"""
DenseNet in pytorch
see the details in papaer
[1] Gao Huang, Zhuang Liu, Laurens van der Maaten, Kilian Q. Weinberger.
    Densely Connected Convolutional Networks
    https://arxiv.org/abs/1608.06993v5
"""
import torch
import torch.nn as nn
import math

class Bottleneck(nn.Module):
    """
    Dense Block
    这里的growth_rate=out_channels, 就是每个Block自己输出的通道数。
    先通过1x1卷积层，将通道数缩小为4 * growth_rate，然后再通过3x3卷积层降低到growth_rate。
    """
    # 通常1×1卷积的通道数为GrowthRate的4倍
    expansion = 4
    
    def __init__(self, in_channels, growth_rate):
        super(Bottleneck, self).__init__()
        zip_channels = self.expansion * growth_rate
        self.features = nn.Sequential(
            nn.BatchNorm2d(in_channels),
            nn.ReLU(True),
            nn.Conv2d(in_channels, zip_channels, kernel_size=1, bias=False),
            nn.BatchNorm2d(zip_channels),
            nn.ReLU(True),
            nn.Conv2d(zip_channels, growth_rate, kernel_size=3, padding=1, bias=False)
        )
        
    def forward(self, x):
        out = self.features(x)
        out = torch.cat([out, x], 1)
        return out        


class Transition(nn.Module):
    """
    改变维数的Transition层 具体包括BN、ReLU、1×1卷积（Conv）、2×2平均池化操作
    先通过1x1的卷积层减少channels，再通过2x2的平均池化层缩小feature-map
    """
    # 1×1卷积的作用是降维，起到压缩模型的作用，而平均池化则是降低特征图的尺寸。
    def __init__(self, in_channels, out_channels):
        super(Transition, self).__init__()
        self.features = nn.Sequential(
            nn.BatchNorm2d(in_channels),
            nn.ReLU(True),
            nn.Conv2d(in_channels, out_channels, kernel_size=1, bias=False),
            nn.AvgPool2d(2)
        )
        
    def forward(self, x):
        out = self.features(x)
        return out

class DenseNet(nn.Module):
    """
    Dense Net
    paper中growth_rate取12，维度压缩的参数θ，即reduction取0.5
    且初始化方法为kaiming_normal()
    num_blocks为每段网络中的DenseBlock数量
    DenseNet和ResNet一样也是六段式网络（一段卷积+四段Dense+平均池化层），最后FC层。
    第一段将维数从3变到2 * growth_rate
    
    (3, 32, 32) -> [Conv2d] -> (24, 32, 32) -> [layer1] -> (48, 16, 16) -> [layer2]
  ->(96, 8, 8) -> [layer3] -> (192, 4, 4) -> [layer4] -> (384, 4, 4) -> [AvgPool]
  ->(384, 1, 1) -> [Linear] -> (10)
    """
    def __init__(self, num_blocks, growth_rate=12, reduction=0.5, num_classes=10, init_weights=True):
        super(DenseNet, self).__init__()
        self.growth_rate = growth_rate
        self.reduction = reduction
        
        num_channels = 2 * growth_rate
        
        self.features = nn.Conv2d(3, num_channels, kernel_size=3, padding=1, bias=False)
        self.layer1, num_channels = self._make_dense_layer(num_channels, num_blocks[0])
        self.layer2, num_channels = self._make_dense_layer(num_channels, num_blocks[1])
        self.layer3, num_channels = self._make_dense_layer(num_channels, num_blocks[2])
        self.layer4, num_channels = self._make_dense_layer(num_channels, num_blocks[3], transition=False)
        self.avg_pool = nn.Sequential(
            nn.BatchNorm2d(num_channels),
            nn.ReLU(True),
            nn.AvgPool2d(4),
        )
        self.classifier = nn.Linear(num_channels, num_classes)
        
        if init_weights:
            self._initialize_weights()
        
    def _make_dense_layer(self, in_channels, nblock, transition=True):
        layers = []
        for i in range(nblock):
            layers += [Bottleneck(in_channels, self.growth_rate)]
            in_channels += self.growth_rate
        out_channels = in_channels
        if transition:
            out_channels = int(math.floor(in_channels * self.reduction))
            layers += [Transition(in_channels, out_channels)]
        return nn.Sequential(*layers), out_channels
    
    def _initialize_weights(self):
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(m.weight, mode='fan_out', nonlinearity='relu')
                if m.bias is not None:
                    nn.init.constant_(m.bias, 0)
            elif isinstance(m, nn.BatchNorm2d):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)
            elif isinstance(m, nn.Linear):
                nn.init.normal_(m.weight, 0, 0.01)
                nn.init.constant_(m.bias, 0)
    
    def forward(self, x):
        out = self.features(x)
        out = self.layer1(out)
        out = self.layer2(out)
        out = self.layer3(out)
        out = self.layer4(out)
        out = self.avg_pool(out)
        out = out.view(out.size(0), -1)
        out = self.classifier(out)
        return out

def DenseNet121():
    return DenseNet([6,12,24,16], growth_rate=32)

def DenseNet169():
    return DenseNet([6,12,32,32], growth_rate=32)

def DenseNet201():
    return DenseNet([6,12,48,32], growth_rate=32)

def DenseNet161():
    return DenseNet([6,12,36,24], growth_rate=48)

def densenet_cifar():
    return DenseNet([6,12,24,16], growth_rate=12)


def test():
    net = densenet_cifar()
    x = torch.randn(1,3,32,32)
    y = net(x)
    print(y.size())
    from torchinfo import summary
    device = 'cuda' if torch.cuda.is_available() else 'cpu'
    net = net.to(device)
    summary(net,(1,3,32,32))