MODULE RandomSubtractiveFibo2;

Gnu CopyLefted.

Abstract: Pseudo-random number generator by Warren D. Smith.

IMPORT RandomBasic, Word;
IMPORT RandomRep;

<* UNUSED *>
CONST
  Module = "RandomSubtractiveFibo2.";

CONST
  asf2 = 95;
  bsf2 = 17;

TwoTo32Minus5 = 16_fffffffb; (* prime

  TwoTo32Minus1 = 16_ffffffff;
*)

REVEAL
  T = TPublic BRANDED OBJECT
        isf2                                       := asf2;
        jsf2                                       := bsf2;
        arrsf2: ARRAY [0 .. asf2 - 1] OF INTEGER;
        (* initialize to random Word.Ts mod TwoTo32Minus5 *)
      OVERRIDES
        init   := Init;
        engine := Engine;
      END;

PROCEDURE Init (SELF: T; initrng: RandomBasic.T; ): T =
  VAR
  BEGIN
    FOR i := asf2 - 1 TO 0 BY -1 DO
      SELF.arrsf2[i] := initrng.generateWord();
    END;
    SELF.arrsf2[0] := Word.Or(initrng.generateWord(), 2_1);
    RETURN SELF;
  END Init;

PROCEDURE Engine (SELF: T; ): Word.T =
  VAR x, y: Word.T;
  BEGIN
    <* ASSERT Word.Size = 32 *>
    DEC(SELF.isf2);
    DEC(SELF.jsf2);
    IF SELF.isf2 < 0 THEN
      SELF.isf2 := asf2 - 1;     (* wraparound *)
    ELSIF SELF.jsf2 < 0 THEN
      SELF.jsf2 := asf2 - 1;     (* wraparound *)
    END;
    y := SELF.arrsf2[SELF.isf2];
    x := Word.Minus(y, SELF.arrsf2[SELF.jsf2]);
    IF Word.GT(x, y) THEN        (* subtraction "wrapped" *)
      x := x + 5;
    END;
    SELF.arrsf2[SELF.isf2] := x;
    RETURN x;
  END Engine;

BEGIN
END RandomSubtractiveFibo2.