{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE GeneralizedNewtypeDeriving
           , NoImplicitPrelude
           , BangPatterns
  #-}

-----------------------------------------------------------------------------
-- |
-- A binding to the epoll I/O event notification facility
--
-- epoll is a variant of poll that can be used either as an edge-triggered or
-- a level-triggered interface and scales well to large numbers of watched file
-- descriptors.
--
-- epoll decouples monitor an fd from the process of registering it.
--
-----------------------------------------------------------------------------

module GHC.Event.EPoll
    (
      new
    , available
    ) where

import qualified GHC.Event.Internal as E

#include "EventConfig.h"
#if !defined(HAVE_EPOLL)
import GHC.Base

new :: IO E.Backend
new = error "EPoll back end not implemented for this platform"

available :: Bool
available = False
{-# INLINE available #-}
#else

#include <sys/epoll.h>

import Data.Bits (Bits, FiniteBits, (.|.), (.&.))
import Data.Word (Word32)
import Foreign.C.Error (eNOENT, getErrno, throwErrno,
                        throwErrnoIfMinus1, throwErrnoIfMinus1_)
import Foreign.C.Types (CInt(..))
import Foreign.Marshal.Utils (with)
import Foreign.Ptr (Ptr)
import Foreign.Storable (Storable(..))
import GHC.Base
import GHC.Num (Num(..))
import GHC.Real (ceiling, fromIntegral)
import GHC.Show (Show)
import System.Posix.Internals (c_close)
import System.Posix.Internals (setCloseOnExec)
import System.Posix.Types (Fd(..))

import qualified GHC.Event.Array    as A
import           GHC.Event.Internal (Timeout(..))

available :: Bool
available = True
{-# INLINE available #-}

data EPoll = EPoll {
      epollFd     :: {-# UNPACK #-} !EPollFd
    , epollEvents :: {-# UNPACK #-} !(A.Array Event)
    }

-- | Create a new epoll backend.
new :: IO E.Backend
new = do
  epfd <- epollCreate
  evts <- A.new 64
  let !be = E.backend poll modifyFd modifyFdOnce delete (EPoll epfd evts)
  return be

delete :: EPoll -> IO ()
delete be = do
  _ <- c_close . fromEPollFd . epollFd $ be
  return ()

-- | Change the set of events we are interested in for a given file
-- descriptor.
modifyFd :: EPoll -> Fd -> E.Event -> E.Event -> IO Bool
modifyFd ep fd oevt nevt =
  with (Event (fromEvent nevt) fd) $ \evptr -> do
    epollControl (epollFd ep) op fd evptr
    return True
  where op | oevt == mempty = controlOpAdd
           | nevt == mempty = controlOpDelete
           | otherwise      = controlOpModify

modifyFdOnce :: EPoll -> Fd -> E.Event -> IO Bool
modifyFdOnce ep fd evt =
  do let !ev = fromEvent evt .|. epollOneShot
     res <- with (Event ev fd) $
            epollControl_ (epollFd ep) controlOpModify fd
     if res == 0
       then return True
       else do err <- getErrno
               if err == eNOENT
                 then with (Event ev fd) $ \evptr -> do
                        epollControl (epollFd ep) controlOpAdd fd evptr
                        return True
                 else throwErrno "modifyFdOnce"

-- | Select a set of file descriptors which are ready for I/O
-- operations and call @f@ for all ready file descriptors, passing the
-- events that are ready.
poll :: EPoll                     -- ^ state
     -> Maybe Timeout             -- ^ timeout in milliseconds
     -> (Fd -> E.Event -> IO ())  -- ^ I/O callback
     -> IO Int
poll ep mtimeout f = do
  let events = epollEvents ep
      fd = epollFd ep

  -- Will return zero if the system call was interupted, in which case
  -- we just return (and try again later.)
  n <- A.unsafeLoad events $ \es cap -> case mtimeout of
    Just timeout -> epollWait fd es cap $ fromTimeout timeout
    Nothing      -> epollWaitNonBlock fd es cap

  when (n > 0) $ do
    A.forM_ events $ \e -> f (eventFd e) (toEvent (eventTypes e))
    cap <- A.capacity events
    when (cap == n) $ A.ensureCapacity events (2 * cap)
  return n

newtype EPollFd = EPollFd {
      fromEPollFd :: CInt
    } deriving (Eq, Show)

data Event = Event {
      eventTypes :: EventType
    , eventFd    :: Fd
    } deriving (Show)

instance Storable Event where
    sizeOf    _ = #size struct epoll_event
    alignment _ = alignment (undefined :: CInt)

    peek ptr = do
        ets <- #{peek struct epoll_event, events} ptr
        ed  <- #{peek struct epoll_event, data.fd}   ptr
        let !ev = Event (EventType ets) ed
        return ev

    poke ptr e = do
        #{poke struct epoll_event, events} ptr (unEventType $ eventTypes e)
        #{poke struct epoll_event, data.fd}   ptr (eventFd e)

newtype ControlOp = ControlOp CInt

#{enum ControlOp, ControlOp
 , controlOpAdd    = EPOLL_CTL_ADD
 , controlOpModify = EPOLL_CTL_MOD
 , controlOpDelete = EPOLL_CTL_DEL
 }

newtype EventType = EventType {
      unEventType :: Word32
    } deriving (Show, Eq, Num, Bits, FiniteBits)

#{enum EventType, EventType
 , epollIn  = EPOLLIN
 , epollOut = EPOLLOUT
 , epollErr = EPOLLERR
 , epollHup = EPOLLHUP
 , epollOneShot = EPOLLONESHOT
 }

-- | Create a new epoll context, returning a file descriptor associated with the context.
-- The fd may be used for subsequent calls to this epoll context.
--
-- The size parameter to epoll_create is a hint about the expected number of handles.
--
-- The file descriptor returned from epoll_create() should be destroyed via
-- a call to close() after polling is finished
--
epollCreate :: IO EPollFd
epollCreate = do
  fd <- throwErrnoIfMinus1 "epollCreate" $
        c_epoll_create 256 -- argument is ignored
  setCloseOnExec fd
  let !epollFd' = EPollFd fd
  return epollFd'

epollControl :: EPollFd -> ControlOp -> Fd -> Ptr Event -> IO ()
epollControl epfd op fd event =
    throwErrnoIfMinus1_ "epollControl" $ epollControl_ epfd op fd event

epollControl_ :: EPollFd -> ControlOp -> Fd -> Ptr Event -> IO CInt
epollControl_ (EPollFd epfd) (ControlOp op) (Fd fd) event =
    c_epoll_ctl epfd op fd event

epollWait :: EPollFd -> Ptr Event -> Int -> Int -> IO Int
epollWait (EPollFd epfd) events numEvents timeout =
    fmap fromIntegral .
    E.throwErrnoIfMinus1NoRetry "epollWait" $
    c_epoll_wait epfd events (fromIntegral numEvents) (fromIntegral timeout)

epollWaitNonBlock :: EPollFd -> Ptr Event -> Int -> IO Int
epollWaitNonBlock (EPollFd epfd) events numEvents =
  fmap fromIntegral .
  E.throwErrnoIfMinus1NoRetry "epollWaitNonBlock" $
  c_epoll_wait_unsafe epfd events (fromIntegral numEvents) 0

fromEvent :: E.Event -> EventType
fromEvent e = remap E.evtRead  epollIn .|.
              remap E.evtWrite epollOut
  where remap evt to
            | e `E.eventIs` evt = to
            | otherwise         = 0

toEvent :: EventType -> E.Event
toEvent e = remap (epollIn  .|. epollErr .|. epollHup) E.evtRead `mappend`
            remap (epollOut .|. epollErr .|. epollHup) E.evtWrite
  where remap evt to
            | e .&. evt /= 0 = to
            | otherwise      = mempty

fromTimeout :: Timeout -> Int
fromTimeout Forever     = -1
fromTimeout (Timeout s) = ceiling $ 1000 * s

foreign import ccall unsafe "sys/epoll.h epoll_create"
    c_epoll_create :: CInt -> IO CInt

foreign import ccall unsafe "sys/epoll.h epoll_ctl"
    c_epoll_ctl :: CInt -> CInt -> CInt -> Ptr Event -> IO CInt

foreign import ccall safe "sys/epoll.h epoll_wait"
    c_epoll_wait :: CInt -> Ptr Event -> CInt -> CInt -> IO CInt

foreign import ccall unsafe "sys/epoll.h epoll_wait"
    c_epoll_wait_unsafe :: CInt -> Ptr Event -> CInt -> CInt -> IO CInt
#endif /* defined(HAVE_EPOLL) */

