"""Layer primitives for the ELF transformer."""

import math
from typing import Optional

import torch
import torch.nn as nn
import torch.nn.functional as F
from einops import rearrange, repeat


# Init defaults:
# - Linear weights: xavier_uniform; biases: 0
# - TimestepEmbedder MLPs and learned tokens: normal(0.02)
# - final_layer.linear: 0 (zero init)
def DEFAULT_KERNEL_INIT(weight: torch.Tensor) -> None:
    nn.init.xavier_uniform_(weight)


def DEFAULT_BIAS_INIT(bias: torch.Tensor) -> None:
    nn.init.zeros_(bias)


def ZERO_INIT(t: torch.Tensor) -> None:
    nn.init.zeros_(t)


def NORMAL_INIT_002(t: torch.Tensor) -> None:
    nn.init.normal_(t, mean=0.0, std=0.02)


def _make_linear(in_features: int, out_features: int, bias: bool = True,
                 kernel_init=DEFAULT_KERNEL_INIT, bias_init=DEFAULT_BIAS_INIT) -> nn.Linear:
    """nn.Linear with explicit initializers."""
    layer = nn.Linear(in_features, out_features, bias=bias)
    kernel_init(layer.weight)
    if bias and bias_init is not None:
        bias_init(layer.bias)
    return layer


def rotate_half(x: torch.Tensor) -> torch.Tensor:
    """Rotate half the hidden dims of the input."""
    x = rearrange(x, '... (d r) -> ... d r', r=2)
    x1, x2 = x.unbind(dim=-1)
    x = torch.stack((-x2, x1), dim=-1)
    return rearrange(x, '... d r -> ... (d r)')


class TextRotaryEmbeddingFast(nn.Module):
    """1D Rotary Position Embedding for text/sequence models."""

    def __init__(self, dim: int, pt_seq_len: int = 512,
                 ft_seq_len: Optional[int] = None, theta: float = 10000.0,
                 num_empty_token: int = 0):
        super().__init__()
        self.dim = dim
        self.pt_seq_len = pt_seq_len
        self.ft_seq_len = ft_seq_len if ft_seq_len is not None else pt_seq_len
        self.theta = theta
        self.num_empty_token = num_empty_token
        freqs_cos, freqs_sin = self._compute_freqs()
        self.register_buffer("freqs_cos", freqs_cos, persistent=False)
        self.register_buffer("freqs_sin", freqs_sin, persistent=False)

    def _compute_freqs(self) -> tuple:
        dim = self.dim
        ft_seq_len = self.ft_seq_len
        pt_seq_len = self.pt_seq_len

        freqs = 1.0 / (self.theta ** (
            torch.arange(0, dim, 2, dtype=torch.float32)[: dim // 2] / dim
        ))
        pos = torch.arange(ft_seq_len, dtype=torch.float32) / ft_seq_len * pt_seq_len

        freqs_main = torch.einsum('..., f -> ... f', pos, freqs)
        freqs_main = repeat(freqs_main, '... n -> ... (n r)', r=2)

        D = freqs_main.shape[-1]
        cos_parts, sin_parts = [], []
        # 1. Empty tokens (no rotation): cos=1, sin=0
        if self.num_empty_token > 0:
            cos_parts.append(torch.ones((self.num_empty_token, D), dtype=freqs.dtype))
            sin_parts.append(torch.zeros((self.num_empty_token, D), dtype=freqs.dtype))
        # 2. Main tokens (RoPE positions 0 to pt_seq_len-1)
        cos_parts.append(torch.cos(freqs_main))
        sin_parts.append(torch.sin(freqs_main))

        freqs_cos = torch.cat(cos_parts, dim=0) if len(cos_parts) > 1 else cos_parts[0]
        freqs_sin = torch.cat(sin_parts, dim=0) if len(sin_parts) > 1 else sin_parts[0]
        return freqs_cos, freqs_sin

    def forward(self, t: torch.Tensor) -> torch.Tensor:
        freqs_cos = self.freqs_cos.to(t.dtype)
        freqs_sin = self.freqs_sin.to(t.dtype)
        return t * freqs_cos + rotate_half(t) * freqs_sin


class RMSNorm(nn.Module):
    """RMS Normalization layer."""

    def __init__(self, hidden_size: int, eps: float = 1e-6):
        super().__init__()
        self.hidden_size = hidden_size
        self.eps = eps
        self.weight = nn.Parameter(torch.ones(hidden_size))

    def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
        input_dtype = hidden_states.dtype
        variance = hidden_states.float().pow(2).mean(dim=-1, keepdim=True)
        inv_std = torch.rsqrt(variance + self.eps).to(input_dtype)
        return self.weight.to(input_dtype) * (hidden_states * inv_std)


class BottleneckTextProj(nn.Module):
    """Text projection with bottleneck."""

    def __init__(self, text_encoder_dim: int, hidden_size: int, bottleneck_dim: int):
        super().__init__()
        self.proj1 = _make_linear(text_encoder_dim, bottleneck_dim, bias=False)
        self.proj2 = _make_linear(bottleneck_dim, hidden_size, bias=True)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        return self.proj2(self.proj1(x))


class TimestepEmbedder(nn.Module):
    """Embeds scalar timesteps into vector representations."""

    def __init__(self, hidden_size: int, frequency_embedding_size: int = 256):
        super().__init__()
        self.hidden_size = hidden_size
        self.frequency_embedding_size = frequency_embedding_size
        self.mlp_0 = _make_linear(
            frequency_embedding_size, hidden_size, bias=True,
            kernel_init=NORMAL_INIT_002, bias_init=DEFAULT_BIAS_INIT,
        )
        self.mlp_2 = _make_linear(
            hidden_size, hidden_size, bias=True,
            kernel_init=NORMAL_INIT_002, bias_init=DEFAULT_BIAS_INIT,
        )

    @staticmethod
    def timestep_embedding(t: torch.Tensor, dim: int, max_period: int = 10000) -> torch.Tensor:
        """Sinusoidal timestep embeddings: (N,) ints -> (N, dim) floats."""
        half = dim // 2
        freqs = torch.exp(
            -math.log(max_period)
            * torch.arange(0, half, dtype=torch.float32, device=t.device)
            / half
        )
        args = t[:, None].to(torch.float32) * freqs[None]
        embedding = torch.cat([torch.cos(args), torch.sin(args)], dim=-1)
        if dim % 2:
            embedding = torch.cat([embedding, torch.zeros_like(embedding[:, :1])], dim=-1)
        return embedding

    def forward(self, t: torch.Tensor) -> torch.Tensor:
        t_emb = self.mlp_0(self.timestep_embedding(t, self.frequency_embedding_size))
        return self.mlp_2(F.silu(t_emb))


def scaled_dot_product_attention(
    query: torch.Tensor, key: torch.Tensor, value: torch.Tensor,
    attn_mask: Optional[torch.Tensor] = None,
) -> torch.Tensor:
    """Scaled dot-product attention.

    query/key/value: (B, num_heads, L|S, head_dim).
    attn_mask: optional int mask (B, S) or (B, L, S); 1=valid, 0=masked.
    Returns: (B, num_heads, L, head_dim).
    """
    bool_mask: Optional[torch.Tensor] = None
    if attn_mask is not None:
        if attn_mask.dim() == 2:
            bool_mask = attn_mask[:, None, None, :]
        elif attn_mask.dim() == 3:
            bool_mask = attn_mask[:, None, :, :]
        else:
            bool_mask = attn_mask
        bool_mask = bool_mask.bool()
    return F.scaled_dot_product_attention(query, key, value, attn_mask=bool_mask)


class Attention(nn.Module):
    """Multi-head self-attention."""

    def __init__(self, dim: int, num_heads: int = 8, qkv_bias: bool = True,
                 qk_norm: bool = True, attn_drop: float = 0.0, proj_drop: float = 0.0):
        super().__init__()
        self.dim = dim
        self.num_heads = num_heads
        self.qk_norm = qk_norm
        self.attn_drop = attn_drop
        self.proj_drop = proj_drop
        head_dim = dim // num_heads
        self.qkv = _make_linear(dim, dim * 3, bias=qkv_bias)
        self.q_norm = RMSNorm(head_dim) if qk_norm else nn.Identity()
        self.k_norm = RMSNorm(head_dim) if qk_norm else nn.Identity()
        self.proj = _make_linear(dim, dim, bias=True)

    def forward(self, x: torch.Tensor, rope_fn: Optional[nn.Module],
                attention_mask: Optional[torch.Tensor] = None,
                deterministic: bool = True) -> torch.Tensor:
        """x: (B, N, C). attention_mask: optional int mask (B, N), 1=valid, 0=padded."""
        B, N, C = x.shape
        head_dim = self.dim // self.num_heads
        qkv = self.qkv(x).reshape(B, N, 3, self.num_heads, head_dim).permute(2, 0, 3, 1, 4)
        q, k, v = qkv[0], qkv[1], qkv[2]
        if self.qk_norm:
            q = self.q_norm(q)
            k = self.k_norm(k)
        if rope_fn is not None:
            q = rope_fn(q)
            k = rope_fn(k)
        x = scaled_dot_product_attention(q, k, v, attn_mask=attention_mask)
        x = x.permute(0, 2, 1, 3).reshape(B, N, C)
        x = self.proj(x)
        if self.proj_drop > 0.0:
            x = F.dropout(x, p=self.proj_drop, training=not deterministic)
        return x


class SwiGLUFFN(nn.Module):
    """SwiGLU Feed-Forward Network."""

    def __init__(self, dim: int, hidden_dim: int, drop: float = 0.0, bias: bool = True):
        super().__init__()
        hidden_dim_eff = int(hidden_dim * 2 / 3)
        self.drop = drop
        self.w12 = _make_linear(dim, 2 * hidden_dim_eff, bias=bias)
        self.w3 = _make_linear(hidden_dim_eff, dim, bias=bias)

    def forward(self, x: torch.Tensor, deterministic: bool = True) -> torch.Tensor:
        x12 = self.w12(x)
        x1, x2 = x12.chunk(2, dim=-1)
        hidden = F.silu(x1) * x2
        if self.drop > 0.0:
            hidden = F.dropout(hidden, p=self.drop, training=not deterministic)
        return self.w3(hidden)


class FinalLayer(nn.Module):
    """The final layer of ELF."""

    def __init__(self, hidden_size: int, patch_size: int, out_channels: int):
        super().__init__()
        self.norm_final = RMSNorm(hidden_size)
        # Zero-init linear (kernel & bias both zero).
        self.linear = _make_linear(
            hidden_size, patch_size * patch_size * out_channels, bias=True,
            kernel_init=ZERO_INIT, bias_init=ZERO_INIT,
        )

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        return self.linear(self.norm_final(x))