Add Random.int63

Behaves equivalently on 64-bit and JavaScript and allows 62-bit
upperbound on 64-bit systems.

Changes

@@ -220,8 +220,9 @@ OCaml 4.12.0
 
 ### Standard library:
 
-- #9487, #9489: Random.int now accepts 62-bit bounds on 64-bit systems.
-  (David Allsopp, request by Francois Berenger, review by Xavier Leroy)
+- #9487, #9489: Add Random.int63 which allows 62-bit bounds on 64-bit systems.
+  (David Allsopp, request by Francois Berenger, review by Xavier Leroy and
+   Damien Doligez)
 
 - #9865: add Format.pp_print_seq
   (Raphaël Proust, review by Nicolás Ojeda Bär)

stdlib/random.ml

@@ -88,17 +88,31 @@ module State = struct
     let v = r mod n in
     if r - v > 0x3FFFFFFF - n + 1 then intaux s n else v
 
+  let int s bound =
+    if bound > 0x3FFFFFFF || bound <= 0
+    then invalid_arg "Random.int"
+    else intaux s bound
+
   let rec int63aux s n =
+    let max_int_32 = (1 lsl 30) + 0x3FFFFFFF in (* 0x7FFFFFFF *)
     let b1 = bits s in
-    let b2 = bits s lsl 30 in
-    let b3 = (bits s land 3) lsl 60 in
-    let r = b1 lor b2 lor b3 in
+    let b2 = bits s in
+    let (b, max_int) =
+      if n <= max_int_32 then
+        (* 32 random bits on both 64-bit OCaml and JavaScript *)
+        ((((b2 land 0x30000000) lsl 2) lor b1) land max_int_32, max_int_32)
+      else
+        let b3 = bits s in
+        (* 63 random bits on 64-bit OCaml; unreachable on JavaScript *)
+        (((((b3 land 0x38000000) lsl 3) lor b2) lsl 30) lor b1, max_int)
+    in
+    let r = b lsr 1 in (* Random non-negative int *)
     let v = r mod n in
     if r - v > max_int - n + 1 then int63aux s n else v
 
-  let int s bound =
+  let int63 s bound =
     if bound <= 0 then
-      invalid_arg "Random.int"
+      invalid_arg "Random.int63"
     else if bound > 0x3FFFFFFF then
       int63aux s bound
     else
@@ -176,6 +190,7 @@ let default = {
 
 let bits () = State.bits default
 let int bound = State.int default bound
+let int63 bound = State.int63 default bound
 let int32 bound = State.int32 default bound
 let nativeint bound = State.nativeint default bound
 let int64 bound = State.int64 default bound

stdlib/random.mli

@@ -39,10 +39,19 @@ val bits : unit -> int
 
 val int : int -> int
 (** [Random.int bound] returns a random integer between 0 (inclusive)
-    and [bound] (exclusive).  [bound] must be greater than 0 and less
-    than {!Stdlib.max_int}
+     and [bound] (exclusive).  [bound] must be greater than 0 and less
+     than 2{^30}. *)
 
-    @before 4.12.0 [bound] had to be less than 2{^30}. *)
+val int63 : int -> int
+(** [Random.int63 bound] returns a random integer between 0 (inclusive)
+     and [bound] (exclusive). [bound] may be any positive integer.
+
+     If [bound] is less than 2{^30}, [Random.int63 bound] is equal to
+     {!Random.int}[ bound]. If [bound] is greater than 2{^30} (on 64-bit systems
+     or non-standard environments, such as JavaScript), [Random.int63] returns
+     a value, where {!Random.int} raises {!Invalid_argument}.
+
+    @since 4.12.0 *)
 
 val int32 : Int32.t -> Int32.t
 (** [Random.int32 bound] returns a random integer between 0 (inclusive)
@@ -91,6 +100,7 @@ module State : sig
 
   val bits : t -> int
   val int : t -> int -> int
+  val int63 : t -> int -> int
   val int32 : t -> Int32.t -> Int32.t
   val nativeint : t -> Nativeint.t -> Nativeint.t
   val int64 : t -> Int64.t -> Int64.t