'''
VGG Networks in PyTorch

VGG是由牛津大学Visual Geometry Group提出的一个深度卷积神经网络模型。
主要特点:
1. 使用小卷积核(3x3)代替大卷积核,降低参数量
2. 深层网络结构,多个卷积层叠加
3. 使用多个3x3卷积层的组合来代替大的感受野
4. 结构规整,易于扩展

网络结构示例(VGG16):
input
  └─> [(Conv3x3, 64) × 2, MaxPool] 
      └─> [(Conv3x3, 128) × 2, MaxPool]
          └─> [(Conv3x3, 256) × 3, MaxPool]
              └─> [(Conv3x3, 512) × 3, MaxPool]
                  └─> [(Conv3x3, 512) × 3, MaxPool]
                      └─> [AvgPool, Flatten]
                          └─> FC(512, num_classes)

参考论文:
[1] K. Simonyan and A. Zisserman, "Very Deep Convolutional Networks for Large-Scale Image Recognition," 
    arXiv preprint arXiv:1409.1556, 2014.
'''

import torch
import torch.nn as nn


# VGG配置参数
# M表示MaxPool层,数字表示输出通道数
cfg = {
    'VGG11': [64, 'M', 128, 'M', 256, 256, 'M', 512, 512, 'M', 512, 512, 'M'],
    'VGG13': [64, 64, 'M', 128, 128, 'M', 256, 256, 'M', 512, 512, 'M', 512, 512, 'M'],
    'VGG16': [64, 64, 'M', 128, 128, 'M', 256, 256, 256, 'M', 512, 512, 512, 'M', 512, 512, 512, 'M'],
    'VGG19': [64, 64, 'M', 128, 128, 'M', 256, 256, 256, 256, 'M', 512, 512, 512, 512, 'M', 512, 512, 512, 512, 'M'],
}


class ConvBlock(nn.Module):
    """VGG的基本卷积块
    
    包含: Conv2d -> BatchNorm -> ReLU
    使用3x3卷积核,步长为1,padding为1以保持特征图大小不变
    
    Args:
        in_channels (int): 输入通道数
        out_channels (int): 输出通道数
        batch_norm (bool): 是否使用BatchNorm,默认为True
    """
    def __init__(self, in_channels, out_channels, batch_norm=True):
        super(ConvBlock, self).__init__()
        
        layers = []
        # 3x3卷积,padding=1保持特征图大小不变
        layers.append(
            nn.Conv2d(
                in_channels=in_channels,
                out_channels=out_channels,
                kernel_size=3,
                stride=1,
                padding=1
            )
        )
        
        # 添加BatchNorm
        if batch_norm:
            layers.append(nn.BatchNorm2d(out_channels))
            
        # ReLU激活函数
        layers.append(nn.ReLU(inplace=True))
        
        self.block = nn.Sequential(*layers)
        
    def forward(self, x):
        """前向传播
        
        Args:
            x (torch.Tensor): 输入特征图
            
        Returns:
            torch.Tensor: 输出特征图
        """
        return self.block(x)


class VGG(nn.Module):
    """VGG网络模型
    
    Args:
        vgg_name (str): VGG变体名称,可选VGG11/13/16/19
        num_classes (int): 分类数量,默认为10
        batch_norm (bool): 是否使用BatchNorm,默认为True
        init_weights (bool): 是否初始化权重,默认为True
    """
    def __init__(self, vgg_name='VGG16', num_classes=10, batch_norm=True, init_weights=True):
        super(VGG, self).__init__()
        
        # 特征提取层
        self.features = self._make_layers(cfg[vgg_name], batch_norm)
        
        # 全局平均池化
        self.avgpool = nn.AdaptiveAvgPool2d((1, 1))
        
        # 分类器
        self.classifier = nn.Sequential(
            nn.Linear(512, num_classes)
        )
        
        # 初始化权重
        if init_weights:
            self._initialize_weights()
    
    def _make_layers(self, cfg, batch_norm=True):
        """构建VGG的特征提取层
        
        Args:
            cfg (List): 网络配置参数
            batch_norm (bool): 是否使用BatchNorm
            
        Returns:
            nn.Sequential: 特征提取层序列
        """
        layers = []
        in_channels = 3
        
        for x in cfg:
            if x == 'M':  # 最大池化层
                layers.append(nn.MaxPool2d(kernel_size=2, stride=2))
            else:  # 卷积块
                layers.append(ConvBlock(in_channels, x, batch_norm))
                in_channels = x
                
        return nn.Sequential(*layers)

    def forward(self, x):
        """前向传播
        
        Args:
            x (torch.Tensor): 输入图像张量,[N,3,H,W]
            
        Returns:
            torch.Tensor: 输出预测张量,[N,num_classes]
        """
        # 特征提取
        x = self.features(x)
        
        # 全局平均池化
        x = self.avgpool(x)
        
        # 展平
        x = torch.flatten(x, 1)
        
        # 分类
        x = self.classifier(x)
        return x
    
    def _initialize_weights(self):
        """初始化模型权重
        
        采用论文中的初始化方法:
        - 卷积层: xavier初始化
        - BatchNorm: weight=1, bias=0
        - 线性层: 正态分布初始化(std=0.01)
        """
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                # VGG论文中使用了xavier初始化
                nn.init.xavier_normal_(m.weight)
                if m.bias is not None:
                    nn.init.zeros_(m.bias)
            elif isinstance(m, nn.BatchNorm2d):
                nn.init.ones_(m.weight)
                nn.init.zeros_(m.bias)
            elif isinstance(m, nn.Linear):
                nn.init.normal_(m.weight, 0, 0.01)
                nn.init.zeros_(m.bias)


def test():
    """测试函数
    
    创建VGG模型并进行前向传播测试,打印模型结构和参数信息
    """
    # 创建模型
    net = VGG('VGG16')
    print('Model Structure:')
    print(net)
    
    # 测试前向传播
    x = torch.randn(2, 3, 32, 32)
    y = net(x)
    print('\nInput Shape:', x.shape)
    print('Output Shape:', y.shape)
    
    # 打印模型信息
    from torchinfo import summary
    device = 'cuda' if torch.cuda.is_available() else 'cpu'
    net = net.to(device)
    summary(net, (2, 3, 32, 32))


if __name__ == '__main__':
    test()