import asyncio
import os
from pathlib import Path
from typing import Callable, Awaitable, Optional


class LogWatcher:
    POLL_INTERVAL = 0.1
    FILE_RETRY_INTERVAL = 1.0

    def __init__(
        self,
        path: str,
        on_line: Callable[[str], Awaitable[None]],
        on_offline: Optional[Callable[[], Awaitable[None]]] = None,
        offline_timeout: float = 60.0,
    ):
        self._path = Path(path)
        self._on_line = on_line
        self._on_offline = on_offline
        self._offline_timeout = offline_timeout
        self._task: Optional[asyncio.Task] = None

    def start(self) -> asyncio.Task:
        self._task = asyncio.create_task(self._run())
        return self._task

    def stop(self):
        if self._task:
            self._task.cancel()

    async def _run(self):
        offline_since: Optional[float] = None

        while True:
            try:
                with open(self._path, 'r', errors='replace') as f:
                    f.seek(0, 2)  # start at end of file (tail -f behaviour)
                    offline_since = None
                    while True:
                        line = f.readline()
                        if line:
                            await self._on_line(line)
                        else:
                            await asyncio.sleep(self.POLL_INTERVAL)
            except OSError:
                now = asyncio.get_event_loop().time()
                if offline_since is None:
                    offline_since = now
                elif now - offline_since >= self._offline_timeout and self._on_offline:
                    await self._on_offline()
                    offline_since = None  # reset so we don't spam
                await asyncio.sleep(min(self.FILE_RETRY_INTERVAL, self._offline_timeout))
            except asyncio.CancelledError:
                return