void drawLine(SDL_Surface *screen, const struct XYcoord *pos1,
    const struct XYcoord *pos2, Uint8 r, Uint8 g, Uint8 b)
{
  struct XYcoord  add,
                  start,
                  end;

  // Add contains the actual increment to each coordinate to move
  // from start to end. first we divide end by start to determine
  // a ratio of how much to move each pixel of the line

  add.x = end.x / start.x;
  add.y = end.y / start.y;

  // Next, we need to make sure that at least one of the add's is
  // 1, so that we don't write several pixels to the same place on
  // the screen.  We do this by taking the largest value, and
  // divide by it's positive value (to preserve sign)

  if(add.x > add.y) {
    add.y /= abs(add.x);  // should be between 0.0 and 1.0
    add.x /= abs(add.x);  // should be 1.0
  }
  else {
    add.x /= abs(add.y); // should be between 0.0 and 1.0
    add.y /= abs(add.y); // should be 1.0
  }

  // Lastly, draw a pixel at start and increment it by add.  We
  // test for the (int) equality, since matching two doubles in this
  // manner would probably never be true

  while((int)start.x != (int)end.x && (int)start.y != (int)end.y) {
    drawPixel(screen, (int)start.x, (int)start.y, r, g, b);
    start.x += add.x;
    start.y += add.y;
  }
}