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+{-# LANGUAGE FlexibleInstances #-}
+
+module Hash where
+
+import Types
+
+import qualified Data.ByteString as B
+import qualified Data.ByteString.Char8 as C8
+import qualified Crypto.Hash as H
+
+-- | Encode a hash pointer using base16 format.
+encodeHashPointer :: HashPointer -> B.ByteString
+encodeHashPointer (HashPointer d) = C8.pack (show d)
+
+class Hashable a where
+	hash :: a -> HashPointer
+
+instance Hashable B.ByteString where
+	hash = HashPointer . H.hash
+
+-- | Hash the concacenation of the hashes, encoding them in base16 format.
+instance Hashable [HashPointer] where
+	hash = hash . B.concat . map encodeHashPointer
+
+instance Hashable a => Hashable (Activity a) where
+	hash (Activity a p s) = hash [hash a, p, hash s]
+	hash (StartActivity a s) = hash [hash a, hash s]
+
+instance Hashable Entered where
+	hash v = hash [hash (enteredData v), hash (echoData v)]
+
+instance Hashable Seen where
+	hash v = hash [hash (seenData v)]
+
+instance Hashable Signature where
+	hash (Signature s) = hash s