lösung der hausaufgabe. kurz aber eher unpraktikabel.

void setup() {
  size(200, 200);
  background(0); 
  noStroke(); 
  frameRate(60);
}

int position = 0;
int RECT_SIZE = 30;


void draw() {
  background(0);
  
  position = (position + 4) % ((width + height) * 2 - RECT_SIZE * 4);
  
  if ( position < width + height - RECT_SIZE * 2 ) {
    rect(constrain(position, 0, width - RECT_SIZE), constrain(position - width + RECT_SIZE, 0, height - RECT_SIZE), RECT_SIZE, RECT_SIZE);
  }
  else {
    rect(width - RECT_SIZE - constrain(position - width - height + RECT_SIZE * 2, 0, width - RECT_SIZE), height - RECT_SIZE - constrain(position - width * 2 - height + RECT_SIZE * 3, 0, height - RECT_SIZE), RECT_SIZE, RECT_SIZE);
  }
}