# Copyright (c) OpenMMLab. All rights reserved.
import torch
import torch.nn as nn
import torch.nn.functional as F

from mmdet.registry import MODELS


def ae_loss_per_image(tl_preds, br_preds, match):
    """Associative Embedding Loss in one image.

    Associative Embedding Loss including two parts: pull loss and push loss.
    Pull loss makes embedding vectors from same object closer to each other.
    Push loss distinguish embedding vector from different objects, and makes
        the gap between them is large enough.

    During computing, usually there are 3 cases:
        - no object in image: both pull loss and push loss will be 0.
        - one object in image: push loss will be 0 and pull loss is computed
            by the two corner of the only object.
        - more than one objects in image: pull loss is computed by corner pairs
            from each object, push loss is computed by each object with all
            other objects. We use confusion matrix with 0 in diagonal to
            compute the push loss.

    Args:
        tl_preds (tensor): Embedding feature map of left-top corner.
        br_preds (tensor): Embedding feature map of bottim-right corner.
        match (list): Downsampled coordinates pair of each ground truth box.
    """

    tl_list, br_list, me_list = [], [], []
    if len(match) == 0:  # no object in image
        pull_loss = tl_preds.sum() * 0.
        push_loss = tl_preds.sum() * 0.
    else:
        for m in match:
            [tl_y, tl_x], [br_y, br_x] = m
            tl_e = tl_preds[:, tl_y, tl_x].view(-1, 1)
            br_e = br_preds[:, br_y, br_x].view(-1, 1)
            tl_list.append(tl_e)
            br_list.append(br_e)
            me_list.append((tl_e + br_e) / 2.0)

        tl_list = torch.cat(tl_list)
        br_list = torch.cat(br_list)
        me_list = torch.cat(me_list)

        assert tl_list.size() == br_list.size()

        # N is object number in image, M is dimension of embedding vector
        N, M = tl_list.size()

        pull_loss = (tl_list - me_list).pow(2) + (br_list - me_list).pow(2)
        pull_loss = pull_loss.sum() / N

        margin = 1  # exp setting of CornerNet, details in section 3.3 of paper

        # confusion matrix of push loss
        conf_mat = me_list.expand((N, N, M)).permute(1, 0, 2) - me_list
        conf_weight = 1 - torch.eye(N).type_as(me_list)
        conf_mat = conf_weight * (margin - conf_mat.sum(-1).abs())

        if N > 1:  # more than one object in current image
            push_loss = F.relu(conf_mat).sum() / (N * (N - 1))
        else:
            push_loss = tl_preds.sum() * 0.

    return pull_loss, push_loss


@MODELS.register_module()
class AssociativeEmbeddingLoss(nn.Module):
    """Associative Embedding Loss.

    More details can be found in
    `Associative Embedding <https://arxiv.org/abs/1611.05424>`_ and
    `CornerNet <https://arxiv.org/abs/1808.01244>`_ .
    Code is modified from `kp_utils.py <https://github.com/princeton-vl/CornerNet/blob/master/models/py_utils/kp_utils.py#L180>`_  # noqa: E501

    Args:
        pull_weight (float): Loss weight for corners from same object.
        push_weight (float): Loss weight for corners from different object.
    """

    def __init__(self, pull_weight=0.25, push_weight=0.25):
        super(AssociativeEmbeddingLoss, self).__init__()
        self.pull_weight = pull_weight
        self.push_weight = push_weight

    def forward(self, pred, target, match):
        """Forward function."""
        batch = pred.size(0)
        pull_all, push_all = 0.0, 0.0
        for i in range(batch):
            pull, push = ae_loss_per_image(pred[i], target[i], match[i])

            pull_all += self.pull_weight * pull
            push_all += self.push_weight * push

        return pull_all, push_all
