function setup() {

  createCanvas(400, 400);

  box = new Box(width / 2, height / 2, 100);

}

​

function draw() {

  background(220);

  box.draw();

}

​

class Box {

  constructor(x, y, w) {

    this.width = w;

    this.half = createVector(w / 2, w / 2);

    this.center = createVector(x, y);

    this.p1 = p5.Vector.sub(this.center, this.half);

    this.p2 = p5.Vector.add(this.center, this.half);

​

    // inside

    if (random() < 0.5) {

      this.start = 0;

      this.stop = PI;

    } else {

      // outside

      this.start = PI;

      this.stop = 0;

    }

  }

​

  draw() {

    stroke("green");

    strokeWeight(4);

    noFill();

    line(this.p2.x, this.p2.y, this.p2.x, this.p1.y);

    line(this.p1.x, this.p1.y, this.p1.x, this.p2.y);

    line(this.p1.x, this.p2.y, this.p2.x, this.p2.y);

    arc(

      this.center.x,

      this.center.y - this.half.y,

      this.width,

      this.width,

      this.start,

      this.stop

    );

  }

}

​