import logging
from asyncio import gather
from asyncio.coroutines import iscoroutinefunction
from asyncio.exceptions import CancelledError
from asyncio.locks import Event, Semaphore
from asyncio.tasks import Task, create_task
from math import inf
from typing import Any, Awaitable, Callable, Dict, Iterable, Iterator, List, Optional, Tuple

from . import exceptions
from .types import ArgsT, KwArgsT, CoroutineFunc, EndCallbackT, CancelCallbackT


log = logging.getLogger(__name__)


class BaseTaskPool:
    """The base class for task pools. Not intended to be used directly."""
    _pools: List['BaseTaskPool'] = []

    @classmethod
    def _add_pool(cls, pool: 'BaseTaskPool') -> int:
        """Adds a `pool` (instance of any subclass) to the general list of pools and returns it's index in the list."""
        cls._pools.append(pool)
        return len(cls._pools) - 1

    def __init__(self, pool_size: int = inf, name: str = None) -> None:
        """Initializes the necessary internal attributes and adds the new pool to the general pools list."""
        self._enough_room: Semaphore = Semaphore()
        self.pool_size = pool_size
        self._open: bool = True
        self._counter: int = 0
        self._running: Dict[int, Task] = {}
        self._cancelled: Dict[int, Task] = {}
        self._ended: Dict[int, Task] = {}
        self._num_cancelled: int = 0
        self._num_ended: int = 0
        self._idx: int = self._add_pool(self)
        self._name: str = name
        self._all_tasks_known_flag: Event = Event()
        self._all_tasks_known_flag.set()
        self._interrupt_flag: Event = Event()
        log.debug("%s initialized", str(self))

    def __str__(self) -> str:
        return f'{self.__class__.__name__}-{self._name or self._idx}'

    @property
    def pool_size(self) -> int:
        """Returns the maximum number of concurrently running tasks currently set in the pool."""
        return self._pool_size

    @pool_size.setter
    def pool_size(self, value: int) -> None:
        """
        Sets the maximum number of concurrently running tasks in the pool.

        Args:
            value:
                A non-negative integer.
                NOTE: Increasing the pool size will immediately start tasks that are awaiting enough room to run.

        Raises:
            `ValueError` if `value` is less than 0.
        """
        if value < 0:
            raise ValueError("Pool size can not be less than 0")
        self._enough_room._value = value
        self._pool_size = value

    @property
    def is_open(self) -> bool:
        """Returns `True` if more the pool has not been closed yet."""
        return self._open

    @property
    def num_running(self) -> int:
        """
        Returns the number of tasks in the pool that are (at that moment) still running.
        At the moment a task's `end_callback` is fired, it is no longer considered to be running.
        """
        return len(self._running)

    @property
    def num_cancelled(self) -> int:
        """
        Returns the number of tasks in the pool that have been cancelled through the pool (up until that moment).
        At the moment a task's `cancel_callback` is fired, it is considered cancelled and no longer running.
        """
        return self._num_cancelled

    @property
    def num_ended(self) -> int:
        """
        Returns the number of tasks started through the pool that have stopped running (up until that moment).
        At the moment a task's `end_callback` is fired, it is considered ended.
        When a task is cancelled, it is not immediately considered ended; only after its `cancel_callback` has returned,
        does it then actually end.
        """
        return self._num_ended

    @property
    def num_finished(self) -> int:
        """
        Returns the number of tasks in the pool that have actually finished running (without having been cancelled).
        """
        return self._num_ended - self._num_cancelled + len(self._cancelled)

    @property
    def is_full(self) -> bool:
        """
        Returns `False` only if (at that moment) the number of running tasks is below the pool's specified size.
        When the pool is full, any call to start a new task within it will block.
        """
        return self._enough_room.locked()

    # TODO: Consider adding task group names
    def _task_name(self, task_id: int) -> str:
        """Returns a standardized name for a task with a specific `task_id`."""
        return f'{self}_Task-{task_id}'

    async def _task_cancellation(self, task_id: int, custom_callback: CancelCallbackT = None) -> None:
        """
        Universal callback to be run upon any task in the pool being cancelled.
        Required for keeping track of running/cancelled tasks and proper logging.

        Args:
            task_id:
                The ID of the task that has been cancelled.
            custom_callback (optional):
                A callback to execute after cancellation of the task.
                It is run at the end of this function with the `task_id` as its only positional argument.
        """
        log.debug("Cancelling %s ...", self._task_name(task_id))
        self._cancelled[task_id] = self._running.pop(task_id)
        self._num_cancelled += 1
        log.debug("Cancelled %s", self._task_name(task_id))
        await _execute_function(custom_callback, args=(task_id, ))

    async def _task_ending(self, task_id: int, custom_callback: EndCallbackT = None) -> None:
        """
        Universal callback to be run upon any task in the pool ending its work.
        Required for keeping track of running/cancelled/ended tasks and proper logging.
        Also releases room in the task pool for potentially waiting tasks.

        Args:
            task_id:
                The ID of the task that has reached its end.
            custom_callback (optional):
                A callback to execute after the task has ended.
                It is run at the end of this function with the `task_id` as its only positional argument.
        """
        try:
            self._ended[task_id] = self._running.pop(task_id)
        except KeyError:
            self._ended[task_id] = self._cancelled.pop(task_id)
        self._num_ended += 1
        self._enough_room.release()
        log.info("Ended %s", self._task_name(task_id))
        await _execute_function(custom_callback, args=(task_id, ))

    async def _task_wrapper(self, awaitable: Awaitable, task_id: int, end_callback: EndCallbackT = None,
                            cancel_callback: CancelCallbackT = None) -> Any:
        """
        Universal wrapper around every task to be run in the pool.
        Returns/raises whatever the wrapped coroutine does.

        Args:
            awaitable:
                The actual coroutine to be run within the task pool.
            task_id:
                The ID of the newly created task.
            end_callback (optional):
                A callback to execute after the task has ended.
                It is run with the `task_id` as its only positional argument.
            cancel_callback (optional):
                A callback to execute after cancellation of the task.
                It is run with the `task_id` as its only positional argument.
        """
        log.info("Started %s", self._task_name(task_id))
        try:
            return await awaitable
        except CancelledError:
            await self._task_cancellation(task_id, custom_callback=cancel_callback)
        finally:
            await self._task_ending(task_id, custom_callback=end_callback)

    async def _start_task(self, awaitable: Awaitable, ignore_closed: bool = False, end_callback: EndCallbackT = None,
                          cancel_callback: CancelCallbackT = None) -> int:
        """
        Starts a coroutine as a new task in the pool.
        This method blocks, **only if** the pool is full.
        Returns/raises whatever the wrapped coroutine does.

        Args:
            awaitable:
                The actual coroutine to be run within the task pool.
            ignore_closed (optional):
                If `True`, even if the pool is closed, the task will still be started.
            end_callback (optional):
                A callback to execute after the task has ended.
                It is run with the `task_id` as its only positional argument.
            cancel_callback (optional):
                A callback to execute after cancellation of the task.
                It is run with the `task_id` as its only positional argument.

        Raises:
            `asyncio_taskpool.exceptions.PoolIsClosed` if the pool has been closed and `ignore_closed` is `False`.
        """
        if not (self.is_open or ignore_closed):
            raise exceptions.PoolIsClosed("Cannot start new tasks")
        await self._enough_room.acquire()
        task_id = self._counter
        self._counter += 1
        try:
            self._running[task_id] = create_task(
                self._task_wrapper(awaitable, task_id, end_callback, cancel_callback),
                name=self._task_name(task_id)
            )
        except Exception as e:
            self._enough_room.release()
            raise e
        return task_id

    def _get_running_task(self, task_id: int) -> Task:
        """
        Gets a running task by its task ID.

        Args:
            task_id: The ID of a task still running within the pool.

        Raises:
            `asyncio_taskpool.exceptions.AlreadyCancelled` if the task with `task_id` has been (recently) cancelled.
            `asyncio_taskpool.exceptions.AlreadyEnded` if the task with `task_id` has ended (recently).
            `asyncio_taskpool.exceptions.InvalidTaskID` if no task with `task_id` is known to the pool.
        """
        try:
            return self._running[task_id]
        except KeyError:
            if self._cancelled.get(task_id):
                raise exceptions.AlreadyCancelled(f"{self._task_name(task_id)} has already been cancelled")
            if self._ended.get(task_id):
                raise exceptions.AlreadyEnded(f"{self._task_name(task_id)} has finished running")
            raise exceptions.InvalidTaskID(f"No task with ID {task_id} found in {self}")

    def cancel(self, *task_ids: int, msg: str = None) -> None:
        """
        Cancels the tasks with the specified IDs.

        Each task ID must belong to a task still running within the pool. Otherwise one of the following exceptions will
        be raised:
        - `AlreadyCancelled` if one of the `task_ids` belongs to a task that has been (recently) cancelled.
        - `AlreadyEnded` if one of the `task_ids` belongs to a task that has ended (recently).
        - `InvalidTaskID` if any of the `task_ids` is not known to the pool.
        Note that once a pool has been flushed, any IDs of tasks that have ended previously will be forgotten.

        Args:
            task_ids:
                Arbitrary number of integers. Each must be an ID of a task still running within the pool.
            msg (optional):
                Passed to the `Task.cancel()` method of every task specified by the `task_ids`.
        """
        tasks = [self._get_running_task(task_id) for task_id in task_ids]
        for task in tasks:
            task.cancel(msg=msg)

    def cancel_all(self, msg: str = None) -> None:
        """
        Cancels all tasks still running within the pool.

        Note that there may be an unknown number of coroutine functions "queued" to be run as tasks.
        This can happen, if for example the `TaskPool.map` method was called with `num_tasks` set to a number smaller
        than the number of arguments from `args_iter`.
        In this case, those already running will be cancelled, while the following will **never even start**.

        Args:
            msg (optional):
                Passed to the `Task.cancel()` method of every task specified by the `task_ids`.
        """
        log.warning("%s cancelling all tasks!", str(self))
        self._interrupt_flag.set()
        for task in self._running.values():
            task.cancel(msg=msg)

    async def flush(self, return_exceptions: bool = False):
        """
        Calls `asyncio.gather` on all ended/cancelled tasks from the pool, returns their results, and forgets the tasks.
        This method blocks, **only if** any of the tasks block while catching a `asyncio.CancelledError` or any of the
        callbacks registered for the tasks block.

        Args:
            return_exceptions (optional): Passed directly into `gather`.
        """
        results = await gather(*self._ended.values(), *self._cancelled.values(), return_exceptions=return_exceptions)
        self._ended = self._cancelled = {}
        if self._interrupt_flag.is_set():
            self._interrupt_flag.clear()
        return results

    def close(self) -> None:
        """Disallows any more tasks to be started in the pool."""
        self._open = False
        log.info("%s is closed!", str(self))

    async def gather(self, return_exceptions: bool = False):
        """
        Calls `asyncio.gather` on **all** tasks from the pool, returns their results, and forgets the tasks.

        The `close()` method must have been called prior to this.

        Note that there may be an unknown number of coroutine functions "queued" to be run as tasks.
        This can happen, if for example the `TaskPool.map` method was called with `num_tasks` set to a number smaller
        than the number of arguments from `args_iter`.
        In this case, calling `cancel_all()` prior to this, will prevent those tasks from starting and potentially
        blocking this method. Otherwise it will wait until they all have started.

        This method may also block, if any task blocks while catching a `asyncio.CancelledError` or if any of the
        callbacks registered for a task blocks.

        Args:
            return_exceptions (optional): Passed directly into `gather`.

        Raises:
            `asyncio_taskpool.exceptions.PoolStillOpen` if the pool has not been closed yet.
        """
        if self._open:
            raise exceptions.PoolStillOpen("Pool must be closed, before tasks can be gathered")
        await self._all_tasks_known_flag.wait()
        results = await gather(*self._ended.values(), *self._cancelled.values(), *self._running.values(),
                               return_exceptions=return_exceptions)
        self._ended = self._cancelled = self._running = {}
        if self._interrupt_flag.is_set():
            self._interrupt_flag.clear()
        return results


class TaskPool(BaseTaskPool):
    async def _apply_one(self, func: CoroutineFunc, args: ArgsT = (), kwargs: KwArgsT = None,
                         end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> int:
        if kwargs is None:
            kwargs = {}
        return await self._start_task(func(*args, **kwargs), end_callback=end_callback, cancel_callback=cancel_callback)

    async def apply(self, func: CoroutineFunc, args: ArgsT = (), kwargs: KwArgsT = None, num: int = 1,
                    end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> Tuple[int]:
        return tuple(await self._apply_one(func, args, kwargs, end_callback, cancel_callback) for _ in range(num))

    @staticmethod
    def _get_next_coroutine(func: CoroutineFunc, args_iter: Iterator[Any], arg_stars: int = 0) -> Optional[Awaitable]:
        try:
            arg = next(args_iter)
        except StopIteration:
            return
        if arg_stars == 0:
            return func(arg)
        if arg_stars == 1:
            return func(*arg)
        if arg_stars == 2:
            return func(**arg)
        raise ValueError

    async def _map(self, func: CoroutineFunc, args_iter: ArgsT, arg_stars: int = 0, num_tasks: int = 1,
                   end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:

        if self._all_tasks_known_flag.is_set():
            self._all_tasks_known_flag.clear()
        args_iter = iter(args_iter)

        async def _start_next_coroutine() -> bool:
            cor = self._get_next_coroutine(func, args_iter, arg_stars)
            if cor is None or self._interrupt_flag.is_set():
                self._all_tasks_known_flag.set()
                return True
            await self._start_task(cor, ignore_closed=True, end_callback=_start_next, cancel_callback=cancel_callback)
            return False

        async def _start_next(task_id: int) -> None:
            await _start_next_coroutine()
            await _execute_function(end_callback, args=(task_id, ))

        for _ in range(num_tasks):
            reached_end = await _start_next_coroutine()
            if reached_end:
                break

    async def map(self, func: CoroutineFunc, args_iter: ArgsT, num_tasks: int = 1,
                  end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
        await self._map(func, args_iter, arg_stars=0, num_tasks=num_tasks,
                        end_callback=end_callback, cancel_callback=cancel_callback)

    async def starmap(self, func: CoroutineFunc, args_iter: Iterable[ArgsT], num_tasks: int = 1,
                      end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
        await self._map(func, args_iter, arg_stars=1, num_tasks=num_tasks,
                        end_callback=end_callback, cancel_callback=cancel_callback)

    async def doublestarmap(self, func: CoroutineFunc, kwargs_iter: Iterable[KwArgsT], num_tasks: int = 1,
                            end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
        await self._map(func, kwargs_iter, arg_stars=2, num_tasks=num_tasks,
                        end_callback=end_callback, cancel_callback=cancel_callback)


class SimpleTaskPool(BaseTaskPool):
    def __init__(self, func: CoroutineFunc, args: ArgsT = (), kwargs: KwArgsT = None,
                 end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None,
                 name: str = None) -> None:
        self._func: CoroutineFunc = func
        self._args: ArgsT = args
        self._kwargs: KwArgsT = kwargs if kwargs is not None else {}
        self._end_callback: EndCallbackT = end_callback
        self._cancel_callback: CancelCallbackT = cancel_callback
        super().__init__(name=name)

    @property
    def func_name(self) -> str:
        return self._func.__name__

    @property
    def size(self) -> int:
        return self.num_running

    async def _start_one(self) -> int:
        return await self._start_task(self._func(*self._args, **self._kwargs),
                                      end_callback=self._end_callback, cancel_callback=self._cancel_callback)

    async def start(self, num: int = 1) -> List[int]:
        return [await self._start_one() for _ in range(num)]

    def stop(self, num: int = 1) -> List[int]:
        num = min(num, self.size)
        ids = []
        for i, task_id in enumerate(reversed(self._running)):
            if i >= num:
                break
            ids.append(task_id)
        self.cancel(*ids)
        return ids

    def stop_all(self) -> List[int]:
        return self.stop(self.size)


async def _execute_function(func: Callable, args: ArgsT = (), kwargs: KwArgsT = None) -> None:
    if kwargs is None:
        kwargs = {}
    if callable(func):
        if iscoroutinefunction(func):
            await func(*args, **kwargs)
        else:
            func(*args, **kwargs)