{$X+ extensions}  program VTwalk(output:'tt:');

(* random walk.  this is intended to display on a VT100 scope.
   maybe a teleray or 713 could be adapted....

   relax

*)
(* converted to P8, jte 81/8/24, 81-12-06*)
(* random numbers fixed & got rid of reals;  jfm  80/08/29 *)
  const
    maxvert=24; maxhoriz=80;
  type
    char=ascii;
    direction = (up, down, left, right);
    directset = set of direction;
  var
    (* position variables *)
    vert, horiz: integer;
    (* move related variables *)
    moves, validmoves: directset;
    (* screen control characters *)
    escape, downmove,rightmove: char;
    upmove,leftmove: char;
    (* random number related variables *)
    r: real;  seed: integer;
    (* miscelaneous variables *)
    i: integer;
    cs: array [' '..'~'] of char;
    c: char;

  function random(var seed: integer) : integer;

    const
      pshift = 262144;	(* 2 ^ (23-5) *)
      qshift = 32;	(* 2 ^ 5 *)
    var
      a, b:
	record case boolean of
	  true:  (i: integer);
	  false: (s: packed set of 0..23);
	end;
  begin
    a.i := abs(seed);  b.i := a.i div qshift; (*right shift 5*)
    a.s := (a.s - b.s) + (b.s - a.s);	(* xor *)
    b.i := (a.i mod qshift) * pshift; (*left shift 18*)
    a.s := [0..11, 13..23] * ( (a.s - b.s) + (b.s - a.s) ) ;
    seed := a.i;  random := a.i
  end (*random*) ;

  procedure initialize;
    var k : integer; ch: char;
  begin
(*  write(' seed??? :'); read(seed); *)
(*  seed hardwired for conversion *)
    seed := 2179;
    escape := chr(27);
    upmove := 'A'; (* escape 'A' *)
    downmove := 'B';  (* escape 'B' *)
    leftmove := 'D';  (* escape 'D' *)
    rightmove := 'C';  (* escape 'C' *)
    write(output,escape,'H',escape,'J'); (* clear VT100 scope*)

    vert := maxvert div 2 + 1;	horiz := maxhoriz div 2;
    for k := 1 to vert do write(output, escape, downmove);
    for k := 1 to horiz do write(output, escape, rightmove);
    validmoves := [up, down, left, right];
    for ch := ' ' to '~' do cs[ch] := ch;
    c := ' ';
  end (*initialize*) ;
  procedure selectmove;
    var
      r: 0..7;
  begin
    repeat
      r := random(seed) mod 8;
      case r of
	0:  moves := [up];
	1:  moves := [down];
	2:  moves := [right];
	3:  moves := [left];
	4:  moves := [up,right];
	5:  moves := [up,left];
	6:  moves := [down,right];
	7:  moves := [down,left]
      end;
      moves := moves * validmoves;
    until moves <> [];
  end (*selectmove*) ;

  procedure makemove;
    var
      d: direction;
  begin
    if right in moves then
      begin horiz := horiz + 1; moves := moves - [right] end
    else  write(output, escape, leftmove);
    d := up;
    while moves <> [ ] do
      begin
	if d in moves then
	  begin
	    case d of
	      up:    begin write(output,escape,upmove); vert := vert -1 end;
	      down:  begin write(output,escape,downmove); vert := vert + 1 end;
	      left:  begin write(output,escape,leftmove); horiz := horiz - 1 end;
	    end (*case*) ;
	    moves := moves - [d]
	  end (*d in moves*) ;
	d := succ(d)
      end (*while*) ;
    c := succ(c);
    write(output,cs[c]);
    if c = '~' then c := ' ';
    validmoves := [ ];
    if vert < maxvert then validmoves := validmoves + [down];
    if vert > 1 then validmoves := validmoves + [up];
    if horiz < maxhoriz then validmoves := validmoves + [right];
    if horiz > 1 then validmoves := validmoves + [left];
  end (*makemove*) ;

begin
  initialize;
  repeat  selectmove;  makemove  until false
end.