(* This is a port of the A2 RNG.
Although not suitable for cryptographic applications, naclcl.lisp used the built-in RNG so it's good enough to get started *)
MODULE Random;
  IMPORT SYSTEM, Texts, Oberon;

  CONST max = 2147483647;
    msbit = 40000000H;
    allbits = 7FFFFFFFH;
    halfrange = 20000000H;
    step = 7;

  VAR buffer: ARRAY 250 OF SET;
    index: INTEGER;
    Z: INTEGER;
    W: Texts.Writer;

  PROCEDURE Rand(): INTEGER;
    CONST a = 16807; q = 127773; r = 2836;
    VAR t: INTEGER;
  BEGIN t := a * (Z MOD q) - r * (Z DIV q);
    IF t > 0 THEN Z := t ELSE Z := t + max END;
    RETURN Z
  END Rand;

  PROCEDURE InitSeed*(seed: INTEGER);
    VAR k, i: INTEGER;
      mask, msb: INTEGER;
  BEGIN Z := seed; index := 0;
    FOR i := 0 TO 249 DO
      buffer[i] := SYSTEM.VAL(SET, Rand())
    END;
    FOR i := 0 TO 249 DO
      IF Rand() > halfrange THEN
        buffer[i] := buffer[i] + SYSTEM.VAL(SET, msbit)
      END
    END;
    msb := msbit; mask := allbits;
    FOR i := 0 TO 30 DO
      k := step * i + 3;
      buffer[k] := buffer[k] * SYSTEM.VAL(SET, mask);
      buffer[k] := buffer[k] * SYSTEM.VAL(SET, msb);
      msb := msb DIV 2;
      mask := mask DIV 2;
    END
  END InitSeed;

  PROCEDURE Integer*(): INTEGER;
    VAR newRand, j: INTEGER;
  BEGIN IF index > 147 THEN j := index - 147 ELSE j := index + 103 END;
    buffer[index] := buffer[index] / buffer[j];
    newRand := SYSTEM.VAL(INTEGER, buffer[index]);
    IF index >= 249 THEN index := 0 ELSE INC(index) END;
    RETURN newRand
  END Integer;

  PROCEDURE Test*;
  BEGIN (* Texts.WriteHex(W, Integer()); Texts.WriteLn(W); *)
    Texts.WriteHex(W, LSL(16, 2)); Texts.WriteLn(W);
    Texts.WriteHex(W, LSL(16, -2)); Texts.WriteLn(W);
    Texts.Append(Oberon.Log, W.buf)
  END Test;

(* TODO: really need to seed this,
or we always get the same sequence.
As I probably should expect. *)
BEGIN InitSeed(1);
  Texts.OpenWriter(W)
END Random.