Refactor: use the language's references

Suggested-by: xvw <[email protected]>

src/index-format/union_find.ml

@@ -1,12 +1,12 @@
 type 'a content =
   | Root of { mutable value : 'a; mutable rank : int }
   | Link of { mutable parent : 'a element }
-and 'a element = { mutable content : 'a content }
+and 'a element = 'a content ref
 
-let make value = { content = Root { value; rank = 0 } }
+let make value = ref (Root { value; rank = 0 })
 
 let rec find x =
-  match x.content with
+  match !x with
   | Root _ -> x
   | Link ({ parent; _ } as link) ->
     let root = find parent in
@@ -16,23 +16,25 @@ let rec find x =
 let union ~f x y =
   let x = find x in
   let y = find y in
-  match (x, y) with
-  | x, y when x == y -> x
-  | ( { content = Root ({ rank = rank_x; value = value_x } as root_x); _ },
-      { content = Root ({ rank = rank_y; value = value_y } as root_y); _ } ) ->
-    let new_value = f value_x value_y in
-    if rank_x < rank_y then (
-      x.content <- Link { parent = y };
-      root_y.value <- new_value;
-      y)
-    else (
-      y.content <- Link { parent = x };
-      root_x.value <- new_value;
-      if rank_x = rank_y then root_x.rank <- root_x.rank + 1;
-      x)
-  | _ -> assert false
+  if x == y then x
+  else begin
+    match (!x, !y) with
+    | ( Root ({ rank = rank_x; value = value_x } as root_x),
+        Root ({ rank = rank_y; value = value_y } as root_y) ) ->
+      let new_value = f value_x value_y in
+      if rank_x < rank_y then (
+        x := Link { parent = y };
+        root_y.value <- new_value;
+        y)
+      else (
+        y := Link { parent = x };
+        root_x.value <- new_value;
+        if rank_x = rank_y then root_x.rank <- root_x.rank + 1;
+        x)
+    | _ -> assert false
+  end
 
 let get elt =
-  match (find elt).content with
+  match !(find elt) with
   | Root { value; _ } -> value
   | Link _ -> assert false