    {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
    {-# LANGUAGE CPP, DeriveDataTypeable #-}

    #if __GLASGOW_HASKELL__ >= 701
    {-# LANGUAGE Trustworthy #-}
    #endif

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Concurrent.STM.TBQueue
    -- Copyright   :  (c) The University of Glasgow 2012
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (requires STM)
    --
    -- 'TBQueue' is a bounded version of 'TQueue'. The queue has a maximum
    -- capacity set when it is created.  If the queue already contains the
    -- maximum number of elements, then 'writeTBQueue' blocks until an
    -- element is removed from the queue.
    --
    -- The implementation is based on the traditional purely-functional
    -- queue representation that uses two lists to obtain amortised /O(1)/
    -- enqueue and dequeue operations.
    --
    -----------------------------------------------------------------------------

    module Control.Concurrent.STM.TBQueue (
            -- * TBQueue
    	TBQueue,
    	newTBQueue,
    	newTBQueueIO,
    	readTBQueue,
    	tryReadTBQueue,
    	peekTBQueue,
    	tryPeekTBQueue,
    	writeTBQueue,
            unGetTBQueue,
            isEmptyTBQueue,
      ) where

    import Data.Typeable
    import GHC.Conc

    #define _UPK_(x) {-# UNPACK #-} !(x)

    -- | 'TBQueue' is an abstract type representing a bounded FIFO channel.
    data TBQueue a
       = TBQueue _UPK_(TVar Int)  -- CR: read capacity
                 _UPK_(TVar [a])  -- R:  elements waiting to be read
                 _UPK_(TVar Int)  -- CW: write capacity
                 _UPK_(TVar [a])  -- W:  elements written (head is most recent)
      deriving Typeable

    instance Eq (TBQueue a) where
      TBQueue a _ _ _ == TBQueue b _ _ _ = a == b

    -- Total channel capacity remaining is CR + CW. Reads only need to
    -- access CR, writes usually need to access only CW but sometimes need
    -- CR.  So in the common case we avoid contention between CR and CW.
    --
    --   - when removing an element from R:
    --     CR := CR + 1
    --
    --   - when adding an element to W:
    --     if CW is non-zero
    --         then CW := CW - 1
    --         then if CR is non-zero
    --                 then CW := CR - 1; CR := 0
    --                 else **FULL**

    -- |Build and returns a new instance of 'TBQueue'
    newTBQueue :: Int   -- ^ maximum number of elements the queue can hold
               -> STM (TBQueue a)
    newTBQueue size = do
      read  <- newTVar []
      write <- newTVar []
      rsize <- newTVar 0
      wsize <- newTVar size
      return (TBQueue rsize read wsize write)

    -- |@IO@ version of 'newTBQueue'.  This is useful for creating top-level
    -- 'TBQueue's using 'System.IO.Unsafe.unsafePerformIO', because using
    -- 'atomically' inside 'System.IO.Unsafe.unsafePerformIO' isn't
    -- possible.
    newTBQueueIO :: Int -> IO (TBQueue a)
    newTBQueueIO size = do
      read  <- newTVarIO []
      write <- newTVarIO []
      rsize <- newTVarIO 0
      wsize <- newTVarIO size
      return (TBQueue rsize read wsize write)

    -- |Write a value to a 'TBQueue'; blocks if the queue is full.
    writeTBQueue :: TBQueue a -> a -> STM ()
    writeTBQueue (TBQueue rsize _read wsize write) a = do
      w <- readTVar wsize
      if (w /= 0)
         then do writeTVar wsize (w - 1)
         else do
              r <- readTVar rsize
              if (r /= 0)
                 then do writeTVar rsize 0
                         writeTVar wsize (r - 1)
                 else retry
      listend <- readTVar write
      writeTVar write (a:listend)

    -- |Read the next value from the 'TBQueue'.
    readTBQueue :: TBQueue a -> STM a
    readTBQueue (TBQueue rsize read _wsize write) = do
      xs <- readTVar read
      r <- readTVar rsize
      writeTVar rsize (r + 1)
      case xs of
        (x:xs') -> do
          writeTVar read xs'
          return x
        [] -> do
          ys <- readTVar write
          case ys of
            [] -> retry
            _  -> do
              let (z:zs) = reverse ys -- NB. lazy: we want the transaction to be
                                      -- short, otherwise it will conflict
              writeTVar write []
              writeTVar read zs
              return z

    -- | A version of 'readTBQueue' which does not retry. Instead it
    -- returns @Nothing@ if no value is available.
    tryReadTBQueue :: TBQueue a -> STM (Maybe a)
    tryReadTBQueue c = fmap Just (readTBQueue c) `orElse` return Nothing

    -- | Get the next value from the @TBQueue@ without removing it,
    -- retrying if the channel is empty.
    peekTBQueue :: TBQueue a -> STM a
    peekTBQueue c = do
      x <- readTBQueue c
      unGetTBQueue c x
      return x

    -- | A version of 'peekTBQueue' which does not retry. Instead it
    -- returns @Nothing@ if no value is available.
    tryPeekTBQueue :: TBQueue a -> STM (Maybe a)
    tryPeekTBQueue c = do
      m <- tryReadTBQueue c
      case m of
        Nothing -> return Nothing
        Just x  -> do
          unGetTBQueue c x
          return m

    -- |Put a data item back onto a channel, where it will be the next item read.
    -- Blocks if the queue is full.
    unGetTBQueue :: TBQueue a -> a -> STM ()
    unGetTBQueue (TBQueue rsize read wsize _write) a = do
      r <- readTVar rsize
      if (r > 0)
         then do writeTVar rsize (r - 1)
         else do
              w <- readTVar wsize
              if (w > 0)
                 then writeTVar wsize (w - 1)
                 else retry
      xs <- readTVar read
      writeTVar read (a:xs)

    -- |Returns 'True' if the supplied 'TBQueue' is empty.
    isEmptyTBQueue :: TBQueue a -> STM Bool
    isEmptyTBQueue (TBQueue _rsize read _wsize write) = do
      xs <- readTVar read
      case xs of
        (_:_) -> return False
        [] -> do ys <- readTVar write
                 case ys of
                   [] -> return True
                   _  -> return False