# Copyright (c) 2021 PaddlePaddle Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import paddle
import paddle.nn as nn
import paddle.nn.functional as F

from ppdet.core.workspace import register, serializable, load_config
from ppdet.core.workspace import create
from ppdet.utils.logger import setup_logger
from ppdet.modeling import ops
from ppdet.utils.checkpoint import load_pretrain_weight
from ppdet.modeling.losses import YOLOv3Loss
logger = setup_logger(__name__)


class DistillModel(nn.Layer):
    def __init__(self, cfg, slim_cfg):
        super(DistillModel, self).__init__()

        self.student_model = create(cfg.architecture)
        logger.debug('Load student model pretrain_weights:{}'.format(
            cfg.pretrain_weights))
        load_pretrain_weight(self.student_model, cfg.pretrain_weights)

        slim_cfg = load_config(slim_cfg)
        self.teacher_model = create(slim_cfg.architecture)
        self.distill_loss = create(slim_cfg.distill_loss)
        logger.debug('Load teacher model pretrain_weights:{}'.format(
            slim_cfg.pretrain_weights))
        load_pretrain_weight(self.teacher_model, slim_cfg.pretrain_weights)

        for param in self.teacher_model.parameters():
            param.trainable = False

    def parameters(self):
        return self.student_model.parameters()

    def forward(self, inputs):
        if self.training:
            teacher_loss = self.teacher_model(inputs)
            student_loss = self.student_model(inputs)
            loss = self.distill_loss(self.teacher_model, self.student_model)
            student_loss['distill_loss'] = loss
            student_loss['teacher_loss'] = teacher_loss['loss']
            student_loss['loss'] += student_loss['distill_loss']
            return student_loss
        else:
            return self.student_model(inputs)


@register
class DistillYOLOv3Loss(nn.Layer):
    def __init__(self, weight=1000):
        super(DistillYOLOv3Loss, self).__init__()
        self.weight = weight

    def obj_weighted_reg(self, sx, sy, sw, sh, tx, ty, tw, th, tobj):
        loss_x = ops.sigmoid_cross_entropy_with_logits(sx, F.sigmoid(tx))
        loss_y = ops.sigmoid_cross_entropy_with_logits(sy, F.sigmoid(ty))
        loss_w = paddle.abs(sw - tw)
        loss_h = paddle.abs(sh - th)
        loss = paddle.add_n([loss_x, loss_y, loss_w, loss_h])
        weighted_loss = paddle.mean(loss * F.sigmoid(tobj))
        return weighted_loss

    def obj_weighted_cls(self, scls, tcls, tobj):
        loss = ops.sigmoid_cross_entropy_with_logits(scls, F.sigmoid(tcls))
        weighted_loss = paddle.mean(paddle.multiply(loss, F.sigmoid(tobj)))
        return weighted_loss

    def obj_loss(self, sobj, tobj):
        obj_mask = paddle.cast(tobj > 0., dtype="float32")
        obj_mask.stop_gradient = True
        loss = paddle.mean(
            ops.sigmoid_cross_entropy_with_logits(sobj, obj_mask))
        return loss

    def forward(self, teacher_model, student_model):
        teacher_distill_pairs = teacher_model.yolo_head.loss.distill_pairs
        student_distill_pairs = student_model.yolo_head.loss.distill_pairs
        distill_reg_loss, distill_cls_loss, distill_obj_loss = [], [], []
        for s_pair, t_pair in zip(student_distill_pairs, teacher_distill_pairs):
            distill_reg_loss.append(
                self.obj_weighted_reg(s_pair[0], s_pair[1], s_pair[2], s_pair[
                    3], t_pair[0], t_pair[1], t_pair[2], t_pair[3], t_pair[4]))
            distill_cls_loss.append(
                self.obj_weighted_cls(s_pair[5], t_pair[5], t_pair[4]))
            distill_obj_loss.append(self.obj_loss(s_pair[4], t_pair[4]))
        distill_reg_loss = paddle.add_n(distill_reg_loss)
        distill_cls_loss = paddle.add_n(distill_cls_loss)
        distill_obj_loss = paddle.add_n(distill_obj_loss)
        loss = (distill_reg_loss + distill_cls_loss + distill_obj_loss
                ) * self.weight
        return loss
