Fixes after last review

Data/ByteString/Internal/Pure.hs

@@ -79,6 +79,7 @@ count_occ !plast !w !count !p = do
     then pure count'
     else count_occ plast w count' (plusPtr p 1)
 
+-- | Haskell equivalent of C `sbs_elem_index`
 elemIndex :: ByteArray# -> Word8 -> Int -> IO Int
 elemIndex !ba !w !len = pure (go 0)
   where
@@ -95,10 +96,6 @@ reverseBytes !dst !src !n
 
 -- | Note that reverse_bytes reverses at least one byte.
 -- Then it loops if necessary until the destination buffer is full
---
--- Compared to the C version, we use pointer themselves as counters instead of
--- carrying around the remaining number of bytes to reverse: one less operation
--- per iteration.
 reverse_bytes :: Ptr Word8 -> Ptr Word8 -> Ptr Word8 -> IO ()
 reverse_bytes orig_dst dst src = do
   c <- peekByteOff src 0
@@ -152,8 +149,9 @@ partition !p !low !high = do
   -- traverse through all elements.
   -- swap element smaller than pivot at index j with leftmost element at
   -- index i greater than pivot (can be itself if no greater element read yet)
+  let !high_is_maxbound = high == maxBound
   let go !i !j
-        | j > high = pure (i-1)
+        | j > high || (high_is_maxbound && j == minBound) = pure (i-1)
         | otherwise = do
           jv <- peekByteOff p j
           if (jv <= pivot)