implemented unescapeString

src/Network/GemServ.hs

@@ -24,13 +24,22 @@ License along with this program.  If not, see
 
 -}
 
+{-# LANGUAGE LambdaCase #-}
+
 module Network.GemServ (
   encodeRequest,
-  escapeString
+  escapeString,
+  unescapeString
 ) where
 
-import Data.Char (ord)
-import Data.List (intercalate)
+import qualified Data.ByteString as BS
+import Data.ByteString.Builder (charUtf8, toLazyByteString)
+import qualified Data.ByteString.Lazy as BSL
+import Data.Char (ord, toLower)
+import Data.List (find, intercalate)
+import Data.Maybe (fromJust)
+import qualified Data.Text as T
+import Data.Text.Encoding (decodeUtf8')
 
 import Network.GemServ.Types
 
@@ -61,6 +70,26 @@ escapeString = concatMap $ \ch ->
       low  = n `mod` 16
       in [hexDigits !! high, hexDigits !! low]
 
+-- | decode an escaped string back to its original value
+unescapeString :: String -> Maybe String
+unescapeString str = case decodeUtf8' $ BS.pack $ toBytes str of
+    Right t -> Just $ T.unpack t
+    _       -> Nothing
+  where
+    toBytes = \case
+      "" -> []
+      '%':h:l:sub -> let
+        h' = toLower h
+        l' = toLower l
+        in if h' `elem` hexDigits && l' `elem` hexDigits
+        then toByte h' l' : toBytes sub
+        else fromIntegral (ord '%') : toBytes (h : l : sub)
+      ch:sub ->
+        BSL.unpack (toLazyByteString $ charUtf8 ch) ++ toBytes sub
+    toByte h l = toNum h * 16 + toNum l
+    toNum ch = fst $ fromJust $
+      find (\x -> snd x == ch) $ zip [0..] hexDigits
+
 hexDigits :: String
 hexDigits = ['0'..'9'] ++ ['a'..'f']