<script src="https://anuraghazra.github.io/CanvasFun/src/lib/Vector.js"></script>
<canvas id="c">

* {
  box-sizing: border-box;
}
body {
  margin: 0;
  padding: 0;
}

let canvas = document.getElementById("c");
let ctx = canvas.getContext("2d");
let CANVAS_WIDTH = window.innerWidth-5;
let CANVAS_HEIGHT = window.innerHeight-5;
canvas.width = CANVAS_WIDTH;
canvas.height = CANVAS_HEIGHT;

class Dot {
  constructor(x, y, vx, vy) {
    this.pos = new Vector(x, y);
    this.oldpos = new Vector(x + (vx||0), y + (vy||0)); // velocity x, y

    this.friction = 0.97;
    this.groundFriction = 0.7;

    this.gravity = new Vector(0, 1);

    this.radius = 5;
    this.color = "#e62a4f";
    this.mass = 1;
  }

  update() {
    let vel = Vector.sub(this.pos, this.oldpos);
    vel.mult(this.friction);

    // if the point touches the ground set groundFriction
    if (this.pos.y >= CANVAS_HEIGHT - this.radius && vel.magSq() > 0.000001) {
      var m = vel.mag();
      vel.x /= m;
      vel.y /= m;
      vel.mult(m * this.groundFriction);
    }
    this.oldpos.setXY(this.pos.x, this.pos.y);
    this.pos.add(vel);
    this.pos.add(this.gravity);
  }

   constrain() {
    if (this.pos.x > CANVAS_WIDTH - this.radius) {
      this.pos.x = CANVAS_WIDTH - this.radius;
    }
    if (this.pos.x < this.radius) {
      this.pos.x = this.radius;
    }
    if (this.pos.y > CANVAS_HEIGHT - this.radius) {
      this.pos.y = CANVAS_HEIGHT - this.radius;
    }
    if (this.pos.y < this.radius) {
      this.pos.y = this.radius;
    }
  };
  
  render(ctx) {
    ctx.beginPath();
    ctx.fillStyle = this.color;
    ctx.arc(this.pos.x, this.pos.y, this.radius, 0, Math.PI * 2);
    ctx.fill();
    ctx.closePath();
  }
}
 
class Stick {
  constructor(p1, p2, length) {
    this.startPoint = p1;
    this.endPoint = p2;
    this.stiffness = 2;
    this.color = '#f5476a';
    // if the length is not given then calculate the distance based on position
    if (!length) {
      this.length = this.startPoint.pos.dist(this.endPoint.pos);
    } else {
      this.length = length;
    }

  }
  
  update() {
    // calculate the distance between two dots
    let dx = this.endPoint.pos.x - this.startPoint.pos.x;
    let dy = this.endPoint.pos.y - this.startPoint.pos.y;
    // pythagoras theorem
    let dist = Math.sqrt(dx * dx + dy * dy);
    // calculate the resting distance betwen the dots
    let diff = (this.length - dist) / dist * this.stiffness;

    // getting the offset of the points
    let offsetx = dx * diff * 0.5;
    let offsety = dy * diff * 0.5;

    // calculate mass
    let m1 = this.startPoint.mass + this.endPoint.mass;
    let m2 = this.startPoint.mass / m1;
    m1 = this.endPoint.mass / m1;

    // and finally apply the offset with calculated mass
    if (!this.startPoint.pinned) {
      this.startPoint.pos.x -= offsetx * m1;
      this.startPoint.pos.y -= offsety * m1;
    }
    if (!this.endPoint.pinned) {
      this.endPoint.pos.x += offsetx * m2;
      this.endPoint.pos.y += offsety * m2;
    }

  }
  
  render(ctx) {
    ctx.beginPath();
    ctx.strokeStyle = this.color;
    ctx.moveTo(this.startPoint.pos.x, this.startPoint.pos.y);
    ctx.lineTo(this.endPoint.pos.x, this.endPoint.pos.y);
    ctx.stroke();
    ctx.closePath();
  }
}


let dots = [];
let sticks = [];

dots.push(new Dot(100, 100, (Math.random() - 0.5) * 100.0));
dots.push(new Dot(200, 100));
dots.push(new Dot(200, 200));
dots.push(new Dot(100, 200));

sticks.push(new Stick(dots[0], dots[1]))
sticks.push(new Stick(dots[1], dots[2]))
sticks.push(new Stick(dots[2], dots[3]))
sticks.push(new Stick(dots[3], dots[0]))
sticks.push(new Stick(dots[3], dots[1]))

const ITERATION = 100;

function animate() {
  ctx.clearRect(0, 0, CANVAS_WIDTH, CANVAS_HEIGHT);

  // update 
  for (let i = 0; i < ITERATION; i++) {
    for (let d of dots) {
      d.constrain();        
    }
    for (let s of sticks) {
      s.update();
    }
  }
  
  for (let d of dots) {
    d.update();
    d.render(ctx);
  }
  for (let s of sticks) {
    s.update();
    s.render(ctx);
  }

  requestAnimationFrame(animate);
}
animate();