ocaml: phase 5.1 stable_unique.ml baseline (Hashtbl dedupe preserving order, 8+38 = 46)

Walk input with Hashtbl.mem + Hashtbl.add seen x () (unit-payload
turns the table into a set); on first occurrence cons to the result
list; reverse at the end:

  let stable_unique xs =
    let seen = Hashtbl.create 8 in
    let result = ref [] in
    List.iter (fun x ->
      if not (Hashtbl.mem seen x) then begin
        Hashtbl.add seen x ();
        result := x :: !result
      end
    ) xs;
    List.rev !result

For [3;1;4;1;5;9;2;6;5;3;5;8;9]:
  result = [3;1;4;5;9;2;6;8]   (input order, dupes dropped)
  length = 8, sum = 38         total = 46

Tests Hashtbl as a set abstraction (unit-payload), the rev-build
idiom, and 'not (Hashtbl.mem seen x)' membership negation.

82 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -70,6 +70,7 @@
   "run_length.ml": 11,
   "safe_div.ml": 20,
   "shuffle.ml": 55,
+  "stable_unique.ml": 46,
   "subset_sum.ml": 8,
   "tic_tac_toe.ml": 1,
   "word_freq.ml": 8,

lib/ocaml/baseline/stable_unique.ml

@@ -0,0 +1,15 @@
+let stable_unique xs =
+  let seen = Hashtbl.create 8 in
+  let result = ref [] in
+  List.iter (fun x ->
+    if not (Hashtbl.mem seen x) then begin
+      Hashtbl.add seen x ();
+      result := x :: !result
+    end
+  ) xs;
+  List.rev !result
+
+;;
+
+List.length (stable_unique [3;1;4;1;5;9;2;6;5;3;5;8;9])
++ List.fold_left (+) 0 (stable_unique [3;1;4;1;5;9;2;6;5;3;5;8;9])