diff --git a/src/core/tests/dune b/src/core/tests/dune
index 3aa1e4f8..4d56aece 100644
--- a/src/core/tests/dune
+++ b/src/core/tests/dune
@@ -12,6 +12,12 @@
   (flags :standard -warn-error -a+8)
   (libraries containers))
 
+(executable
+  (name test_random)
+  (flags :standard -warn-error -a+8)
+  (modules test_random)
+  (libraries containers))
+
 (alias
   (name runtest)
   (locks ctest)
diff --git a/src/core/tests/test_hash.ml b/src/core/tests/test_hash.ml
new file mode 100644
index 00000000..4ebb50ea
--- /dev/null
+++ b/src/core/tests/test_hash.ml
@@ -0,0 +1,92 @@
+
+
+(* test hash functions a bit *)
+
+module H = CCHash
+
+module Hist = struct
+  type t = {
+    tbl: (int,int) Hashtbl.t;
+    mutable n_samples: int;
+  }
+  let create() : t = {tbl=Hashtbl.create 32; n_samples=0}
+  let add self x =
+    Hashtbl.replace self.tbl x (1 + try Hashtbl.find self.tbl x with _ ->0);
+    self.n_samples <- 1 + self.n_samples
+
+  let pp out (self:t) : unit =
+    let max = Hashtbl.fold (fun k _ n -> max k n) self.tbl 0 in
+    let min = Hashtbl.fold (fun k _ n -> min k n) self.tbl max in
+    for i=min to max do
+      let n = try Hashtbl.find self.tbl i with _ -> 0 in
+      Format.fprintf out "[v=%-4d, n-inputs %-6d] %s@." i n
+        (String.make (int_of_float @@ ceil (log (float n))) '#');
+    done
+end
+
+(* how long does it take to get back to [n0]?
+   @param max if provided, stop when reaching it *)
+let orbit_size ?(max=max_int) n0 : int =
+  let size = ref 1 in
+  let n = ref (H.int n0) in
+  while !n <> n0 && !size < max do
+    n := H.int !n;
+    incr size;
+  done;
+  !size
+
+let reset_line = "\x1b[2K\r"
+
+let t_int n1 n2 =
+  Printf.printf "test hash_int on %d--%d\n" n1 n2;
+  let hist = Hist.create() in
+  for i=n1 to n2 do
+    Printf.printf "%sorbit for %d…%!" reset_line i;
+    let orb = orbit_size ~max:30_000 i in
+    Hist.add hist orb;
+  done;
+  Printf.printf "%s%!" reset_line;
+  Format.printf "histogram:@.%a@." Hist.pp hist;
+  (*assert (Hist.check_uniform hist);*)
+  ()
+
+(* how long does it take to get back to [n0] by
+   hashing [n] with [m0] ?
+   @param max if provided, stop when reaching it *)
+let left_orbit_size ?(max=max_int) n0 m0 : int =
+  let size = ref 1 in
+  let n = ref (H.combine2 (H.int n0) (H.int m0)) in
+  while !n <> n0 && !size < max do
+    n := H.combine2 (H.int !n) (H.int m0);
+    incr size;
+  done;
+  !size
+
+let t_int2 n1 n2 n3 n4 =
+  Printf.printf "test hash combine2 on %d--%d x %d--%d\n" n1 n2 n3 n4;
+  let hist = Hist.create() in
+  for i=n1 to n2 do
+    for j=n3 to n4 do
+      Printf.printf "%sleft-orbit for %d x %d…%!" reset_line i j;
+      let orb = left_orbit_size ~max:20_000 i j in
+      Hist.add hist orb;
+    done;
+  done;
+  Printf.printf "%s%!" reset_line;
+  Format.printf "histogram:@.%a@." Hist.pp hist;
+  ()
+
+let help =
+{|Test the hash function on integers, by computing orbits for inputs.
+An orbit is a cycle [n -> h(n) -> h(h(n)) -> h^3(n) -> … h^k(n)]
+where [h^k(n) == n]. For a good hash functions, orbits should be as long as
+possible.
+|}
+
+let () =
+  print_endline help;
+  t_int 0 80_000;
+  t_int 3_000_000 3_040_000;
+  t_int (-40_000) (-17_000);
+  t_int2 10 300 600 800;
+  ()