Derive scalar changes and composition laws

test/test_harness.ml

@@ -92,3 +92,17 @@ let check_message name expected thunk =
   match error_of thunk with
   | None -> fail name "expected a diagnostic"
   | Some diagnostic -> check_equal_string name expected diagnostic.Diagnostic.message
+
+type rng = { mutable state : int }
+
+let rng seed = { state = seed land 0x3FFFFFFF }
+
+let next rng =
+  rng.state <- (rng.state * 1103515245 + 12345) land 0x3FFFFFFF;
+  rng.state
+
+let range rng bound = if bound <= 0 then 0 else next rng mod bound
+
+let pick rng items = List.nth items (range rng (List.length items))
+
+let string_of_int_list items = Util.join "," (List.map string_of_int items)