'''
EfficientNet in PyTorch.

Paper: "EfficientNet: Rethinking Model Scaling for Convolutional Neural Networks"
Reference: https://github.com/keras-team/keras-applications/blob/master/keras_applications/efficientnet.py

主要特点：
1. 使用MBConv作为基本模块，包含SE注意力机制
2. 通过复合缩放方法(compound scaling)同时调整网络的宽度、深度和分辨率
3. 使用Swish激活函数和DropConnect正则化
'''
import torch
import torch.nn as nn
import torch.nn.functional as F
import math

def swish(x):
    """Swish激活函数: x * sigmoid(x)"""
    return x * x.sigmoid()

def drop_connect(x, drop_ratio):
    """DropConnect正则化
    
    Args:
        x: 输入tensor
        drop_ratio: 丢弃率
    
    Returns:
        经过DropConnect处理的tensor
    """
    keep_ratio = 1.0 - drop_ratio
    mask = torch.empty([x.shape[0], 1, 1, 1], dtype=x.dtype, device=x.device)
    mask.bernoulli_(keep_ratio)
    x.div_(keep_ratio)
    x.mul_(mask)
    return x

class SE(nn.Module):
    '''Squeeze-and-Excitation注意力模块
    
    Args:
        in_channels: 输入通道数
        se_channels: SE模块中间层的通道数
    '''
    def __init__(self, in_channels, se_channels):
        super(SE, self).__init__()
        self.se1 = nn.Conv2d(in_channels, se_channels, kernel_size=1, bias=True)
        self.se2 = nn.Conv2d(se_channels, in_channels, kernel_size=1, bias=True)

    def forward(self, x):
        out = F.adaptive_avg_pool2d(x, (1, 1))  # 全局平均池化
        out = swish(self.se1(out))
        out = self.se2(out).sigmoid()
        return x * out  # 特征重标定

class MBConv(nn.Module):
    '''MBConv模块: Mobile Inverted Bottleneck Convolution
    
    Args:
        in_channels: 输入通道数
        out_channels: 输出通道数
        kernel_size: 卷积核大小
        stride: 步长
        expand_ratio: 扩展比率
        se_ratio: SE模块的压缩比率
        drop_rate: DropConnect的丢弃率
    '''
    def __init__(self,
                 in_channels,
                 out_channels,
                 kernel_size,
                 stride,
                 expand_ratio=1,
                 se_ratio=0.25,
                 drop_rate=0.):
        super(MBConv, self).__init__()
        self.stride = stride
        self.drop_rate = drop_rate
        self.expand_ratio = expand_ratio

        # Expansion phase
        channels = expand_ratio * in_channels
        self.conv1 = nn.Conv2d(in_channels, channels, kernel_size=1, stride=1, padding=0, bias=False)
        self.bn1 = nn.BatchNorm2d(channels)

        # Depthwise conv
        self.conv2 = nn.Conv2d(channels, channels, kernel_size=kernel_size, stride=stride,
                              padding=(1 if kernel_size == 3 else 2), groups=channels, bias=False)
        self.bn2 = nn.BatchNorm2d(channels)

        # SE layers
        se_channels = int(in_channels * se_ratio)
        self.se = SE(channels, se_channels)

        # Output phase
        self.conv3 = nn.Conv2d(channels, out_channels, kernel_size=1, stride=1, padding=0, bias=False)
        self.bn3 = nn.BatchNorm2d(out_channels)

        # Shortcut connection
        self.has_skip = (stride == 1) and (in_channels == out_channels)

    def forward(self, x):
        # Expansion
        out = x if self.expand_ratio == 1 else swish(self.bn1(self.conv1(x)))
        # Depthwise convolution
        out = swish(self.bn2(self.conv2(out)))
        # Squeeze-and-excitation
        out = self.se(out)
        # Pointwise convolution
        out = self.bn3(self.conv3(out))
        # Shortcut
        if self.has_skip:
            if self.training and self.drop_rate > 0:
                out = drop_connect(out, self.drop_rate)
            out = out + x
        return out

class EfficientNet(nn.Module):
    '''EfficientNet模型
    
    Args:
        width_coefficient: 宽度系数
        depth_coefficient: 深度系数
        dropout_rate: 分类层的dropout率
        num_classes: 分类数量
    '''
    def __init__(self,
                 width_coefficient=1.0,
                 depth_coefficient=1.0,
                 dropout_rate=0.2,
                 num_classes=10):
        super(EfficientNet, self).__init__()
        
        # 模型配置
        cfg = {
            'num_blocks': [1, 2, 2, 3, 3, 4, 1],  # 每个stage的block数量
            'expansion': [1, 6, 6, 6, 6, 6, 6],    # 扩展比率
            'out_channels': [16, 24, 40, 80, 112, 192, 320],  # 输出通道数
            'kernel_size': [3, 3, 5, 3, 5, 5, 3],  # 卷积核大小
            'stride': [1, 2, 2, 2, 1, 2, 1],       # 步长
            'dropout_rate': dropout_rate,
            'drop_connect_rate': 0.2,
        }
        
        self.cfg = cfg
        self.width_coefficient = width_coefficient
        self.depth_coefficient = depth_coefficient

        # Stem layer
        self.conv1 = nn.Conv2d(3, 32, kernel_size=3, stride=1, padding=1, bias=False)
        self.bn1 = nn.BatchNorm2d(32)

        # Build blocks
        self.layers = self._make_layers(in_channels=32)

        # Head layer
        final_channels = cfg['out_channels'][-1] * int(width_coefficient)
        self.linear = nn.Linear(final_channels, num_classes)

    def _make_layers(self, in_channels):
        layers = []
        cfg = [self.cfg[k] for k in ['expansion', 'out_channels', 'num_blocks', 'kernel_size', 'stride']]
        blocks = sum(self.cfg['num_blocks'])
        b = 0  # 用于计算drop_connect_rate
        
        for expansion, out_channels, num_blocks, kernel_size, stride in zip(*cfg):
            out_channels = int(out_channels * self.width_coefficient)
            num_blocks = int(math.ceil(num_blocks * self.depth_coefficient))
            
            for i in range(num_blocks):
                stride_i = stride if i == 0 else 1
                drop_rate = self.cfg['drop_connect_rate'] * b / blocks
                layers.append(
                    MBConv(in_channels,
                          out_channels,
                          kernel_size,
                          stride_i,
                          expansion,
                          se_ratio=0.25,
                          drop_rate=drop_rate))
                in_channels = out_channels
                b += 1
                
        return nn.Sequential(*layers)

    def forward(self, x):
        # Stem
        out = swish(self.bn1(self.conv1(x)))
        # Blocks
        out = self.layers(out)
        # Head
        out = F.adaptive_avg_pool2d(out, 1)
        out = out.view(out.size(0), -1)
        if self.training and self.cfg['dropout_rate'] > 0:
            out = F.dropout(out, p=self.cfg['dropout_rate'])
        out = self.linear(out)
        return out

def EfficientNetB0(num_classes=10):
    """EfficientNet-B0"""
    return EfficientNet(width_coefficient=1.0,
                       depth_coefficient=1.0,
                       dropout_rate=0.2,
                       num_classes=num_classes)

def EfficientNetB1(num_classes=10):
    """EfficientNet-B1"""
    return EfficientNet(width_coefficient=1.0,
                       depth_coefficient=1.1,
                       dropout_rate=0.2,
                       num_classes=num_classes)

def EfficientNetB2(num_classes=10):
    """EfficientNet-B2"""
    return EfficientNet(width_coefficient=1.1,
                       depth_coefficient=1.2,
                       dropout_rate=0.3,
                       num_classes=num_classes)

def EfficientNetB3(num_classes=10):
    """EfficientNet-B3"""
    return EfficientNet(width_coefficient=1.2,
                       depth_coefficient=1.4,
                       dropout_rate=0.3,
                       num_classes=num_classes)

def EfficientNetB4(num_classes=10):
    """EfficientNet-B4"""
    return EfficientNet(width_coefficient=1.4,
                       depth_coefficient=1.8,
                       dropout_rate=0.4,
                       num_classes=num_classes)

def EfficientNetB5(num_classes=10):
    """EfficientNet-B5"""
    return EfficientNet(width_coefficient=1.6,
                       depth_coefficient=2.2,
                       dropout_rate=0.4,
                       num_classes=num_classes)

def EfficientNetB6(num_classes=10):
    """EfficientNet-B6"""
    return EfficientNet(width_coefficient=1.8,
                       depth_coefficient=2.6,
                       dropout_rate=0.5,
                       num_classes=num_classes)

def EfficientNetB7(num_classes=10):
    """EfficientNet-B7"""
    return EfficientNet(width_coefficient=2.0,
                       depth_coefficient=3.1,
                       dropout_rate=0.5,
                       num_classes=num_classes)

def test():
    """测试函数"""
    net = EfficientNetB0()
    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))

if __name__ == '__main__':
    test()