content: {-# 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.
  --
  -----------------------------------------------------------------------------
  
  {-
  The Glasgow Haskell Compiler License
  
  Copyright 2002, The University Court of the University of Glasgow. 
  All rights reserved.
  
  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are met:
  
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  this list of conditions and the following disclaimer.
   
  - Redistributions in binary form must reproduce the above copyright notice,
  this list of conditions and the following disclaimer in the documentation
  and/or other materials provided with the distribution.
   
  - Neither name of the University nor the names of its contributors may be
  used to endorse or promote products derived from this software without
  specific prior written permission. 
  
  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
  UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW OR THE CONTRIBUTORS BE LIABLE
  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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  DAMAGE.
  -}
  
  
  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)
  