# api/rag_engine.py
"""
RAG engine:
- build_rag_chunks_from_file(path, doc_type) -> List[chunk]
- retrieve_relevant_chunks(query, chunks, ...) -> (context_text, used_chunks)

Chunk format (MVP):
{
  "text": str,
  "source_file": str,
  "section": str,
  "doc_type": str
}

✅ Update in this version:
- retrieve_relevant_chunks now supports optional scoping:
  - allowed_source_files: Optional[List[str]]  (match by basename)
  - allowed_doc_types: Optional[List[str]]
- Scoping happens BEFORE scoring, so refs returned are guaranteed to be the true used chunks.
"""

import os
import re
from typing import Dict, List, Tuple, Optional

from pypdf import PdfReader
from docx import Document
from pptx import Presentation


# ============================
# Token helpers (optional tiktoken)
# ============================
def _safe_import_tiktoken():
    try:
        import tiktoken  # type: ignore
        return tiktoken
    except Exception:
        return None


def _approx_tokens(text: str) -> int:
    if not text:
        return 0
    return max(1, int(len(text) / 4))


def _count_text_tokens(text: str, model: str = "") -> int:
    tk = _safe_import_tiktoken()
    if tk is None:
        return _approx_tokens(text)

    try:
        enc = tk.encoding_for_model(model) if model else tk.get_encoding("cl100k_base")
    except Exception:
        enc = tk.get_encoding("cl100k_base")

    return len(enc.encode(text or ""))


def _truncate_to_tokens(text: str, max_tokens: int, model: str = "") -> str:
    """
    Deterministic truncation. Uses tiktoken if available; otherwise approximates by char ratio.
    """
    if not text:
        return text

    tk = _safe_import_tiktoken()
    if tk is None:
        total = _approx_tokens(text)
        if total <= max_tokens:
            return text
        ratio = max_tokens / max(1, total)
        cut = max(50, min(len(text), int(len(text) * ratio)))
        s = text[:cut]
        while _approx_tokens(s) > max_tokens and len(s) > 50:
            s = s[: int(len(s) * 0.9)]
        return s

    try:
        enc = tk.encoding_for_model(model) if model else tk.get_encoding("cl100k_base")
    except Exception:
        enc = tk.get_encoding("cl100k_base")

    ids = enc.encode(text or "")
    if len(ids) <= max_tokens:
        return text
    return enc.decode(ids[:max_tokens])


# ============================
# RAG hard limits
# ============================
RAG_TOPK_LIMIT = 4
RAG_CHUNK_TOKEN_LIMIT = 500
RAG_CONTEXT_TOKEN_LIMIT = 2000  # 4 * 500


# ----------------------------
# Helpers
# ----------------------------
def _clean_text(s: str) -> str:
    s = (s or "").replace("\r", "\n")
    s = re.sub(r"\n{3,}", "\n\n", s)
    return s.strip()


def _split_into_chunks(text: str, max_chars: int = 1400) -> List[str]:
    """
    Simple deterministic chunker:
    - split by blank lines
    - then pack into <= max_chars
    """
    text = _clean_text(text)
    if not text:
        return []

    paras = [p.strip() for p in text.split("\n\n") if p.strip()]
    chunks: List[str] = []
    buf = ""

    for p in paras:
        if not buf:
            buf = p
            continue

        if len(buf) + 2 + len(p) <= max_chars:
            buf = buf + "\n\n" + p
        else:
            chunks.append(buf)
            buf = p

    if buf:
        chunks.append(buf)

    return chunks


def _file_label(path: str) -> str:
    return os.path.basename(path) if path else "uploaded_file"


def _basename(x: str) -> str:
    try:
        return os.path.basename(x or "")
    except Exception:
        return x or ""


# ----------------------------
# Parsers
# ----------------------------
def _parse_pdf_to_text(path: str) -> List[Tuple[str, str]]:
    """
    Returns list of (section_label, text)
    section_label uses page numbers.
    """
    reader = PdfReader(path)
    out: List[Tuple[str, str]] = []
    for i, page in enumerate(reader.pages):
        t = page.extract_text() or ""
        t = _clean_text(t)
        if t:
            out.append((f"p{i+1}", t))
    return out


def _parse_docx_to_text(path: str) -> List[Tuple[str, str]]:
    doc = Document(path)
    paras = [p.text.strip() for p in doc.paragraphs if p.text and p.text.strip()]
    if not paras:
        return []
    full = "\n\n".join(paras)
    return [("docx", _clean_text(full))]


def _parse_pptx_to_text(path: str) -> List[Tuple[str, str]]:
    prs = Presentation(path)
    out: List[Tuple[str, str]] = []
    for idx, slide in enumerate(prs.slides, start=1):
        lines: List[str] = []
        for shape in slide.shapes:
            if hasattr(shape, "text") and shape.text:
                txt = shape.text.strip()
                if txt:
                    lines.append(txt)
        if lines:
            out.append((f"slide{idx}", _clean_text("\n".join(lines))))
    return out


# ----------------------------
# Public API
# ----------------------------
def build_rag_chunks_from_file(path: str, doc_type: str) -> List[Dict]:
    """
    Build RAG chunks from a local file path.
    Supports: .pdf / .docx / .pptx / .txt
    """
    if not path or not os.path.exists(path):
        return []

    ext = os.path.splitext(path)[1].lower()
    source_file = _file_label(path)

    sections: List[Tuple[str, str]] = []
    try:
        if ext == ".pdf":
            sections = _parse_pdf_to_text(path)
        elif ext == ".docx":
            sections = _parse_docx_to_text(path)
        elif ext == ".pptx":
            sections = _parse_pptx_to_text(path)
        elif ext in [".txt", ".md"]:
            with open(path, "r", encoding="utf-8", errors="ignore") as f:
                sections = [("text", _clean_text(f.read()))]
        else:
            print(f"[rag_engine] unsupported file type: {ext}")
            return []
    except Exception as e:
        print(f"[rag_engine] parse error for {source_file}: {repr(e)}")
        return []

    chunks: List[Dict] = []
    for section, text in sections:
        for j, piece in enumerate(_split_into_chunks(text), start=1):
            chunks.append(
                {
                    "text": piece,
                    "source_file": source_file,
                    "section": f"{section}#{j}",
                    "doc_type": doc_type,
                }
            )

    return chunks


def retrieve_relevant_chunks(
    query: str,
    chunks: List[Dict],
    k: int = RAG_TOPK_LIMIT,
    max_context_chars: int = 600,  # kept for backward compatibility (still used as a safety cap)
    min_score: int = 6,
    chunk_token_limit: int = RAG_CHUNK_TOKEN_LIMIT,
    max_context_tokens: int = RAG_CONTEXT_TOKEN_LIMIT,
    model_for_tokenizer: str = "",
    # ✅ NEW: scoping controls
    allowed_source_files: Optional[List[str]] = None,
    allowed_doc_types: Optional[List[str]] = None,
) -> Tuple[str, List[Dict]]:
    """
    Deterministic lightweight retrieval (no embeddings):
    - score by token overlap
    - return top-k chunks concatenated as context

    ✅ Scoping:
    - If allowed_source_files provided: only consider chunks whose source_file basename is in the allowlist
    - If allowed_doc_types provided: only consider chunks whose doc_type is in the allowlist
    Scoping is applied BEFORE scoring; returned used_chunks are the true sources for refs.

    Hard limits implemented:
    - top-k <= 4 (default)
    - each chunk <= 500 tokens
    - total context <= 2000 tokens (default)
    """
    query = _clean_text(query)
    if not query or not chunks:
        return "", []

    # ----------------------------
    # ✅ Apply scoping BEFORE scoring
    # ----------------------------
    filtered = chunks or []

    if allowed_source_files:
        allow_files = {_basename(str(x)).strip() for x in allowed_source_files if str(x).strip()}
        if allow_files:
            filtered = [
                c
                for c in filtered
                if _basename(str(c.get("source_file", ""))).strip() in allow_files
            ]

    if allowed_doc_types:
        allow_dt = {str(x).strip() for x in allowed_doc_types if str(x).strip()}
        if allow_dt:
            filtered = [c for c in filtered if str(c.get("doc_type", "")).strip() in allow_dt]

    if not filtered:
        return "", []

    # ✅ Short query gate: avoid wasting time on RAG for greetings / tiny inputs
    q_tokens_list = re.findall(r"[a-zA-Z0-9]+", query.lower())
    if (len(q_tokens_list) < 3) and (len(query) < 20):
        return "", []

    q_tokens = set(q_tokens_list)
    if not q_tokens:
        return "", []

    scored: List[Tuple[int, Dict]] = []
    for c in filtered:
        text = (c.get("text") or "")
        if not text:
            continue
        t_tokens = set(re.findall(r"[a-zA-Z0-9]+", text.lower()))
        score = len(q_tokens.intersection(t_tokens))
        if score >= min_score:
            scored.append((score, c))

    if not scored:
        return "", []

    scored.sort(key=lambda x: x[0], reverse=True)

    # hard cap k
    k = min(int(k or RAG_TOPK_LIMIT), RAG_TOPK_LIMIT)
    top = [c for _, c in scored[:k]]

    # truncate each chunk to <= chunk_token_limit
    used: List[Dict] = []
    truncated_texts: List[str] = []
    total_tokens = 0

    for c in top:
        raw = c.get("text") or ""
        if not raw:
            continue

        t = _truncate_to_tokens(raw, max_tokens=chunk_token_limit, model=model_for_tokenizer)

        # enforce total context tokens cap
        t_tokens = _count_text_tokens(t, model=model_for_tokenizer)
        if total_tokens + t_tokens > max_context_tokens:
            remaining = max_context_tokens - total_tokens
            if remaining <= 0:
                break
            t = _truncate_to_tokens(t, max_tokens=remaining, model=model_for_tokenizer)
            t_tokens = _count_text_tokens(t, model=model_for_tokenizer)

        # legacy char cap safety (keep your previous behavior as extra guard)
        if max_context_chars and max_context_chars > 0:
            current_chars = sum(len(x) for x in truncated_texts)
            if current_chars + len(t) > max_context_chars:
                t = t[: max(0, max_context_chars - current_chars)]

        t = _clean_text(t)
        if not t:
            continue

        truncated_texts.append(t)
        used.append(c)
        total_tokens += t_tokens

        if total_tokens >= max_context_tokens:
            break

    if not truncated_texts:
        return "", []

    context = "\n\n---\n\n".join(truncated_texts)
    return context, used