Remove the unnecessary extra bit

stdlib/random.ml

@@ -97,24 +97,24 @@ module State = struct
     let max_int_32 = (1 lsl 30) + 0x3FFFFFFF in (* 0x7FFFFFFF *)
     let b1 = bits s in
     let b2 = bits s in
-    let (b, max_int) =
+    let (r, max_int) =
       if n <= max_int_32 then
-        (* 32 random bits on both 64-bit OCaml and JavaScript.
-           Use upper 16 bits of b1 and b2. *)
+        (* 31 random bits on both 64-bit OCaml and JavaScript.
+           Use upper 15 bits of b1 and 16 bits of b2. *)
         let bpos =
-          (((b2 land 0x3FFFC000) lsl 2) lor (b1 lsr 14))
+          (((b2 land 0x3FFFC000) lsl 1) lor (b1 lsr 15))
         in
           (bpos, max_int_32)
       else
         let b3 = bits s in
-        (* 63 random bits on 64-bit OCaml; unreachable on JavaScript.
-           Use upper 21 bits of b1, b2 and b3. *)
+        (* 62 random bits on 64-bit OCaml; unreachable on JavaScript.
+           Use upper 20 bits of b1 and 21 bits of b2 and b3. *)
         let bpos =
-          ((((b3 land 0x3FFFFE00) lsl 12) lor (b2 lsr 9)) lsl 21) lor (b1 lsr 9)
+          ((((b3 land 0x3FFFFE00) lsl 12) lor (b2 lsr 9)) lsl 20)
+            lor (b1 lsr 10)
         in
           (bpos, 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