[sledge] Simplify Term.Sum.to_term

Reviewed By: jvillard

Differential Revision: D21042514

fbshipit-source-id: 9e0365e21

sledge/lib/import/qset.ml

@@ -80,6 +80,13 @@ struct
   let pop = M.pop
   let min_elt = M.min_elt
   let pop_min_elt = M.pop_min_elt
+
+  let classify s =
+    match pop s with
+    | None -> `Zero
+    | Some (elt, q, s') when is_empty s' -> `One (elt, q)
+    | _ -> `Many
+
   let to_list m = M.to_alist m
   let iter m ~f = M.iteri ~f:(fun ~key ~data -> f key data) m
   let exists m ~f = M.existsi ~f:(fun ~key ~data -> f key data) m

sledge/lib/import/qset_intf.ml

@@ -65,6 +65,9 @@ module type S = sig
   val pop_min_elt : t -> (elt * Q.t * t) option
   (** Find and remove minimum element. [O(log n)]. *)
 
+  val classify : t -> [`Zero | `One of elt * Q.t | `Many]
+  (** Classify a set as either empty, singleton, or otherwise. *)
+
   val find_and_remove : t -> elt -> (Q.t * t) option
   (** Find and remove an element. *)
 

sledge/lib/term.ml

@@ -480,16 +480,16 @@ module Sum = struct
     else Qset.map_counts ~f:(fun _ -> Q.mul const) sum
 
   let to_term sum =
-    match Qset.pop sum with
-    | None -> zero
-    | Some (arg, q, sum') when Qset.is_empty sum' -> (
+    match Qset.classify sum with
+    | `Zero -> zero
+    | `One (arg, q) -> (
       match arg with
       | Integer {data} ->
           assert (Z.equal Z.one data) ;
           rational q
       | _ when Q.equal Q.one q -> arg
       | _ -> Add sum )
-    | _ -> Add sum
+    | `Many -> Add sum
 end
 
 (* Products of indeterminants represented by multisets. A product ∏ᵢ xᵢ^nᵢ