program Gas;

const
  x_size = 30;
  y_size = 18;
  nrMol  = 10;

var
  s: semaphore;

procedure DrawChamber;
var
  i: integer;
begin
  clrscr;
  gotoxy(2, 2);
  for i := 1 to x_size+2 do write('*');
  for i := 1 to y_size do begin
    gotoxy(2, 2+i); write('*', ' ':x_size, '*');
  end;
  gotoxy(2, 3+y_size);
  for i := 1 to x_size+2 do write('*');
end;

process type Molecule(id: char);
var
  x, y, px, py,
  dx, dy, i, j: integer;
begin
  px := 0; py := 0;
  x := random(x_size) + 1; y := random(y_size) + 1;
  dx := random(2) - 2; dy := random(2) - 1;
  i := 0;
  repeat
    if (x + dx) < 1 then dx := 1;
    if (x + dx) > x_size then dx := -1;
    if (y + dy) < 1 then dy := 1;
    if (y + dy) > y_size then dy := -1;
    px := x; x := x + dx;
    py := y; y := y + dy;

    wait(s);
{       gotoxy(px+2, py+2); write(chr(ord(id)+32)); }
      gotoxy(px+2, py+2); write(' ');
      gotoxy(x+2, y+2); write(id);
    signal(s);
    sleep(5);
  forever;
end;

var
  Mol: array [1..nrMol] of Molecule;
  id: integer;

begin
  DrawChamber;
  initial(s, 1);
  cobegin
    for id := 1 to nrMol do
      Mol[id](chr(64+id));
  coend;
end.