{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE RecordWildCards #-}

{-| Device for running an action at (i.e. shortly after) a certain time, which
can be used to implement things like time-based cache expiry.

This implementation avoids the use of polling and leans on Haskell's scheduler
to achieve low-latency without lots of computational overhead.

The alarm can be set multiple times, and in this case the alarm will go off at
the earliest requested time. If the alarm is set in the past, the action will
run immediately. When the action runs, it clears all future alarms; the action
can itself return the time at which it should run again.

To perform time-based cache expiry, create an 'AlarmClock' whose action flushes
any stale entries from the cache and then calls `setAlarm` for the next time
that an entry will expire (if there are any). When expiring entries are added
to the cache, call 'setAlarm' to ensure that they will expire in a timely
fashion.

-}

module Control.Concurrent.AlarmClock
  ( AlarmClock()
  , newAlarmClock
  , destroyAlarmClock
  , setAlarm
  , setAlarmSTM
  , setAlarmNow
  , isAlarmSet
  , isAlarmSetSTM
  ) where

import Control.Applicative ((<$>), (<*>))
import Control.Concurrent (forkIO, newEmptyMVar, readMVar, putMVar)
import Control.Concurrent.STM (STM, atomically, retry, TVar, newTVar, writeTVar, readTVar, modifyTVar')
import Control.Concurrent.Timeout (timeout)
import Control.Exception (finally)
import Control.Monad (void)
import Data.Time (UTCTime, diffUTCTime, getCurrentTime)
import GHC.Conc (labelThread, myThreadId)

{-| An 'AlarmClock' is a device for running an action at (or shortly after) a certain time. -}
data AlarmClock = AlarmClock
  { acWaitForExit :: IO ()
  , acNewSetting  :: TVar AlarmSetting
  , acIsSet       :: TVar Bool
  }

{-| Create a new 'AlarmClock' that runs the given action. Initially, there is
no wakeup time set: you must call 'setAlarm' for anything else to happen. -}
newAlarmClock
  :: (AlarmClock -> IO ())
    -- ^ Action to run when the alarm goes off. The action is provided the alarm clock
    -- so it can set a new alarm if desired. Note that `setAlarm` must be called once
    -- the alarm has gone off to cause it to go off again.
  -> IO AlarmClock
newAlarmClock onWakeUp = do
  joinVar <- newEmptyMVar
  ac <- atomically $ AlarmClock (readMVar joinVar) <$> newTVar AlarmNotSet <*> newTVar False
  void $ forkIO $ runAlarmClock ac (onWakeUp ac) `finally` putMVar joinVar ()
  return ac

{-| Destroy the 'AlarmClock' so no further alarms will occur. If the alarm is currently going off
then this will block until the action is finished. -}
destroyAlarmClock :: AlarmClock -> IO ()
destroyAlarmClock AlarmClock{..} = atomically (writeTVar acNewSetting AlarmDestroyed) >> acWaitForExit

{-| Make the 'AlarmClock' go off at (or shortly after) the given time.  This
can be called more than once; in which case, the alarm will go off at the
earliest given time. -}
setAlarm :: AlarmClock -> UTCTime -> IO ()
setAlarm ac t = atomically $ setAlarmSTM ac t

{-| Make the 'AlarmClock' go off at (or shortly after) the given time.  This
can be called more than once; in which case, the alarm will go off at the
earliest given time. -}
setAlarmSTM :: AlarmClock -> UTCTime -> STM ()
setAlarmSTM AlarmClock{..} t = modifyTVar' acNewSetting $ \case
  AlarmDestroyed -> AlarmDestroyed
  AlarmNotSet    -> AlarmSet t
  AlarmSet t'    -> AlarmSet $! min t t'

{-| Make the 'AlarmClock' go off right now. -}
setAlarmNow :: AlarmClock -> IO ()
setAlarmNow alarm = getCurrentTime >>= setAlarm alarm

{-| Is the alarm set - i.e. will it go off at some point in the future even if `setAlarm` is not called? -}
isAlarmSet :: AlarmClock -> IO Bool
isAlarmSet = atomically . isAlarmSetSTM

{-| Is the alarm set - i.e. will it go off at some point in the future even if `setAlarm` is not called? -}
isAlarmSetSTM :: AlarmClock -> STM Bool
isAlarmSetSTM AlarmClock{..} = readTVar acNewSetting
  >>= \case { AlarmNotSet -> readTVar acIsSet; _ -> return True }

data AlarmSetting = AlarmNotSet | AlarmSet UTCTime | AlarmDestroyed

labelMyThread :: String -> IO ()
labelMyThread threadLabel = myThreadId >>= flip labelThread threadLabel

runAlarmClock :: AlarmClock -> IO () -> IO ()
runAlarmClock AlarmClock{..} wakeUpAction = labelMyThread "alarmclock" >> loop
  where
  loop = readNextSetting >>= go

  readNextSetting = atomically $ readTVar acNewSetting >>= \case
    AlarmNotSet    -> retry
    AlarmDestroyed -> return Nothing
    AlarmSet t     -> do
      writeTVar acNewSetting AlarmNotSet
      writeTVar acIsSet True
      return $ Just t

  go Nothing           = return ()
  go (Just wakeUpTime) = wakeNoLaterThan wakeUpTime

  wakeNoLaterThan wakeUpTime = do
    dt <- ceiling <$> (1000000 *) <$> diffUTCTime wakeUpTime <$> getCurrentTime
    if dt <= 0
      then actAndContinue
      else timeout dt readNextSetting >>= \case
            Nothing -> actAndContinue
            Just newSetting -> go newSetting

  actAndContinue = do
    atomically $ writeTVar acIsSet False
    wakeUpAction
    loop