(function () {
  const Tracker = function () {
    sessionId = null

    this.getSrc = function () {
      var checker = document.getElementById('acChecker')
      if (checker) {
        // Cleanup pixel tag, some sites rewrite it causing broken display
        checker.style.display = 'none'
        checker.src = ''
        return checker.getAttribute('data-src')
      }

      // Fallback to get src from noscript pixel tag
      var noscriptElems = document.getElementsByTagName('noscript')
      for (elem of noscriptElems) {
        // <noscript> content is not available via DOM, access as text
        var html = elem.textContent || elem.innerHTML
        if (!html) {
          console.error('There is no noscript (pixel) on the page')
        }
        if (html.trim().startsWith('<img')) {
          return html.split('"')[1]
        }
      }
    }

    this.sendReq = function (method, url, data, cb) {
      const request = new XMLHttpRequest()
      // set a request
      if (cb) {
        request.addEventListener('load', cb)
      }
      request.open(method, url, false)
      request.setRequestHeader('Content-Type', 'application/json')
      // request body
      request.send(JSON.stringify(data))
      return request
    }

    this.getUrlParams = function () {
      // parse pixel data in order to check a correct pixel
      const params = this.src.split('?')[1]
      const pixelData = params ? params.split('&')
        .reduce((acc, elem) => {
          const parsed = elem.split('=');
          return Object.assign(acc, { [parsed[0]]: parsed[1] })
        }, {}) : []
      const elems = this.src.split('/')
      return {
        scheme: elems[0].concat('//'),
        host: elems[2],
        queryParams: pixelData,
        id: elems[4]
      }
    }

    this.handlePixelResp = function () {
      if (this.status === 200) {
        sessionId = JSON.parse(this.responseText).session_id
      } else {
        console.error('Cannot handle request response')
      }
    }

    this.handlePixel = function () {
      this.src = this.getSrc()
      const urlData = this.getUrlParams()
      const url = [urlData.scheme, urlData.host, '/api/pixels/calculate_pixel/'].join('')
      // request body
      let location
      try {
        // if we get any browser's restriction exception just get the current window loc href
        location = window.parent.location.href
      } catch (e) {
        location = window.location.href
      }
      const data = {
        source_url: location,
        referrer_url: document.referrer,
        id: urlData.id
      }
      const method = 'POST'  // explicit better than implicit (args below)
      this.sendReq(method, url, data, this.handlePixelResp)
    }

    this.checkTimeOnPage = function () {
      const urlData = this.getUrlParams()
      const url = [urlData.scheme, urlData.host, '/api/change_session_data/'].join('')
      const method = 'PUT'  // explicit better than implicit (args below)
      // send time on page duration to the server
      this.sendReq(method, url, { session_id: sessionId })
    }

    this.run = function () {
      // run js script only after getting pixel successfully
      var afterPixelFetched = function () {
        this.handlePixel()
      }
      afterPixelFetched = afterPixelFetched.bind(this)
      window.addEventListener('load', afterPixelFetched)
      window.onbeforeunload = function () {
        this.checkTimeOnPage()
        return null
      }
      window.onbeforeunload = window.onbeforeunload.bind(this)
    }
  }

  const tracker = new Tracker()
  tracker.run()
})()