<!-- https://css-tricks.com/playing-with-particles-using-the-web-animations-api/ -->
<article>
  <h1>Title</h1>

  <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Voluptates voluptatibus dicta quis assumenda dolore necessitatibus voluptate quibusdam fugit eius omnis. Recusandae, ab rem repellendus molestiae nemo animi quia expedita modi!
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  Quia recusandae nostrum facere voluptates soluta sint maxime mollitia quaerat a architecto excepturi odit, repudiandae culpa vero praesentium eaque assumenda necessitatibus laborum quod iusto ipsum, explicabo incidunt beatae? Quisquam, quod?
  Voluptas deleniti possimus tempora sapiente aliquam placeat accusantium. Illo minima doloribus, repellat laboriosam facilis dicta. Itaque, voluptates deleniti ipsam quibusdam facere aliquid vel recusandae, sapiente sint maiores soluta, veritatis consequatur.
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  <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Natus at optio, cum impedit hic voluptate ex. Repellat officia facilis labore est omnis praesentium et nesciunt suscipit voluptatum. Iure, expedita asperiores.</p>

  <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Quasi quisquam dolore odit dignissimos. Cum perferendis assumenda iusto, non ipsum adipisci nesciunt similique recusandae omnis maxime culpa praesentium placeat corrupti natus.
  Reprehenderit voluptatum perferendis laudantium blanditiis odit pariatur quis corrupti quo! Inventore aperiam delectus quas porro velit quis, mollitia quia veritatis repudiandae. Totam recusandae accusantium animi fugiat cum harum, suscipit dignissimos!
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</article>

// https://css-tricks.com/playing-with-particles-using-the-web-animations-api/
particle {
  left: 0;
  pointer-events: none;
  position: fixed;
  top: 0;
  opacity: 0;
}

article {
  max-width: 65ch;
  margin: 1rem auto;
  font-size: 1.4rem;
  line-height: 1.6;
}

// https://css-tricks.com/playing-with-particles-using-the-web-animations-api/
(function () {
  const TextSelectionParticles = {
    supportsAnimate: !!document.body.animate,
    left: 0,
    right: 0,
    initialTopStart: null,

    init() {
      if (!this.supportsAnimate) return

      this.animateSelection = this.animateSelection.bind(this)
      this.resetTop = this.resetTop.bind(this)

      this.listen()
    },

    listen() {
      document.addEventListener('selectstart', this.resetTop)
      document.addEventListener('selectionchange', this.animateSelection)
    },

    resetTop() {
      this.initialTopStart = null
    },

    animateSelection() {
      const selection = window.getSelection()
      const range = selection.getRangeAt(0)
      const bounds = range.getClientRects()
      const initialBounds = bounds[0]
      const recentBounds = bounds[bounds.length - 1]

      if (!this.initialTopStart) {
        this.initialTopStart = initialBounds.top
      }

      const hasLeftChanged = this.left !== recentBounds.left
      const hasMovedDown = recentBounds.top > this.initialTopStart 
      const boundsToUse = hasMovedDown ? recentBounds : initialBounds

      this.left = boundsToUse.left
      this.right = boundsToUse.right

      const x = hasLeftChanged ? this.left : this.right
      const y = boundsToUse.y + boundsToUse.height

      createParticle(x, y)
    },
  }

  function createParticle(x, y) {
    const particle = document.createElement('particle')

    document.body.appendChild(particle)

    const size = Math.floor(Math.random() * 10 + 15)

    particle.style.width = `${size}px`
    particle.style.height = `${size / 5}px`
    particle.style.background = `hsl(${Math.random() * 60 + 180}, 70%, 60%)`

    const destinationX = x
    const destinationY = y + 10

    const animation = particle.animate([{
        transform: `translate(${x - (size / 2)}px, ${y - (size / 2)}px) rotate(0deg)`,
        opacity: 1
      },
      {
        transform: `translate(${destinationX}px, ${destinationY}px) rotate(-10deg)`,
        opacity: 0
      }
    ], {
      duration: 500 + Math.random() * 1000,
      easing: 'cubic-bezier(0, .9, .57, 1)',
      delay: Math.random() * 100
    })

    animation.addEventListener('finish', () => {
      particle.remove()
    }, { once: true })
  }

  TextSelectionParticles.init()
})()