'''
ShuffleNet in PyTorch.

ShuffleNet是一个专门为移动设备设计的高效卷积神经网络。其主要创新点在于使用了两个新操作:
1. 逐点组卷积(pointwise group convolution)
2. 通道重排(channel shuffle)
这两个操作大大降低了计算复杂度,同时保持了良好的准确率。

主要特点:
1. 使用组卷积减少参数量和计算量
2. 使用通道重排操作使不同组之间的信息可以流通
3. 使用深度可分离卷积进一步降低计算复杂度
4. 设计了多个计算复杂度版本以适应不同的设备

Reference:
[1] Xiangyu Zhang, Xinyu Zhou, Mengxiao Lin, Jian Sun
    ShuffleNet: An Extremely Efficient Convolutional Neural Network for Mobile Devices. CVPR 2018.
'''
import torch
import torch.nn as nn
import torch.nn.functional as F


class ShuffleBlock(nn.Module):
    """通道重排模块
    
    通过重新排列通道的顺序来实现不同组之间的信息交流。
    
    Args:
        groups (int): 分组数量
    """
    def __init__(self, groups):
        super(ShuffleBlock, self).__init__()
        self.groups = groups

    def forward(self, x):
        """通道重排的前向传播
        
        步骤:
        1. [N,C,H,W] -> [N,g,C/g,H,W]  # 重塑为g组
        2. [N,g,C/g,H,W] -> [N,C/g,g,H,W]  # 转置g维度
        3. [N,C/g,g,H,W] -> [N,C,H,W]  # 重塑回原始形状
        
        Args:
            x: 输入张量,[N,C,H,W]
            
        Returns:
            out: 通道重排后的张量,[N,C,H,W]
        """
        N, C, H, W = x.size()
        g = self.groups
        return x.view(N,g,C//g,H,W).permute(0,2,1,3,4).reshape(N,C,H,W)


class Bottleneck(nn.Module):
    """ShuffleNet的基本模块
    
    结构:
    x -> 1x1 GConv -> BN -> Shuffle -> 3x3 DWConv -> BN -> 1x1 GConv -> BN -> (+) -> ReLU
         |---------------------|
         
    Args:
        in_channels (int): 输入通道数
        out_channels (int): 输出通道数
        stride (int): 步长,用于下采样
        groups (int): 组卷积的分组数
    """
    def __init__(self, in_channels, out_channels, stride, groups):
        super(Bottleneck, self).__init__()
        self.stride = stride
        
        # 确定中间通道数和分组数
        mid_channels = out_channels // 4
        g = 1 if in_channels == 24 else groups
        
        # 第一个1x1组卷积
        self.conv1 = nn.Conv2d(in_channels, mid_channels, 
                              kernel_size=1, groups=g, bias=False)
        self.bn1 = nn.BatchNorm2d(mid_channels)
        self.shuffle1 = ShuffleBlock(groups=g)
        
        # 3x3深度可分离卷积
        self.conv2 = nn.Conv2d(mid_channels, mid_channels,
                              kernel_size=3, stride=stride, padding=1, 
                              groups=mid_channels, bias=False)
        self.bn2 = nn.BatchNorm2d(mid_channels)
        
        # 第二个1x1组卷积
        self.conv3 = nn.Conv2d(mid_channels, out_channels,
                              kernel_size=1, groups=groups, bias=False)
        self.bn3 = nn.BatchNorm2d(out_channels)

        # 残差连接
        self.shortcut = nn.Sequential()
        if stride == 2:
            self.shortcut = nn.Sequential(
                nn.AvgPool2d(3, stride=2, padding=1)
            )

    def forward(self, x):
        # 主分支
        out = F.relu(self.bn1(self.conv1(x)))
        out = self.shuffle1(out)
        out = F.relu(self.bn2(self.conv2(out)))
        out = self.bn3(self.conv3(out))
        
        # 残差连接
        res = self.shortcut(x)
        
        # 如果是下采样层,拼接残差;否则相加
        out = F.relu(torch.cat([out, res], 1)) if self.stride == 2 else F.relu(out + res)
        return out


class ShuffleNet(nn.Module):
    """ShuffleNet模型
    
    网络结构:
    1. 一个卷积层进行特征提取
    2. 三个阶段,每个阶段包含多个带重排的残差块
    3. 平均池化和全连接层进行分类
    
    Args:
        cfg (dict): 配置字典,包含:
            - out_channels (list): 每个阶段的输出通道数
            - num_blocks (list): 每个阶段的块数
            - groups (int): 组卷积的分组数
    """
    def __init__(self, cfg):
        super(ShuffleNet, self).__init__()
        out_channels = cfg['out_channels']
        num_blocks = cfg['num_blocks']
        groups = cfg['groups']

        # 第一层卷积
        self.conv1 = nn.Conv2d(3, 24, kernel_size=1, bias=False)
        self.bn1 = nn.BatchNorm2d(24)
        self.in_channels = 24
        
        # 三个阶段
        self.layer1 = self._make_layer(out_channels[0], num_blocks[0], groups)
        self.layer2 = self._make_layer(out_channels[1], num_blocks[1], groups)
        self.layer3 = self._make_layer(out_channels[2], num_blocks[2], groups)
        
        # 分类层
        self.avg_pool = nn.AdaptiveAvgPool2d(1)
        self.classifier = nn.Linear(out_channels[2], 10)
        
        # 初始化权重
        self._initialize_weights()

    def _make_layer(self, out_channels, num_blocks, groups):
        """构建ShuffleNet的一个阶段
        
        Args:
            out_channels (int): 输出通道数
            num_blocks (int): 块的数量
            groups (int): 分组数
            
        Returns:
            nn.Sequential: 一个阶段的层序列
        """
        layers = []
        for i in range(num_blocks):
            stride = 2 if i == 0 else 1
            cat_channels = self.in_channels if i == 0 else 0
            layers.append(
                Bottleneck(
                    self.in_channels, 
                    out_channels - cat_channels,
                    stride=stride, 
                    groups=groups
                )
            )
            self.in_channels = out_channels
        return nn.Sequential(*layers)

    def forward(self, x):
        """前向传播
        
        Args:
            x: 输入张量,[N,3,32,32]
            
        Returns:
            out: 输出张量,[N,num_classes]
        """
        # 特征提取
        out = F.relu(self.bn1(self.conv1(x)))
        
        # 三个阶段
        out = self.layer1(out)
        out = self.layer2(out)
        out = self.layer3(out)
        
        # 分类
        out = self.avg_pool(out)
        out = out.view(out.size(0), -1)
        out = self.classifier(out)
        return out
    
    def _initialize_weights(self):
        """初始化模型权重
        
        采用kaiming初始化方法:
        - 卷积层权重采用kaiming_normal_初始化
        - BN层参数采用常数初始化
        - 线性层采用正态分布初始化
        """
        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 ShuffleNetG2():
    """返回groups=2的ShuffleNet模型"""
    cfg = {
        'out_channels': [200,400,800],
        'num_blocks': [4,8,4],
        'groups': 2
    }
    return ShuffleNet(cfg)


def ShuffleNetG3():
    """返回groups=3的ShuffleNet模型"""
    cfg = {
        'out_channels': [240,480,960],
        'num_blocks': [4,8,4],
        'groups': 3
    }
    return ShuffleNet(cfg)


def test():
    """测试函数"""
    # 创建模型
    net = ShuffleNetG2()
    print('Model Structure:')
    print(net)
    
    # 测试前向传播
    x = torch.randn(1,3,32,32)
    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, (1,3,32,32))


if __name__ == '__main__':
    test()