"""
Backfill `device_serials` on Firestore `users` docs from the devices' `user_list`.

`device_serials` is what the MQTT ACL uses to decide which vesper/{serial}/...
topics an app user may reach (see mqtt/app_users.py). New assignments keep it
in sync automatically; this script builds it for assignments that predate that.

The devices' user_list is treated as the source of truth: each user's
device_serials is set to exactly the serials of the devices that list them.
Running it twice is a no-op the second time.

Also reports:
  - users whose document ID != their `uid` field (or who have no uid) — the
    MQTT layer resolves users by the uid field, so this is informational,
    but a user without a uid can never connect via the app
  - devices that have users but no serial (can't be reached over MQTT)
  - user_list entries that point to users that don't exist

Run from backend/ (locally or inside the backend container):

    python scripts/backfill_user_device_serials.py           # dry run
    python scripts/backfill_user_device_serials.py --apply   # write changes
"""

import argparse
import sys
from collections import defaultdict
from pathlib import Path

sys.path.insert(0, str(Path(__file__).resolve().parent.parent))

from google.cloud.firestore_v1 import DocumentReference  # noqa: E402

from shared.firebase import init_firebase, get_db  # noqa: E402
from users.service import device_serial_of  # noqa: E402

BATCH_SIZE = 400  # Firestore batch limit is 500 writes


def _entry_user_id(entry) -> str:
    """user_list entries are DocumentReferences, "users/{id}" paths or raw doc IDs."""
    if isinstance(entry, DocumentReference):
        return entry.id
    if isinstance(entry, str) and entry.strip():
        return entry.strip().split("/")[-1]
    return ""


def main() -> int:
    parser = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
    parser.add_argument("--apply", action="store_true", help="write changes (default: dry run)")
    args = parser.parse_args()

    init_firebase()
    db = get_db()
    if db is None:
        print("[ERROR] Firebase not initialized — check FIREBASE_SERVICE_ACCOUNT_PATH")
        return 1

    mode = "APPLY" if args.apply else "DRY RUN"
    print(f"=== backfill device_serials ({mode}) ===\n")

    # 1. devices -> {user_doc_id: {serials}}
    wanted: dict[str, set[str]] = defaultdict(set)
    devices_without_serial = []
    device_count = 0
    for doc in db.collection("devices").stream():
        device_count += 1
        data = doc.to_dict() or {}
        user_ids = [u for u in (_entry_user_id(e) for e in (data.get("user_list") or [])) if u]
        if not user_ids:
            continue
        serial = device_serial_of(data)
        if not serial:
            devices_without_serial.append(doc.id)
            continue
        for uid in user_ids:
            wanted[uid].add(serial)

    # 2. users
    users = {doc.id: (doc.reference, doc.to_dict() or {}) for doc in db.collection("users").stream()}
    by_uid_field = {data.get("uid"): doc_id for doc_id, (_, data) in users.items() if data.get("uid")}

    # user_list entries that don't match a doc ID but do match a uid field
    dangling = []
    for ref_id in list(wanted):
        if ref_id in users:
            continue
        if ref_id in by_uid_field:
            wanted[by_uid_field[ref_id]].update(wanted.pop(ref_id))
        else:
            dangling.append((ref_id, sorted(wanted.pop(ref_id))))

    id_mismatch = []
    no_uid = []
    changes = []
    for doc_id, (ref, data) in sorted(users.items()):
        uid = data.get("uid") or ""
        if not uid:
            no_uid.append(doc_id)
        elif uid != doc_id:
            id_mismatch.append((doc_id, uid))

        current = data.get("device_serials")
        desired = sorted(wanted.get(doc_id, set()))
        if current is None and not desired:
            continue  # nothing assigned and field absent — leave the doc alone
        if list(current or []) != desired:
            changes.append((doc_id, ref, list(current or []), desired))

    # 3. report
    print(f"Scanned {device_count} devices, {len(users)} users.\n")

    print(f"Users to update: {len(changes)}")
    for doc_id, _, current, desired in changes:
        print(f"  users/{doc_id}: {current} -> {desired}")

    print(f"\nUsers whose doc ID != uid: {len(id_mismatch)}")
    for doc_id, uid in id_mismatch:
        print(f"  users/{doc_id}  uid={uid}")

    print(f"\nUsers with no uid field (cannot use app MQTT login): {len(no_uid)}")
    for doc_id in no_uid:
        print(f"  users/{doc_id}")

    print(f"\nDevices with users but no serial_number/device_id: {len(devices_without_serial)}")
    for doc_id in devices_without_serial:
        print(f"  devices/{doc_id}")

    print(f"\nuser_list entries pointing to missing users: {len(dangling)}")
    for ref_id, serials in dangling:
        print(f"  {ref_id}  (on devices {serials})")

    # 4. write
    if not args.apply:
        print("\nDry run — nothing written. Re-run with --apply to write.")
        return 0

    for i in range(0, len(changes), BATCH_SIZE):
        batch = db.batch()
        for _, ref, _, desired in changes[i:i + BATCH_SIZE]:
            batch.update(ref, {"device_serials": desired})
        batch.commit()
    print(f"\nWrote device_serials on {len(changes)} user docs.")
    return 0


if __name__ == "__main__":
    sys.exit(main())