module Quasar.ResourceManager (
  -- * MonadResourceManager
  MonadResourceManager(..),
  FailedToRegisterResource,
  registerNewResource,
  registerDisposable,
  registerDisposeAction,
  registerSimpleDisposeAction,
  disposeEventually,
  withSubResourceManagerM,
  onResourceManager,
  captureDisposable,
  captureDisposable_,
  captureTask,

  -- ** ResourceManager
  IsResourceManager(..),
  ResourceManager,
  newResourceManager,
  attachDisposeAction,
  attachDisposeAction_,

  -- ** Initialization
  CombinedException,
  withRootResourceManager,

  -- ** Linking computations to a resource manager
  linkExecution,
  CancelLinkedExecution,

  -- ** Resource manager implementations
  newUnmanagedRootResourceManager,
  --newUnmanagedDefaultResourceManager,
) where


import Control.Concurrent (ThreadId, forkIOWithUnmask, myThreadId, throwTo)
import Control.Concurrent.STM
import Control.Monad.Catch
import Control.Monad.Reader
import Data.Foldable (toList)
import Data.List.NonEmpty (NonEmpty(..), nonEmpty)
import Data.List.NonEmpty qualified as NonEmpty
import Data.Sequence
import Data.Sequence qualified as Seq
import Quasar.Awaitable
import Quasar.Disposable
import Quasar.Prelude
import Quasar.Utils.Concurrent


data FailedToRegisterResource = FailedToRegisterResource
  deriving stock (Eq, Show)

instance Exception FailedToRegisterResource where
  displayException FailedToRegisterResource =
    "FailedToRegisterResource: Failed to register a resource to a resource manager. This might result in leaked resources if left unhandled."

-- | Internal entry of `ResourceManager`. The `TMVar` will be set to `Nothing` when the disposable has completed disposing.
newtype ResourceManagerEntry = ResourceManagerEntry (TMVar (Awaitable (), Disposable))

instance IsAwaitable () ResourceManagerEntry where
  toAwaitable (ResourceManagerEntry var) = do
    varContents <- unsafeAwaitSTM $ tryReadTMVar var
    case varContents of
      -- If the var is empty the Entry has already been disposed
      Nothing -> pure ()
      Just (awaitable, _) -> awaitable


newEntry :: IsDisposable a => a -> IO ResourceManagerEntry
newEntry disposable = do
  disposedAwaitable <- cacheAwaitable (isDisposed disposable)
  ResourceManagerEntry <$> newTMVarIO (disposedAwaitable, toDisposable disposable)

checkEntries :: Seq ResourceManagerEntry -> IO ()
checkEntries = mapM_ checkEntry

checkEntry :: ResourceManagerEntry -> IO ()
checkEntry (ResourceManagerEntry var) = do
  atomically (tryReadTMVar var) >>= \case
    Nothing -> pure ()
    Just (awaitable, _) -> do
      completed <- isJust <$> peekAwaitable awaitable
      when completed $ atomically $ void $ tryTakeTMVar var

entryIsEmpty :: ResourceManagerEntry -> STM Bool
entryIsEmpty (ResourceManagerEntry var) = isEmptyTMVar var


class IsDisposable a => IsResourceManager a where
  toResourceManager :: a -> ResourceManager
  toResourceManager = ResourceManager

  -- | Attaches an `Disposable` to a ResourceManager. It will automatically be disposed when the resource manager is disposed.
  attachDisposable :: (IsDisposable b, MonadIO m) => a -> b -> m ()
  attachDisposable self = attachDisposable (toResourceManager self)

  --subResourceManager :: MonadResourceManager m => m (DisposableResourceThingy)

  -- | Forward an exception that happened asynchronously.
  throwToResourceManager :: Exception e => a -> e -> IO ()
  throwToResourceManager = throwToResourceManager . toResourceManager

  {-# MINIMAL toResourceManager | (attachDisposable, throwToResourceManager) #-}


data ResourceManager = forall a. IsResourceManager a => ResourceManager a
instance IsResourceManager ResourceManager where
  toResourceManager = id
  attachDisposable (ResourceManager x) = attachDisposable x
  throwToResourceManager (ResourceManager x) = throwToResourceManager x
instance IsDisposable ResourceManager where
  toDisposable (ResourceManager x) = toDisposable x

class (MonadAwait m, MonadMask m, MonadIO m, MonadFix m) => MonadResourceManager m where
  -- | Get the underlying resource manager.
  askResourceManager :: m ResourceManager

  -- | Replace the resource manager for a computation.
  localResourceManager :: IsResourceManager a => a -> m r -> m r


registerDisposable :: (IsDisposable a, MonadResourceManager m) => a -> m ()
registerDisposable disposable = do
  resourceManager <- askResourceManager
  attachDisposable resourceManager disposable


registerDisposeAction :: MonadResourceManager m => IO (Awaitable ()) -> m ()
registerDisposeAction disposeAction = mask_ $ registerDisposable =<< newDisposable disposeAction

registerSimpleDisposeAction :: MonadResourceManager m => IO () -> m ()
registerSimpleDisposeAction disposeAction = registerDisposeAction (pure () <$ disposeAction)

registerNewResource :: (IsDisposable a, MonadResourceManager m) => m a -> m a
registerNewResource action = mask_ do
  afix \awaitable -> do
    registerDisposeAction $ either (\(_ :: SomeException) -> mempty) dispose =<< try (await awaitable)
    action


-- TODO rename to withResourceScope?
withSubResourceManagerM :: MonadResourceManager m => m a -> m a
withSubResourceManagerM action =
  bracket newResourceManager (await <=< dispose) \scope -> localResourceManager scope action


instance (MonadAwait m, MonadMask m, MonadIO m, MonadFix m) => MonadResourceManager (ReaderT ResourceManager m) where
  localResourceManager resourceManager = local (const (toResourceManager resourceManager))

  askResourceManager = ask


instance {-# OVERLAPPABLE #-} MonadResourceManager m => MonadResourceManager (ReaderT r m) where
  askResourceManager = lift askResourceManager

  localResourceManager resourceManager action = do
    x <- ask
    lift $ localResourceManager resourceManager $ runReaderT action x


-- TODO MonadResourceManager instances for StateT, WriterT, RWST, MaybeT, ...


onResourceManager :: (IsResourceManager a, MonadIO m) => a -> ReaderT ResourceManager IO r -> m r
onResourceManager target action = liftIO $ runReaderT action (toResourceManager target)


captureDisposable :: MonadResourceManager m => m a -> m (a, Disposable)
captureDisposable action = do
  -- TODO improve performance by only creating a new resource manager when two or more disposables are attached
  resourceManager <- newResourceManager
  result <- localResourceManager resourceManager action
  pure $ (result, toDisposable resourceManager)

captureDisposable_ :: MonadResourceManager m => m () -> m Disposable
captureDisposable_ = snd <<$>> captureDisposable

captureTask :: MonadResourceManager m => m (Awaitable a) -> m (Task a)
captureTask action = do
  (awaitable, disposable) <- captureDisposable action
  pure $ Task disposable awaitable



-- | A computation bound to a resource manager with 'linkThread' should be canceled.
data CancelLinkedExecution = CancelLinkedExecution Unique
  deriving anyclass Exception

instance Show CancelLinkedExecution where
  show _ = "CancelLinkedExecution"


data LinkState = LinkStateLinked ThreadId | LinkStateThrowing | LinkStateCompleted
  deriving stock Eq


-- | Links the execution of a computation to a resource manager.
--
-- The computation is executed on the current thread. When the resource manager is disposed before the computation
-- is completed, a `CancelLinkedExecution`-exception is thrown to the current thread.
linkExecution :: MonadResourceManager m => m a -> m (Maybe a)
linkExecution action = do
  key <- liftIO $ newUnique
  var <- liftIO $ newTVarIO =<< LinkStateLinked <$> myThreadId
  registerSimpleDisposeAction $ do
    atomically (swapTVar var LinkStateThrowing) >>= \case
      LinkStateLinked threadId -> throwTo threadId $ CancelLinkedExecution key
      LinkStateThrowing -> pure () -- Dispose called twice
      LinkStateCompleted -> pure () -- Thread has already left link

  catch
    do
      result <- action
      state <- liftIO $ atomically $ swapTVar var LinkStateCompleted
      when (state == LinkStateThrowing) $ sleepForever -- Wait for exception to arrive
      pure $ Just result

    \ex@(CancelLinkedExecution exceptionKey) ->
      if key == exceptionKey
        then return Nothing
        else throwM ex



-- * Resource manager implementations

-- ** Root resource manager

newtype CombinedException = CombinedException (NonEmpty SomeException)
  deriving stock Show

instance Exception CombinedException where
  displayException (CombinedException exceptions) = intercalate "\n" (header : exceptionMessages)
    where
      header = mconcat ["CombinedException with ", show (NonEmpty.length exceptions), "exceptions:"]
      exceptionMessages = (displayException <$> toList exceptions)


data RootResourceManagerState
  = RootResourceManagerNormal
  | RootResourceManagerDisposing
  | RootResourceManagerDisposed
  deriving stock Eq

data RootResourceManager
  = RootResourceManager
      ResourceManager
      (TVar RootResourceManagerState)
      (TVar (Seq SomeException))
      (Awaitable ())

instance IsResourceManager RootResourceManager where
  attachDisposable (RootResourceManager child _ _ _) disposable = attachDisposable child disposable
  throwToResourceManager (RootResourceManager _ stateVar exceptionsVar _) ex = do
    -- TODO only log exceptions when disposing does not finish in time
    traceIO $ "Exception thrown to root resource manager: " <> displayException ex
    disposed <- liftIO $ atomically do
      state <- readTVar stateVar
      -- Start disposing
      when (state == RootResourceManagerNormal) $ writeTVar stateVar RootResourceManagerDisposing
      let disposed = state == RootResourceManagerDisposed

      unless disposed $ modifyTVar exceptionsVar (|> toException ex)
      pure disposed

    when disposed $ fail "Could not throw to resource manager: RootResourceManager is already disposed"


instance IsDisposable RootResourceManager where
  dispose (RootResourceManager _ stateVar _ isDisposedAwaitable) = do
    liftIO $ atomically do
      state <- readTVar stateVar
      -- Start disposing
      when (state == RootResourceManagerNormal) $ writeTVar stateVar RootResourceManagerDisposing
    pure isDisposedAwaitable
  isDisposed (RootResourceManager _ _ _ isDisposedAwaitable) = isDisposedAwaitable

newUnmanagedRootResourceManager :: MonadIO m => m ResourceManager
newUnmanagedRootResourceManager = liftIO $ toResourceManager <$> do
  stateVar <- newTVarIO RootResourceManagerNormal
  exceptionsVar <- newTVarIO Empty
  mfix \root -> do
    isDisposedAwaitable <- toAwaitable <$> unmanagedFork (disposeThread root)
    child <- newUnmanagedDefaultResourceManager (toResourceManager root)
    pure $ RootResourceManager child stateVar exceptionsVar isDisposedAwaitable
  where
    disposeThread :: RootResourceManager -> IO ()
    disposeThread (RootResourceManager child stateVar exceptionsVar _) = do
      atomically do
        state <- readTVar stateVar
        when (state == RootResourceManagerNormal) retry
      -- TODO start thread: wait for timeout, then report exceptions or report hang
      await =<< dispose child
      atomically do
        exceptions <- nonEmpty . toList <$> readTVar exceptionsVar
        mapM_ (throwM . CombinedException) exceptions


withRootResourceManager :: (MonadAwait m, MonadMask m, MonadIO m) => ReaderT ResourceManager IO a -> m a
withRootResourceManager action =
  bracket
    newUnmanagedRootResourceManager
    (await <=< dispose)
    (`onResourceManager` action)


-- ** Default resource manager

data DefaultResourceManager = DefaultResourceManager {
  parentResourceManager :: ResourceManager,
  disposingVar :: TVar Bool,
  disposedVar :: TVar Bool,
  entriesVar :: TVar (Seq ResourceManagerEntry)
}

instance IsResourceManager DefaultResourceManager where
  throwToResourceManager DefaultResourceManager{parentResourceManager} = throwToResourceManager parentResourceManager

  attachDisposable resourceManager disposable = liftIO $ mask_ do
    entry <- newEntry disposable

    join $ atomically do
      disposing <- readTVar (disposingVar resourceManager)

      unless disposing $ modifyTVar (entriesVar resourceManager) (|> entry)

      pure do
        -- IO that is run after the STM transaction is completed
        when disposing $
          throwM FailedToRegisterResource `catchAll` throwToResourceManager resourceManager

instance IsDisposable DefaultResourceManager where
  dispose resourceManager = liftIO $ mask_ do
    entries <- atomically do
      isAlreadyDisposing <- swapTVar (disposingVar resourceManager) True
      if not isAlreadyDisposing
        then readTVar (entriesVar resourceManager)
        else pure Empty

    mapM_ entryStartDispose entries
    pure $ isDisposed resourceManager
    where
      entryStartDispose :: ResourceManagerEntry -> IO ()
      entryStartDispose (ResourceManagerEntry var) =
        atomically (tryReadTMVar var) >>= \case
          Nothing -> pure ()
          Just (_, disposable) ->
            catchAll
              do void (dispose disposable)
              \ex -> do
                -- Disposable failed so it should be removed
                atomically (void $ tryTakeTMVar var)
                -- This will only throw if the parent is disposed, which would be an illegal state
                -- TODO wrap in a 'DisposeException'
                throwToResourceManager resourceManager ex


  isDisposed resourceManager =
    unsafeAwaitSTM do
      disposed <- readTVar (disposedVar resourceManager)
      unless disposed retry

newResourceManager :: MonadResourceManager m => m ResourceManager
newResourceManager = mask_ do
  parent <- askResourceManager
  -- TODO: return efficent resource manager
  resourceManager <- newUnmanagedDefaultResourceManager parent
  registerDisposable resourceManager
  pure resourceManager

newUnmanagedDefaultResourceManager :: MonadIO m => ResourceManager -> m ResourceManager
newUnmanagedDefaultResourceManager parentResourceManager = liftIO do
  disposingVar <- newTVarIO False
  disposedVar <- newTVarIO False
  entriesVar <- newTVarIO Empty

  let resourceManager = DefaultResourceManager {
    parentResourceManager,
    disposingVar,
    disposedVar,
    entriesVar
  }

  void $ mask_ $ forkIOWithUnmask \unmask ->
    unmask (freeGarbage resourceManager)
      `catchAll`
        \ex -> throwToResourceManager resourceManager (userError ("freeGarbage failed for DefaultResourceManager: " <> displayException ex))

  pure $ toResourceManager resourceManager


freeGarbage :: DefaultResourceManager -> IO ()
freeGarbage resourceManager = go
  where
    go :: IO ()
    go = do
      snapshot <- atomically $ readTVar entriesVar'

      let listChanged = unsafeAwaitSTM do
            newLength <- Seq.length <$> readTVar entriesVar'
            when (newLength == Seq.length snapshot) retry

          isDisposing = unsafeAwaitSTM do
            disposing <- readTVar (disposingVar resourceManager)
            unless disposing retry

      -- Wait for any entry to complete or until a new entry is added
      let awaitables = (toAwaitable <$> toList snapshot)
      -- GC fails here when an waitable throws an exception
      void if Quasar.Prelude.null awaitables
        then awaitAny2 listChanged isDisposing
        else awaitAny (listChanged :| awaitables)

      -- Checking entries for completion has to be done in IO.
      -- Completion is queried with `entryIsEmpty` during the following STM transaction for legacy reasons (the resource
      -- manager once did allow to add resources while disposing). This could be simplified now.
      checkEntries =<< atomically (readTVar entriesVar')

      join $ atomically $ do
        disposing <- readTVar (disposingVar resourceManager)

        -- Filter completed entries
        allEntries <- readTVar entriesVar'
        filteredEntries <- foldM (\acc entry -> entryIsEmpty entry >>= \isEmpty -> pure if isEmpty then acc else acc |> entry) Empty allEntries
        writeTVar entriesVar' filteredEntries

        if disposing && Seq.null filteredEntries
           then do
             writeTVar (disposedVar resourceManager) True
             pure $ pure ()
           else pure go

    entriesVar' :: TVar (Seq ResourceManagerEntry)
    entriesVar' = entriesVar resourceManager


-- * Utilities

-- | Creates an `Disposable` that is bound to a ResourceManager. It will automatically be disposed when the resource manager is disposed.
attachDisposeAction :: MonadIO m => ResourceManager -> IO (Awaitable ()) -> m Disposable
attachDisposeAction resourceManager action = liftIO $ mask_ $ do
  disposable <- newDisposable action
  attachDisposable resourceManager disposable
  pure disposable

-- | Attaches a dispose action to a ResourceManager. It will automatically be run when the resource manager is disposed.
attachDisposeAction_ :: MonadIO m => ResourceManager -> IO (Awaitable ()) -> m ()
attachDisposeAction_ resourceManager action = void $ attachDisposeAction resourceManager action

-- | Start disposing a resource but instead of waiting for the operation to complete, pass the responsibility to a
-- `MonadResourceManager`.
--
-- The synchronous part of the `dispose`-Function will be run immediately but the resulting `Awaitable` will be passed
-- to the resource manager.
disposeEventually :: (IsDisposable a, MonadResourceManager m) => a -> m ()
disposeEventually disposable = do
  disposeCompleted <- dispose disposable
  peekAwaitable disposeCompleted >>= \case
    Just () -> pure ()
    Nothing -> registerDisposable disposable