"""
Threading resolution.

Given a freshly-inserted emails row (or its about-to-be-inserted parsed dict),
figure out which email_threads row it belongs to. If none exists, create one.

Priority order (see architecture doc §10):
  1. Existing email in our DB that shares any RFC Message-ID with this one's
     References/In-Reply-To chain — inherit its thread.
  2. Existing thread with the same gmail_thread_id.
  3. Existing thread with the same rfc_thread_root_id.
  4. Create a new thread.
"""

import re
import sqlite3
from typing import Optional

from . import db as _db


SUBJECT_PREFIX_RE = re.compile(r"^\s*(re|fwd?|aw|sv|antw|回复|fw)\s*:\s*", re.IGNORECASE)


def normalize_subject(s: Optional[str]) -> Optional[str]:
    if not s:
        return None
    out = s
    # Strip up to 5 nested Re:/Fwd: prefixes.
    for _ in range(5):
        new = SUBJECT_PREFIX_RE.sub("", out, count=1)
        if new == out:
            break
        out = new
    return out.strip().lower()


def resolve_thread_id(conn: sqlite3.Connection, parsed: dict) -> str:
    """Returns a thread_id — either an existing one or a newly created one."""
    # Step 1: RFC cross-link.
    candidates = list(parsed.get("references") or [])
    if parsed.get("in_reply_to"):
        candidates.append(parsed["in_reply_to"])

    if candidates:
        existing_email_id = _db.find_email_id_by_any_rfc_id(conn, candidates)
        if existing_email_id:
            cur = conn.cursor()
            cur.execute("SELECT thread_id FROM emails WHERE id = ?", (existing_email_id,))
            row = cur.fetchone()
            if row and row["thread_id"]:
                return row["thread_id"]

    # Step 2: gmail_thread_id match.
    gt = parsed.get("gmail_thread_id")
    if gt:
        existing = _db.find_thread_by_gmail_id(conn, gt)
        if existing:
            return existing["id"]

    # Step 3: RFC thread-root match.
    rfc_root = parsed.get("rfc_thread_root_id")
    if rfc_root:
        existing = _db.find_thread_by_rfc_root(conn, rfc_root)
        if existing:
            return existing["id"]

    # Step 4: create.
    return _db.create_thread(
        conn,
        gmail_thread_id=gt,
        rfc_thread_root_id=rfc_root,
        subject_normalized=normalize_subject(parsed.get("subject")),
        first_message_at=parsed.get("sent_at"),
    )