ocaml: phase 6 Random module (deterministic LCG) (+4 tests, 549 total)

Linear-congruential PRNG with mutable seed (_state ref). API:

  init s        seed the PRNG
  self_init ()  default seed (1)
  int bound     0 <= n < bound
  bool ()       fair coin
  float bound   uniform in [0, bound)
  bits ()       30 bits

Stepping rule:
  state := (state * 1103515245 + 12345) mod 2147483647
  result := |state| mod bound

Same seed reproduces the same sequence. Real OCaml's Random uses
Lagged Fibonacci; ours is simpler but adequate for shuffles and
Monte Carlo demos in baseline programs.

  Random.init 42; Random.int 100  = 48
  Random.init 1;  Random.int 10   = 0

lib/ocaml/runtime.sx

@@ -609,6 +609,30 @@
      let asprintf fmt = Printf.sprintf fmt
    end ;;
 
+   module Random = struct
+     (* Linear-congruential PRNG. Deterministic for testing — same
+        seed reproduces the same sequence. Real OCaml's Random uses
+        Lagged Fibonacci; ours is simpler but adequate for shuffles
+        and Monte Carlo demos in baseline programs. *)
+     let _state = ref 1
+
+     let init s = _state := s
+
+     let self_init () = _state := 1
+
+     let int bound =
+       _state := (!_state * 1103515245 + 12345) mod 2147483647;
+       Int.abs (!_state) mod bound
+
+     let bool () = int 2 = 1
+
+     let float bound =
+       let n = int 1000000 in
+       float_of_int n /. 1000000.0 *. bound
+
+     let bits () = int 1073741824
+   end ;;
+
    module Lazy = struct
      let force lz = _lazy_force lz
    end ;;