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#
# 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
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#    http://www.apache.org/licenses/LICENSE-2.0
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# distributed under the License is distributed on an "AS IS" BASIS,
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import paddle
import paddle.nn as nn
import paddle.nn.functional as F

from .layers import make_norm, Conv3x3, CBAM
from .stanet import Backbone, Decoder


class DSAMNet(nn.Layer):
    """
    The DSAMNet implementation based on PaddlePaddle.

    The original article refers to
        Q. Shi, et al., "A Deeply Supervised Attention Metric-Based Network and an 
        Open Aerial Image Dataset for Remote Sensing Change Detection"
        (https://ieeexplore.ieee.org/document/9467555).

    Note that this implementation differs from the original work in two aspects:
    1. We do not use multiple dilation rates in layer 4 of the ResNet backbone.
    2. A classification head is used in place of the original metric learning-based 
        head to stablize the training process.

    Args:
        in_channels (int): Number of bands of the input images.
        num_classes (int): Number of target classes.
        ca_ratio (int, optional): Channel reduction ratio for the channel 
            attention module. Default: 8.
        sa_kernel (int, optional): Size of the convolutional kernel used in the 
            spatial attention module. Default: 7.
    """

    def __init__(self, in_channels, num_classes, ca_ratio=8, sa_kernel=7):
        super(DSAMNet, self).__init__()

        WIDTH = 64

        self.backbone = Backbone(
            in_ch=in_channels, arch='resnet18', strides=(1, 1, 2, 2, 1))
        self.decoder = Decoder(WIDTH)

        self.cbam1 = CBAM(64, ratio=ca_ratio, kernel_size=sa_kernel)
        self.cbam2 = CBAM(64, ratio=ca_ratio, kernel_size=sa_kernel)

        self.dsl2 = DSLayer(64, num_classes, 32, stride=2, output_padding=1)
        self.dsl3 = DSLayer(128, num_classes, 32, stride=4, output_padding=3)

        self.conv_out = nn.Sequential(
            Conv3x3(
                WIDTH, WIDTH, norm=True, act=True),
            Conv3x3(WIDTH, num_classes))

        self.init_weight()

    def forward(self, t1, t2):
        f1 = self.backbone(t1)
        f2 = self.backbone(t2)

        y1 = self.decoder(f1)
        y2 = self.decoder(f2)

        y1 = self.cbam1(y1)
        y2 = self.cbam2(y2)

        out = paddle.abs(y1 - y2)
        out = F.interpolate(
            out, size=paddle.shape(t1)[2:], mode='bilinear', align_corners=True)
        pred = self.conv_out(out)

        if not self.training:
            return [pred]
        else:
            ds2 = self.dsl2(paddle.abs(f1[0] - f2[0]))
            ds3 = self.dsl3(paddle.abs(f1[1] - f2[1]))
            return [pred, ds2, ds3]

    def init_weight(self):
        pass


class DSLayer(nn.Sequential):
    def __init__(self, in_ch, out_ch, itm_ch, **convd_kwargs):
        super(DSLayer, self).__init__(
            nn.Conv2DTranspose(
                in_ch, itm_ch, kernel_size=3, padding=1, **convd_kwargs),
            make_norm(itm_ch),
            nn.ReLU(),
            nn.Dropout2D(p=0.2),
            nn.Conv2DTranspose(
                itm_ch, out_ch, kernel_size=3, padding=1))