    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE BangPatterns
               , CPP
               , ExistentialQuantification
               , NoImplicitPrelude
               , TypeSynonymInstances
               , FlexibleInstances
      #-}

    module GHC.Event.TimerManager
        ( -- * Types
          TimerManager

          -- * Creation
        , new
        , newWith
        , newDefaultBackend
        , emControl

          -- * Running
        , finished
        , loop
        , step
        , shutdown
        , cleanup
        , wakeManager

          -- * Registering interest in timeout events
        , TimeoutCallback
        , TimeoutKey
        , registerTimeout
        , updateTimeout
        , unregisterTimeout
        ) where

    #include "EventConfig.h"

    ------------------------------------------------------------------------
    -- Imports

    import Control.Exception (finally)
    import Data.Foldable (sequence_)
    import Data.IORef (IORef, atomicModifyIORef', mkWeakIORef, newIORef, readIORef,
                       writeIORef)
    import GHC.Base
    import GHC.Conc.Signal (runHandlers)
    import GHC.Num (Num(..))
    import GHC.Real ((/), fromIntegral )
    import GHC.Show (Show(..))
    import GHC.Event.Clock (getMonotonicTime)
    import GHC.Event.Control
    import GHC.Event.Internal (Backend, Event, evtRead, Timeout(..))
    import GHC.Event.Unique (Unique, UniqueSource, newSource, newUnique)
    import System.Posix.Types (Fd)

    import qualified GHC.Event.Internal as I
    import qualified GHC.Event.PSQ as Q

    #if defined(HAVE_POLL)
    import qualified GHC.Event.Poll   as Poll
    #else
    # error not implemented for this operating system
    #endif

    ------------------------------------------------------------------------
    -- Types

    -- | A timeout registration cookie.
    newtype TimeoutKey   = TK Unique
        deriving (Eq)

    -- | Callback invoked on timeout events.
    type TimeoutCallback = IO ()

    data State = Created
               | Running
               | Dying
               | Finished
                 deriving (Eq, Show)

    -- | A priority search queue, with timeouts as priorities.
    type TimeoutQueue = Q.PSQ TimeoutCallback

    -- | An edit to apply to a 'TimeoutQueue'.
    type TimeoutEdit = TimeoutQueue -> TimeoutQueue

    -- | The event manager state.
    data TimerManager = TimerManager
        { emBackend      :: !Backend
        , emTimeouts     :: {-# UNPACK #-} !(IORef TimeoutQueue)
        , emState        :: {-# UNPACK #-} !(IORef State)
        , emUniqueSource :: {-# UNPACK #-} !UniqueSource
        , emControl      :: {-# UNPACK #-} !Control
        }

    ------------------------------------------------------------------------
    -- Creation

    handleControlEvent :: TimerManager -> Fd -> Event -> IO ()
    handleControlEvent mgr fd _evt = do
      msg <- readControlMessage (emControl mgr) fd
      case msg of
        CMsgWakeup      -> return ()
        CMsgDie         -> writeIORef (emState mgr) Finished
        CMsgSignal fp s -> runHandlers fp s

    newDefaultBackend :: IO Backend
    #if defined(HAVE_POLL)
    newDefaultBackend = Poll.new
    #else
    newDefaultBackend = error "no back end for this platform"
    #endif

    -- | Create a new event manager.
    new :: IO TimerManager
    new = newWith =<< newDefaultBackend

    newWith :: Backend -> IO TimerManager
    newWith be = do
      timeouts <- newIORef Q.empty
      ctrl <- newControl True
      state <- newIORef Created
      us <- newSource
      _ <- mkWeakIORef state $ do
                   st <- atomicModifyIORef' state $ \s -> (Finished, s)
                   when (st /= Finished) $ do
                     I.delete be
                     closeControl ctrl
      let mgr = TimerManager { emBackend = be
                             , emTimeouts = timeouts
                             , emState = state
                             , emUniqueSource = us
                             , emControl = ctrl
                             }
      _ <- I.modifyFd be (controlReadFd ctrl) mempty evtRead
      _ <- I.modifyFd be (wakeupReadFd ctrl) mempty evtRead
      return mgr

    -- | Asynchronously shuts down the event manager, if running.
    shutdown :: TimerManager -> IO ()
    shutdown mgr = do
      state <- atomicModifyIORef' (emState mgr) $ \s -> (Dying, s)
      when (state == Running) $ sendDie (emControl mgr)

    finished :: TimerManager -> IO Bool
    finished mgr = (== Finished) `liftM` readIORef (emState mgr)

    cleanup :: TimerManager -> IO ()
    cleanup mgr = do
      writeIORef (emState mgr) Finished
      I.delete (emBackend mgr)
      closeControl (emControl mgr)

    ------------------------------------------------------------------------
    -- Event loop

    -- | Start handling events.  This function loops until told to stop,
    -- using 'shutdown'.
    --
    -- /Note/: This loop can only be run once per 'TimerManager', as it
    -- closes all of its control resources when it finishes.
    loop :: TimerManager -> IO ()
    loop mgr = do
      state <- atomicModifyIORef' (emState mgr) $ \s -> case s of
        Created -> (Running, s)
        _       -> (s, s)
      case state of
        Created -> go `finally` cleanup mgr
        Dying   -> cleanup mgr
        _       -> do cleanup mgr
                      error $ "GHC.Event.Manager.loop: state is already " ++
                          show state
     where
      go = do running <- step mgr
              when running go

    step :: TimerManager -> IO Bool
    step mgr = do
      timeout <- mkTimeout
      _ <- I.poll (emBackend mgr) (Just timeout) (handleControlEvent mgr)
      state <- readIORef (emState mgr)
      state `seq` return (state == Running)
     where

      -- | Call all expired timer callbacks and return the time to the
      -- next timeout.
      mkTimeout :: IO Timeout
      mkTimeout = do
          now <- getMonotonicTime
          (expired, timeout) <- atomicModifyIORef' (emTimeouts mgr) $ \tq ->
               let (expired, tq') = Q.atMost now tq
                   timeout = case Q.minView tq' of
                     Nothing             -> Forever
                     Just (Q.E _ t _, _) ->
                         -- This value will always be positive since the call
                         -- to 'atMost' above removed any timeouts <= 'now'
                         let t' = t - now in t' `seq` Timeout t'
               in (tq', (expired, timeout))
          sequence_ $ map Q.value expired
          return timeout

    -- | Wake up the event manager.
    wakeManager :: TimerManager -> IO ()
    wakeManager mgr = sendWakeup (emControl mgr)

    ------------------------------------------------------------------------
    -- Registering interest in timeout events

    -- | Register a timeout in the given number of microseconds.  The
    -- returned 'TimeoutKey' can be used to later unregister or update the
    -- timeout.  The timeout is automatically unregistered after the given
    -- time has passed.
    registerTimeout :: TimerManager -> Int -> TimeoutCallback -> IO TimeoutKey
    registerTimeout mgr us cb = do
      !key <- newUnique (emUniqueSource mgr)
      if us <= 0 then cb
        else do
          now <- getMonotonicTime
          let expTime = fromIntegral us / 1000000.0 + now

          editTimeouts mgr (Q.insert key expTime cb)
          wakeManager mgr
      return $ TK key

    -- | Unregister an active timeout.
    unregisterTimeout :: TimerManager -> TimeoutKey -> IO ()
    unregisterTimeout mgr (TK key) = do
      editTimeouts mgr (Q.delete key)
      wakeManager mgr

    -- | Update an active timeout to fire in the given number of
    -- microseconds.
    updateTimeout :: TimerManager -> TimeoutKey -> Int -> IO ()
    updateTimeout mgr (TK key) us = do
      now <- getMonotonicTime
      let expTime = fromIntegral us / 1000000.0 + now

      editTimeouts mgr (Q.adjust (const expTime) key)
      wakeManager mgr

    editTimeouts :: TimerManager -> TimeoutEdit -> IO ()
    editTimeouts mgr g = atomicModifyIORef' (emTimeouts mgr) $ \tq -> (g tq, ())