<canvas id="canvas"></canvas>

body, html {
  margin: 0;
  overflow: hidden;
}

canvas {
  display: block;
  cursor: pointer;
}

/*
  Johan Karlsson, 2020
  https://twitter.com/DonKarlssonSan
  MIT License, see Details View
*/

class Circle {
  constructor(x, y) {
    this.x = x;
    this.y = y;
    this.r = 3;
    this.done = false;
  }
  
  draw() {    
    ctx.beginPath();
    ctx.arc(this.x, this.y, this.r, 0, Math.PI*2);
    ctx.stroke();
  }
}

let canvas;
let ctx;
let w, h;
let circles;

function setup() {
  canvas = document.querySelector("#canvas");
  ctx = canvas.getContext("2d");
  reset();
  window.addEventListener("resize", () => {
    reset();
    draw();
  });
  canvas.addEventListener("click", draw);
}

function reset() {
  w = canvas.width = window.innerWidth;
  h = canvas.height = window.innerHeight;
}

function clear() {
  ctx.fillStyle = "black";
  ctx.fillRect(0, 0, w, h);
  ctx.strokeStyle = "white";
}

function dist(x1, y1, x2, y2) {
  return Math.hypot(x1 - x2, y1 - y2);
}

function addCircles() {
  let nrOfTries = 0;
  let wasAdded;
  do {
    wasAdded = false;
    let x = Math.random() * w;
    let y = Math.random() * h;
    if(validPos(x, y)) {
      wasAdded = true;
      let c = new Circle(x, y);
      circles.push(c);
    }
    nrOfTries++;
  } while (!wasAdded && nrOfTries < 90)
}

function validPos(x, y) {
  for(let i = 0; i < circles.length; i++) {
    let current = circles[i];
    let d = dist(x, y, current.x, current.y);
    if(d - 4 < current.r) {
      return false;
    }
  }
  return true;
}

function canGrow(circle) {
  for(let i = 0; i < circles.length; i++) {
    let current = circles[i];
    if(circle !== current) {
      let d = dist(circle.x, circle.y, current.x, current.y);
      if(d - 4 <= circle.r + current.r) {
        return false;
      } 
    }
  }
  return true;
}

function resetCircles() {
  circles = [];
}
  
function packCircles() {
  let nrOfTries = w * h / 100;
  for(let i = 0; i < nrOfTries; i++) {
    if(i % 4 === 0) {
      addCircles();
    }
    circles.filter(c => !c.done).forEach(c => {
      if(canGrow(c)) {
        c.r += 2;
      } else {
        c.done = true;
      }
    });
  }
}
  
function drawCircles() {
  circles.forEach(c => c.draw());
}
  
function draw() {
  clear();
  resetCircles();
  packCircles();
  drawCircles();
}

setup();
draw();