import asyncio
import datetime
import logging
from typing import Awaitable, Callable

import discord

from bot.discovery import Target
from bot.storage import Storage

log = logging.getLogger(__name__)

ReactFn = Callable[[discord.Message, Target], Awaitable[bool]]


async def scan_channel_backlog(
    storage: Storage,
    channel: discord.abc.Messageable,
    target: Target,
    friend_id: int,
    cutoff: datetime.datetime,
    react_fn: ReactFn,
) -> None:
    """Scan a channel's history once, resumably.

    First run: walk forward from `cutoff` (now - days_back) to the present.
    Subsequent runs: only diff forward from the newest message already seen,
    since the days_back window only ever moves forward in time.
    """
    channel_id = getattr(channel, "id", None)
    if channel_id is None:
        return

    state = await asyncio.to_thread(storage.get_scan_state, channel_id)
    newest_id_seen = int(state["newest_id_seen"]) if state and state.get("newest_id_seen") else None

    after = discord.Object(id=newest_id_seen) if newest_id_seen else cutoff

    checkpoint_every = 25
    seen_since_checkpoint = 0

    try:
        async for message in channel.history(after=after, oldest_first=True, limit=None):
            if newest_id_seen is None or message.id > newest_id_seen:
                newest_id_seen = message.id

            await react_fn(message, target)

            seen_since_checkpoint += 1
            if seen_since_checkpoint >= checkpoint_every:
                seen_since_checkpoint = 0
                await asyncio.to_thread(
                    storage.set_scan_state, channel_id, target.target_id, newest_id_seen, False
                )

        await asyncio.to_thread(
            storage.set_scan_state, channel_id, target.target_id, newest_id_seen, True
        )
    except discord.Forbidden:
        log.warning("No access to channel %s (%s), skipping", channel_id, getattr(channel, "name", ""))
    except discord.HTTPException:
        log.exception("HTTP error scanning channel %s", channel_id)