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

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

import qualified Data.MinMaxPSQ as MM
import Data.Wrapper.PSQ         as PSQ
import Network.Kademlia.Routing as R
import Network.Kademlia.Search

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

newtype AnnounceKey = AnnounceKey ByteString
 deriving (Hashable,Ord,Eq)

instance Show AnnounceKey where
    show (AnnounceKey bs) = "AnnounceKey " ++ show (Char8.unpack bs)

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

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

-- | 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
    = DeleteAnnouncement
    | ScheduledItem (Announcer -> AnnounceKey -> POSIXTime -> STM (IO ()))
    | StopAnnouncer
 -- Can't use Data because STM and IO. :(
 --   deriving Data {- itemStatusNum sch = constrIndex $ toConstr sch -}
 -- Using Generic works with Lyxia's generic-data package...
 --                 {- itemStatusNum sch = Lyxia.conIdToInt $ Lyxia.conId sch -}
 -- For now, make sure to keep 'itemStatusNum' up to date with 'ScheduledItem'.
 deriving Generics.Generic


itemStatusNum :: ScheduledItem -> Int
itemStatusNum (DeleteAnnouncement   ) = 0
itemStatusNum (ScheduledItem      {}) = 1
itemStatusNum (StopAnnouncer        ) = 2
itemStatusNum _                       = error "itemStatusNum not in sync with ScheduledItem declaration."

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.  Do not write to this TVar ever.
      scheduled       :: TVar Schedule
      -- | This TVar is False when the Announcer thread has finished.
    , announcerActive :: TVar Bool
      -- | This delay is used to wait until it's time to act on the earliest
      -- scheduled item.  It will be interrupted whenever a new item is
      -- scheduled.
    , commander :: TChan SchedulerCommand
    }

-- | Schedules an event to occur long ago at the epoch (which effectively makes
-- the event happen as soon as possible).  Note that the caller will usually
-- also want to interrupt the 'interrutible' delay so that it finds this item
-- immediately.
scheduleImmediately :: Announcer -> AnnounceKey -> ScheduledItem -> STM ()
scheduleImmediately announcer k item = scheduleAbs announcer k item 0

scheduleAbs :: Announcer -> AnnounceKey -> ScheduledItem -> POSIXTime -> STM ()
scheduleAbs announcer k item absTime =
    writeTChan (commander announcer) $ ScheduleAction (k, absTime, item)

runAction :: Announcer -> IO () -> STM ()
runAction announcer = writeTChan (commander announcer) . RunAction

-- | 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

unschedule :: Announcer -> AnnounceKey -> STM ()
unschedule announcer k = writeTChan (commander announcer) $ UnscheduleAction k

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
    fork $ announceThread announcer
    return announcer

readTChanTimeout :: TVar Bool -> TChan a -> STM (Maybe a)
readTChanTimeout delay pktChannel = do
  -- delay <- registerDelay (toMicroseconds timeout)
  Just <$> readTChan pktChannel <|> pure Nothing <* (readTVar >=> check) delay

toMicroseconds :: POSIXTime -> Int
toMicroseconds = round . (* 1000) . (* 1000)

data SchedulerCommand = ShutdownScheduler | ScheduleAction KPS | RunAction (IO ()) | UnscheduleAction AnnounceKey

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, s), queue') -> do
          now <- getPOSIXTime
          note $ "queue full - 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')
                case s of
                  DeleteAnnouncement -> return relisten
                  StopAnnouncer      -> declareInactive >> return (return ())
                  ScheduledItem f    -> (fmap (>> relisten) (fmap fork (f announcer k now)))

      where
        modifyScheduled f = modifyTVar (scheduled announcer) (Schedule . f . unSchedule)
        declareInactive = writeTVar (announcerActive announcer) False
        handleCommand =
          \case
            ShutdownScheduler -> atomically declareInactive
            RunAction io -> io >> relisten
            UnscheduleAction k -> do
              atomically $ modifyScheduled $ PSQ.delete k
              relisten
            ScheduleAction (k, p, s) -> do
              atomically $ modifyScheduled $ PSQ.insert' k s p
              relisten

announceThread :: Announcer -> IO ()
announceThread announcer = do
    myThreadId >>= flip labelThread "announcer"
    listener announcer

-- | Returns 'Nothing' to stop the announcer thread (when the event is
-- StopAnnouncer).  Otherwise, returns an action that will be performed in the
-- announcer thread before looping for the next scheduled item.
--
-- Note that the returned action is responsible for re-scheduled another event
-- for periodic tasks.  Otherwise, as is the case for the 'DeleteAnnouncement'
-- event, the item associated with a particular announce key will be removed
-- from the queue.
performScheduledItem :: Announcer -> POSIXTime -> Binding' AnnounceKey POSIXTime ScheduledItem -> STM (Maybe (IO ()))
performScheduledItem announcer now = \case

    (Binding _ StopAnnouncer _)          -> return Nothing

    -- announcement removed:
    (Binding _ DeleteAnnouncement _)     -> return $ Just $ return ()

    (Binding k (ScheduledItem action) _) -> Just <$> action announcer k now