//example 5-8, square following edge, uses a x, y, speed and state variables
int x = 0; //x location of square
int y = 0; //y location of square
int speed = 5;  //speed of square
int state = 0; //track square's state

void setup() {
  size(400,400);
}

void draw() {
  background(100);
  
  //display the square
  noStroke();
  rect(x, y, 10, 10);
  if (state == 0) {
    x = x + speed;
    if (x > width-10) {
      x = width-10;
      state = 1;
    }
  } 
  else if (state == 1) {
    y = y + speed;
    if (y > height-10) {
      y = height-10;
      state = 2;
    }
  }
  else if (state == 2) {
    x = x - speed;
    if (x < 0) {
      x = 0;
      state = 3;
    }
  } 
  else if (state == 3) {
    y = y - speed;
    if (y < 0) {
      y = 0;
      state = 0;
    }
  }
}