/**
 * Animate scroll to top button in/off view
 */

export default (() => {
  let button = document.querySelector('.btn-scroll-top'),
    scrollOffset = 500

  if (button === null) return

  let offsetFromTop = parseInt(scrollOffset, 10),
    progress = button.querySelector('svg rect'),
    length = progress.getTotalLength()

  progress.style.strokeDasharray = length
  progress.style.strokeDashoffset = length

  const showProgress = () => {
    let scrollPosition = window.pageYOffset,
      scrollHeight =
        document.documentElement.scrollHeight -
        document.documentElement.clientHeight,
      scrollPercent = scrollPosition / scrollHeight || 0,
      draw = length * scrollPercent
    progress.style.strokeDashoffset = length - draw
  }

  const handleScroll = () => {
    const toggleButtonVisibility = () => {
      if (window.pageYOffset > offsetFromTop) {
        button.classList.add('show')
      } else {
        button.classList.remove('show')
      }
    }

    const throttledToggleButtonVisibility = throttle(
      toggleButtonVisibility,
      100
    )

    throttledToggleButtonVisibility()
    showProgress()
  }

  const throttle = (callback, delay) => {
    let timeoutId = null
    return function () {
      if (timeoutId === null) {
        timeoutId = setTimeout(() => {
          callback.apply(null, arguments)
          timeoutId = null
        }, delay)
      }
    }
  }

  window.addEventListener('scroll', handleScroll)
})()