"""One ``sendMessageDraft`` stream per running turn (§3.8).

A draft is ephemeral and lives 30 s, Telegram throttles edits to about one
per second per chat, and thinking or a tool call would otherwise look like a
hang - so the draft opens with a status line straight away, is refreshed on
a timer rather than on every delta, and is kept alive while nothing changes.
"""

from __future__ import annotations

import asyncio
import contextlib
import logging
import time
import zlib
from typing import TYPE_CHECKING

from aiogram.exceptions import TelegramAPIError, TelegramBadRequest

from beaver_gateway.frontends.telegram.render import to_html

if TYPE_CHECKING:
    from aiogram import Bot

__all__ = ["Draft"]

_log = logging.getLogger("beaver_gateway.frontends.telegram.drafts")

_TAIL = 3500
_KEEPALIVE = 20.0


class Draft:
    def __init__(
        self,
        bot: Bot,
        *,
        chat_id: int,
        thread_id: int | None,
        turn_id: str,
        interval: float = 0.7,
        status: str = "⏳ думаю",
    ) -> None:
        self._bot = bot
        self._chat_id = chat_id
        self._thread_id = thread_id
        self._draft_id = (zlib.crc32(turn_id.encode()) & 0x7FFFFFFF) or 1
        self._interval = interval
        self.status = status
        self.text = ""
        self._dirty = True
        self._broken = False
        self._last_sent = 0.0
        self._task: asyncio.Task[None] | None = None
        self._inflight: asyncio.Future[None] | None = None

    def start(self) -> None:
        if self._task is None:
            self._task = asyncio.create_task(self._run())

    def set_status(self, status: str) -> None:
        if status != self.status:
            self.status = status
            self._dirty = True

    def append(self, text: str) -> None:
        if text:
            self.text += text
            self._dirty = True

    async def stop(self) -> None:
        """Stop pushing and wait for the push in flight.

        The final ``sendMessage`` must reach Telegram after the last draft,
        or the draft lands on top of the message and lingers for its 30 s.
        """
        if self._task is None:
            return
        self._task.cancel()
        with contextlib.suppress(asyncio.CancelledError):
            await self._task
        self._task = None
        if self._inflight is not None:
            with contextlib.suppress(Exception):
                await self._inflight

    async def _run(self) -> None:
        while not self._broken:
            if self._dirty or time.monotonic() - self._last_sent > _KEEPALIVE:
                self._inflight = asyncio.ensure_future(self._push())
                await asyncio.shield(self._inflight)
            await asyncio.sleep(self._interval)

    async def _push(self) -> None:
        self._dirty = False
        self._last_sent = time.monotonic()
        text = self._render()
        try:
            try:
                await self._send(to_html(text), "HTML")
            except TelegramBadRequest:
                await self._send(text, None)
        except TelegramAPIError as exc:
            self._broken = True
            _log.warning(
                "draft to %s/%s stopped: %s", self._chat_id, self._thread_id, exc
            )

    async def _send(self, text: str, parse_mode: str | None) -> None:
        await self._bot.send_message_draft(
            chat_id=self._chat_id,
            draft_id=self._draft_id,
            message_thread_id=self._thread_id,
            text=text,
            parse_mode=parse_mode,
        )

    def _render(self) -> str:
        tail = self.text[-_TAIL:]
        return f"{self.status}\n\n{tail}" if tail.strip() else self.status