    {-
    Copyright (C) 2010-2015 John MacFarlane <jgm@berkeley.edu>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    -}

    {- |
       Module      : Text.Pandoc.UUID
       Copyright   : Copyright (C) 2010-2015 John MacFarlane
       License     : GNU GPL, version 2 or above

       Maintainer  : John MacFarlane <jgm@berkeley.edu>
       Stability   : alpha
       Portability : portable

    UUID generation using Version 4 (random method) described
    in RFC4122. See http://tools.ietf.org/html/rfc4122
    -}

    module Text.Pandoc.UUID ( UUID, getRandomUUID ) where

    import Text.Printf ( printf )
    import System.Random ( randomIO )
    import Data.Word
    import Data.Bits ( setBit, clearBit )
    import Control.Monad ( liftM )

    data UUID = UUID Word8 Word8 Word8 Word8 Word8 Word8 Word8 Word8
                     Word8 Word8 Word8 Word8 Word8 Word8 Word8 Word8

    instance Show UUID where
      show (UUID a b c d e f g h i j k l m n o p) =
       "urn:uuid:" ++
       printf "%02x" a ++
       printf "%02x" b ++
       printf "%02x" c ++
       printf "%02x" d ++
       "-" ++
       printf "%02x" e ++
       printf "%02x" f ++
       "-" ++
       printf "%02x" g ++
       printf "%02x" h ++
       "-" ++
       printf "%02x" i ++
       printf "%02x" j ++
       "-" ++
       printf "%02x" k ++
       printf "%02x" l ++
       printf "%02x" m ++
       printf "%02x" n ++
       printf "%02x" o ++
       printf "%02x" p

    getRandomUUID :: IO UUID
    getRandomUUID = do
      let getRN :: a -> IO Word8
          getRN _ = liftM fromIntegral (randomIO :: IO Int)
      [a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p] <- mapM getRN ([1..16] :: [Int])
      -- set variant
      let i' = i `setBit` 7 `clearBit` 6
      -- set version (0100 for random)
      let g' = g `clearBit` 7 `setBit` 6 `clearBit` 5 `clearBit` 4
      return $ UUID a b c d e f g' h i' j k l m n o p