    {-# LANGUAGE Safe #-}
    {-# LANGUAGE AutoDeriveTypeable, BangPatterns #-}
    {-# 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   :  experimental
    -- 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, tryTakeMVar
                              , putMVar, newMVar
                              , tryPutMVar, isEmptyMVar)
    import Data.Typeable
    import Control.Exception
    import Data.Maybe

    -- | 'QSemN' is a quantity semaphore in which the resource is aqcuired
    -- 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 !(MVar (Int, [(Int, MVar ())], [(Int, MVar ())]))
      deriving Typeable

    -- 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; the MVar lock on the
    -- semaphore itself resolves race conditions between signalQSemN and a
    -- thread attempting to dequeue itself.

    -- |Build a new 'QSemN' with a supplied initial quantity.
    --  The initial quantity must be at least 0.
    newQSemN :: Int -> IO QSemN
    newQSemN initial
      | initial < 0 = fail "newQSemN: Initial quantity must be non-negative"
      | otherwise   = do
          sem <- newMVar (initial, [], [])
          return (QSemN sem)

    -- |Wait for the specified quantity to become available
    waitQSemN :: QSemN -> Int -> IO ()
    waitQSemN (QSemN m) sz =
      mask_ $ do
        (i,b1,b2) <- takeMVar m
        let z = i-sz
        if z < 0
           then do
             b <- newEmptyMVar
             putMVar m (i, b1, (sz,b):b2)
             wait b
           else do
             putMVar m (z, b1, b2)
             return ()
      where
        wait b = do
            takeMVar b `onException`
                    (uninterruptibleMask_ $ do -- Note [signal uninterruptible]
                       (i,b1,b2) <- takeMVar m
                       r <- tryTakeMVar b
                       r' <- if isJust r
                                then signal sz (i,b1,b2)
                                else do putMVar b (); return (i,b1,b2)
                       putMVar m r')

    -- |Signal that a given quantity is now available from the 'QSemN'.
    signalQSemN :: QSemN -> Int -> IO ()
    signalQSemN (QSemN m) sz = uninterruptibleMask_ $ do
      r <- takeMVar m
      r' <- signal sz r
      putMVar m r'

    signal :: Int
           -> (Int,[(Int,MVar ())],[(Int,MVar ())])
           -> IO (Int,[(Int,MVar ())],[(Int,MVar ())])

    signal sz0 (i,a1,a2) = loop (sz0 + i) a1 a2
     where
       loop 0  bs b2 = return (0,  bs, b2)
       loop sz [] [] = return (sz, [], [])
       loop sz [] b2 = loop sz (reverse b2) []
       loop sz ((j,b):bs) b2
         | j > sz = do
           r <- isEmptyMVar b
           if r then return (sz, (j,b):bs, b2)
                else loop sz bs b2
         | otherwise = do
           r <- tryPutMVar b ()
           if r then loop (sz-j) bs b2
                else loop sz bs b2