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(* Title: Tools/random_word.ML
Author: Makarius
Simple generator for pseudo-random numbers, using unboxed word
arithmetic only. Unprotected concurrency introduces some true
randomness.
*)
structure Random :> Random =
struct
(* random words: 0w0 <= result <= max_word *)
(*minimum length of unboxed words on all supported ML platforms*)
val _ = Word.wordSize >= 30
orelse raise Fail ("Bad platform word size");
val max_word = 0wx3FFFFFFF;
val top_bit = 0wx20000000;
(*multiplier according to Borosh and Niederreiter (for modulus = 2^30),
see http://random.mat.sbg.ac.at/~charly/server/server.html*)
val a = 0w777138309;
fun step x = Word.andb (a * x + 0w1, max_word);
fun change r f = r := f (!r);
local val rand = (*Unsynchronized.*)ref 0w1
in fun nextWord () = ((*Unsynchronized.*)change rand step; ! rand) end;
(*NB: higher bits are more random than lower ones*)
fun nextBool () = Word.andb (nextWord (), top_bit) = 0w0;
(* random integers: 0 <= result < k *)
val max_int = Word.toInt max_word;
fun nextInt k =
if k <= 0 orelse k > max_int then raise Fail ("next_int: out of range")
else if k = max_int then Word.toInt (nextWord ())
else Word.toInt (Word.mod (nextWord (), Word.fromInt k));
(* random reals: 0.0 <= result < 1.0 *)
val scaling = real max_int + 1.0;
fun nextReal () = real (Word.toInt (nextWord ())) / scaling;
end;
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