    {-# LANGUAGE BangPatterns, CPP, ExistentialQuantification, NoImplicitPrelude,
        RecordWildCards, TypeSynonymInstances #-}
    module System.Event.Manager
        ( -- * Types
          EventManager

          -- * Creation
        , new
        , newWith
        , newDefaultBackend

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

          -- * Registering interest in I/O events
        , Event
        , evtRead
        , evtWrite
        , IOCallback
        , FdKey(keyFd)
        , registerFd_
        , registerFd
        , unregisterFd_
        , unregisterFd
        , closeFd

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

    #include "EventConfig.h"

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

    import Control.Concurrent.MVar (MVar, modifyMVar, newMVar, readMVar)
    import Control.Exception (finally)
    import Control.Monad ((=<<), forM_, liftM, sequence_, when)
    import Data.IORef (IORef, atomicModifyIORef, mkWeakIORef, newIORef, readIORef,
                       writeIORef)
    import Data.Maybe (Maybe(..))
    import Data.Monoid (mappend, mconcat, mempty)
    import GHC.Base
    import GHC.Conc.Signal (runHandlers)
    import GHC.List (filter)
    import GHC.Num (Num(..))
    import GHC.Real ((/), fromIntegral )
    import GHC.Show (Show(..))
    import System.Event.Clock (getCurrentTime)
    import System.Event.Control
    import System.Event.Internal (Backend, Event, evtClose, evtRead, evtWrite,
                                  Timeout(..))
    import System.Event.Unique (Unique, UniqueSource, newSource, newUnique)
    import System.Posix.Types (Fd)

    import qualified System.Event.IntMap as IM
    import qualified System.Event.Internal as I
    import qualified System.Event.PSQ as Q

    #if defined(HAVE_KQUEUE)
    import qualified System.Event.KQueue as KQueue
    #elif defined(HAVE_EPOLL)
    import qualified System.Event.EPoll  as EPoll
    #elif defined(HAVE_POLL)
    import qualified System.Event.Poll   as Poll
    #else
    # error not implemented for this operating system
    #endif

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

    data FdData = FdData {
          fdKey       :: {-# UNPACK #-} !FdKey
        , fdEvents    :: {-# UNPACK #-} !Event
        , _fdCallback :: !IOCallback
        } deriving (Show)

    -- | A file descriptor registration cookie.
    data FdKey = FdKey {
          keyFd     :: {-# UNPACK #-} !Fd
        , keyUnique :: {-# UNPACK #-} !Unique
        } deriving (Eq, Show)

    -- | Callback invoked on I/O events.
    type IOCallback = FdKey -> Event -> IO ()

    instance Show IOCallback where
        show _ = "IOCallback"

    -- | 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

    {-
    Instead of directly modifying the 'TimeoutQueue' in
    e.g. 'registerTimeout' we keep a list of edits to perform, in the form
    of a chain of function closures, and have the I/O manager thread
    perform the edits later.  This exist to address the following GC
    problem:

    Since e.g. 'registerTimeout' doesn't force the evaluation of the
    thunks inside the 'emTimeouts' IORef a number of thunks build up
    inside the IORef.  If the I/O manager thread doesn't evaluate these
    thunks soon enough they'll get promoted to the old generation and
    become roots for all subsequent minor GCs.

    When the thunks eventually get evaluated they will each create a new
    intermediate 'TimeoutQueue' that immediately becomes garbage.  Since
    the thunks serve as roots until the next major GC these intermediate
    'TimeoutQueue's will get copied unnecesarily in the next minor GC,
    increasing GC time.  This problem is known as "floating garbage".

    Keeping a list of edits doesn't stop this from happening but makes the
    amount of data that gets copied smaller.

    TODO: Evaluate the content of the IORef to WHNF on each insert once
    this bug is resolved: http://hackage.haskell.org/trac/ghc/ticket/3838
    -}

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

    -- | The event manager state.
    data EventManager = EventManager
        { emBackend      :: !Backend
        , emFds          :: {-# UNPACK #-} !(MVar (IM.IntMap [FdData]))
        , emTimeouts     :: {-# UNPACK #-} !(IORef TimeoutEdit)
        , emState        :: {-# UNPACK #-} !(IORef State)
        , emUniqueSource :: {-# UNPACK #-} !UniqueSource
        , emControl      :: {-# UNPACK #-} !Control
        }

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

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

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

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

    newWith :: Backend -> IO EventManager
    newWith be = do
      iofds <- newMVar IM.empty
      timeouts <- newIORef id
      ctrl <- newControl
      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 = EventManager { emBackend = be
                             , emFds = iofds
                             , emTimeouts = timeouts
                             , emState = state
                             , emUniqueSource = us
                             , emControl = ctrl
                             }
      _ <- registerFd_ mgr (handleControlEvent mgr) (controlReadFd ctrl) evtRead
      _ <- registerFd_ mgr (handleControlEvent mgr) (wakeupReadFd ctrl) evtRead
      return mgr

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

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

    cleanup :: EventManager -> IO ()
    cleanup EventManager{..} = do
      writeIORef emState Finished
      I.delete emBackend
      closeControl emControl

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

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

    step :: EventManager -> TimeoutQueue -> IO (Bool, TimeoutQueue)
    step mgr@EventManager{..} tq = do
      (timeout, q') <- mkTimeout tq
      I.poll emBackend timeout (onFdEvent mgr)
      state <- readIORef emState
      state `seq` return (state == Running, q')
     where

      -- | Call all expired timer callbacks and return the time to the
      -- next timeout.
      mkTimeout :: TimeoutQueue -> IO (Timeout, TimeoutQueue)
      mkTimeout q = do
          now <- getCurrentTime
          applyEdits <- atomicModifyIORef emTimeouts $ \f -> (id, f)
          let (expired, q'') = let q' = applyEdits q in q' `seq` Q.atMost now q'
          sequence_ $ map Q.value expired
          let timeout = case Q.minView q'' 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'
          return (timeout, q'')

    ------------------------------------------------------------------------
    -- Registering interest in I/O events

    -- | Register interest in the given events, without waking the event
    -- manager thread.  The 'Bool' return value indicates whether the
    -- event manager ought to be woken.
    registerFd_ :: EventManager -> IOCallback -> Fd -> Event
                -> IO (FdKey, Bool)
    registerFd_ EventManager{..} cb fd evs = do
      u <- newUnique emUniqueSource
      modifyMVar emFds $ \oldMap -> do
        let fd'  = fromIntegral fd
            reg  = FdKey fd u
            !fdd = FdData reg evs cb
            (!newMap, (oldEvs, newEvs)) =
                case IM.insertWith (++) fd' [fdd] oldMap of
                  (Nothing,   n) -> (n, (mempty, evs))
                  (Just prev, n) -> (n, pairEvents prev newMap fd')
            modify = oldEvs /= newEvs
        when modify $ I.modifyFd emBackend fd oldEvs newEvs
        return (newMap, (reg, modify))
    {-# INLINE registerFd_ #-}

    -- | @registerFd mgr cb fd evs@ registers interest in the events @evs@
    -- on the file descriptor @fd@.  @cb@ is called for each event that
    -- occurs.  Returns a cookie that can be handed to 'unregisterFd'.
    registerFd :: EventManager -> IOCallback -> Fd -> Event -> IO FdKey
    registerFd mgr cb fd evs = do
      (r, wake) <- registerFd_ mgr cb fd evs
      when wake $ wakeManager mgr
      return r
    {-# INLINE registerFd #-}

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

    eventsOf :: [FdData] -> Event
    eventsOf = mconcat . map fdEvents

    pairEvents :: [FdData] -> IM.IntMap [FdData] -> Int -> (Event, Event)
    pairEvents prev m fd = let l = eventsOf prev
                               r = case IM.lookup fd m of
                                     Nothing  -> mempty
                                     Just fds -> eventsOf fds
                           in (l, r)

    -- | Drop a previous file descriptor registration, without waking the
    -- event manager thread.  The return value indicates whether the event
    -- manager ought to be woken.
    unregisterFd_ :: EventManager -> FdKey -> IO Bool
    unregisterFd_ EventManager{..} (FdKey fd u) =
      modifyMVar emFds $ \oldMap -> do
        let dropReg cbs = case filter ((/= u) . keyUnique . fdKey) cbs of
                              []   -> Nothing
                              cbs' -> Just cbs'
            fd' = fromIntegral fd
            (!newMap, (oldEvs, newEvs)) =
                case IM.updateWith dropReg fd' oldMap of
                  (Nothing,   _)    -> (oldMap, (mempty, mempty))
                  (Just prev, newm) -> (newm, pairEvents prev newm fd')
            modify = oldEvs /= newEvs
        when modify $ I.modifyFd emBackend fd oldEvs newEvs
        return (newMap, modify)

    -- | Drop a previous file descriptor registration.
    unregisterFd :: EventManager -> FdKey -> IO ()
    unregisterFd mgr reg = do
      wake <- unregisterFd_ mgr reg
      when wake $ wakeManager mgr

    -- | Close a file descriptor in a race-safe way.
    closeFd :: EventManager -> (Fd -> IO ()) -> Fd -> IO ()
    closeFd mgr close fd = do
      fds <- modifyMVar (emFds mgr) $ \oldMap -> do
        close fd
        case IM.delete (fromIntegral fd) oldMap of
          (Nothing,  _)       -> return (oldMap, [])
          (Just fds, !newMap) -> do
            when (eventsOf fds /= mempty) $ wakeManager mgr
            return (newMap, fds)
      forM_ fds $ \(FdData reg ev cb) -> cb reg (ev `mappend` evtClose)

    ------------------------------------------------------------------------
    -- 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 :: EventManager -> Int -> TimeoutCallback -> IO TimeoutKey
    registerTimeout mgr us cb = do
      !key <- newUnique (emUniqueSource mgr)
      if us <= 0 then cb
        else do
          now <- getCurrentTime
          let expTime = fromIntegral us / 1000000.0 + now

          -- We intentionally do not evaluate the modified map to WHNF here.
          -- Instead, we leave a thunk inside the IORef and defer its
          -- evaluation until mkTimeout in the event loop.  This is a
          -- workaround for a nasty IORef contention problem that causes the
          -- thread-delay benchmark to take 20 seconds instead of 0.2.
          atomicModifyIORef (emTimeouts mgr) $ \f ->
              let f' = (Q.insert key expTime cb) . f in (f', ())
          wakeManager mgr
      return $ TK key

    -- | Unregister an active timeout.
    unregisterTimeout :: EventManager -> TimeoutKey -> IO ()
    unregisterTimeout mgr (TK key) = do
      atomicModifyIORef (emTimeouts mgr) $ \f ->
          let f' = (Q.delete key) . f in (f', ())
      wakeManager mgr

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

      atomicModifyIORef (emTimeouts mgr) $ \f ->
          let f' = (Q.adjust (const expTime) key) . f in (f', ())
      wakeManager mgr

    ------------------------------------------------------------------------
    -- Utilities

    -- | Call the callbacks corresponding to the given file descriptor.
    onFdEvent :: EventManager -> Fd -> Event -> IO ()
    onFdEvent mgr fd evs = do
      fds <- readMVar (emFds mgr)
      case IM.lookup (fromIntegral fd) fds of
          Just cbs -> forM_ cbs $ \(FdData reg ev cb) ->
                        when (evs `I.eventIs` ev) $ cb reg evs
          Nothing  -> return ()