# Residual Dense Network for Image Super-Resolution
# https://arxiv.org/abs/1802.08797

import torch
import torch.nn as nn


class RDB_Conv(nn.Module):
    def __init__(self, inChannels, growRate, kSize=3):
        super(RDB_Conv, self).__init__()
        Cin = inChannels
        G = growRate
        self.conv = nn.Sequential(*[
            nn.Conv2d(Cin, G, kSize, padding=(kSize - 1) // 2, stride=1),
            nn.ReLU()
        ])

    def forward(self, x):
        out = self.conv(x)
        return torch.cat((x, out), 1)


class RDB(nn.Module):
    def __init__(self, growRate0, growRate, nConvLayers, kSize=3):
        super(RDB, self).__init__()
        G0 = growRate0
        G = growRate
        C = nConvLayers

        convs = []
        for c in range(C):
            convs.append(RDB_Conv(G0 + c * G, G))
        self.convs = nn.Sequential(*convs)

        # Local Feature Fusion
        self.LFF = nn.Conv2d(G0 + C * G, G0, 1, padding=0, stride=1)

    def forward(self, x):
        return self.LFF(self.convs(x)) * 0.2 + x


class RDN(nn.Module):
    def __init__(self, in_channels, out_channels, num_features, num_blocks, num_layers, upscale_factor):
        super(RDN, self).__init__()
        r = upscale_factor
        G0 = 32
        kSize = 3

        # number of RDB blocks, conv layers, out channels
        self.D, C, G = [num_blocks, num_layers, num_features]

        # Shallow feature extraction net
        self.SFENet1 = nn.Conv2d(in_channels, G0, kSize, padding=(kSize - 1) // 2, stride=1)
        self.SFENet2 = nn.Conv2d(G0, G0, kSize, padding=(kSize - 1) // 2, stride=1)

        # Redidual dense blocks and dense feature fusion
        self.RDBs = nn.ModuleList()
        for i in range(self.D):
            self.RDBs.append(
                RDB(growRate0=G0, growRate=G, nConvLayers=C)
            )

        # Global Feature Fusion
        self.GFF = nn.Sequential(*[
            nn.Conv2d(self.D * G0, G0, 1, padding=0, stride=1),
            nn.Conv2d(G0, G0, kSize, padding=(kSize - 1) // 2, stride=1)
        ])

        # Up-sampling net
        if r == 2 or r == 3:
            self.UPNet = nn.Sequential(*[
                nn.Conv2d(G0, G * r * r, kSize, padding=(kSize - 1) // 2, stride=1),
                nn.PixelShuffle(r),
                nn.Conv2d(G, out_channels, kSize, padding=(kSize - 1) // 2, stride=1)
            ])
        elif r == 4:
            self.UPNet = nn.Sequential(*[
                nn.Conv2d(G0, G * 4, kSize, padding=(kSize - 1) // 2, stride=1),
                nn.PixelShuffle(2),
                nn.Conv2d(G, G * 4, kSize, padding=(kSize - 1) // 2, stride=1),
                nn.PixelShuffle(2),
                nn.Conv2d(G, out_channels, kSize, padding=(kSize - 1) // 2, stride=1)
            ])
        else:
            raise ValueError("scale must be 2 or 3 or 4.")

    def forward(self, x):
        f__1 = self.SFENet1(x)
        x = self.SFENet2(f__1)

        RDBs_out = []
        for i in range(self.D):
            x = self.RDBs[i](x)
            RDBs_out.append(x)

        x = self.GFF(torch.cat(RDBs_out, 1))
        x += f__1

        return self.UPNet(x)
