""" Global Context Attention Block

Paper: `GCNet: Non-local Networks Meet Squeeze-Excitation Networks and Beyond`
    - https://arxiv.org/abs/1904.11492

Official code consulted as reference: https://github.com/xvjiarui/GCNet

Hacked together by / Copyright 2021 Ross Wightman
"""
from typing import Optional, Tuple, Type, Union

from torch import nn as nn
import torch.nn.functional as F

from .create_act import create_act_layer, get_act_layer
from .helpers import make_divisible
from .mlp import ConvMlp
from .norm import LayerNorm2d


class GlobalContext(nn.Module):

    def __init__(
            self,
            channels: int,
            use_attn: bool = True,
            fuse_add: bool = False,
            fuse_scale: bool = True,
            init_last_zero: bool = False,
            rd_ratio: float = 1./8,
            rd_channels: Optional[int] = None,
            rd_divisor: int = 1,
            act_layer: Type[nn.Module] = nn.ReLU,
            gate_layer: Union[str, Type[nn.Module]] = 'sigmoid',
            device=None,
            dtype=None
    ):
        dd = {'device': device, 'dtype': dtype}
        super().__init__()
        act_layer = get_act_layer(act_layer)

        self.conv_attn = nn.Conv2d(channels, 1, kernel_size=1, bias=True, **dd) if use_attn else None

        if rd_channels is None:
            rd_channels = make_divisible(channels * rd_ratio, rd_divisor, round_limit=0.)
        if fuse_add:
            self.mlp_add = ConvMlp(channels, rd_channels, act_layer=act_layer, norm_layer=LayerNorm2d, **dd)
        else:
            self.mlp_add = None
        if fuse_scale:
            self.mlp_scale = ConvMlp(channels, rd_channels, act_layer=act_layer, norm_layer=LayerNorm2d, **dd)
        else:
            self.mlp_scale = None

        self.gate = create_act_layer(gate_layer)
        self.init_last_zero = init_last_zero

        self.reset_parameters()

    def reset_parameters(self):
        if self.conv_attn is not None:
            nn.init.kaiming_normal_(self.conv_attn.weight, mode='fan_in', nonlinearity='relu')
        if self.mlp_add is not None:
            nn.init.zeros_(self.mlp_add.fc2.weight)

    def forward(self, x):
        B, C, H, W = x.shape

        if self.conv_attn is not None:
            attn = self.conv_attn(x).reshape(B, 1, H * W)  # (B, 1, H * W)
            attn = F.softmax(attn, dim=-1).unsqueeze(3)  # (B, 1, H * W, 1)
            context = x.reshape(B, C, H * W).unsqueeze(1) @ attn
            context = context.view(B, C, 1, 1)
        else:
            context = x.mean(dim=(2, 3), keepdim=True)

        if self.mlp_scale is not None:
            mlp_x = self.mlp_scale(context)
            x = x * self.gate(mlp_x)
        if self.mlp_add is not None:
            mlp_x = self.mlp_add(context)
            x = x + mlp_x

        return x