"""Split-Attention"""

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

from cvpods.layers import Conv2d, get_norm


__all__ = ['SplAtConv2d']


class RFConv2d(object):
    def __init__(self, *args, **kwargs):
        raise NotImplementedError


class DropBlock2D(RFConv2d):
    pass


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=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 = RFConv2d(in_channels,
                                 channels * radix,
                                 kernel_size,
                                 stride,
                                 padding,
                                 dilation,
                                 groups=groups * radix,
                                 bias=bias,
                                 average_mode=rectify_avg,
                                 **kwargs)
        else:
            self.conv = Conv2d(in_channels,
                               channels * radix,
                               kernel_size,
                               stride,
                               padding,
                               dilation,
                               groups=groups * radix,
                               bias=bias,
                               **kwargs)
        self.use_bn = norm is not None
        self.bn0 = get_norm(norm, channels * radix)
        self.relu = ReLU(inplace=True)
        self.fc1 = Conv2d(channels, inter_channels, 1, groups=self.cardinality)
        self.bn1 = get_norm(norm, inter_channels)
        self.fc2 = Conv2d(inter_channels, channels * radix, 1, groups=self.cardinality)
        if dropblock_prob > 0.0:
            self.dropblock = DropBlock2D(dropblock_prob, 3)

    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, channel = x.shape[:2]
        if self.radix > 1:
            splited = torch.split(x, channel // 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).view((batch, self.radix, self.channels))
        if self.radix > 1:
            atten = F.softmax(atten, dim=1).view(batch, -1, 1, 1)
        else:
            atten = F.sigmoid(atten, dim=1).view(batch, -1, 1, 1)

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