Rewrite countOcc as countOccBA

Data/ByteString/Internal/Pure.hs

@@ -68,17 +68,15 @@ countOccBA ba len w = pure (go 0 0)
       | otherwise = go n (i+1)
 
 countOcc :: Ptr Word8 -> Int -> Word8 -> IO Int
-countOcc p len w
-  | len == 0  = pure 0
-  | otherwise = count_occ (plusPtr p (len - 1)) w 0 p
-
-count_occ :: Ptr Word8 -> Word8 -> Int -> Ptr Word8 -> IO Int
-count_occ !plast !w !count !p = do
-  c <- peek p
-  let !count' = if c == w then count+1 else count
-  if p == plast
-    then pure count'
-    else count_occ plast w count' (plusPtr p 1)
+countOcc p len w = go 0 0
+  where
+    go !n !i
+      | i == len  = pure n
+      | otherwise = do
+          c <- peekByteOff p i
+          if c == w
+            then go (n+1) (i+1)
+            else go n     (i+1)
 
 -- | Haskell equivalent of C `sbs_elem_index`
 elemIndex :: ByteArray# -> Word8 -> Int -> IO Int