{-# LANGUAGE MagicHash #-}

-----------------------------------------------------------------------------
-- |
-- Module      : Berp.Base
-- Copyright   : (c) Copyright 2002, The University Court of the University 
--               of Glasgow.  All rights reserved.  
-- License     : See the GHC license below. 
-- Maintainer  : (of this branch) florbitous@gmail.com
-- Stability   : experimental
-- Portability : ghc
--
-- A thread-safe supply of unique values.
-- Same as the version in GHC but with added show instance.
--
-----------------------------------------------------------------------------

{-
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Copyright 2002, The University Court of the University of Glasgow. 
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used to endorse or promote products derived from this software without
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THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY COURT OF THE UNIVERSITY OF
GLASGOW AND THE CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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-}


module Berp.Base.Unique (
   -- * Unique objects
   Unique (..),         -- instance (Eq, Ord)
   newUnique,           -- :: IO Unique
   hashUnique           -- :: Unique -> Int
 ) where

import Prelude

import Control.Concurrent.MVar
import System.IO.Unsafe (unsafePerformIO)

import GHC.Base
import GHC.Num

-- | An abstract unique object.  Objects of type 'Unique' may be
-- compared for equality and ordering and hashed into 'Int'.
newtype Unique = Unique { uniqueInteger :: Integer } deriving (Eq,Ord)

-- not safe, but needed for printing purposes. 
instance Show Unique where
   show (Unique i) = show i

{-# NOINLINE uniqSource #-}
uniqSource :: MVar Integer
uniqSource = unsafePerformIO (newMVar 0)

-- | Creates a new object of type 'Unique'.  The value returned will
-- not compare equal to any other value of type 'Unique' returned by
-- previous calls to 'newUnique'.  There is no limit on the number of
-- times 'newUnique' may be called.
newUnique :: IO Unique
newUnique = do
   val <- takeMVar uniqSource
   let next = val+1
   putMVar uniqSource next
   return (Unique next)

-- | Hashes a 'Unique' into an 'Int'.  Two 'Unique's may hash to the
-- same value, although in practice this is unlikely.  The 'Int'
-- returned makes a good hash key.
hashUnique :: Unique -> Int
hashUnique (Unique i) = I# (hashInteger i)