Create Stamped_hashtable structure

src/utils/stamped_hashtable.ml

@@ -0,0 +1,51 @@
+type ('a, 'b) t = {
+  table: ('a, 'b) Hashtbl.t;
+  mutable recent: (int * 'a) list;
+  mutable sorted: (int * 'a) list;
+}
+
+let create n = {
+  table = Hashtbl.create n;
+  recent = [];
+  sorted = [];
+}
+
+let add t ~stamp a b =
+  Hashtbl.add t.table a b;
+  t.recent <- (stamp, a) :: t.recent
+
+let mem t a =
+  Hashtbl.mem t.table a
+
+let find t a =
+  Hashtbl.find t.table a
+
+(* Sort by decreasing stamps *)
+let order (i1, _) (i2, _) =
+  Int.compare i2 i1
+
+let rec filter_prefix pred = function
+  | x :: xs when not (pred x) ->
+    filter_prefix pred xs
+  | xs -> xs
+
+let backtrack t ~stamp =
+  let process (stamp', path) =
+    if stamp' > stamp then (
+      Hashtbl.remove t.table path;
+      false
+    ) else
+      true
+  in
+  let recent =
+    t.recent
+    |> List.filter process
+    |> List.fast_sort order
+  in
+  t.recent <- [];
+  let sorted =
+    t.sorted
+    |> filter_prefix process
+    |> List.merge order recent
+  in
+  t.sorted <- sorted

src/utils/stamped_hashtable.mli

@@ -0,0 +1,11 @@
+type ('a, 'b) t
+type changes
+
+val create : int -> ('a, 'b) t
+val add : ('a, 'b) t -> stamp:int -> 'a -> 'b -> unit
+val mem : ('a, 'b) t -> 'a -> bool
+val find : ('a, 'b) t -> 'a -> 'b
+
+(* [backtrack table ~stamp] remove all items of [table] with a stamp strictly
+   greater than [stamp] *)
+val backtrack : ('a, 'b) t -> stamp:int -> unit