import torch
import torch.nn as nn
import torch.nn.functional as F


class TaskAlignedAssigner(nn.Module):
    def __init__(self,
                 topk=10,
                 num_classes=80,
                 alpha=0.5,
                 beta=6.0, 
                 eps=1e-9):
        super(TaskAlignedAssigner, self).__init__()
        self.topk = topk
        self.num_classes = num_classes
        self.bg_idx = num_classes
        self.alpha = alpha
        self.beta = beta
        self.eps = eps

    @torch.no_grad()
    def forward(self,
                pd_scores,
                pd_bboxes,
                anc_points,
                gt_labels,
                gt_bboxes):
        """This code referenced to
           https://github.com/Nioolek/PPYOLOE_pytorch/blob/master/ppyoloe/assigner/tal_assigner.py
        Args:
            pd_scores (Tensor): shape(bs, num_total_anchors, num_classes)
            pd_bboxes (Tensor): shape(bs, num_total_anchors, 4)
            anc_points (Tensor): shape(num_total_anchors, 2)
            gt_labels (Tensor): shape(bs, n_max_boxes, 1)
            gt_bboxes (Tensor): shape(bs, n_max_boxes, 4)
        Returns:
            target_labels (Tensor): shape(bs, num_total_anchors)
            target_bboxes (Tensor): shape(bs, num_total_anchors, 4)
            target_scores (Tensor): shape(bs, num_total_anchors, num_classes)
            fg_mask (Tensor): shape(bs, num_total_anchors)
        """
        self.bs = pd_scores.size(0)
        self.n_max_boxes = gt_bboxes.size(1)

        mask_pos, align_metric, overlaps = self.get_pos_mask(
            pd_scores, pd_bboxes, gt_labels, gt_bboxes, anc_points)

        target_gt_idx, fg_mask, mask_pos = select_highest_overlaps(
            mask_pos, overlaps, self.n_max_boxes)

        # assigned target
        target_labels, target_bboxes, target_scores = self.get_targets(
            gt_labels, gt_bboxes, target_gt_idx, fg_mask)

        # normalize
        align_metric *= mask_pos
        pos_align_metrics = align_metric.max(axis=-1, keepdim=True)[0]
        pos_overlaps = (overlaps * mask_pos).max(axis=-1, keepdim=True)[0]
        norm_align_metric = (align_metric * pos_overlaps / (pos_align_metrics + self.eps)).max(-2)[0].unsqueeze(-1)
        target_scores = target_scores * norm_align_metric

        return target_labels, target_bboxes, target_scores, fg_mask.bool()


    def get_pos_mask(self,
                     pd_scores,
                     pd_bboxes,
                     gt_labels,
                     gt_bboxes,
                     anc_points):

        # get anchor_align metric
        align_metric, overlaps = self.get_box_metrics(pd_scores, pd_bboxes, gt_labels, gt_bboxes)
        # get in_gts mask
        mask_in_gts = select_candidates_in_gts(anc_points, gt_bboxes)
        # get topk_metric mask
        mask_topk = self.select_topk_candidates(align_metric * mask_in_gts)
        # merge all mask to a final mask
        mask_pos = mask_topk * mask_in_gts

        return mask_pos, align_metric, overlaps


    def get_box_metrics(self,
                        pd_scores,
                        pd_bboxes,
                        gt_labels,
                        gt_bboxes):

        pd_scores = pd_scores.permute(0, 2, 1)
        gt_labels = gt_labels.long()
        ind = torch.zeros([2, self.bs, self.n_max_boxes], dtype=torch.long)
        ind[0] = torch.arange(end=self.bs).view(-1, 1).repeat(1, self.n_max_boxes)
        ind[1] = gt_labels.squeeze(-1)
        bbox_scores = pd_scores[ind[0], ind[1]]

        overlaps = iou_calculator(gt_bboxes, pd_bboxes)
        align_metric = bbox_scores.pow(self.alpha) * overlaps.pow(self.beta)

        return align_metric, overlaps


    def select_topk_candidates(self, metrics, largest=True):
        num_anchors = metrics.shape[-1]
        topk_metrics, topk_idxs = torch.topk(
            metrics, self.topk, axis=-1, largest=largest)
        topk_mask = (topk_metrics.max(axis=-1, keepdim=True)[0] > self.eps).tile(
            [1, 1, self.topk])
        topk_idxs = torch.where(topk_mask, topk_idxs, torch.zeros_like(topk_idxs))
        is_in_topk = F.one_hot(topk_idxs, num_anchors).sum(axis=-2)
        is_in_topk = torch.where(is_in_topk > 1,
            torch.zeros_like(is_in_topk), is_in_topk)
        return is_in_topk.to(metrics.dtype)


    def get_targets(self,
                    gt_labels,
                    gt_bboxes,
                    target_gt_idx,
                    fg_mask):

        # assigned target labels
        batch_ind = torch.arange(end=self.bs, dtype=torch.int64, device=gt_labels.device)[...,None]
        target_gt_idx = target_gt_idx + batch_ind * self.n_max_boxes
        target_labels = gt_labels.long().flatten()[target_gt_idx]

        # assigned target boxes
        target_bboxes = gt_bboxes.reshape([-1, 4])[target_gt_idx]

        # assigned target scores
        target_labels[target_labels<0] = 0
        target_scores = F.one_hot(target_labels, self.num_classes)
        fg_scores_mask  = fg_mask[:, :, None].repeat(1, 1, self.num_classes)
        target_scores = torch.where(fg_scores_mask > 0, target_scores,
                                        torch.full_like(target_scores, 0))

        return target_labels, target_bboxes, target_scores
    

def select_candidates_in_gts(xy_centers, gt_bboxes, eps=1e-9):
    """select the positive anchors's center in gt
    Args:
        xy_centers (Tensor): shape(bs*n_max_boxes, num_total_anchors, 4)
        gt_bboxes (Tensor): shape(bs, n_max_boxes, 4)
    Return:
        (Tensor): shape(bs, n_max_boxes, num_total_anchors)
    """
    n_anchors = xy_centers.size(0)
    bs, n_max_boxes, _ = gt_bboxes.size()
    _gt_bboxes = gt_bboxes.reshape([-1, 4])
    xy_centers = xy_centers.unsqueeze(0).repeat(bs * n_max_boxes, 1, 1)
    gt_bboxes_lt = _gt_bboxes[:, 0:2].unsqueeze(1).repeat(1, n_anchors, 1)
    gt_bboxes_rb = _gt_bboxes[:, 2:4].unsqueeze(1).repeat(1, n_anchors, 1)
    b_lt = xy_centers - gt_bboxes_lt
    b_rb = gt_bboxes_rb - xy_centers
    bbox_deltas = torch.cat([b_lt, b_rb], dim=-1)
    bbox_deltas = bbox_deltas.reshape([bs, n_max_boxes, n_anchors, -1])
    return (bbox_deltas.min(axis=-1)[0] > eps).to(gt_bboxes.dtype)


def select_highest_overlaps(mask_pos, overlaps, n_max_boxes):
    """if an anchor box is assigned to multiple gts,
        the one with the highest iou will be selected.
    Args:
        mask_pos (Tensor): shape(bs, n_max_boxes, num_total_anchors)
        overlaps (Tensor): shape(bs, n_max_boxes, num_total_anchors)
    Return:
        target_gt_idx (Tensor): shape(bs, num_total_anchors)
        fg_mask (Tensor): shape(bs, num_total_anchors)
        mask_pos (Tensor): shape(bs, n_max_boxes, num_total_anchors)
    """
    fg_mask = mask_pos.sum(axis=-2)
    if fg_mask.max() > 1:
        mask_multi_gts = (fg_mask.unsqueeze(1) > 1).repeat([1, n_max_boxes, 1])
        max_overlaps_idx = overlaps.argmax(axis=1)
        is_max_overlaps = F.one_hot(max_overlaps_idx, n_max_boxes)
        is_max_overlaps = is_max_overlaps.permute(0, 2, 1).to(overlaps.dtype)
        mask_pos = torch.where(mask_multi_gts, is_max_overlaps, mask_pos)
        fg_mask = mask_pos.sum(axis=-2)
    target_gt_idx = mask_pos.argmax(axis=-2)
    return target_gt_idx, fg_mask , mask_pos


def iou_calculator(box1, box2, eps=1e-9):
    """Calculate iou for batch
    Args:
        box1 (Tensor): shape(bs, n_max_boxes, 1, 4)
        box2 (Tensor): shape(bs, 1, num_total_anchors, 4)
    Return:
        (Tensor): shape(bs, n_max_boxes, num_total_anchors)
    """
    box1 = box1.unsqueeze(2)  # [N, M1, 4] -> [N, M1, 1, 4]
    box2 = box2.unsqueeze(1)  # [N, M2, 4] -> [N, 1, M2, 4]
    px1y1, px2y2 = box1[:, :, :, 0:2], box1[:, :, :, 2:4]
    gx1y1, gx2y2 = box2[:, :, :, 0:2], box2[:, :, :, 2:4]
    x1y1 = torch.maximum(px1y1, gx1y1)
    x2y2 = torch.minimum(px2y2, gx2y2)
    overlap = (x2y2 - x1y1).clip(0).prod(-1)
    area1 = (px2y2 - px1y1).clip(0).prod(-1)
    area2 = (gx2y2 - gx1y1).clip(0).prod(-1)
    union = area1 + area2 - overlap + eps

    return overlap / union