GUID.hs

-- | Globally Unique IDentifiers.
module Data.CQRS.GUID
       ( GUID
       , fromByteString
       , hexDecode
       , hexEncode
       , newGUID
       , toByteString
       ) where

import           Control.Applicative ((<$>))
import           Data.ByteString (ByteString)
import qualified Data.ByteString as B
import qualified Data.ByteString.Base16 as B16
import           Data.ByteString.Char8 ()
import           Data.Default (Default(..))
import           Data.SafeCopy (SafeCopy(..), contain, safePut, safeGet)
import           Data.Typeable (Typeable)
import           Data.Word (Word8)
import           System.Random (randomRIO)

-- | A Globally Unique IDentifier.
newtype GUID = GUID ByteString
              deriving (Show, Typeable, Eq, Ord)

-- | Default GUID value.
instance Default GUID where
    def = GUID $ B.pack $ replicate 32 0

-- | Get a random byte.
randomWord8 :: IO Word8
randomWord8 = fmap fromInteger $ randomRIO (0,255)

-- | Create a new random GUID.
newGUID :: IO GUID
newGUID = do
  uuid <- sequence $ replicate 32 randomWord8
  return $ GUID $ B.pack uuid

-- | Serialize instance.
instance SafeCopy GUID where
    putCopy = contain . safePut . toByteString
    getCopy = contain $ fromByteString <$> safeGet

-- | Hex encode a GUID.
hexEncode :: GUID -> ByteString
hexEncode (GUID s) = B16.encode s

-- | Decode a GUID from hex representation.
hexDecode :: ByteString -> Maybe GUID
hexDecode s =
  case B16.decode s of
    (a,b) | B.length b == 0 -> Just $ GUID a
    _                       -> Nothing

-- | Convert from ByteString.
fromByteString :: ByteString -> GUID
fromByteString = GUID

-- | Convert to ByteString.
toByteString :: GUID -> ByteString
toByteString (GUID g) = g