"""
Grammar-Constrained Decoding (GCD) for Spectra-Reason-GCD inference.
Ensures generated SMILES are syntactically valid using an EBNF grammar.
"""
from __future__ import annotations

from pathlib import Path
from typing import Any, List, Optional

import torch


def get_smiles_constraint(tokenizer, grammar_path: Optional[Path] = None):
    """
    Return a logits processor for SMILES grammar if transformers_cfg is available.
    Otherwise return None (unconstrained).
    """
    try:
        from transformers_cfg.grammar_utils import IncrementalGrammarConstraint
    except ImportError:
        return None
    if grammar_path is None:
        grammar_path = Path(__file__).resolve().parents[1] / "grammars" / "smiles.ebnf"
    if not grammar_path.exists():
        return None
    grammar_str = grammar_path.read_text()
    try:
        return IncrementalGrammarConstraint(grammar_str, "root", tokenizer)
    except Exception:
        return None


def generate_with_gcd(
    model,
    tokenizer,
    inputs_embeds: torch.Tensor,
    attention_mask: Optional[torch.Tensor] = None,
    position_ids: Optional[torch.Tensor] = None,
    max_new_tokens: int = 200,
    grammar_path: Optional[Path] = None,
    **generate_kwargs: Any,
) -> List[str]:
    """
    Generate from model with optional SMILES grammar constraint.
    inputs_embeds: (B, 64 + prefix_len, dim) or (B, total_len, dim) including soft tokens.
    """
    logits_processor = []
    constraint = get_smiles_constraint(tokenizer, grammar_path)
    if constraint is not None:
        logits_processor.append(constraint)

    gen_kw = {
        "max_new_tokens": max_new_tokens,
        "do_sample": False,
        "pad_token_id": tokenizer.pad_token_id or tokenizer.eos_token_id,
        **generate_kwargs,
    }
    if logits_processor:
        gen_kw["logits_processor"] = logits_processor

    with torch.no_grad():
        out = model.generate(
            inputs_embeds=inputs_embeds,
            attention_mask=attention_mask,
            position_ids=position_ids,
            **gen_kw,
        )

    # Decode only the generated part (skip input length)
    input_len = inputs_embeds.shape[1]
    generated = out[:, input_len:]
    return tokenizer.batch_decode(generated, skip_special_tokens=True)


def predict_smiles_from_spectrum(
    encoder,
    model,
    tokenizer,
    peaks: torch.Tensor,
    device: torch.device,
    max_new_tokens: int = 200,
    use_gcd: bool = True,
    grammar_path: Optional[Path] = None,
) -> str:
    """
    Single spectrum -> SMILES. Encoder produces 64 soft tokens; we build prompt
    (user message), prepend soft tokens, then generate with optional GCD.
    """
    encoder.eval()
    model.eval()
    meta = {"peaks": peaks.to(device)}
    if peaks.dim() == 2:
        peaks = peaks.unsqueeze(0)
        meta["peaks"] = peaks.to(device)
    with torch.no_grad():
        soft = encoder(peaks=peaks.to(device), meta=meta)
    
    B = soft.shape[0]
    llm_dim = soft.shape[2]
    embed_layer = model.get_input_embeddings()

    messages = [
        {"role": "system", "content": "You are an expert mass spectrometrist. Analyze the input spectrum, deduce the substructures, and generate the valid SMILES string."},
        {"role": "user", "content": "Analyze the spectrum and predict the molecule structure."},
    ]
    prompt = tokenizer.apply_chat_template(
        messages,
        tokenize=True,
        add_generation_prompt=True,
        return_tensors="pt",
    )
    prompt = prompt.to(device)
    if prompt.dim() == 1:
        prompt = prompt.unsqueeze(0).expand(B, -1)
    text_emb = embed_layer(prompt)
    soft = soft.to(dtype=text_emb.dtype) # Ensure dtype match
    inputs_embeds = torch.cat([soft, text_emb], dim=1)
    attention_mask = torch.ones(inputs_embeds.shape[0], inputs_embeds.shape[1], dtype=torch.long, device=device)
    position_ids = torch.arange(inputs_embeds.shape[1], device=device).unsqueeze(0).expand(B, -1)

    constraint = get_smiles_constraint(tokenizer, grammar_path) if use_gcd else None
    logits_processor = [constraint] if constraint is not None else []

    with torch.no_grad():
        out = model.generate(
            inputs_embeds=inputs_embeds,
            attention_mask=attention_mask,
            position_ids=position_ids,
            max_new_tokens=max_new_tokens,
            do_sample=False,
            pad_token_id=tokenizer.pad_token_id or tokenizer.eos_token_id,
            logits_processor=logits_processor,
        )
    # When using inputs_embeds without input_ids, generate returns ONLY new tokens.
    generated = out
    decoded = tokenizer.batch_decode(generated, skip_special_tokens=True)
    return decoded[0].strip() if decoded else ""