{-# LANGUAGE DeriveDataTypeable         #-}
{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE ExistentialQuantification  #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE LambdaCase                 #-}
{-# LANGUAGE NamedFieldPuns             #-}
{-# LANGUAGE NondecreasingIndentation   #-}
module Announcer
    ( Announcer()
    , AnnounceKey
    , scheduleAbs
    , scheduleToList
    , packAnnounceKey
    , unpackAnnounceKey
    , encodeAnnounceKey
    , decodeAnnounceKey
    , forkAnnouncer
    , stopAnnouncer
    , cancel
    , runAction
    , unschedule
    , delayAction
    , scheduleRel

    -- lower level, Announcer.Tox needs these.
    , scheduleImmediately
    , ScheduledItem(ScheduledItem)
    ) where

import Data.Wrapper.PSQ         as PSQ

import Control.Applicative
import Control.Concurrent.Lifted.Instrument
import Control.Concurrent.STM
import Control.Monad
import Data.ByteString                      (ByteString)
import qualified Data.ByteString.Char8      as Char8
import Data.Hashable
import Data.Time.Clock                      (NominalDiffTime)
import Data.Time.Clock.POSIX
import qualified GHC.Generics               as Generics
-- import Generic.Data.Internal.Meta as Lyxia

-- | Actions are scheduled and canceled via this key.
newtype AnnounceKey = AnnounceKey ByteString
 deriving (Hashable,Ord,Eq)

packAnnounceKey :: Announcer -> String -> AnnounceKey
packAnnounceKey _ = AnnounceKey . Char8.pack

unpackAnnounceKey :: Announcer -> AnnounceKey -> String
unpackAnnounceKey _ (AnnounceKey bs) = Char8.unpack bs

encodeAnnounceKey :: Announcer -> AnnounceKey -> ByteString
encodeAnnounceKey _ (AnnounceKey k) = k

decodeAnnounceKey :: Announcer -> ByteString -> AnnounceKey
decodeAnnounceKey _ k = AnnounceKey k

-- | Actions that can be scheduled to occur at some particular time in the
-- future.  Since periodic event handlers are responsible for re-scheduling
-- themselves, they are typically bootstrapped using 'scheduleImmediately' with
-- 'NewAnnouncement' which triggers the ordinary recurring scheduling of
-- 'Announce'.
data ScheduledItem = ScheduledItem (Announcer -> AnnounceKey -> POSIXTime -> STM (IO ()))
 deriving Generics.Generic


newtype Schedule = Schedule { unSchedule :: PSQ' AnnounceKey POSIXTime ScheduledItem }

emptySchedule :: Schedule
emptySchedule = Schedule PSQ.empty

type KPS = (AnnounceKey, POSIXTime, ScheduledItem)

scheduleToList :: Announcer -> STM [KPS]
scheduleToList announcer = PSQ.toList . unSchedule <$> readTVar (scheduled announcer)

data Announcer = Announcer
    { -- | The queue of scheduled events.  The priority is the time at which an
      -- event is supposed to occur.
      scheduled       :: TVar Schedule
      -- | This TVar is False when the Announcer thread has finished.
    , announcerActive :: TVar Bool
      -- | This channel is used to communicate instructions to the timing
      -- thread.  It can be used to schedule actions or shutdown the thread.
    , commander :: TChan SchedulerCommand
    }

-- | Schedule an action to happen immediately.
scheduleImmediately :: Announcer -> AnnounceKey -> ScheduledItem -> STM ()
scheduleImmediately announcer k item =
    writeTChan (commander announcer) $ RunActionSTM k item

-- | Schedule an action to happen at a specific time.
scheduleAbs :: Announcer -> AnnounceKey -> ScheduledItem -> POSIXTime -> STM ()
scheduleAbs announcer k item absTime =
    writeTChan (commander announcer) $ ScheduleAction (k, absTime, item)

-- | Schedule an action to occur s specified delta into the future from now.
scheduleRel, delayAction :: Announcer -> AnnounceKey -> ScheduledItem -> NominalDiffTime -> STM ()
scheduleRel announcer k item relTime =
    writeTChan (commander announcer) $ DelayAction (k, relTime, item)

-- | Schedule an action to occur s specified delta into the future from now.
-- This is an alias for 'scheduleRel'.
delayAction = scheduleRel

-- | Fork an action immediately.  The String argument is a thread label.
runAction :: Announcer -> String -> IO () -> STM ()
runAction announcer lbl = writeTChan (commander announcer) . RunAction lbl

-- | Terminate the 'Announcer' thread.  Don't use the Announcer after this.
stopAnnouncer :: Announcer -> IO ()
stopAnnouncer announcer = do
    atomically $ writeTChan (commander announcer) ShutdownScheduler
    atomically $ readTVar (announcerActive announcer) >>= check . not

-- | Cancel a scheduled action (STM).
unschedule :: Announcer -> AnnounceKey -> STM ()
unschedule announcer k = writeTChan (commander announcer) $ UnscheduleAction k

-- | Cancel a scheduled action (IO).
cancel :: Announcer -> AnnounceKey -> IO ()
cancel = ((.).(.)) atomically unschedule

-- | Construct an 'Announcer' object and fork a thread in which to perform the
-- Kademlia searches and announces.
forkAnnouncer :: IO Announcer
forkAnnouncer = do
    announcer <- atomically $ Announcer <$> newTVar emptySchedule
                                        <*> newTVar True
                                        <*> newTChan
    tid <- forkIO $ listener announcer
    labelThread tid "announcer"
    return announcer

-- Wait for an item to occur on the given channel, or for the given TVar to be
-- True.
readTChanTimeout :: TVar Bool -> TChan a -> STM (Maybe a)
readTChanTimeout delay pktChannel =
    orElse (Just <$> readTChan pktChannel)
           (pure Nothing <* (readTVar >=> check) delay)

data SchedulerCommand
  = ShutdownScheduler
  | ScheduleAction KPS -- run an action at an absolute time
  | DelayAction KPS    -- run an action at a time relative to the present
  | RunAction String (IO ())
  | RunActionSTM AnnounceKey ScheduledItem
  | UnscheduleAction AnnounceKey

-- This is the main scheduling thread that polls the commander TChan and forks
-- scheduled actions.
listener :: Announcer -> IO ()
listener announcer = relisten
  -- This function assumes it is the only writer to the (scheduled announcer :: TVar Schedule).
  -- If that TVar is written in another thread, the changes may be overwritten here.
  where
    chan = commander announcer
    note :: String -> IO ()
    note = if False then print else const (return ())
    relisten = do
      queue <- fmap unSchedule $ atomically $ readTVar $ scheduled announcer
      case minView queue of
        Nothing -> do
          note "queue empty - listening indefinitely"
          atomically (readTChan chan) >>= handleCommand
        Just ((k, p, ScheduledItem f), queue') -> do
          now <- getPOSIXTime
          note $ "queue non-empty - listening with timeout - " ++ show (p, now)
          delay <- registerDelay (toMicroseconds (p - now))
          join $ atomically $ do
            readTChanTimeout delay chan >>= \case
              Just cmd -> return $ handleCommand cmd
              Nothing  -> do
                writeTVar (scheduled announcer) (Schedule queue')
                (fmap (>> relisten) (fmap fork (f announcer k now)))
      where
        modifyScheduled f               = modifyTVar (scheduled announcer) (Schedule . f . unSchedule)
        declareInactive                 = writeTVar (announcerActive announcer) False
        handleCommand ShutdownScheduler = atomically declareInactive
        handleCommand cmd               = (>> relisten) $
          case cmd of
            ScheduleAction (k, p, s) -> atomically $ modifyScheduled $ PSQ.insert' k s p
            UnscheduleAction k       -> atomically $ modifyScheduled $ PSQ.delete k
            RunAction lbl io -> void $ forkLabeled lbl io
            RunActionSTM k (ScheduledItem f) -> do
              now <- getPOSIXTime
              void . fork . join . atomically $ modifyScheduled (PSQ.delete k) >> f announcer k now
            DelayAction (k, p, s) -> do
              now <- getPOSIXTime
              atomically $ modifyScheduled $ PSQ.insert' k s (now + p)