(function (doc, win) {
  var docEl = doc.documentElement,
    resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
    recalc = function () {
      var viewportWidth = window.innerWidth

      let baseWidth // 基准宽度
      if (!viewportWidth) return
      switch (true) {
        case (viewportWidth >= 1200):
          baseWidth = 1920
          break
        case (viewportWidth >= 648 && viewportWidth < 1200):
          baseWidth = 768
          break
        case (viewportWidth < 648):
          baseWidth = 375
          break
      }

      docEl.style.fontSize = 10 * (viewportWidth / baseWidth) + 'px'
    }

  // 防抖函数
  var debounce = function (func, wait) {
    var timeout
    return function () {
      var context = this, args = arguments
      clearTimeout(timeout)
      timeout = setTimeout(function () {
        func.apply(context, args)
      }, wait)
    }
  }

  if (!doc.addEventListener) return

  // 给 recalc 函数加上防抖
  var optimizedRecalc = debounce(recalc, 10)

  // 事件监听
  win.addEventListener(resizeEvt, optimizedRecalc, false)
  doc.addEventListener('DOMContentLoaded', recalc, false)
})(document, window)