Export a md5DigestBytes function to get access to the raw bytes

The existing Show instance provides a convenient way to get the hex
representation and the Binary and Serialise instances also provide
access to the raw bytes. That said, the documentation does not make
clear that this is what the Binary and Serialise instances do.

So this patch adds md5DigestBytes :: MD5Digest -> B.ByteString
and haddock docs to point out the other options, of Show, Binary or
Serialise.

Author: dcoutts

Data/Digest/Pure/MD5.hs

@@ -31,6 +31,7 @@ module Data.Digest.Pure.MD5
         , md5
         , md5Update
         , md5Finalize
+        , md5DigestBytes
         -- * Crypto-API interface
         , Hash(..)
         ) where
@@ -39,6 +40,9 @@ import qualified Data.ByteString as B
 import qualified Data.ByteString.Lazy as L
 import Data.ByteString.Unsafe (unsafeDrop,unsafeUseAsCString)
 import Data.ByteString.Internal
+#if MIN_VERSION_binary(0,8,3)
+import Data.ByteString.Builder.Extra as B
+#endif
 import Data.Bits
 import Data.List
 import Data.Word
@@ -249,19 +253,42 @@ getNthWord n = right . G.runGet G.getWord32le . B.drop (n * sizeOf (undefined ::
   right x = case x of Right y -> y
 #endif
 
+-- | The raw bytes of an 'MD5Digest'. It is always 16 bytes long.
+--
+-- You can also use the 'Binary' or 'S.Serialize' instances to output the raw
+-- bytes. Alternatively you can use 'show' to prodce the standard hex
+-- representation.
+--
+md5DigestBytes :: MD5Digest -> B.ByteString
+md5DigestBytes (MD5Digest h) = md5PartialBytes h
+
+md5PartialBytes :: MD5Partial -> B.ByteString
+md5PartialBytes =
+    toBs . (put :: MD5Partial -> Put)
+  where
+    toBs :: Put -> B.ByteString
+#if MIN_VERSION_binary(0,8,3)
+    -- with later binary versions we can control the buffer size precisely:
+    toBs = L.toStrict
+         . B.toLazyByteStringWith (B.untrimmedStrategy 16 0) L.empty
+         . execPut
+#else
+    toBs = B.concat . L.toChunks . runPut
+    -- note L.toStrict is only in newer bytestring versions
+#endif
+
 ----- Some quick and dirty instances follow -----
 
 instance Show MD5Digest where
     show (MD5Digest h) = show h
 
 instance Show MD5Partial where
-  show (MD5Par a b c d) =
-    let bs = runPut $ putWord32be d >> putWord32be c >>
-                      putWord32be b >> putWord32be a
+  show md5par =
+    let bs = md5PartialBytes md5par
     in foldl' (\str w -> let cx = showHex w str
                          in if length cx < length str + 2
                                  then '0':cx
-                                else cx) "" (L.unpack bs)
+                                else cx) "" (B.unpack (B.reverse bs))
 
 instance Binary MD5Digest where
     put (MD5Digest p) = put p