Implement pure-Haskell reverse

src/Data/Text/Internal/Reverse.hs

@@ -7,13 +7,14 @@ module Data.Text.Internal.Reverse where
 
 #if !defined(PURE_HASKELL)
 import GHC.Exts as Exts
-import Control.Monad.ST (ST, runST)
 import Control.Monad.ST.Unsafe (unsafeIOToST)
 import Foreign.C.Types (CSize(..))
 #else
 import Data.Text.Internal (Text(..))
-import qualified Data.Text.Array as A
+import Data.Word (Word8)
 #endif
+import Control.Monad.ST (ST, runST)
+import qualified Data.Text.Array as A
 
 -- | /O(n)/ Reverse the characters of a string.
 --
@@ -27,7 +28,36 @@ reverse ::
 #endif
   Text -> Text
 #if defined(PURE_HASKELL)
-reverse t@(Text ba off len) = t
+reverse (Text ba off len) = runST $ do
+    dest <- A.new len
+    let bytes = A.toList ba off len
+    _ <- reversePoints bytes (len - 1) dest
+    result <- A.unsafeFreeze dest
+    pure $ Text result 0 len
+
+reversePoints :: [Word8] -> Int -> A.MArray s -> ST s ()
+reversePoints bytes off dest =
+    case bytes of
+        [] -> pure ()
+        (leadByte:rest) | leadByte < 0x80 -> do
+            A.unsafeWrite dest (off - 0) leadByte
+            reversePoints rest (off - 1) dest
+        (leadByte:b2:rest) | leadByte < 0xe0 -> do
+            A.unsafeWrite dest (off - 1) leadByte
+            A.unsafeWrite dest (off - 0) b2
+            reversePoints rest (off - 2) dest
+        (leadByte:b2:b3:rest) | leadByte < 0xf0 -> do
+            A.unsafeWrite dest (off - 2) leadByte
+            A.unsafeWrite dest (off - 1) b2
+            A.unsafeWrite dest (off - 0) b3
+            reversePoints rest (off - 3) dest
+        (leadByte:b2:b3:b4:rest) -> do
+            A.unsafeWrite dest (off - 3) leadByte
+            A.unsafeWrite dest (off - 2) b2
+            A.unsafeWrite dest (off - 1) b3
+            A.unsafeWrite dest (off - 0) b4
+            reversePoints rest (off - 4) dest
+        _ -> error ("Not valid UTF-8: " <> show bytes)
 #else
 reverse (Text (A.ByteArray ba) off len) = runST $ do
     marr@(A.MutableByteArray mba) <- A.new len