content: {-# 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
  