"""
Transformer components for CodonTranslator.
Includes RMSNorm, self-attention (SDPA/Flash) with optional mask,
cross-attention for conditioning memory, SwiGLU FFN, and a basic block.
"""

import math
from typing import Optional, Tuple

import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.nn.attention import SDPBackend, sdpa_kernel  # Require recent PyTorch


class RMSNorm(nn.Module):
    """Root Mean Square Layer Normalization."""
    
    def __init__(self, dim: int, eps: float = 1e-6):
        super().__init__()
        self.eps = eps
        self.weight = nn.Parameter(torch.ones(dim))
    
    def forward(self, x: torch.Tensor) -> torch.Tensor:
        """
        Apply RMS normalization.
        
        Args:
            x: Input tensor of any shape ending in dim
            
        Returns:
            Normalized tensor of same shape
        """
        norm = torch.rsqrt(x.pow(2).mean(-1, keepdim=True) + self.eps)
        return x * norm * self.weight


def _apply_rope(x: torch.Tensor, cos: torch.Tensor, sin: torch.Tensor) -> torch.Tensor:
    """Apply rotary embeddings to x: [B,H,T,D]; cos/sin: [1,1,T,D]."""
    x1 = x[..., ::2]
    x2 = x[..., 1::2]
    x_rot = torch.zeros_like(x)
    x_rot[..., ::2] = -x2
    x_rot[..., 1::2] = x1
    return x * cos + x_rot * sin


class MultiHeadAttention(nn.Module):
    """Self-attention using PyTorch SDPA kernels (Flash/MemEff/Math) + RoPE.
    - attn_mask: bool [B, T, T] with True = keep, False = block
    - is_causal: whether to apply causal masking internally
    """

    def __init__(
        self,
        dim: int,
        num_heads: int,
        dropout: float = 0.0,
        use_rope: bool = True,
    ):
        super().__init__()
        assert dim % num_heads == 0, f"dim {dim} must be divisible by num_heads {num_heads}"
        self.dim = dim
        self.num_heads = num_heads
        self.head_dim = dim // num_heads
        self.dropout = dropout
        self.use_rope = use_rope

        self.qkv = nn.Linear(dim, 3 * dim, bias=False)
        self.out_proj = nn.Linear(dim, dim, bias=False)
        self.resid_dropout = nn.Dropout(dropout)

        # RoPE cache
        self._rope_cache: dict[tuple[int, torch.device, torch.dtype], tuple[torch.Tensor, torch.Tensor]] = {}

    def _rope_cos_sin(self, T: int, device: torch.device, dtype: torch.dtype) -> tuple[torch.Tensor, torch.Tensor]:
        key = (T, device, dtype)
        cached = self._rope_cache.get(key)
        if cached is not None:
            return cached
        dim_half = self.head_dim // 2
        inv_freq = 1.0 / (10000 ** (torch.arange(0, dim_half, device=device, dtype=torch.float32) / dim_half))
        t = torch.arange(T, device=device, dtype=torch.float32)
        freqs = torch.outer(t, inv_freq)
        cos = torch.cos(freqs).repeat_interleave(2, dim=-1)
        sin = torch.sin(freqs).repeat_interleave(2, dim=-1)
        cos = cos.to(dtype).unsqueeze(0).unsqueeze(0)  # [1,1,T,D]
        sin = sin.to(dtype).unsqueeze(0).unsqueeze(0)
        self._rope_cache[key] = (cos, sin)
        return cos, sin

    def forward(
        self,
        x: torch.Tensor,
        past_kv: Optional[Tuple[torch.Tensor, torch.Tensor]] = None,
        return_kv: bool = False,
        position_offset: int = 0,
    ) -> "torch.Tensor | Tuple[torch.Tensor, Tuple[torch.Tensor, torch.Tensor]]":
        """
        Self-attention with optional KV cache support.

        Args:
            x: [B, T_new, H]
            past_kv: Optional tuple (k, v), each [B, nH, T_past, Hd]
            return_kv: If True, also return updated (k, v)
            position_offset: Starting position index for RoPE (past length)

        Returns:
            out or (out, present_kv)
        """
        B, T_new, _ = x.shape

        # QKV projections and reshape (ensure contiguous for SDPA kernels)
        qkv = self.qkv(x).view(B, T_new, 3, self.num_heads, self.head_dim).permute(2, 0, 3, 1, 4)
        q, k_new, v_new = qkv[0].contiguous(), qkv[1].contiguous(), qkv[2].contiguous()

        # RoPE for new tokens only
        if self.use_rope:
            # Compute cos/sin up to (offset + T_new), then slice the tail for new positions
            cos, sin = self._rope_cos_sin(position_offset + T_new, x.device, q.dtype)
            if position_offset > 0:
                cos = cos[:, :, position_offset: position_offset + T_new, :]
                sin = sin[:, :, position_offset: position_offset + T_new, :]
            # Apply to q and k_new
            q = _apply_rope(q, cos, sin)
            k_new = _apply_rope(k_new, cos, sin)

        # Concatenate with cache if provided
        if past_kv is not None:
            k_past, v_past = past_kv
            k = torch.cat([k_past, k_new], dim=2)
            v = torch.cat([v_past, v_new], dim=2)
            is_causal = False  # No future tokens present; avoid unnecessary masking
        else:
            k, v = k_new, v_new
            is_causal = True

        # Prefer FlashAttention; fall back to MemEff then Math. Autocast to half/bfloat16 on CUDA.
        backends = [SDPBackend.FLASH_ATTENTION]#, SDPBackend.EFFICIENT_ATTENTION, SDPBackend.MATH]
        with sdpa_kernel(backends):
            if x.device.type == "cuda" and q.dtype not in (torch.float16, torch.bfloat16):
                amp_dtype = torch.bfloat16 if torch.cuda.is_bf16_supported() else torch.float16
                with torch.amp.autocast(device_type="cuda", dtype=amp_dtype):
                    out = F.scaled_dot_product_attention(
                        q, k, v,
                        dropout_p=self.dropout if self.training else 0.0,
                        is_causal=is_causal,
                    )
            else:
                out = F.scaled_dot_product_attention(
                    q, k, v,
                    dropout_p=self.dropout if self.training else 0.0,
                    is_causal=is_causal,
                )

        out = out.transpose(1, 2).contiguous().view(B, T_new, self.dim)
        # Align dtype with residual/Linear weights to avoid bf16/float mismatches
        if out.dtype != x.dtype:
            out = out.to(x.dtype)
        out = self.out_proj(out)
        out = self.resid_dropout(out)

        if return_kv:
            return out, (k, v)
        return out



class GroupedQueryAttention(nn.Module):
    """Grouped-Query Attention (GQA) using Flash Attention via PyTorch SDPA.

    - num_heads total query heads
    - num_kv_groups shared K/V groups (num_heads must be divisible by num_kv_groups)
    - Optional q/k RMSNorm
    - Supports RoPE with a scalar or per-sample position_offset (like MHA)
    - Optional KV cache compatible with the existing interface (stores expanded per-head K/V)
    """

    def __init__(
        self,
        dim: int,
        num_heads: int,
        num_kv_groups: int,
        dropout: float = 0.0,
        qk_norm: bool = False,
    ) -> None:
        super().__init__()
        assert num_heads % max(1, num_kv_groups) == 0, "num_heads must be divisible by num_kv_groups"
        self.dim = dim
        self.num_heads = int(num_heads)
        self.num_kv_groups = max(1, int(num_kv_groups))
        self.group_size = self.num_heads // self.num_kv_groups

        assert dim % num_heads == 0, "dim must be divisible by num_heads"
        self.head_dim = dim // num_heads
        self.dropout = dropout

        self.Wq = nn.Linear(dim, self.num_heads * self.head_dim, bias=False)
        self.Wk = nn.Linear(dim, self.num_kv_groups * self.head_dim, bias=False)
        self.Wv = nn.Linear(dim, self.num_kv_groups * self.head_dim, bias=False)
        self.out_proj = nn.Linear(self.num_heads * self.head_dim, dim, bias=False)

        self.q_norm = RMSNorm(self.head_dim) if qk_norm else None
        self.k_norm = RMSNorm(self.head_dim) if qk_norm else None

        # RoPE cache
        self._rope_cache: dict[tuple[int, torch.device, torch.dtype], tuple[torch.Tensor, torch.Tensor]] = {}

    def _rope_cos_sin(self, T: int, device: torch.device, dtype: torch.dtype) -> tuple[torch.Tensor, torch.Tensor]:
        key = (T, device, dtype)
        cached = self._rope_cache.get(key)
        if cached is not None:
            return cached
        dim_half = self.head_dim // 2
        inv_freq = 1.0 / (10000 ** (torch.arange(0, dim_half, device=device, dtype=torch.float32) / dim_half))
        t = torch.arange(T, device=device, dtype=torch.float32)
        freqs = torch.outer(t, inv_freq)
        cos = torch.cos(freqs).repeat_interleave(2, dim=-1)
        sin = torch.sin(freqs).repeat_interleave(2, dim=-1)
        cos = cos.to(dtype).unsqueeze(0).unsqueeze(0)  # [1,1,T,D]
        sin = sin.to(dtype).unsqueeze(0).unsqueeze(0)
        self._rope_cache[key] = (cos, sin)
        return cos, sin

    def forward(
        self,
        x: torch.Tensor,
        past_kv: Optional[Tuple[torch.Tensor, torch.Tensor]] = None,
        return_kv: bool = False,
        position_offset: int | torch.Tensor = 0,
    ) -> "torch.Tensor | Tuple[torch.Tensor, Tuple[torch.Tensor, torch.Tensor]]":
        B, T_new, _ = x.shape

        # Project to Q, K, V
        q = self.Wq(x).view(B, T_new, self.num_heads, self.head_dim).transpose(1, 2).contiguous()         # [B,H,T,Hd]
        k = self.Wk(x).view(B, T_new, self.num_kv_groups, self.head_dim).transpose(1, 2).contiguous()     # [B,G,T,Hd]
        v = self.Wv(x).view(B, T_new, self.num_kv_groups, self.head_dim).transpose(1, 2).contiguous()     # [B,G,T,Hd]

        # Optional RMSNorm on q/k
        if self.q_norm is not None:
            q = self.q_norm(q)
        if self.k_norm is not None:
            k = self.k_norm(k)

        # RoPE for new tokens only
        if isinstance(position_offset, int):
            cos, sin = self._rope_cos_sin(position_offset + T_new, x.device, q.dtype)
            if position_offset > 0:
                cos = cos[:, :, position_offset: position_offset + T_new, :]
                sin = sin[:, :, position_offset: position_offset + T_new, :]
            q = _apply_rope(q, cos, sin)
            k = _apply_rope(k, cos, sin)
        else:
            off = position_offset.to(device=x.device, dtype=torch.long)
            max_off = int(off.max().item())
            cos_all, sin_all = self._rope_cos_sin(max_off + T_new, x.device, q.dtype)
            ar = torch.arange(T_new, device=x.device, dtype=torch.long)
            idx = (off.unsqueeze(1) + ar.unsqueeze(0))  # [B, T_new]
            cos_b = cos_all.squeeze(0).squeeze(0)[idx].unsqueeze(1)  # [B,1,T,D]
            sin_b = sin_all.squeeze(0).squeeze(0)[idx].unsqueeze(1)
            q = _apply_rope(q, cos_b, sin_b)
            # k has groups dimension [B,G,T,D]; share same offsets per batch
            k = _apply_rope(k, cos_b, sin_b)

        # Expand grouped K/V to per-head by repeating groups
        if self.group_size > 1:
            k_exp = k.repeat_interleave(self.group_size, dim=1)  # [B,H,T,Hd]
            v_exp = v.repeat_interleave(self.group_size, dim=1)  # [B,H,T,Hd]
        else:
            k_exp, v_exp = k, v  # already per-head

        # KV cache: concatenate past along sequence dim
        if past_kv is not None:
            k_past, v_past = past_kv
            k_cat = torch.cat([k_past, k_exp], dim=2)
            v_cat = torch.cat([v_past, v_exp], dim=2)
            is_causal = False
        else:
            k_cat, v_cat = k_exp, v_exp
            is_causal = True

        # Prefer FlashAttention; fall back to MemEff/Math. Ensure CUDA autocast to half/bfloat16 so kernels are available
        with sdpa_kernel([SDPBackend.FLASH_ATTENTION, SDPBackend.EFFICIENT_ATTENTION, SDPBackend.MATH]):
            if x.device.type == "cuda" and q.dtype not in (torch.float16, torch.bfloat16):
                amp_dtype = torch.bfloat16 if torch.cuda.is_bf16_supported() else torch.float16
                with torch.amp.autocast(device_type="cuda", dtype=amp_dtype):
                    out = torch.nn.functional.scaled_dot_product_attention(
                        q, k_cat, v_cat,
                        dropout_p=self.dropout if self.training else 0.0,
                        is_causal=is_causal,
                    )  # [B,H,T,Hd]
            else:
                out = torch.nn.functional.scaled_dot_product_attention(
                    q, k_cat, v_cat,
                    dropout_p=self.dropout if self.training else 0.0,
                    is_causal=is_causal,
                )  # [B,H,T,Hd]

        out = out.transpose(1, 2).contiguous().view(B, T_new, self.num_heads * self.head_dim)
        # Ensure dtype compatibility for Linear / residual path
        if out.dtype != x.dtype:
            out = out.to(x.dtype)
        out = self.out_proj(out)

        if return_kv:
            return out, (k_cat, v_cat)
        return out



class FeedForward(nn.Module):
    """Feed-forward network with optional GLU activation."""
    
    def __init__(
        self,
        dim: int,
        hidden_dim: int,
        dropout: float = 0.0,
    ):
        super().__init__()

        self.w1 = nn.Linear(dim, hidden_dim, bias=False)
        self.w2 = nn.Linear(hidden_dim, dim, bias=False)
        self.w3 = nn.Linear(dim, hidden_dim, bias=False)
        
        self.dropout = nn.Dropout(dropout)
    
    def forward(self, x: torch.Tensor) -> torch.Tensor:
        """
        Apply feed-forward network.
        
        Args:
            x: Input tensor [B, T, dim]
            
        Returns:
            Output tensor [B, T, dim]
        """
        
        return self.dropout(self.w2(F.silu(self.w1(x)) * self.w3(x)))


class TransformerBlock(nn.Module):
    """Pre-norm Transformer block using self-attn + SwiGLU FFN (no cross-attention)."""

    def __init__(
        self,
        dim: int,
        num_heads: int,
        mlp_ratio: float = 4.0,
        dropout: float = 0.0,
        num_kv_groups: int | None = None,
        qk_norm: bool = False,
        attn_type: str = "gqa",  # "gqa" or "mha"
    ):
        super().__init__()
        self.norm1 = RMSNorm(dim)
        if attn_type == "mha":
            self.attn = MultiHeadAttention(dim=dim, num_heads=num_heads, dropout=dropout)
            self._attn_is_gqa = False
        else:
            # Use Grouped-Query Attention (defaults to no grouping when num_kv_groups is None)
            kv_groups = num_heads if (num_kv_groups is None) else max(1, int(num_kv_groups))
            self.attn = GroupedQueryAttention(dim=dim, num_heads=num_heads, num_kv_groups=kv_groups, dropout=dropout, qk_norm=qk_norm)
            self._attn_is_gqa = True
        self.norm2 = RMSNorm(dim)
        self.ffn = FeedForward(dim=dim, hidden_dim=int(dim * mlp_ratio), dropout=dropout)

    def forward(
        self,
        x: torch.Tensor,
        past_kv: Optional[Tuple[torch.Tensor, torch.Tensor]] = None,
        use_cache: bool = False,
        position_offset: int = 0,
    ) -> "torch.Tensor | Tuple[torch.Tensor, Tuple[torch.Tensor, torch.Tensor]]":
        """Forward pass with optional KV caching."""
        if use_cache or (past_kv is not None):
            attn_out = self.attn(self.norm1(x), past_kv=past_kv, return_kv=True, position_offset=position_offset)
            x = x + attn_out[0]
            x = x + self.ffn(self.norm2(x))
            return x, attn_out[1]
        else:
            x = x + self.attn(self.norm1(x))
            x = x + self.ffn(self.norm2(x))
            return x