from abc import abstractmethod

import torch
import torch.nn as nn
from mmcv.cnn import ConvModule, bias_init_with_prob, normal_init

from mmdet.core import force_fp32, multi_apply
from ..builder import HEADS, build_loss
from .base_dense_head import BaseDenseHead


@HEADS.register_module()
class AnchorFreeHead(BaseDenseHead):
    """Anchor-free head (FCOS, Fovea, RepPoints, etc.).

    Args:
        num_classes (int): Number of categories excluding the background
            category.
        in_channels (int): Number of channels in the input feature map.
        feat_channels (int): Number of hidden channels. Used in child classes.
        stacked_convs (int): Number of stacking convs of the head.
        strides (tuple): Downsample factor of each feature map.
        dcn_on_last_conv (bool): If true, use dcn in the last layer of
            towers. Default: False.
        conv_bias (bool | str): If specified as `auto`, it will be decided by
            the norm_cfg. Bias of conv will be set as True if `norm_cfg` is
            None, otherwise False. Default: "auto".
        background_label (int | None): Label ID of background, set as 0 for
            RPN and num_classes for other heads. It will automatically set as
            num_classes if None is given.
        loss_cls (dict): Config of classification loss.
        loss_bbox (dict): Config of localization loss.
        conv_cfg (dict): Config dict for convolution layer. Default: None.
        norm_cfg (dict): Config dict for normalization layer. Default: None.
        train_cfg (dict): Training config of anchor head.
        test_cfg (dict): Testing config of anchor head.
    """  # noqa: W605

    _version = 1

    def __init__(self,
                 num_classes,
                 in_channels,
                 feat_channels=256,
                 stacked_convs=4,
                 strides=(4, 8, 16, 32, 64),
                 dcn_on_last_conv=False,
                 conv_bias='auto',
                 background_label=None,
                 loss_cls=dict(
                     type='FocalLoss',
                     use_sigmoid=True,
                     gamma=2.0,
                     alpha=0.25,
                     loss_weight=1.0),
                 loss_bbox=dict(type='IoULoss', loss_weight=1.0),
                 conv_cfg=None,
                 norm_cfg=None,
                 train_cfg=None,
                 test_cfg=None):
        super(AnchorFreeHead, self).__init__()
        self.num_classes = num_classes
        self.cls_out_channels = num_classes
        self.in_channels = in_channels
        self.feat_channels = feat_channels
        self.stacked_convs = stacked_convs
        self.strides = strides
        self.dcn_on_last_conv = dcn_on_last_conv
        assert conv_bias == 'auto' or isinstance(conv_bias, bool)
        self.conv_bias = conv_bias
        self.loss_cls = build_loss(loss_cls)
        self.loss_bbox = build_loss(loss_bbox)
        self.train_cfg = train_cfg
        self.test_cfg = test_cfg
        self.conv_cfg = conv_cfg
        self.norm_cfg = norm_cfg
        self.fp16_enabled = False
        self.background_label = (
            num_classes if background_label is None else background_label)
        # background_label should be either 0 or num_classes
        assert (self.background_label == 0
                or self.background_label == num_classes)

        self._init_layers()

    def _init_layers(self):
        """Initialize layers of the head."""
        self._init_cls_convs()
        self._init_reg_convs()
        self._init_predictor()

    def _init_cls_convs(self):
        """Initialize classification conv layers of the head."""
        self.cls_convs = nn.ModuleList()
        for i in range(self.stacked_convs):
            chn = self.in_channels if i == 0 else self.feat_channels
            if self.dcn_on_last_conv and i == self.stacked_convs - 1:
                conv_cfg = dict(type='DCNv2')
            else:
                conv_cfg = self.conv_cfg
            self.cls_convs.append(
                ConvModule(
                    chn,
                    self.feat_channels,
                    3,
                    stride=1,
                    padding=1,
                    conv_cfg=conv_cfg,
                    norm_cfg=self.norm_cfg,
                    bias=self.conv_bias))

    def _init_reg_convs(self):
        """Initialize bbox regression conv layers of the head."""
        self.reg_convs = nn.ModuleList()
        for i in range(self.stacked_convs):
            chn = self.in_channels if i == 0 else self.feat_channels
            if self.dcn_on_last_conv and i == self.stacked_convs - 1:
                conv_cfg = dict(type='DCNv2')
            else:
                conv_cfg = self.conv_cfg
            self.reg_convs.append(
                ConvModule(
                    chn,
                    self.feat_channels,
                    3,
                    stride=1,
                    padding=1,
                    conv_cfg=conv_cfg,
                    norm_cfg=self.norm_cfg,
                    bias=self.conv_bias))

    def _init_predictor(self):
        """Initialize predictor layers of the head."""
        self.conv_cls = nn.Conv2d(
            self.feat_channels, self.cls_out_channels, 3, padding=1)
        self.conv_reg = nn.Conv2d(self.feat_channels, 4, 3, padding=1)

    def init_weights(self):
        """Initialize weights of the head."""
        for m in self.cls_convs:
            if isinstance(m.conv, nn.Conv2d):
                normal_init(m.conv, std=0.01)
        for m in self.reg_convs:
            if isinstance(m.conv, nn.Conv2d):
                normal_init(m.conv, std=0.01)
        bias_cls = bias_init_with_prob(0.01)
        normal_init(self.conv_cls, std=0.01, bias=bias_cls)
        normal_init(self.conv_reg, std=0.01)

    def _load_from_state_dict(self, state_dict, prefix, local_metadata, strict,
                              missing_keys, unexpected_keys, error_msgs):
        """Hack some keys of the model state dict so that can load checkpoints
        of previous version."""
        version = local_metadata.get('version', None)
        if version is None:
            # the key is different in early versions
            # for example, 'fcos_cls' become 'conv_cls' now
            bbox_head_keys = [
                k for k in state_dict.keys() if k.startswith(prefix)
            ]
            ori_predictor_keys = []
            new_predictor_keys = []
            # e.g. 'fcos_cls' or 'fcos_reg'
            for key in bbox_head_keys:
                ori_predictor_keys.append(key)
                key = key.split('.')
                conv_name = None
                if key[1].endswith('cls'):
                    conv_name = 'conv_cls'
                elif key[1].endswith('reg'):
                    conv_name = 'conv_reg'
                elif key[1].endswith('centerness'):
                    conv_name = 'conv_centerness'
                else:
                    assert NotImplementedError
                if conv_name is not None:
                    key[1] = conv_name
                    new_predictor_keys.append('.'.join(key))
                else:
                    ori_predictor_keys.pop(-1)
            for i in range(len(new_predictor_keys)):
                state_dict[new_predictor_keys[i]] = state_dict.pop(
                    ori_predictor_keys[i])
        super()._load_from_state_dict(state_dict, prefix, local_metadata,
                                      strict, missing_keys, unexpected_keys,
                                      error_msgs)

    def forward(self, feats):
        """Forward features from the upstream network.

        Args:
            feats (tuple[Tensor]): Features from the upstream network, each is
                a 4D-tensor.

        Returns:
            tuple: Usually contain classification scores and bbox predictions.
                cls_scores (list[Tensor]): Box scores for each scale level,
                    each is a 4D-tensor, the channel number is
                    num_points * num_classes.
                bbox_preds (list[Tensor]): Box energies / deltas for each scale
                    level, each is a 4D-tensor, the channel number is
                    num_points * 4.
        """
        return multi_apply(self.forward_single, feats)[:2]

    def forward_single(self, x):
        """Forward features of a single scale levle.

        Args:
            x (Tensor): FPN feature maps of the specified stride.

        Returns:
            tuple: Scores for each class, bbox predictions, features
                after classification and regression conv layers, some
                models needs these features like FCOS.
        """
        cls_feat = x
        reg_feat = x

        for cls_layer in self.cls_convs:
            cls_feat = cls_layer(cls_feat)
        cls_score = self.conv_cls(cls_feat)

        for reg_layer in self.reg_convs:
            reg_feat = reg_layer(reg_feat)
        bbox_pred = self.conv_reg(reg_feat)
        return cls_score, bbox_pred, cls_feat, reg_feat

    @abstractmethod
    @force_fp32(apply_to=('cls_scores', 'bbox_preds'))
    def loss(self,
             cls_scores,
             bbox_preds,
             gt_bboxes,
             gt_labels,
             img_metas,
             gt_bboxes_ignore=None):
        """Compute loss of the head.

        Args:
            cls_scores (list[Tensor]): Box scores for each scale level,
                each is a 4D-tensor, the channel number is
                num_points * num_classes.
            bbox_preds (list[Tensor]): Box energies / deltas for each scale
                level, each is a 4D-tensor, the channel number is
                num_points * 4.
            gt_bboxes (list[Tensor]): Ground truth bboxes for each image with
                shape (num_gts, 4) in [tl_x, tl_y, br_x, br_y] format.
            gt_labels (list[Tensor]): class indices corresponding to each box
            img_metas (list[dict]): Meta information of each image, e.g.,
                image size, scaling factor, etc.
            gt_bboxes_ignore (None | list[Tensor]): specify which bounding
                boxes can be ignored when computing the loss.
        """

        raise NotImplementedError

    @abstractmethod
    @force_fp32(apply_to=('cls_scores', 'bbox_preds'))
    def get_bboxes(self,
                   cls_scores,
                   bbox_preds,
                   img_metas,
                   cfg=None,
                   rescale=None):
        """ Transform network output for a batch into bbox predictions.

        Args:
            cls_scores (list[Tensor]): Box scores for each scale level
                Has shape (N, num_points * num_classes, H, W)
            bbox_preds (list[Tensor]): Box energies / deltas for each scale
                level with shape (N, num_points * 4, H, W)
            img_metas (list[dict]): Meta information of each image, e.g.,
                image size, scaling factor, etc.
            cfg (mmcv.Config): Test / postprocessing configuration,
                if None, test_cfg would be used
            rescale (bool): If True, return boxes in original image space
        """

        raise NotImplementedError

    @abstractmethod
    def get_targets(self, points, gt_bboxes_list, gt_labels_list):
        """Compute regression, classification and centerss targets for points
            in multiple images.

        Args:
            points (list[Tensor]): Points of each fpn level, each has shape
                (num_points, 2).
            gt_bboxes_list (list[Tensor]): Ground truth bboxes of each image,
                each has shape (num_gt, 4).
            gt_labels_list (list[Tensor]): Ground truth labels of each box,
                each has shape (num_gt,).
        """
        raise NotImplementedError

    def _get_points_single(self,
                           featmap_size,
                           stride,
                           dtype,
                           device,
                           flatten=False):
        """Get points of a single scale level."""
        h, w = featmap_size
        x_range = torch.arange(w, dtype=dtype, device=device)
        y_range = torch.arange(h, dtype=dtype, device=device)
        y, x = torch.meshgrid(y_range, x_range)
        if flatten:
            y = y.flatten()
            x = x.flatten()
        return y, x

    def get_points(self, featmap_sizes, dtype, device, flatten=False):
        """Get points according to feature map sizes.

        Args:
            featmap_sizes (list[tuple]): Multi-level feature map sizes.
            dtype (torch.dtype): Type of points.
            device (torch.device): Device of points.

        Returns:
            tuple: points of each image.
        """
        mlvl_points = []
        for i in range(len(featmap_sizes)):
            mlvl_points.append(
                self._get_points_single(featmap_sizes[i], self.strides[i],
                                        dtype, device, flatten))
        return mlvl_points
