    {-# LANGUAGE Trustworthy #-}
    {-# OPTIONS_GHC -funbox-strict-fields #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Concurrent.QSemN
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  stable
    -- Portability :  non-portable (concurrency)
    --
    -- Quantity semaphores in which each thread may wait for an arbitrary
    -- \"amount\".
    --
    -----------------------------------------------------------------------------

    module Control.Concurrent.QSemN
            (  -- * General Quantity Semaphores
              QSemN,        -- abstract
              newQSemN,     -- :: Int   -> IO QSemN
              waitQSemN,    -- :: QSemN -> Int -> IO ()
              signalQSemN   -- :: QSemN -> Int -> IO ()
          ) where

    import Control.Concurrent.MVar ( MVar, newEmptyMVar, takeMVar
                              , tryPutMVar, isEmptyMVar)
    import Control.Exception
    import Control.Monad (when)
    import Data.IORef (IORef, newIORef, atomicModifyIORef)
    import System.IO.Unsafe (unsafePerformIO)

    -- | 'QSemN' is a quantity semaphore in which the resource is acquired
    -- and released in units of one. It provides guaranteed FIFO ordering
    -- for satisfying blocked `waitQSemN` calls.
    --
    -- The pattern
    --
    -- >   bracket_ (waitQSemN n) (signalQSemN n) (...)
    --
    -- is safe; it never loses any of the resource.
    --
    data QSemN = QSemN !(IORef (Int, [(Int, MVar ())], [(Int, MVar ())]))

    -- The semaphore state (i, xs, ys):
    --
    --   i is the current resource value
    --
    --   (xs,ys) is the queue of blocked threads, where the queue is
    --           given by xs ++ reverse ys.  We can enqueue new blocked threads
    --           by consing onto ys, and dequeue by removing from the head of xs.
    --
    -- A blocked thread is represented by an empty (MVar ()).  To unblock
    -- the thread, we put () into the MVar.
    --
    -- A thread can dequeue itself by also putting () into the MVar, which
    -- it must do if it receives an exception while blocked in waitQSemN.
    -- This means that when unblocking a thread in signalQSemN we must
    -- first check whether the MVar is already full.

    -- |Build a new 'QSemN' with a supplied initial quantity.
    --  The initial quantity must be at least 0.
    newQSemN :: Int -> IO QSemN
    newQSemN :: Int -> IO QSemN
    newQSemN Int
    initial
      | Int
    initial Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    < Int
    0 = String -> IO QSemN
    forall a. String -> IO a
    forall (m :: * -> *) a. MonadFail m => String -> m a
    fail String
    "newQSemN: Initial quantity must be non-negative"
      | Bool
    otherwise   = do
          IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    sem <- (Int, [(Int, MVar ())], [(Int, MVar ())])
    -> IO (IORef (Int, [(Int, MVar ())], [(Int, MVar ())]))
    forall a. a -> IO (IORef a)
    newIORef (Int
    initial, [], [])
          QSemN -> IO QSemN
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (IORef (Int, [(Int, MVar ())], [(Int, MVar ())]) -> QSemN
    QSemN IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    sem)

    -- An unboxed version of Maybe (MVar a)
    data MaybeMV a = JustMV !(MVar a) | NothingMV

    -- |Wait for the specified quantity to become available
    waitQSemN :: QSemN -> Int -> IO ()
    -- We need to mask here. Once we've enqueued our MVar, we need
    -- to be sure to wait for it. Otherwise, we could lose our
    -- allocated resource.
    waitQSemN :: QSemN -> Int -> IO ()
    waitQSemN qs :: QSemN
    qs@(QSemN IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    m) Int
    sz = IO () -> IO ()
    forall a. IO a -> IO a
    mask_ (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $ do
        -- unsafePerformIO and not unsafeDupablePerformIO. We must
        -- be sure to wait on the same MVar that gets enqueued.
      MaybeMV ()
    mmvar <- IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    -> ((Int, [(Int, MVar ())], [(Int, MVar ())])
        -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ()))
    -> IO (MaybeMV ())
    forall a b. IORef a -> (a -> (a, b)) -> IO b
    atomicModifyIORef IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    m (((Int, [(Int, MVar ())], [(Int, MVar ())])
      -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ()))
     -> IO (MaybeMV ()))
    -> ((Int, [(Int, MVar ())], [(Int, MVar ())])
        -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ()))
    -> IO (MaybeMV ())
    forall a b. (a -> b) -> a -> b
    $ \ (Int
    i,[(Int, MVar ())]
    b1,[(Int, MVar ())]
    b2) -> IO ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    forall a. IO a -> a
    unsafePerformIO (IO ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
     -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ()))
    -> IO ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    forall a b. (a -> b) -> a -> b
    $ do
        let z :: Int
    z = Int
    iInt -> Int -> Int
    forall a. Num a => a -> a -> a
    -Int
    sz
        if Int
    z Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    < Int
    0
          then do
            MVar ()
    b <- IO (MVar ())
    forall a. IO (MVar a)
    newEmptyMVar
            ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    -> IO ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ((Int
    i, [(Int, MVar ())]
    b1, (Int
    sz,MVar ()
    b)(Int, MVar ()) -> [(Int, MVar ())] -> [(Int, MVar ())]
    forall a. a -> [a] -> [a]
    :[(Int, MVar ())]
    b2), MVar () -> MaybeMV ()
    forall a. MVar a -> MaybeMV a
    JustMV MVar ()
    b)
          else ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    -> IO ((Int, [(Int, MVar ())], [(Int, MVar ())]), MaybeMV ())
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ((Int
    z, [(Int, MVar ())]
    b1, [(Int, MVar ())]
    b2), MaybeMV ()
    forall a. MaybeMV a
    NothingMV)

      -- Note: this case match actually allocates the MVar if necessary.
      case MaybeMV ()
    mmvar of
        MaybeMV ()
    NothingMV -> () -> IO ()
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
        JustMV MVar ()
    b -> MVar () -> IO ()
    wait MVar ()
    b
      where
        wait :: MVar () -> IO ()
        wait :: MVar () -> IO ()
    wait MVar ()
    b =
          MVar () -> IO ()
    forall a. MVar a -> IO a
    takeMVar MVar ()
    b IO () -> IO () -> IO ()
    forall a b. IO a -> IO b -> IO a
    `onException` do
            Bool
    already_filled <- Bool -> Bool
    not (Bool -> Bool) -> IO Bool -> IO Bool
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> MVar () -> () -> IO Bool
    forall a. MVar a -> a -> IO Bool
    tryPutMVar MVar ()
    b ()
            Bool -> IO () -> IO ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    when Bool
    already_filled (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $ QSemN -> Int -> IO ()
    signalQSemN QSemN
    qs Int
    sz

    -- |Signal that a given quantity is now available from the 'QSemN'.
    signalQSemN :: QSemN -> Int -> IO ()
    -- We don't need to mask here because we should *already* be masked
    -- here (e.g., by bracket). Indeed, if we're not already masked,
    -- it's too late to do so.
    --
    -- What if the unsafePerformIO thunk is forced in another thread,
    -- and receives an asynchronous exception? That shouldn't be a
    -- problem: when we force it ourselves, presumably masked, we
    -- will resume its execution.
    signalQSemN :: QSemN -> Int -> IO ()
    signalQSemN (QSemN IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    m) Int
    sz0 = do
        -- unsafePerformIO and not unsafeDupablePerformIO. We must not
        -- wake up more threads than we're supposed to.
      ()
    unit <- IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    -> ((Int, [(Int, MVar ())], [(Int, MVar ())])
        -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), ()))
    -> IO ()
    forall a b. IORef a -> (a -> (a, b)) -> IO b
    atomicModifyIORef IORef (Int, [(Int, MVar ())], [(Int, MVar ())])
    m (((Int, [(Int, MVar ())], [(Int, MVar ())])
      -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), ()))
     -> IO ())
    -> ((Int, [(Int, MVar ())], [(Int, MVar ())])
        -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), ()))
    -> IO ()
    forall a b. (a -> b) -> a -> b
    $ \(Int
    i,[(Int, MVar ())]
    a1,[(Int, MVar ())]
    a2) ->
                IO ((Int, [(Int, MVar ())], [(Int, MVar ())]), ())
    -> ((Int, [(Int, MVar ())], [(Int, MVar ())]), ())
    forall a. IO a -> a
    unsafePerformIO (Int
    -> [(Int, MVar ())]
    -> [(Int, MVar ())]
    -> IO ((Int, [(Int, MVar ())], [(Int, MVar ())]), ())
    forall {a}.
    (Num a, Ord a) =>
    a
    -> [(a, MVar ())]
    -> [(a, MVar ())]
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    loop (Int
    sz0 Int -> Int -> Int
    forall a. Num a => a -> a -> a
    + Int
    i) [(Int, MVar ())]
    a1 [(Int, MVar ())]
    a2)

      -- Forcing this will actually wake the necessary threads.
      () -> IO ()
    forall a. a -> IO a
    evaluate ()
    unit
     where
       loop :: a
    -> [(a, MVar ())]
    -> [(a, MVar ())]
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    loop a
    0  [(a, MVar ())]
    bs [(a, MVar ())]
    b2 = ((a, [(a, MVar ())], [(a, MVar ())]), ())
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ((a
    0,  [(a, MVar ())]
    bs, [(a, MVar ())]
    b2), ())
       loop a
    sz [] [] = ((a, [(a, MVar ())], [(a, MVar ())]), ())
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ((a
    sz, [], []), ())
       loop a
    sz [] [(a, MVar ())]
    b2 = a
    -> [(a, MVar ())]
    -> [(a, MVar ())]
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    loop a
    sz ([(a, MVar ())] -> [(a, MVar ())]
    forall a. [a] -> [a]
    reverse [(a, MVar ())]
    b2) []
       loop a
    sz ((a
    j,MVar ()
    b):[(a, MVar ())]
    bs) [(a, MVar ())]
    b2
         | a
    j a -> a -> Bool
    forall a. Ord a => a -> a -> Bool
    > a
    sz = do
           Bool
    r <- MVar () -> IO Bool
    forall a. MVar a -> IO Bool
    isEmptyMVar MVar ()
    b
           if Bool
    r then ((a, [(a, MVar ())], [(a, MVar ())]), ())
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ((a
    sz, (a
    j,MVar ()
    b)(a, MVar ()) -> [(a, MVar ())] -> [(a, MVar ())]
    forall a. a -> [a] -> [a]
    :[(a, MVar ())]
    bs, [(a, MVar ())]
    b2), ())
                else a
    -> [(a, MVar ())]
    -> [(a, MVar ())]
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    loop a
    sz [(a, MVar ())]
    bs [(a, MVar ())]
    b2
         | Bool
    otherwise = do
           Bool
    r <- MVar () -> () -> IO Bool
    forall a. MVar a -> a -> IO Bool
    tryPutMVar MVar ()
    b ()
           if Bool
    r then a
    -> [(a, MVar ())]
    -> [(a, MVar ())]
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    loop (a
    sza -> a -> a
    forall a. Num a => a -> a -> a
    -a
    j) [(a, MVar ())]
    bs [(a, MVar ())]
    b2
                else a
    -> [(a, MVar ())]
    -> [(a, MVar ())]
    -> IO ((a, [(a, MVar ())], [(a, MVar ())]), ())
    loop a
    sz [(a, MVar ())]
    bs [(a, MVar ())]
    b2