{-# LANGUAGE CPP #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE ScopedTypeVariables #-}
#if GHCJS_FAST_WEAK
{-# LANGUAGE ForeignFunctionInterface #-}
{-# LANGUAGE JavaScriptFFI #-}
#endif

-- |
-- Module:
--   Reflex.FastWeak
-- Description:
--   'FastWeak' is a weak pointer to some value, and 'FastWeakTicket' ensures the value
--   referred to by a 'FastWeak' stays live while the ticket is held (live).
--
--   On GHC or GHCJS when not built with the @fast-weak@ cabal flag, 'FastWeak' is a wrapper
--   around the simple version of 'System.Mem.Weak.Weak' where the key and value are the same.
--
--   On GHCJS when built with the @fast-weak@ cabal flag, 'FastWeak' is implemented directly
--   in JS using @h$FastWeak@ and @h$FastWeakTicket@ which are a nonstandard part of the GHCJS RTS.
module Reflex.FastWeak
  ( FastWeakTicket
  , FastWeak
  , mkFastWeakTicket
  , getFastWeakTicketValue
  , getFastWeakTicketWeak
  , getFastWeakValue
  , getFastWeakTicket
  , emptyFastWeak
#ifdef GHCJS_FAST_WEAK
  --TODO: Move these elsewhere
  , unsafeFromRawJSVal
  , unsafeToRawJSVal
  , js_isNull
#endif
  ) where

import GHC.Exts (Any)
import Unsafe.Coerce

#ifdef GHCJS_FAST_WEAK
import GHCJS.Types
#else
import Control.Exception (evaluate)
import System.IO.Unsafe
import System.Mem.Weak
#endif


#ifdef GHCJS_FAST_WEAK
-- | A 'FastWeak' which has been promoted to a strong reference. 'getFastWeakTicketValue'
-- can be used to get the referred to value without fear of @Nothing,
-- and 'getFastWeakTicketWeak' can be used to get the weak version.
--
-- Implemented by way of special support in the GHCJS RTS, @h$FastWeakTicket@.
newtype FastWeakTicket a = FastWeakTicket JSVal

-- | A reference to some value which can be garbage collected if there are only
-- weak references to the value left.
--
-- 'getFastWeakValue' can be used to try and obtain a strong reference to the value.
--
-- The value in a @FastWeak@ can also be kept alive by obtaining a 'FastWeakTicket' using
-- 'getFastWeakTicket' if the value hasn't been collected yet.
--
-- Implemented by way of special support in the GHCJS RTS, @h$FastWeak@.
newtype FastWeak a = FastWeak JSVal

-- Just designed to mirror JSVal, so that we can unsafeCoerce between the two
data Val a = Val { unVal :: a }

-- | Coerce a JSVal that represents the heap object of a value of type @a@ into a value of type @a@
unsafeFromRawJSVal :: JSVal -> a
unsafeFromRawJSVal v = unVal (unsafeCoerce v)

-- | Coerce a heap object of type @a@ into a 'JSVal' which represents that object.
unsafeToRawJSVal :: a -> JSVal
unsafeToRawJSVal v = unsafeCoerce (Val v)
#else
-- | A 'FastWeak' which has been promoted to a strong reference. 'getFastWeakTicketValue'
-- can be used to get the referred to value without fear of @Nothing@,
-- and 'getFastWeakTicketWeak' can be used to get the weak version.
data FastWeakTicket a = FastWeakTicket
  { forall a. FastWeakTicket a -> a
_fastWeakTicket_val :: !a
  , forall a. FastWeakTicket a -> Weak a
_fastWeakTicket_weak :: {-# UNPACK #-} !(Weak a)
  }

-- | A reference to some value which can be garbage collected if there are only weak references to the value left.
--
-- 'getFastWeakValue' can be used to try and obtain a strong reference to the value.
--
-- The value in a @FastWeak@ can also be kept alive by obtaining a 'FastWeakTicket' using 'getFastWeakTicket'
-- if the value hasn't been collected yet.
--
-- Synonymous with 'Weak'.
type FastWeak a = Weak a
#endif

-- | Return the @a@ kept alive by the given 'FastWeakTicket'.
--
-- This needs to be in IO so we know that we've relinquished the ticket.
getFastWeakTicketValue :: FastWeakTicket a -> IO a
#ifdef GHCJS_FAST_WEAK
getFastWeakTicketValue t = do
  v <- js_ticketVal t
  return $ unsafeFromRawJSVal v

foreign import javascript unsafe "$r = $1.val;" js_ticketVal :: FastWeakTicket a -> IO JSVal
#else
getFastWeakTicketValue :: forall a. FastWeakTicket a -> IO a
getFastWeakTicketValue = a -> IO a
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return (a -> IO a) -> (FastWeakTicket a -> a) -> FastWeakTicket a -> IO a
forall b c a. (b -> c) -> (a -> b) -> a -> c
. FastWeakTicket a -> a
forall a. FastWeakTicket a -> a
_fastWeakTicket_val
#endif

-- | Get the value referred to by a 'FastWeak' if it hasn't yet been collected,
-- or @Nothing@ if it has been collected.
getFastWeakValue :: FastWeak a -> IO (Maybe a)
#ifdef GHCJS_FAST_WEAK
getFastWeakValue w = do
  r <- js_weakVal w
  case js_isNull r of
    True -> return Nothing
    False -> return $ Just $ unsafeFromRawJSVal r

foreign import javascript unsafe "$1 === null" js_isNull :: JSVal -> Bool

foreign import javascript unsafe "$r = ($1.ticket === null) ? null : $1.ticket.val;" js_weakVal :: FastWeak a -> IO JSVal
#else
getFastWeakValue :: forall a. FastWeak a -> IO (Maybe a)
getFastWeakValue = Weak a -> IO (Maybe a)
forall a. FastWeak a -> IO (Maybe a)
deRefWeak
#endif

-- | Try to create a 'FastWeakTicket' for the given 'FastWeak' which will ensure the value referred
-- remains alive. Returns @Just@ if the value hasn't been collected
-- and a ticket can therefore be obtained, @Nothing@ if it's been collected.
getFastWeakTicket :: forall a. FastWeak a -> IO (Maybe (FastWeakTicket a))
#ifdef GHCJS_FAST_WEAK
getFastWeakTicket w = do
  r <- js_weakTicket w
  case js_isNull r of
    True -> return Nothing
    False -> return $ Just $ FastWeakTicket r

foreign import javascript unsafe "$r = $1.ticket;" js_weakTicket :: FastWeak a -> IO JSVal
#else
getFastWeakTicket :: forall a. FastWeak a -> IO (Maybe (FastWeakTicket a))
getFastWeakTicket FastWeak a
w = do
  FastWeak a -> IO (Maybe a)
forall a. FastWeak a -> IO (Maybe a)
deRefWeak FastWeak a
w IO (Maybe a)
-> (Maybe a -> IO (Maybe (FastWeakTicket a)))
-> IO (Maybe (FastWeakTicket a))
forall a b. IO a -> (a -> IO b) -> IO b
forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>>= \case
    Maybe a
Nothing -> Maybe (FastWeakTicket a) -> IO (Maybe (FastWeakTicket a))
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return Maybe (FastWeakTicket a)
forall a. Maybe a
Nothing
    Just a
v -> Maybe (FastWeakTicket a) -> IO (Maybe (FastWeakTicket a))
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return (Maybe (FastWeakTicket a) -> IO (Maybe (FastWeakTicket a)))
-> Maybe (FastWeakTicket a) -> IO (Maybe (FastWeakTicket a))
forall a b. (a -> b) -> a -> b
$ FastWeakTicket a -> Maybe (FastWeakTicket a)
forall a. a -> Maybe a
Just (FastWeakTicket a -> Maybe (FastWeakTicket a))
-> FastWeakTicket a -> Maybe (FastWeakTicket a)
forall a b. (a -> b) -> a -> b
$ FastWeakTicket
      { _fastWeakTicket_val :: a
_fastWeakTicket_val = a
v
      , _fastWeakTicket_weak :: FastWeak a
_fastWeakTicket_weak = FastWeak a
w
      }
#endif

-- | Create a 'FastWeakTicket' directly from a value, creating a 'FastWeak' in the process
-- which can be obtained with 'getFastWeakTicketValue'.
--
-- This function is marked NOINLINE so it is opaque to GHC.
-- If we do not do this, then GHC will sometimes fuse the constructor away
-- so any weak references that are attached to the ticket will have their
-- finalizer run. Using the opaque constructor, GHC does not see the
-- constructor application, so it behaves like an IORef and cannot be fused away.
--
-- The result is also evaluated to WHNF, since forcing a thunk invalidates
-- the weak pointer to it in some cases.
{-# NOINLINE mkFastWeakTicket #-}
mkFastWeakTicket :: a -> IO (FastWeakTicket a)
-- I think it's fine if this is lazy - it'll retain the 'a', but so would the output; we just need to make sure it's forced before we start relying on the
-- associated FastWeak to actually be weak
#ifdef GHCJS_FAST_WEAK
mkFastWeakTicket v = js_fastWeakTicket (unsafeToRawJSVal v)

foreign import javascript unsafe "$r = new h$FastWeakTicket($1);" js_fastWeakTicket :: JSVal -> IO (FastWeakTicket a)
#else
mkFastWeakTicket :: forall a. a -> IO (FastWeakTicket a)
mkFastWeakTicket a
v = do
  a
v' <- a -> IO a
forall a. a -> IO a
evaluate a
v
  Weak a
w <- a -> Maybe (IO ()) -> IO (Weak a)
forall k. k -> Maybe (IO ()) -> IO (Weak k)
mkWeakPtr a
v' Maybe (IO ())
forall a. Maybe a
Nothing
  FastWeakTicket a -> IO (FastWeakTicket a)
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return (FastWeakTicket a -> IO (FastWeakTicket a))
-> FastWeakTicket a -> IO (FastWeakTicket a)
forall a b. (a -> b) -> a -> b
$ FastWeakTicket
    { _fastWeakTicket_val :: a
_fastWeakTicket_val = a
v'
    , _fastWeakTicket_weak :: Weak a
_fastWeakTicket_weak = Weak a
w
    }
#endif

-- | Demote a 'FastWeakTicket'; which ensures the value is alive, to a 'FastWeak' which doesn't.
-- Note that unless the ticket for the same 'FastWeak' is held in some other way
-- the value might be collected immediately.
getFastWeakTicketWeak :: FastWeakTicket a -> IO (FastWeak a)
-- Needs IO so that it can force the value - otherwise, could end up with a reference to the Ticket, which would retain the value
#ifdef GHCJS_FAST_WEAK
foreign import javascript unsafe "$r = $1.weak;" getFastWeakTicketWeak' :: FastWeakTicket a -> IO (FastWeak a)
{-# INLINE getFastWeakTicketWeak #-}
getFastWeakTicketWeak = getFastWeakTicketWeak'
#else
getFastWeakTicketWeak :: forall a. FastWeakTicket a -> IO (FastWeak a)
getFastWeakTicketWeak = FastWeak a -> IO (FastWeak a)
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return (FastWeak a -> IO (FastWeak a))
-> (FastWeakTicket a -> FastWeak a)
-> FastWeakTicket a
-> IO (FastWeak a)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. FastWeakTicket a -> FastWeak a
forall a. FastWeakTicket a -> Weak a
_fastWeakTicket_weak
#endif

-- | A weak reference that is always empty
emptyFastWeak :: FastWeak a
emptyFastWeak :: forall a. FastWeak a
emptyFastWeak = FastWeak Any -> FastWeak a
forall a b. a -> b
unsafeCoerce FastWeak Any
w
  where w :: FastWeak Any
#ifdef GHCJS_FAST_WEAK
        w = js_emptyWeak
#else
        w :: FastWeak Any
w = IO (FastWeak Any) -> FastWeak Any
forall a. IO a -> a
unsafePerformIO (IO (FastWeak Any) -> FastWeak Any)
-> IO (FastWeak Any) -> FastWeak Any
forall a b. (a -> b) -> a -> b
$ do
          FastWeak Any
w' <- Any -> Maybe (IO ()) -> IO (FastWeak Any)
forall k. k -> Maybe (IO ()) -> IO (Weak k)
mkWeakPtr Any
forall a. HasCallStack => a
undefined Maybe (IO ())
forall a. Maybe a
Nothing
          FastWeak Any -> IO ()
forall v. Weak v -> IO ()
finalize FastWeak Any
w'
          FastWeak Any -> IO (FastWeak Any)
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return FastWeak Any
w'
#endif
{-# NOINLINE emptyFastWeak #-}

#ifdef GHCJS_FAST_WEAK
foreign import javascript unsafe "$r = new h$FastWeak(null);" js_emptyWeak :: FastWeak Any
#endif