;(function () {
  'use strict'
  const smoothScrollLinks = document.getElementsByClassName('smooth-scroll')

  const easeInOutQuad = function (t) {
    return t < 0.5 ? 2 * t * t : -1 + (4 - 2 * t) * t
  }

  const scrollToEl = (startTime, currentTime, duration, scrollEndElemTop, startScrollOffset) => {
    const runtime = currentTime - startTime
    let progress = runtime / duration

    progress = Math.min(progress, 1)

    const ease = easeInOutQuad(progress)

    window.scroll(0, startScrollOffset + (scrollEndElemTop * ease))
    if (runtime < duration) {
      window.requestAnimationFrame((timestamp) => {
        const currentTime = timestamp || new Date().getTime()
        scrollToEl(startTime, currentTime, duration, scrollEndElemTop, startScrollOffset)
      })
    }
  }

  if (smoothScrollLinks.length > 0) {
    for (let i = 0; i < smoothScrollLinks.length; i++) {
      const smoothScrollLink = smoothScrollLinks[i]

      smoothScrollLink.addEventListener('click', function (e) {
        e.preventDefault()
        const link = e.target.closest('.smooth-scroll')
        const targetId = link.href.split('#')[1]
        const target = document.getElementById(targetId)
        const duration = link.getAttribute('data-duration') || 1000

        if (!target) return

        window.requestAnimationFrame((timestamp) => {
          const stamp = timestamp || new Date().getTime()
          const start = stamp

          const startScrollOffset = window.pageYOffset
          const scrollEndElemTop = target.getBoundingClientRect().top

          scrollToEl(start, stamp, duration, scrollEndElemTop, startScrollOffset)
        })
      })
    }
  }
}())