# The implementation is adopted from Split-Attention Network, A New ResNet Variant,
# made publicly available under the Apache License 2.0 License
# at https://github.com/zhanghang1989/ResNeSt/blob/master/resnest/torch/models/splat.py
"""Split-Attention"""

import torch
import torch.nn.functional as F
from torch import nn
from torch.nn import BatchNorm2d, Conv2d, Linear, Module, ReLU
from torch.nn.modules.utils import _pair

__all__ = ['SplAtConv2d']


class SplAtConv2d(Module):
    """Split-Attention Conv2d
    """

    def __init__(self,
                 in_channels,
                 channels,
                 kernel_size,
                 stride=(1, 1),
                 padding=(0, 0),
                 dilation=(1, 1),
                 groups=1,
                 bias=True,
                 radix=2,
                 reduction_factor=4,
                 rectify=False,
                 rectify_avg=False,
                 norm_layer=None,
                 dropblock_prob=0.0,
                 **kwargs):
        super(SplAtConv2d, self).__init__()
        padding = _pair(padding)
        self.rectify = rectify and (padding[0] > 0 or padding[1] > 0)
        self.rectify_avg = rectify_avg
        inter_channels = max(in_channels * radix // reduction_factor, 32)
        self.radix = radix
        self.cardinality = groups
        self.channels = channels
        self.dropblock_prob = dropblock_prob
        if self.rectify:
            self.conv = Conv2d(
                in_channels,
                channels * radix,
                kernel_size,
                stride,
                padding,
                dilation,
                groups=groups * radix,
                bias=bias,
                **kwargs)
        else:
            self.conv = Conv2d(
                in_channels,
                channels * radix,
                kernel_size,
                stride,
                padding,
                dilation,
                groups=groups * radix,
                bias=bias,
                **kwargs)
        self.use_bn = norm_layer is not None
        if self.use_bn:
            self.bn0 = norm_layer(channels * radix)
        self.relu = ReLU(inplace=True)
        self.fc1 = Conv2d(channels, inter_channels, 1, groups=self.cardinality)
        if self.use_bn:
            self.bn1 = norm_layer(inter_channels)
        self.fc2 = Conv2d(
            inter_channels, channels * radix, 1, groups=self.cardinality)
        if dropblock_prob > 0.0:
            self.dropblock = DropBlock2D(dropblock_prob, 3)
        self.rsoftmax = rSoftMax(radix, groups)

    def forward(self, x):
        x = self.conv(x)
        if self.use_bn:
            x = self.bn0(x)
        if self.dropblock_prob > 0.0:
            x = self.dropblock(x)
        x = self.relu(x)

        batch, rchannel = x.shape[:2]
        if self.radix > 1:
            splited = torch.split(x, rchannel // self.radix, dim=1)
            gap = sum(splited)
        else:
            gap = x
        gap = F.adaptive_avg_pool2d(gap, 1)
        gap = self.fc1(gap)

        if self.use_bn:
            gap = self.bn1(gap)
        gap = self.relu(gap)

        atten = self.fc2(gap)
        atten = self.rsoftmax(atten).view(batch, -1, 1, 1)

        if self.radix > 1:
            attens = torch.split(atten, rchannel // self.radix, dim=1)
            out = sum([att * split for (att, split) in zip(attens, splited)])
        else:
            out = atten * x
        return out.contiguous()


class rSoftMax(nn.Module):

    def __init__(self, radix, cardinality):
        super().__init__()
        self.radix = radix
        self.cardinality = cardinality

    def forward(self, x):
        batch = x.size(0)
        if self.radix > 1:
            x = x.view(batch, self.cardinality, self.radix, -1).transpose(1, 2)
            x = F.softmax(x, dim=1)
            x = x.reshape(batch, -1)
        else:
            x = torch.sigmoid(x)
        return x