// draw levy fractal of given DEGREE over line from (X1,Y1)
//   to (X2,Y2)
void levy(int x1, int y1, int x2, int y2, int degree)
{
   if ( degree <= 0 )
      line(x1, y1, x2, y2);
   else
   {
      double baseAngle = (x1 == x2)
                       ? pi / 2
                       : atan(abs(y2 - y1) / abs(x2 - x1));
      double side      = sqrt(double(sqr(x2 - x1)
                                     + sqr(y2 - y1))) / 2;
      double cos_side  = side * (cos(baseAngle)
                                 - sin(baseAngle));
      double sin_side  = side * (cos(baseAngle)
                                 + sin(baseAngle));
      int    newx, newy;

      if ( x1 < x2 && y1 < y2 )
      {
         newx = iround(x2 - cos_side);
         newy = iround(y2 - sin_side);
      }
      else if ( x1 < x2 )  // && y1 >= y2
      {
         newx = iround(x1 + cos_side);
         newy = iround(y1 - sin_side);
      }
      else if ( y1 < y2 )  // && x1 >= x2
      {
         newx = iround(x1 - cos_side);
         newy = iround(y1 + sin_side);
      }
      else                 // x1 >= x2 && y1 >= y2
      {
         newx = iround(x2 + cos_side);
         newy = iround(y2 + sin_side);
      }
      levy(x1,   y1,   newx, newy, degree - 1);
      levy(newx, newy, x2,   y2,   degree - 1);
   }
}