"""
FlowMatchingTTS – adapts CosyVoice's Conditional Flow Matching pipeline
to the semacs-tts dataset (VQ codes → mel spectrogram).

Architecture mirrors MaskedDiffWithXvec from cosyvoice/flow/flow.py:
  codes  → Embedding → causal Transformer → Linear → InterpolateRegulator
         ↘ ConditionalCFM (ConditionalDecoder UNet1D) ← speaker emb
         → mel spectrogram loss
"""
import math
import random
import sys
import os

sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))

import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.utils.checkpoint as ckpt
from omegaconf import DictConfig

from flow_matching.utils.cfm import ConditionalCFM
from flow_matching.utils.decoder import ConditionalDecoder
from flow_matching.utils.length_regulator import InterpolateRegulator
from flow_matching.utils.mask import make_pad_mask


class SinusoidalPE(nn.Module):
    def __init__(self, d_model: int, dropout: float = 0.0, max_len: int = 8192):
        super().__init__()
        self.dropout = nn.Dropout(dropout)
        pe = torch.zeros(max_len, d_model)
        pos = torch.arange(0, max_len).float().unsqueeze(1)
        div = torch.exp(
            torch.arange(0, d_model, 2).float() * (-math.log(10000.0) / d_model)
        )
        pe[:, 0::2] = torch.sin(pos * div)
        pe[:, 1::2] = torch.cos(pos * div)
        self.register_buffer('pe', pe.unsqueeze(0))   # (1, max_len, d_model)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        return self.dropout(x + self.pe[:, :x.size(1)])


def _forward_layer(layer, x, attn_mask, pad_mask):
    return layer(x, src_mask=attn_mask, src_key_padding_mask=pad_mask)


class CodeEncoder(nn.Module):
    """Causal Transformer encoder that operates on pre-embedded VQ codes."""

    def __init__(
        self,
        hidden_dim: int,
        num_heads: int,
        num_layers: int,
        ffn_dim: int,
        dropout: float,
        causal: bool,
        grad_checkpoint: bool,
    ):
        super().__init__()
        self.causal = causal
        self.grad_checkpoint = grad_checkpoint
        self.pos_enc = SinusoidalPE(hidden_dim, dropout)
        self.layers = nn.ModuleList([
            nn.TransformerEncoderLayer(
                d_model=hidden_dim,
                nhead=num_heads,
                dim_feedforward=ffn_dim,
                dropout=dropout,
                batch_first=True,
                norm_first=True,    # pre-LN for training stability
            )
            for _ in range(num_layers)
        ])
        self.norm = nn.LayerNorm(hidden_dim)
        self._hidden_dim = hidden_dim

    def output_size(self) -> int:
        return self._hidden_dim

    def forward(self, x: torch.Tensor, lengths: torch.Tensor):
        """
        x:       (B, T, hidden_dim) – embedded codes
        lengths: (B,)               – valid code lengths per sample
        Returns: (B, T, hidden_dim), lengths
        """
        B, T, _ = x.shape
        x = self.pos_enc(x)

        pad_mask = make_pad_mask(lengths, T)       # (B, T), True = padded
        attn_mask = None
        if self.causal:
            attn_mask = torch.triu(
                torch.ones(T, T, device=x.device, dtype=torch.bool), diagonal=1
            )

        for layer in self.layers:
            if self.grad_checkpoint and self.training:
                x = ckpt.checkpoint(
                    _forward_layer, layer, x, attn_mask, pad_mask,
                    use_reentrant=False,
                )
            else:
                x = layer(x, src_mask=attn_mask, src_key_padding_mask=pad_mask)

        return self.norm(x), lengths


class FlowMatchingTTS(nn.Module):
    """
    Full TTS flow matching model.

    Training input:
        codes     (B, num_q, T_codes)  VQ code indices
        code_lens (B,)                 valid code lengths
        mel       (B, T_mel, n_mels)   target log-mel spectrogram
        mel_lens  (B,)                 valid mel frame lengths
        spk_emb   (B, 192)             CAM++ speaker embedding (pre-extracted)

    Output:
        {'loss': scalar}               conditional flow matching loss
    """

    def __init__(self, cfg):
        super().__init__()
        m    = cfg.model
        data = cfg.data
        cfm  = cfg.cfm

        self.n_mels    = data.n_mels       # 100
        hidden_dim     = m.hidden_dim      # 768
        spk_emb_dim    = m.spk_emb_dim     # 192

        # ── code embedding: sum across quantizers ────────────────────────
        self.code_embedding = nn.Embedding(m.codebook_size, hidden_dim)

        # ── causal transformer encoder ───────────────────────────────────
        self.encoder = CodeEncoder(
            hidden_dim=hidden_dim,
            num_heads=m.num_heads,
            num_layers=m.num_layers,
            ffn_dim=m.ffn_dim,
            dropout=m.dropout,
            causal=m.causal,
            grad_checkpoint=m.grad_checkpoint,
        )

        # ── project encoder output to mel dimension ──────────────────────
        self.encoder_proj = nn.Linear(hidden_dim, self.n_mels)

        # ── speaker embedding: 192 → n_mels ─────────────────────────────
        self.spk_embed_affine = nn.Linear(spk_emb_dim, self.n_mels)

        # ── length regulator: upsample codes to mel frame rate ───────────
        self.length_regulator = InterpolateRegulator(
            channels=self.n_mels,
            sampling_ratios=(),
        )

        # ── conditional flow matching decoder ────────────────────────────
        # ConditionalDecoder input = concat(x, mu, spks_t, cond) = 4 × n_mels
        cfm_params = DictConfig({
            'sigma_min':          cfm.sigma_min,
            'solver':             'euler',
            't_scheduler':        cfm.t_scheduler,
            'training_cfg_rate':  cfm.training_cfg_rate,
            'inference_cfg_rate': cfm.inference_cfg_rate,
            'reg_loss_type':      'l1',
        })
        estimator = ConditionalDecoder(
            in_channels=4 * self.n_mels,   # x + mu + spks_expanded + cond
            out_channels=self.n_mels,
            channels=(256, 256),
            dropout=0.05,
            attention_head_dim=64,
            n_blocks=4,
            num_mid_blocks=12,
            num_heads=8,
            act_fn='gelu',
        )
        self.decoder = ConditionalCFM(
            in_channels=self.n_mels,
            cfm_params=cfm_params,
            n_spks=1,
            spk_emb_dim=self.n_mels,   # already projected to n_mels
            estimator=estimator,
        )

    # ── forward (training) ───────────────────────────────────────────────────

    def forward(self, batch: dict, device) -> dict:
        """
        Same interface as cosyvoice MaskedDiffWithXvec:  model(batch, device).

        Batch keys (added by Executor before this call):
            codes      (B, num_q, T_codes)
            code_lens  (B,)
            mel        (B, T_mel, n_mels)
            mel_lens   (B,)
            embedding  (B, 192)  L2-normalised CAM++ speaker embedding
        """
        codes     = batch['codes'].to(device)
        code_lens = batch['code_lens'].to(device)
        mel       = batch['mel'].to(device)
        mel_lens  = batch['mel_lens'].to(device)
        embedding = batch['embedding'].to(device)      # (B, 192)

        # Speaker projection
        spk = F.normalize(embedding, dim=-1)
        spk = self.spk_embed_affine(spk)               # (B, n_mels)

        # Code embedding: sum over quantizer axis
        x = self.code_embedding(codes)                 # (B, num_q, T, hidden_dim)
        x = x.sum(dim=1)                               # (B, T_codes, hidden_dim)

        # Encode
        h, _ = self.encoder(x, code_lens)              # (B, T_codes, hidden_dim)
        h    = self.encoder_proj(h)                    # (B, T_codes, n_mels)

        # Upsample to mel frame rate
        h, _ = self.length_regulator(h, mel_lens)      # (B, T_mel, n_mels)

        # Build conditioning: random-length mel prefix (50 % chance per sample)
        conds = torch.zeros_like(mel)
        for i, j in enumerate(mel_lens.tolist()):
            if random.random() < 0.5:
                continue
            idx = random.randint(0, int(0.8 * j))
            conds[i, :idx] = mel[i, :idx]
        conds = conds.transpose(1, 2)                  # (B, n_mels, T_mel)

        # Transpose to (B, n_mels, T) for the CFM decoder
        mel_t = mel.transpose(1, 2).contiguous()       # (B, n_mels, T_mel)
        h_t   = h.transpose(1, 2).contiguous()         # (B, n_mels, T_mel)

        # Safety alignment (no-op when length_regulator works correctly)
        if mel_t.shape[-1] != h_t.shape[-1]:
            mel_t = F.interpolate(mel_t, size=h_t.shape[-1], mode='nearest')
            conds = F.interpolate(conds, size=h_t.shape[-1], mode='nearest')

        mask = (~make_pad_mask(mel_lens)).to(h)        # (B, T_mel)

        loss, _ = self.decoder.compute_loss(
            mel_t,
            mask.unsqueeze(1),
            h_t,
            spk,
            cond=conds,
        )
        return {'loss': loss}

    # ── inference ────────────────────────────────────────────────────────────

    @torch.inference_mode()
    def inference(
        self,
        codes:        torch.Tensor,   # (1, num_q, T_codes)
        code_lens:    torch.Tensor,   # (1,)
        prompt_mel:   torch.Tensor,   # (1, T_prompt, n_mels) or empty
        target_len:   int,            # desired output mel frames
        spk_emb:      torch.Tensor,   # (1, 192)
        n_timesteps:  int = 10,
        temperature:  float = 1.0,
    ) -> torch.Tensor:
        """Returns generated mel: (1, n_mels, T_target)"""
        spk = F.normalize(spk_emb, dim=-1)
        spk = self.spk_embed_affine(spk)               # (1, n_mels)

        x = self.code_embedding(codes).sum(dim=1)      # (1, T_codes, hidden_dim)
        h, _ = self.encoder(x, code_lens)
        h    = self.encoder_proj(h)                    # (1, T_codes, n_mels)

        out_lens = torch.tensor([target_len], device=codes.device)
        h, _     = self.length_regulator(h, out_lens)  # (1, target_len, n_mels)
        h_t      = h.transpose(1, 2).contiguous()      # (1, n_mels, target_len)

        conds = torch.zeros_like(h_t)
        if prompt_mel.shape[1] > 0:
            p = min(prompt_mel.shape[1], target_len)
            conds[:, :, :p] = prompt_mel[:, :p].transpose(1, 2)

        mask = torch.ones(1, 1, target_len, device=codes.device)

        mel_out = self.decoder(
            mu=h_t,
            mask=mask,
            spks=spk,
            cond=conds,
            n_timesteps=n_timesteps,
            temperature=temperature,
        )
        return mel_out   # (1, n_mels, target_len)