"""Adapted from https://github.com/lucidrains/denoising-diffusion-pytorch"""
import math
from collections import namedtuple
from functools import partial
from typing import List, Optional

import torch
import torch.nn.functional as F
from einops import rearrange
from torch import einsum, nn, Tensor

from trainers.utils import default, exists

# constants

ModelPrediction = namedtuple('ModelPrediction', ['pred_noise', 'pred_x_start'])

# helpers functions


def l2norm(t: Tensor) -> Tensor:
    """L2 normalize along last dimension"""
    return F.normalize(t, dim=-1)


# small helper modules


class Residual(nn.Module):
    """Residual of any Module -> x' = f(x) + x"""
    def __init__(self, fn: nn.Module):
        super().__init__()
        self.fn = fn

    def forward(self, x, *args, **kwargs):
        return self.fn(x, *args, **kwargs) + x


def Upsample(dim: int, dim_out: Optional[int] = None) -> nn.Sequential:
    """UpsampleConv with factor 2"""
    return nn.Sequential(
        nn.Upsample(scale_factor=2, mode='nearest'),
        nn.Conv2d(dim, default(dim_out, dim), 3, padding=1)
    )


def Downsample(dim: int, dim_out: Optional[int] = None) -> nn.Conv2d:
    """Strided Conv2d for downsampling"""
    return nn.Conv2d(dim, default(dim_out, dim), 4, 2, 1)


class LayerNorm(nn.Module):
    def __init__(self, dim: int):
        super().__init__()
        self.g = nn.Parameter(torch.ones(1, dim, 1, 1))

    def forward(self, x: Tensor) -> Tensor:
        eps = 1e-5 if x.dtype == torch.float32 else 1e-3
        var = torch.var(x, dim=1, unbiased=False, keepdim=True)
        mean = torch.mean(x, dim=1, keepdim=True)
        return (x - mean) * (var + eps).rsqrt() * self.g


class PreNorm(nn.Module):
    """Apply LayerNorm before any Module"""
    def __init__(self, dim: int, fn: nn.Module):
        super().__init__()
        self.fn = fn
        self.norm = LayerNorm(dim)

    def forward(self, x: Tensor) -> Tensor:
        x = self.norm(x)
        return self.fn(x)


class SinusoidalPosEmb(nn.Module):
    """Classical sinosoidal embedding"""
    def __init__(self, dim: int):
        super().__init__()
        self.dim = dim

    def forward(self, t: Tensor) -> Tensor:
        """
        :param t: Batch of time steps (b,)
        :return emb: Sinusoidal time embedding (b, dim)
        """
        device = t.device
        half_dim = self.dim // 2
        emb = math.log(10000) / (half_dim - 1)
        emb = torch.exp(torch.arange(half_dim, device=device) * -emb)
        emb = t[:, None] * emb[None, :]
        emb = torch.cat((emb.sin(), emb.cos()), dim=-1)
        return emb


class LearnedSinusoidalPosEmb(nn.Module):
    """ following @crowsonkb 's lead with learned sinusoidal pos emb """
    """ https://github.com/crowsonkb/v-diffusion-jax/blob/master/diffusion/models/danbooru_128.py#L8 """
    def __init__(self, dim: int):
        super().__init__()
        assert (dim % 2) == 0
        half_dim = dim // 2
        self.weights = nn.Parameter(torch.randn(half_dim))

    def forward(self, t: Tensor) -> Tensor:
        """
        :param t: Batch of time steps (b,)
        :return fouriered: Concatenation of t and time embedding (b, dim + 1)
        """
        t = rearrange(t, 'b -> b 1')
        freqs = t * rearrange(self.weights, 'd -> 1 d') * 2 * math.pi
        fouriered = torch.cat((freqs.sin(), freqs.cos()), dim=-1)
        fouriered = torch.cat((t, fouriered), dim=-1)
        return fouriered

# building block modules


class Block(nn.Module):
    def __init__(self, dim: int, dim_out: int, groups: int = 8):
        super().__init__()
        self.proj = nn.Conv2d(dim, dim_out, 3, padding=1)
        self.norm = nn.GroupNorm(groups, dim_out)
        self.act = nn.SiLU()

    def forward(self, x: Tensor, scale_shift: Optional[Tensor] = None) -> Tensor:
        x = self.proj(x)
        x = self.norm(x)

        if exists(scale_shift):
            scale, shift = scale_shift
            x = x * (scale + 1) + shift

        x = self.act(x)
        return x


class ResnetBlock(nn.Module):
    def __init__(
        self,
        dim: int,
        dim_out: int,
        *,
        time_emb_dim: Optional[int] = None,
        groups: int = 8
    ):
        super().__init__()

        self.time_mlp = nn.Sequential(
            nn.SiLU(),
            nn.Linear(time_emb_dim, dim_out * 2)
        ) if exists(time_emb_dim) else None

        self.block1 = Block(dim, dim_out, groups=groups)
        self.block2 = Block(dim_out, dim_out, groups=groups)
        if dim != dim_out:
            self.res_conv = nn.Conv2d(dim, dim_out, 1)
        else:
            self.res_conv = nn.Identity()

    def forward(self, x: Tensor, time_emb: Optional[Tensor] = None) -> Tensor:
        """
        :param x: Batch of input images (b, c, h, w)
        :param time_emb: Batch of time embeddings (b, c)
        """
        scale_shift = None

        if exists(self.time_mlp) and exists(time_emb):
            time_emb = self.time_mlp(time_emb)
            time_emb = rearrange(time_emb, 'b c -> b c 1 1')
            scale_shift = time_emb.chunk(2, dim=1)

        h = self.block1(x, scale_shift=scale_shift)
        h = self.block2(h)
        return h + self.res_conv(x)


class LinearAttention(nn.Module):
    """Attention with linear to_qtv"""
    def __init__(self, dim: int, heads: int = 4, dim_head: int = 32):
        super().__init__()
        self.scale = dim_head ** -0.5
        self.heads = heads
        hidden_dim = dim_head * heads
        self.to_qkv = nn.Conv2d(dim, hidden_dim * 3, 1, bias=False)

        self.to_out = nn.Sequential(
            nn.Conv2d(hidden_dim, dim, 1),
            LayerNorm(dim)
        )

    def forward(self, x: Tensor) -> Tensor:
        """
        :param x: Batch of input images (b, c, h, w)
        """
        b, c, h, w = x.shape
        qkv = self.to_qkv(x).chunk(3, dim=1)
        q, k, v = map(lambda t: rearrange(t, 'b (h c) x y -> b h c (x y)', h=self.heads), qkv)

        q = q.softmax(dim=-2)
        k = k.softmax(dim=-1)

        q = q * self.scale
        v = v / (h * w)

        context = torch.einsum('b h d n, b h e n -> b h d e', k, v)

        out = torch.einsum('b h d e, b h d n -> b h e n', context, q)
        out = rearrange(out, 'b h c (x y) -> b (h c) x y', h=self.heads, x=h, y=w)
        return self.to_out(out)


class Attention(nn.Module):
    """Attention with convolutional to_qtv"""
    def __init__(
        self,
        dim: int,
        heads: int = 4,
        dim_head: int = 32,
        scale: int = 16
    ):
        super().__init__()
        self.scale = scale
        self.heads = heads
        hidden_dim = dim_head * heads
        self.to_qkv = nn.Conv2d(dim, hidden_dim * 3, 1, bias=False)
        self.to_out = nn.Conv2d(hidden_dim, dim, 1)

    def forward(self, x: Tensor) -> Tensor:
        b, c, h, w = x.shape
        qkv = self.to_qkv(x).chunk(3, dim=1)
        q, k, v = map(lambda t: rearrange(t, 'b (h c) x y -> b h c (x y)', h=self.heads), qkv)

        q, k = map(l2norm, (q, k))

        sim = einsum('b h d i, b h d j -> b h i j', q, k) * self.scale
        attn = sim.softmax(dim=-1)

        out = einsum('b h i j, b h d j -> b h i d', attn, v)
        out = rearrange(out, 'b h (x y) d -> b (h d) x y', x=h, y=w)
        return self.to_out(out)

# model


class Unet(nn.Module):
    def __init__(
        self,
        dim: int = 64,
        init_dim: Optional[int] = None,
        out_dim: Optional[int] = None,
        dim_mults: List[int] = [1, 2, 4, 8],
        channels: int = 1,
        resnet_block_groups: int = 8,
        learned_variance: bool = False,
        learned_sinusoidal_cond: bool = False,
        learned_sinusoidal_dim: int = 16,
        **kwargs
    ):
        super().__init__()

        # determine dimensions

        self.channels = channels

        init_dim = default(init_dim, dim)
        self.init_conv = nn.Conv2d(channels, init_dim, 7, padding=3)

        dims = [init_dim, *map(lambda m: dim * m, dim_mults)]
        in_out = list(zip(dims[:-1], dims[1:]))

        block_class = partial(ResnetBlock, groups=resnet_block_groups)

        # time embeddings

        time_dim = dim * 4

        self.learned_sinusoidal_cond = learned_sinusoidal_cond

        if learned_sinusoidal_cond:
            sinu_pos_emb = LearnedSinusoidalPosEmb(learned_sinusoidal_dim)
            fourier_dim = learned_sinusoidal_dim + 1
        else:
            sinu_pos_emb = SinusoidalPosEmb(dim)
            fourier_dim = dim

        self.time_mlp = nn.Sequential(
            sinu_pos_emb,
            nn.Linear(fourier_dim, time_dim),
            nn.GELU(),
            nn.Linear(time_dim, time_dim)
        )

        # layers

        self.downs = nn.ModuleList([])
        self.ups = nn.ModuleList([])
        num_resolutions = len(in_out)

        for ind, (dim_in, dim_out) in enumerate(in_out):
            is_last = ind >= (num_resolutions - 1)

            self.downs.append(nn.ModuleList([
                block_class(dim_in, dim_in, time_emb_dim=time_dim),
                block_class(dim_in, dim_in, time_emb_dim=time_dim),
                Residual(PreNorm(dim_in, LinearAttention(dim_in))),
                Downsample(dim_in, dim_out) if not is_last else nn.Conv2d(
                    dim_in, dim_out, 3, padding=1)
            ]))

        mid_dim = dims[-1]
        self.mid_block1 = block_class(mid_dim, mid_dim, time_emb_dim=time_dim)
        self.mid_attn = Residual(PreNorm(mid_dim, Attention(mid_dim)))
        self.mid_block2 = block_class(mid_dim, mid_dim, time_emb_dim=time_dim)

        for ind, (dim_in, dim_out) in enumerate(reversed(in_out)):
            is_last = ind == (len(in_out) - 1)

            self.ups.append(nn.ModuleList([
                block_class(dim_out + dim_in, dim_out, time_emb_dim=time_dim),
                block_class(dim_out + dim_in, dim_out, time_emb_dim=time_dim),
                Residual(PreNorm(dim_out, LinearAttention(dim_out))),
                Upsample(dim_out, dim_in) if not is_last else nn.Conv2d(
                    dim_out, dim_in, 3, padding=1)
            ]))

        default_out_dim = channels * (1 if not learned_variance else 2)
        self.out_dim = default(out_dim, default_out_dim)

        self.final_res_block = block_class(dim * 2, dim, time_emb_dim=time_dim)
        self.final_conv = nn.Conv2d(dim, self.out_dim, 1)

    def forward(self, x: Tensor, timestep: Optional[Tensor]=None, cond: Optional[Tensor]=None) -> Tensor:
        x = self.init_conv(x)
        r = x.clone()

        t = self.time_mlp(timestep) if timestep is not None else None

        h = []

        for block1, block2, attn, downsample in self.downs:
            x = block1(x, t)
            h.append(x)

            x = block2(x, t)
            x = attn(x)
            h.append(x)

            x = downsample(x)

        x = self.mid_block1(x, t)
        x = self.mid_attn(x)
        x = self.mid_block2(x, t)

        for block1, block2, attn, upsample in self.ups:
            x = torch.cat((x, h.pop()), dim=1)
            x = block1(x, t)

            x = torch.cat((x, h.pop()), dim=1)
            x = block2(x, t)
            x = attn(x)

            x = upsample(x)

        x = torch.cat((x, r), dim=1)

        x = self.final_res_block(x, t)
        return self.final_conv(x)


if __name__ == '__main__':
    model = Unet(channels=1)
    x = torch.randn(1, 1, 128, 128)
    y = model(x, timestep=torch.tensor([100]))
    print(y.shape)