 16,
                'feedforward_channels': 5120
            }),
    }
    # not using num_extra_tokens in deit3 because adding cls tokens after
    # adding pos_embed
    num_extra_tokens = 0

    def __init__(self,
                 arch='base',
                 img_size=224,
                 patch_size=16,
                 in_channels=3,
                 out_indices=-1,
                 drop_rate=0.,
                 drop_path_rate=0.,
                 qkv_bias=True,
                 norm_cfg=dict(type='LN', eps=1e-6),
                 final_norm=True,
                 with_cls_token=True,
                 output_cls_token=True,
                 use_layer_scale=True,
                 interpolate_mode='bicubic',
                 patch_cfg=dict(),
                 layer_cfgs=dict(),
                 frozen_stages=-1,
                 norm_eval=False,
                 init_cfg=None,
                 **kwargs):
        super(VisionTransformer, self).__init__(init_cfg)

        if isinstance(arch, str):
            arch = arch.lower()
            assert arch in set(self.arch_zoo), \
                f'Arch {arch} is not in default archs {set(self.arch_zoo)}'
            self.arch_settings = self.arch_zoo[arch]
        else:
            essential_keys = {
                'embed_dims', 'num_layers', 'num_heads', 'feedforward_channels'
            }
            assert isinstance(arch, dict) and essential_keys <= set(arch), \
                f'Custom arch needs a dict with keys {essential_keys}'
            self.arch_settings = arch

        self.embed_dims = self.arch_settings['embed_dims']
        self.num_layers = self.arch_settings['num_layers']
        self.img_size = to_2tuple(img_size)

        # Set patch embedding
        _patch_cfg = dict(
            in_channels=in_channels,
            input_size=img_size,
            embed_dims=self.embed_dims,
            conv_type='Conv2d',
            kernel_size=patch_size,
            stride=patch_size,
        )
        _patch_cfg.update(patch_cfg)
        self.patch_embed = PatchEmbed(**_patch_cfg)
        self.patch_resolution = self.patch_embed.init_out_size
        num_patches = self.patch_resolution[0] * self.patch_resolution[1]

        # Set cls token
        if output_cls_token:
            assert with_cls_token is True, f'with_cls_token must be True if' \
                f'set output_cls_token to True, but got {with_cls_token}'
        self.with_cls_token = with_cls_token
        self.output_cls_token = output_cls_token
        self.cls_token = nn.Parameter(torch.zeros(1, 1, self.embed_dims))

        # Set position embedding
        self.interpolate_mode = interpolate_mode
        self.pos_embed = nn.Parameter(
            torch.zeros(1, num_patches, self.embed_dims))
        self._register_load_state_dict_pre_hook(self._prepare_pos_embed)

        self.drop_after_pos = nn.Dropout(p=drop_rate)

        if isinstance(out_indices, int):
            out_indices = [out_indices]
        assert isinstance(out_indices, Sequence), \
            f'"out_indices" must by a sequence or int, ' \
            f'get {type(out_indices)} instead.'
        for i, index in enumerate(out_indices):
            if index < 0:
                out_indices[i] = self.num_layers + index
            assert 0 <= out_indices[i] <= self.num_layers, \
                f'Invalid out_indices {index}'
        self.out_indices = out_indices
        self.frozen_stages = frozen_stages
        self.norm_eval = norm_eval

        # stochastic depth decay rule
        dpr = np.linspace(0, drop_path_rate, self.num_layers)

        self.layers = ModuleList()
        if isinstance(layer_cfgs, dict):
            layer_cfgs = [layer_cfgs] * self.num_layers
        for i in range(self.num_layers):
            _layer_cfg = dict(
                embed_dims=self.embed_dims,
                num_heads=self.arch_settings['num_heads'],
                feedforward_channels=self.
                arch_settings['feedforward_channels'],
                drop_rate=drop_rate,
                drop_path_rate=dpr[i],
                qkv_bias=qkv_bias,
                norm_cfg=norm_cfg,
                use_layer_scale=use_layer_scale)
            _layer_cfg.update(layer_cfgs[i])
            self.layers.append(DeiT3TransformerEncoderLayer(**_layer_cfg))

        self.final_norm = final_norm
        if final_norm:
            self.norm1_name, norm1 = build_norm_layer(
                norm_cfg, self.embed_dims, postfix=1)
            self.add_module(self.norm1_name, norm1)

    def init_weights(self, pretrained=None):

        if pretrained is None:
            if self.init_cfg is None:
                for m in self.modules():
                    if isinstance(m, (nn.Linear)):
                        trunc_normal_init(m, std=0.02)
                    elif isinstance(m, (
                        nn.LayerNorm, nn.BatchNorm2d, nn.GroupNorm)):
                        constant_init(m, val=1, bias=0)
            # pos_embed & cls_token
            nn.init.trunc_normal_(self.pos_embed, mean=0, std=.02)
            nn.init.trunc_normal_(self.cls_token, mean=0, std=.02)

    def forward(self, x):
        B = x.shape[0]
        x, patch_resolution = self.patch_embed(x)

        x = x + resize_pos_embed(
            self.pos_embed,
            self.patch_resolution,
            patch_resolution,
            mode=self.interpolate_mode,
            num_extra_tokens=self.num_extra_tokens)
        x = self.drop_after_pos(x)

        # stole cls_tokens impl from Phil Wang, thanks
        cls_tokens = self.cls_token.expand(B, -1, -1)
        x = torch.cat((cls_tokens, x), dim=1)

        if not self.with_cls_token:
            # Remove class token for transformer encoder input
            x = x[:, 1:]

        outs = []
        for i, layer in enumerate(self.layers):
            x = layer(x)

            if i == len(self.layers) - 1 and self.final_norm:
                x = self.norm1(x)

            if i in self.out_indices:
                B, _, C = x.shape
                if self.with_cls_token:
                    patch_token = x[:, 1:].reshape(B, *patch_resolution, C)
                    patch_token = patch_token.permute(0, 3, 1, 2)
                    cls_token = x[:, 0]
                else:
                    patch_token = x.reshape(B, *patch_resolution, C)
                    patch_token = patch_token.permute(0, 3, 1, 2)
                    cls_token = None
                if self.output_cls_token:
                    out = [patch_token, cls_token]
                else:
                    out = patch_token
                outs.append(out)

        return outs
