"""Adapted from https://github.com/pytorch/vision/blob/master/torchvision/models/resnet.py.

The key change is that we remove the average pooling and fully connected layer.
"""

import torch
import torch.nn as nn
from torch.hub import load_state_dict_from_url
from torchvision.models.resnet import BasicBlock, Bottleneck, conv1x1


class ResNetFeatureExtractor(nn.Module):

    def __init__(self, block, layers, num_classes=1000, zero_init_residual=False,
                 groups=1, width_per_group=64, replace_stride_with_dilation=None,
                 norm_layer=None):
        super().__init__()
        if norm_layer is None:
            norm_layer = nn.BatchNorm2d
        self._norm_layer = norm_layer

        self.inplanes = 64
        self.dilation = 1
        if replace_stride_with_dilation is None:
            # each element in the tuple indicates if we should replace
            # the 2x2 stride with a dilated convolution instead
            replace_stride_with_dilation = [False, False, False]
        if len(replace_stride_with_dilation) != 3:
            raise ValueError("replace_stride_with_dilation should be None "
                             "or a 3-element tuple, got {}".format(replace_stride_with_dilation))
        self.groups = groups
        self.base_width = width_per_group
        self.conv1 = nn.Conv2d(3, self.inplanes, kernel_size=7, stride=2, padding=3,
                               bias=False)
        self.bn1 = norm_layer(self.inplanes)
        self.relu = nn.ReLU(inplace=True)
        self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)
        self.layer1 = self._make_layer(block, 64, layers[0])
        self.layer2 = self._make_layer(block, 128, layers[1], stride=2,
                                       dilate=replace_stride_with_dilation[0])
        self.layer3 = self._make_layer(block, 256, layers[2], stride=2,
                                       dilate=replace_stride_with_dilation[1])
        self.layer4 = self._make_layer(block, 512, layers[3], stride=2,
                                       dilate=replace_stride_with_dilation[2])
        self.avgpool = nn.AdaptiveAvgPool2d((1, 1))

        # TODO: To save memory, remove this and set strict to False
        self.fc = nn.Linear(512 * block.expansion, num_classes)

        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(
                    m.weight, mode='fan_out', nonlinearity='relu')
            elif isinstance(m, (nn.BatchNorm2d, nn.GroupNorm)):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)

        # Zero-initialize the last BN in each residual branch,
        # so that the residual branch starts with zeros, and each residual block behaves like an identity.
        # This improves the model by 0.2~0.3% according to https://arxiv.org/abs/1706.02677
        if zero_init_residual:
            for m in self.modules():
                if isinstance(m, Bottleneck):
                    nn.init.constant_(m.bn3.weight, 0)
                elif isinstance(m, BasicBlock):
                    nn.init.constant_(m.bn2.weight, 0)

    def _make_layer(self, block, planes, blocks, stride=1, dilate=False):
        norm_layer = self._norm_layer
        downsample = None
        previous_dilation = self.dilation
        if dilate:
            self.dilation *= stride
            stride = 1
        if stride != 1 or self.inplanes != planes * block.expansion:
            downsample = nn.Sequential(
                conv1x1(self.inplanes, planes * block.expansion, stride),
                norm_layer(planes * block.expansion),
            )

        layers = []
        layers.append(block(self.inplanes, planes, stride, downsample, self.groups,
                            self.base_width, previous_dilation, norm_layer))
        self.inplanes = planes * block.expansion
        for _ in range(1, blocks):
            layers.append(block(self.inplanes, planes, groups=self.groups,
                                base_width=self.base_width, dilation=self.dilation,
                                norm_layer=norm_layer))

        return nn.Sequential(*layers)

    def forward(self, x, pool=False):
        # x.shape == [B, 3, 224, 224]
        x = self.conv1(x)
        # x.shape == [B, 64, 112, 112]
        x = self.bn1(x)
        x = self.relu(x)
        x = self.maxpool(x)
        # x.shape == [B, 64, 56, 56]

        x = self.layer1(x)
        # x.shape == [B, 256, 56, 56]
        x = self.layer2(x)
        # x.shape == [B, 512, 28, 28]
        x = self.layer3(x)
        # x.shape == [B, 1024, 14, 14]
        x = self.layer4(x)
        # x.shape == [B, 2048, 7, 7]

        if pool:
            x = self.avgpool(x)
            # x.shape == [B, 2048, 1, 1]

            x = torch.flatten(x, 1)
            # x.shape == [B, 2048]

        return x


model_urls = {
    'resnext101_32x8d': 'https://download.pytorch.org/models/ig_resnext101_32x8-c38310e5.pth',
    'resnext101_32x16d': 'https://download.pytorch.org/models/ig_resnext101_32x16-c6f796b0.pth',
    'resnext101_32x32d': 'https://download.pytorch.org/models/ig_resnext101_32x32-e4b90b00.pth',
    'resnext101_32x48d': 'https://download.pytorch.org/models/ig_resnext101_32x48-3e41cc8a.pth',
    'resnet152': 'https://download.pytorch.org/models/resnet152-b121ed2d.pth',
}


def _resnext(arch, block, layers, pretrained, progress, **kwargs):
    model = ResNetFeatureExtractor(block, layers, **kwargs)
    state_dict = load_state_dict_from_url(model_urls[arch], progress=progress)
    model.load_state_dict(state_dict)
    return model


def resnext101_32x8d_wsl(progress=True, **kwargs):
    """Constructs a ResNeXt-101 32x8 model pre-trained on weakly-supervised data
    and finetuned on ImageNet from Figure 5 in
    `"Exploring the Limits of Weakly Supervised Pretraining" <https://arxiv.org/abs/1805.00932>`_
    Args:
        progress (bool): If True, displays a progress bar of the download to stderr.
    """
    kwargs['groups'] = 32
    kwargs['width_per_group'] = 8
    return _resnext('resnext101_32x8d', Bottleneck, [3, 4, 23, 3], True, progress, **kwargs)


def resnext101_32x16d_wsl(progress=True, **kwargs):
    """Constructs a ResNeXt-101 32x16 model pre-trained on weakly-supervised data
    and finetuned on ImageNet from Figure 5 in
    `"Exploring the Limits of Weakly Supervised Pretraining" <https://arxiv.org/abs/1805.00932>`_
    Args:
        progress (bool): If True, displays a progress bar of the download to stderr.
    """
    kwargs['groups'] = 32
    kwargs['width_per_group'] = 16
    return _resnext('resnext101_32x16d', Bottleneck, [3, 4, 23, 3], True, progress, **kwargs)


def resnext101_32x32d_wsl(progress=True, **kwargs):
    """Constructs a ResNeXt-101 32x32 model pre-trained on weakly-supervised data
    and finetuned on ImageNet from Figure 5 in
    `"Exploring the Limits of Weakly Supervised Pretraining" <https://arxiv.org/abs/1805.00932>`_
    Args:
        progress (bool): If True, displays a progress bar of the download to stderr.
    """
    kwargs['groups'] = 32
    kwargs['width_per_group'] = 32
    return _resnext('resnext101_32x32d', Bottleneck, [3, 4, 23, 3], True, progress, **kwargs)


def resnext101_32x48d_wsl(progress=True, **kwargs):
    """Constructs a ResNeXt-101 32x48 model pre-trained on weakly-supervised data
    and finetuned on ImageNet from Figure 5 in
    `"Exploring the Limits of Weakly Supervised Pretraining" <https://arxiv.org/abs/1805.00932>`_
    Args:
        progress (bool): If True, displays a progress bar of the download to stderr.
    """
    kwargs['groups'] = 32
    kwargs['width_per_group'] = 48
    return _resnext('resnext101_32x48d', Bottleneck, [3, 4, 23, 3], True, progress, **kwargs)


def resnet152(progress=True, **kwargs):
    r"""ResNet-152 model from
    `"Deep Residual Learning for Image Recognition" <https://arxiv.org/pdf/1512.03385.pdf>`_
    Args:
        pretrained (bool): If True, returns a model pre-trained on ImageNet
        progress (bool): If True, displays a progress bar of the download to stderr
    """
    return _resnext('resnet152', Bottleneck, [3, 8, 36, 3], True, progress,
                    **kwargs)
