Merge pull request #150 from xavierleroy/revised-hash

Use standard hash function for `Z.hash` and add `Z.seeded_hash`

caml_z.c

@@ -3350,11 +3350,6 @@ static intnat ml_z_custom_hash(value v)
   return acc;
 }
 
-CAMLprim value ml_z_hash(value v)
-{
-  return Val_long(ml_z_custom_hash(v));
-}
-
 /* serialized format:
    - 1-byte sign (1 for negative, 0 for positive)
    - 4-byte size in bytes

tests/Makefile

@@ -38,7 +38,7 @@ test:: ofstring.exe
 
 test:: chi2.exe
 	@echo "Testing random number generation..."
-	@./chi2.exe
+	@if ./chi2.exe; then echo "chi2: passed"; else echo "chi2: FAILED"; exit 2; fi
 
 bench:: timings.exe
 	./timings.exe

tests/chi2.ml

@@ -1,12 +1,11 @@
 (* Accumulate [n] samples from function [f] and check the chi-square.
-   Only the low 8 bits of the result of [f] are sampled. *)
+   Assumes [f] returns integers in the [0..255] range. *)
 
 let chisquare n f =
   let r = 256 in
   let freq = Array.make r 0 in
   for i = 0 to n - 1 do
-    let t = Z.to_int (Z.logand (f ()) (Z.of_int 0xFF)) in
-    freq.(t) <- freq.(t) + 1
+    let t = f () in freq.(t) <- freq.(t) + 1
   done;
   let expected = float n /. float r in
   let t =
@@ -22,9 +21,19 @@ let chisquare n f =
   *)
   chi2 <= degfree +. 4.0 *. sqrt (2.0 *. degfree)
 
+let failed = ref false
+
+let test_base name f =
+  if not (chisquare 100_000 f) then begin
+    Printf.printf "%s: suspicious result\n%!" name;
+    failed := true
+  end
+
 let test name f =
-  if not (chisquare 100_000 f)
-  then Printf.printf "%s: suspicious result\n%!" name
+  (* Test the low 8 bits of the result of f *)
+  test_base name  (fun () -> Z.to_int (Z.logand (f ()) (Z.of_int 0xFF)))
+
+let p = Z.of_string "35742549198872617291353508656626642567"
 
 let _ =
   test "random_bits 15 (bits 0-7)"
@@ -38,6 +47,14 @@ let _ =
   test "random_int 2^30 (bits 21-28)"
        (fun () -> Z.(shift_right (random_int (shift_left one 30)) 21));
   test "random_int (256 * p) / p"
-       (let p = Z.of_string "35742549198872617291353508656626642567" in
-        let bound = Z.shift_left p 8 in
-        fun () -> Z.(div (random_int bound) p))
+       (let bound = Z.shift_left p 8 in
+        fun () -> Z.(div (random_int bound) p));
+  (* Also test our hash function, why not? *)
+  test_base "hash (random_int p) (bits 0-7)"
+       (fun () -> Z.(hash (random_int p)) land 0xFF);
+  test_base "hash (random_int p) (bits 16-23)"
+       (fun () -> (Z.(hash (random_int p)) lsr 16) land 0xFF);
+  exit (if !failed then 2 else 0)
+
+
+

z.ml

@@ -258,7 +258,8 @@ external perfect_power: t -> bool = "ml_z_perfect_power"
 external perfect_square: t -> bool = "ml_z_perfect_square"
 external probab_prime: t -> int -> int = "ml_z_probab_prime"
 external nextprime: t -> t = "ml_z_nextprime"
-external hash: t -> int = "ml_z_hash" [@@noalloc]
+let hash: t -> int = Stdlib.Hashtbl.hash
+let seeded_hash: int -> t -> int = Stdlib.Hashtbl.seeded_hash
 external to_bits: t -> string = "ml_z_to_bits"
 external of_bits: string -> t = "ml_z_of_bits"
 

z.mli

@@ -486,12 +486,24 @@ val is_odd: t -> bool
     @since 1.4
 *)
 
-external hash: t -> int = "ml_z_hash" [@@noalloc]
+val hash: t -> int
 (** Hashes a number, producing a small integer.
-    The result is consistent with equality: if [a] = [b], then [hash a] =
-    [hash b].
-    OCaml's generic hash function, [Hashtbl.hash], works correctly with
-    numbers, but {!Z.hash} is slightly faster.
+    The result is consistent with equality:
+    if [a] = [b], then [hash a] = [hash b].
+    The result is the same as produced by OCaml's generic hash function,
+    {!Hashtbl.hash}.
+    Together with type {!Z.t}, the function {!Z.hash} makes it possible
+    to pass module {!Z} as argument to the functor {!Hashtbl.Make}.
+    @before 1.14 a different hash algorithm was used.
+*)
+
+val seeded_hash: int -> t -> int
+(** Like {!Z.hash}, but takes a seed as extra argument for diversification.
+    The result is the same as produced by OCaml's generic seeded hash function,
+    {!Hashtbl.seeded_hash}.
+    Together with type {!Z.t}, the function {!Z.hash} makes it possible
+    to pass module {!Z} as argument to the functor {!Hashtbl.MakeSeeded}.
+    @since 1.14
 *)
 
 (** {1 Elementary number theory} *)
@@ -717,6 +729,8 @@ val random_int: ?rng: Random.State.t -> t -> t
     Random numbers produced by this function are not cryptographically
     strong and must not be used in cryptographic or high-security
     contexts.  See {!Z.random_int_gen} for an alternative.
+
+    @since 1.13
 *)
 
 val random_bits: ?rng: Random.State.t -> int -> t
@@ -731,6 +745,8 @@ val random_bits: ?rng: Random.State.t -> int -> t
     Random numbers produced by this function are not cryptographically
     strong and must not be used in cryptographic or high-security
     contexts.  See {!Z.random_bits_gen} for an alternative.
+
+    @since 1.13
 *)
 
 val random_int_gen: fill: (bytes -> int -> int -> unit) -> t -> t
@@ -751,6 +767,7 @@ val random_int_gen: fill: (bytes -> int -> int -> unit) -> t -> t
 <<
     Z.random_int_gen ~fill:Cryptokit.Random.secure_rng#bytes bound
 >>
+    @since 1.13
 *)
 
 val random_bits_gen: fill: (bytes -> int -> int -> unit) -> int -> t
@@ -759,6 +776,7 @@ val random_bits_gen: fill: (bytes -> int -> int -> unit) -> int -> t
     This is a more efficient special case of {!Z.random_int_gen} when the
     bound is a power of two.  The [fill] parameter is as described in
     {!Z.random_int_gen}.
+    @since 1.13
 *)
 
 (** {1 Prefix and infix operators} *)