// arrow of time in "Good and Real"

​

// there is a bug I can't fix: soon after the animation starts,

// the large circle starts jumping as if being displaced or as 

// if a collision goes wrong

​

let circles;

​

function setup() {

  createCanvas(400, 400);

  r = 5; s = 1; // radius and speed of small circles

  R = 50; S = 2; // radius and speed of large circles

  circles = new Group;

  for (let i = 0; i < 1; i++) {

    // big circle(s)  

    circles.add(makeCircle(R, S)); 

  }

  for (let i = 0; i < 100; i++) {

    // lots of little circles  

    circles.add(makeCircle(r, s * random(0, 1)));  

  }

}

​

function makeCircle(r, s) {

  let col = [random(255), random(255), random(255)];

  c = createSprite(random(width), random(height), r, r);

  c.draw = function() { 

    fill(col); 

    ellipse(0, 0, r, r) 

  } 

  c.setCollider("circle");

  c.setSpeed(s, random(0, 360));

  c.mass = PI * r * r;

  c.scale = 1;

  return c;

}

​

function draw() {

  background(220);

  // three versions of the .bounce() call:

  // all have the same problem

  // 1a. nested for-loop version with i < j:

  // for (let i = 0; i < circles.length; i++) {

  //   for (let j = i + 1; j < circles.length; j++) {

  //     circles[i].bounce(circles[j]);

  //   }

  // }

  // 1b. nested for-loop version with i != j

  // for (let i = 0; i < circles.length; i++) {

  //   for (let j = 0; j < circles.length; j++) {

  //     if (i != j) {

  //       circles[i].bounce(circles[j]);

  //     }

  //   }

  // }

  // 2. single for-loop version:

  // for (let c of circles) {

  //  c.bounce(circles);   // 2a singular bounce plural

  //  //circles.bounce(c); // 2b plural bounce singular

  // }

  // original no for-loop version

  circles.bounce(circles);

  bounceGroupWalls(circles);

  for (let c of circles) {

    c.debug = mouseIsPressed;

  }

  drawSprites();

  status();

}

​

function status() {

  textSize(12);

  let s = 0;

  for (let c of circles) {

    s += c.getSpeed() * c.mass;

  }

  s = s.toFixed(0);

  text("Momentum " + s, 30, 30);

}

​

function bounceGroupWalls(g) {

  //all sprites in group g bounce at the screen edges

  for (let s of g) {

    if(s.position.x <= 0 || s.position.x >= width) {

      s.velocity.x *= -1;

    }

    if(s.position.y <= 0 || s.position.y >= height) {

      s.velocity.y *= -1;

    }    

  }

}