let diameter_slider;

let oldX, oldY;

let fresh_id = 0;

let draw_line = false;

let grav_const = 1;

let delete_planets = [];

let ghost_planet;

let vel_factor = 2;

let xsys;

let paused = false, pausing = false;

​

function setup() {

  frameRate(60);

  let theCanvas = createCanvas(600, 600);

  /*theCanvas.parent("p5_container");*/

  diameter_slider = createSlider(10, 100, 10, 5);

}

​

function draw() {

  background(220);

  fill(0);

  text("Diameter: " + diameter_slider.value(), 10, height-10)

  if (draw_line) {

    dashed_line(oldX, oldY, mouseX, mouseY, 50);

    fill(random_color);

    /*ellipse(oldX, oldY, diameter_slider.value());*/

    ghost_planet.draw();

    xsys = ghost_planet.path(8, 120);

    dashed_array(xsys[0], xsys[1]);

  }

  for (planet of planets) {

    if (!paused) {

      planet.vel.add(planet.new_accel(deltaTime / 1000));

      planet.pos.add(planet.new_vel(deltaTime / 1000));

    }

    planet.draw();

  }

  for (collision of find_colliders(planets)) {

    update_colliders(planets[collision[0]], planets[collision[1]]);

    delete_planets.push(collision[0]);

    delete_planets.push(collision[1]);

  }

  delete_planets.sort();

  delete_planets.reverse();

  for (delete_planet of delete_planets) {

    planets.splice(delete_planet, 1);

  }

  delete_planets = [];

}

​

function mousePressed() {

  oldX = mouseX;

  oldY = mouseY;

  if (mouse_in_box(0, 0, width, height)) {

    draw_line = true;

    if (pausing) {paused = true}

    random_color = color(random(100, 255), random(100, 255), random(100, 255));

    ghost_planet = new Planet(oldX, oldY, 0, 0, diameter_slider.value(), random_color);

  }

}

​

function mouseReleased() {

  if (draw_line) {

    let vx = (oldX - mouseX) / vel_factor;

    let vy = (oldY - mouseY) / vel_factor;

    ghost_planet = undefined;

    planets.push(new Planet(oldX, oldY, vx, vy, diameter_slider.value(), random_color));

    draw_line = false;

    paused = false;

  }

}

​

function mouse_in_box(x1, y1, x2, y2) {

  if (mouseX > x1 && mouseX < x2 &&

    mouseY > y1 && mouseY < y2) {

    return true;

  }

  return false;

}

​

function dashed_line(x1, y1, x2, y2, small_length) {

  fill(0);

  let distance = Math.sqrt((x2 - x1) ** 2 + (y2 - y1) ** 2);

  let num_lines = distance / small_length * 2;

  num_lines = 2 * Math.floor(num_lines / 2) + 1;

  let xfrac = (x2 - x1) / num_lines;

  let yfrac = (y2 - y1) / num_lines;

  for (let i = 0; i < num_lines; i += 2) {

    line(x1 + xfrac * i, y1 + yfrac * i,

      x1 + xfrac * (i + 1), y1 + yfrac * (i + 1));

  }

}

​

function dashed_array(xs, ys) {

  fill(0);

  for (i = 0; i < xs.length; i+=2) {

    line(xs[i], ys[i], xs[i+1], ys[i+1]);

  }

}

​

function find_colliders(circles) {

  let colliders = [];

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

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

      if (circle_collision(circles[i], circles[j])) {

        colliders.push([i, j]);

      }

    }

  }

  return colliders;

}

​

function circle_collision(c1, c2) {

  let threshold = (c1.radius + c2.radius) ** 2;

  let sep_vec = p5.Vector.sub(c1.pos, c2.pos);

  let dist_sq = sep_vec.magSq();

  if (dist_sq < threshold) {

    return true;

  }

  return false;

}

​

function update_colliders(c1, c2) {

  let relDist = p5.Vector.sub(c2.pos, c1.pos);

  let mass_frac = 1 / (1 + c1.mass / c2.mass);

  let COM = p5.Vector.add(c1.pos, p5.Vector.mult(relDist, mass_frac));

  let new_vel = p5.Vector.mult(

    p5.Vector.add(

      p5.Vector.mult(c1.vel, c1.mass),

      p5.Vector.mult(c2.vel, c2.mass)

      ),

    1 / (c1.mass + c2.mass));

  let new_mass = c1.mass + c2.mass;

  let new_diameter = new_mass**(1/3);

  planets.push(new Planet(COM.x, COM.y, new_vel.x, new_vel.y, new_diameter, "#abc"));

}

​

class Planet {

  constructor(initX, initY, initVX, initVY, diameter, color) {

    this.pos = createVector(initX, initY);

    this.vel = createVector(initVX, initVY);

    this.diameter = diameter;

    this.radius = this.diameter/2;

    this.mass = this.diameter**3;

    this.color = color;

    this.id = fresh_id;

    fresh_id ++;

  }

  draw() {

    fill(this.color);

    ellipse(this.pos.x, this.pos.y, this.diameter);

  }

  new_vel(dt) {

    return p5.Vector.mult(this.vel, dt);

  }

  new_accel(dt) {

    let accel = createVector(0,0);

    let r, r_norm;

    let accel_mag;

    for (var planet_ of planets) {

      if (this.id == planet_.id) {

        continue;

      }

      r = p5.Vector.sub(planet_.pos, this.pos);

      accel_mag = (grav_const * planet_.mass) / r.magSq();

      r_norm = r.normalize();

      accel.add(r_norm.mult(accel_mag));

    }

    return p5.Vector.mult(accel, dt);

  }

  path(time, steps) {

    let dt = time / steps;

    let vx = (oldX - mouseX) / vel_factor;

    let vy = (oldY - mouseY) / vel_factor;

    let v = createVector(vx, vy);

    let xs = [oldX];

    let ys = [oldY];

    for (i = 1; i < steps; i++) {

      v.add(p5.Vector.mult(

        static_accel(xs[xs.length-1], ys[ys.length-1]),

        dt));

      xs.push(xs[xs.length-1] + v.x * dt);

      ys.push(ys[ys.length-1] + v.y  *dt);

    }

    return [xs, ys];

  }

}

function static_accel(x, y) {

  let accel = createVector(0, 0);

  let pos = createVector(x, y);

  let r, r_norm;

  let accel_mag;

  for (var planet_ of planets) {

    r = p5.Vector.sub(planet_.pos, pos)

    accel_mag = (grav_const * planet_.mass) / r.magSq();

    r_norm = r.normalize();

    accel.add(r_norm.mult(accel_mag));

  }

  return accel;

}