(* Copyright (C) 2009,2017 Matthew Fluet.
 * Copyright (C) 1999-2006 Henry Cejtin, Matthew Fluet, Suresh
 *    Jagannathan, and Stephen Weeks.
 *
 * MLton is released under a BSD-style license.
 * See the file MLton-LICENSE for details.
 *)

structure Random: RANDOM =
struct

local
   open MLton.Random
in
   val alphaNumString = alphaNumString
   val seed = seed
   val useed = useed
   val word = rand
   val srand = srand
end

val word = Trace.trace ("Random.word", Unit.layout, Word.layout) word
val srand = Trace.trace ("Random.srand", Word.layout, Unit.layout) srand

local
   val ri: int ref = ref 0
   val rw = ref (word ())
   val max = Word.wordSize - 1
in
   fun bool () =
      let
         val i = !ri
         val b = 0w1 = Word.andb (0wx1, Word.>> (!rw, Word.fromInt i))
         val _ =
            if i = max
               then (rw := word ()
                     ; ri := 0)
            else ri := 1 + i
      in b
      end
end

fun int () = Word.toIntX (word ())

val int = Trace.trace ("Random.int", Unit.layout, Int.layout) int

local
val maxNat =
   let
      val shft = Option.fold (Int.precision, Word.wordSize, Int.min)
      val shft = Word.fromInt (shft - 1)
   in
      Word.toInt (Word.notb (Word.<< (Word.notb 0w0, shft)))
   end

val maxNatW = Word.fromInt maxNat

fun nat () = Word.toInt (Word.andb (word (), maxNatW))

val nat = Trace.trace ("Random.nat", Unit.layout, Int.layout) nat

val maxNatR = Real.fromInt maxNat

fun scale r = r / maxNatR

val natReal = Real.fromInt o nat

val natReal = Trace.trace0 ("Random.natReal", Real.layout) natReal
in
fun real () = scale (natReal () + scale (natReal ()))

val real = Trace.trace0 ("Random.real", Real.layout) real
end

local
   val r: word ref = ref 0w0
   val max: word ref = ref 0w0
in
   fun wordLessThan (w: word): word =
      if w = 0w0
         then Error.bug "Random.wordLessThan"
      else
         let
            val () =
               if w - 0w1 <= !max
                  then ()
               else (r := word ()
                     ; max := Word.notb 0wx0)
            val w' = !r
            val () = r := Word.div (w', w)
            val () = max := Word.div (!max, w)
         in
            Word.mod (w', w)
         end
end

fun natLessThan (n: int): int =
   if n <= 0
      then Error.bug "Random.natLessThan"
   else Word.toInt (wordLessThan (Word.fromInt n))

fun charFrom (s: string): char =
   Pervasive.String.sub (s, natLessThan (Pervasive.String.size s))

fun nRandom {list, length, n} =
   let
      fun loop (need: int, length: int, xs: 'a list, ac: 'a list): 'a list =
         (Assert.assert ("Random.nRandom", fn () => need <= length)
          ; if need <= 0
               then ac
            else (case xs of
                     [] => Error.bug "nRandom"
                   | x :: xs =>
                        if natLessThan length < need
                           then loop (need - 1, length - 1, xs, x :: ac)
                        else loop (need, length - 1, xs, ac)))
   in loop (n, length, list, [])
   end

val nRandom = fn x =>
   Assert.assertFun
   ("nRandom", nRandom,
    fn {list, length, n} => (length = List.length list
                             andalso 0 <= n
                             andalso n <= length,
                             fn l => n = List.length l))
   x

fun list l =
   let
      val n = List.length l
   in
      if n = 0
         then NONE
      else SOME (List.nth (l, natLessThan n))
   end

end