"""
Media-hash helpers.

Strategy
--------
We never download files just to hash them. Instead we use Telegram's
file_unique_id as a stable, server-side content identifier – two files
that are byte-for-byte identical always share the same file_unique_id,
regardless of who uploaded them or when.

The cache (backup_ids.json) stores:
{ "<file_unique_id>": "<sha256_hex>" }
"""

import hashlib
import logging

from aiogram import Bot

import bot_state as state
from persistence import save_backup_ids

logger = logging.getLogger(__name__)


def _hash(file_unique_id: str) -> str:
    return hashlib.sha256(file_unique_id.encode()).hexdigest()


def register_file(file_unique_id: str) -> bool:
    """Add a file to the in-memory cache and persist it.

    Returns True if the file was new, False if it was already known.
    """
    if file_unique_id in state.backup_hashes:
        return False
    state.backup_hashes[file_unique_id] = _hash(file_unique_id)
    save_backup_ids()
    return True


def is_duplicate(file_unique_id: str) -> bool:
    return file_unique_id in state.backup_hashes


def check_media_list(media: list[dict]) -> tuple[list[dict], list[dict]]:
    """Split a media list into (unique, duplicates)."""
    unique, dupes = [], []
    for item in media:
        if is_duplicate(item.get("file_unique_id", "")):
            dupes.append(item)
        else:
            unique.append(item)
    return unique, dupes


async def preload_backup_hashes(bot: Bot, chat_id: int) -> None:
    """
    Since the standard Bot API does not expose a bulk message history
    endpoint for groups, we:
    1. Load whatever is already in backup_ids.json (done by load_backup_ids).
    2. On each new message in the backup group, register via register_file.

    To do a full historical reindex, admins forward all older media back
    into the backup group — the bot registers each file automatically.
    """
    logger.info(
        "Backup hash cache loaded from disk: %d known files. "
        "New files arriving in the backup group will be indexed automatically.",
        len(state.backup_hashes),
    )