%
% (c) The GRASP/AQUA Project, Glasgow University, 1995
%
\section[Semaphore]{Quantity semaphores}

General/quantity semaphores

\begin{code}
module Semaphore
      (
       {- abstract -}
       QSem,

       newQSem,		-- :: Int  -> IO QSem
       waitQSem,	-- :: QSem -> IO ()
       signalQSem,	-- :: QSem -> IO ()

       {- abstract -}
       QSemN,
       newQSemN,	-- :: Int   -> IO QSemN
       waitQSemN,	-- :: QSemN -> Int -> IO ()
       signalQSemN	-- :: QSemN -> Int -> IO ()
	
      ) where

import PrelConc
\end{code}

General semaphores are also implemented readily in terms of shared
@MVar@s, only have to catch the case when the semaphore is tried
waited on when it is empty (==0). Implement this in the same way as
shared variables are implemented - maintaining a list of @MVar@s
representing threads currently waiting. The counter is a shared
variable, ensuring the mutual exclusion on its access.

\begin{code}
newtype QSem = QSem (MVar (Int, [MVar ()]))

newQSem :: Int -> IO QSem
newQSem init = do
   sem <- newMVar (init,[])
   return (QSem sem)

waitQSem :: QSem -> IO ()
waitQSem (QSem sem) = do
   (avail,blocked) <- takeMVar sem  -- gain ex. access
   if avail > 0 then
     putMVar sem (avail-1,[])
    else do
     block <- newEmptyMVar
      {-
	Stuff the reader at the back of the queue,
	so as to preserve waiting order. A signalling
	process then only have to pick the MVar at the
	front of the blocked list.

	The version of waitQSem given in the paper could
	lead to starvation.
      -}
     putMVar sem (0, blocked++[block])
     takeMVar block

signalQSem :: QSem -> IO ()
signalQSem (QSem sem) = do
   (avail,blocked) <- takeMVar sem
   case blocked of
     [] -> putMVar sem (avail+1,[])

     (block:blocked') -> do
	   putMVar sem (0,blocked')
	   putMVar block ()

\end{code}


\begin{code}
newtype QSemN = QSemN (MVar (Int,[(Int,MVar ())]))

newQSemN :: Int -> IO QSemN 
newQSemN init = do
   sem <- newMVar (init,[])
   return (QSemN sem)

waitQSemN :: QSemN -> Int -> IO ()
waitQSemN (QSemN sem) sz = do
  (avail,blocked) <- takeMVar sem   -- gain ex. access
  if (avail - sz) > 0 then
       -- discharging 'sz' still leaves the semaphore
       -- in an 'unblocked' state.
     putMVar sem (avail-sz,[])
   else do
     block <- newEmptyMVar
     putMVar sem (avail, blocked++[(sz,block)])
     takeMVar block

signalQSemN :: QSemN -> Int  -> IO ()
signalQSemN (QSemN sem) n = do
   (avail,blocked)   <- takeMVar sem
   (avail',blocked') <- free (avail+n) blocked
   putMVar sem (avail',blocked')
 where
   free avail []    = return (avail,[])
   free avail ((req,block):blocked)
     | avail >= req = do
	putMVar block ()
	free (avail-req) blocked
     | otherwise    = do
	(avail',blocked') <- free avail blocked
        return (avail',(req,block):blocked')

\end{code}