optimizes the merge function for OGRE documents

The new implementation will untouch the document if the other one is
an empty document or is the same. When the documents are different and
non-trivial we will take the larger one and update it with the smaller
one, which is also faster. At the end, it makes merge more than 10
times faster under certain scenarios.

Author: ivg

lib/ogre/ogre.ml

@@ -232,7 +232,7 @@ module Doc = struct
               else (packed :: data)) in
     { doc with entries }
 
-  let merge doc {scheme; entries} =
+  let do_merge doc {scheme; entries} =
     Map.to_sequence scheme |>
     Error.Seq.fold ~init:doc ~f:(fun doc (name,sign) ->
         update_scheme name sign doc) >>= fun doc ->
@@ -241,6 +241,14 @@ module Doc = struct
         Error.List.fold values ~init:doc ~f:(fun doc value ->
             update_entries name value doc))
 
+  let merge d1 d2 = match is_empty d1, is_empty d2 with
+    | true,true -> Ok d1
+    | true,false -> Ok d2
+    | false,true -> Ok d1
+    | false,false -> match compare d1 d2 with
+      | 0 -> Ok d1
+      | 1 -> do_merge d1 d2
+      | _ -> do_merge d2 d1
 
   let put k attr =
     let {Attribute.name; save; sign} = attr () in