class Snowflake {
  constructor() {
    this.x = 0;
    this.y = 0;
    this.vx = 0;
    this.vy = 0;
    this.radius = 0;
    this.alpha = 0;

    this.reset();
  }

  reset() {
    this.x = this.randBetween(0, window.innerWidth);
    this.y = this.randBetween(0, -window.innerHeight);
    this.vx = this.randBetween(-3, 3);
    this.vy = this.randBetween(2, 5);
    this.radius = this.randBetween(1, 4);
    this.alpha = this.randBetween(0.1, 0.9);
  }

  randBetween(min, max) {
    return min + Math.random() * (max - min);
  }

  update() {
    this.x += this.vx;
    this.y += this.vy;

    if (this.y + this.radius > window.innerHeight) {
      this.reset();
    }
  }
}

class Snow {
  constructor() {
    this.canvas = document.createElement("canvas");
    this.ctx = this.canvas.getContext("2d");

    document.body.appendChild(this.canvas);

    window.addEventListener("resize", () => this.onResize());
    this.onResize();
    this.updateBound = this.update.bind(this);
    //實現動畫
    requestAnimationFrame(this.updateBound);

    this.createSnowflakes();
  }

  onResize() {
    this.width = window.innerWidth;
    this.height = window.innerHeight;
    this.canvas.width = this.width;
    this.canvas.height = this.height;
  }

  createSnowflakes() {
    const flakes = window.innerWidth / 4;

    this.snowflakes = [];

    for (let s = 0; s < flakes; s++) {
      this.snowflakes.push(new Snowflake());
    }
  }

  update() {
    //清除原來上面的雪花
    this.ctx.clearRect(0, 0, this.width, this.height);

    for (let flake of this.snowflakes) {
      //計算每一片雪花的新坐標
      flake.update();
      //在canvas上畫出雪花
      this.ctx.save();
      this.ctx.fillStyle = "#FFF";
      this.ctx.beginPath();
      this.ctx.arc(flake.x, flake.y, flake.radius, 0, Math.PI * 2);
      this.ctx.closePath();
      this.ctx.globalAlpha = flake.alpha;
      this.ctx.fill();
      this.ctx.restore();
    }
    requestAnimationFrame(this.updateBound);
  }
}

new Snow();