const transformer = options => ({
  apply: 'post',
  transform({ code, isBuild }) {
    if (!isBuild) return code

    return code.replace('</body>', `<script defer type="module">
      const KEY = 'ga:user'
      const UID = (localStorage[KEY] = localStorage[KEY] || Math.random() + '.' + Math.random())
      const onError = err => console.error('[vite vitals] ', err) 
      const onDebug = (label, payload) => console.log(label, payload) 

      function sendToAnalytics (fullPath, metric) {
        const { name, delta, id, entries } = metric
        const opts = {
          ec: '${options.eventCategory}',
          ea: name,
          el: id,
          ev: parseInt(delta),
          dp: fullPath,
          ni: true
        }
        
        if (name === 'TTFB') opts.ev = parseInt(delta - entries[0].requestStart)
        
        const args = { tid: '${options.trackingID}', cid: UID, ...opts }
        const obj = { t: 'event', ...args, ...opts, z: Date.now() }

        const blob = new Blob([new URLSearchParams(obj).toString()], {
          type: 'application/x-www-form-urlencoded'
        })

        const url = 'https://www.google-analytics.com/collect?v=1'
        
        const debug = ${options.debug}
        
        if (debug) onDebug(name, JSON.stringify(obj, null, 2))
          
        ;(navigator.sendBeacon && navigator.sendBeacon(url, blob)) ||
          fetch(url, { 
            body: blob,
            method: 'POST', 
            credentials: 'omit',
            keepalive: true 
          })
      }
      async function webVitals (fullPath) {
        try {
          const { getCLS, getFID, getLCP, getTTFB, getFCP } = await import('https://unpkg.com/web-vitals@0.2.4/dist/web-vitals.es5.min.js?module')
          getFID(metric => sendToAnalytics(fullPath, metric))
          getTTFB(metric => sendToAnalytics(fullPath, metric))
          getLCP(metric => sendToAnalytics(fullPath, metric))
          getCLS(metric => sendToAnalytics(fullPath, metric))
          getFCP(metric => sendToAnalytics(fullPath, metric))
        } catch (err) {
          onError(err)
        }
      }
      webVitals(window.location.pathname + window.location.search)
    </script></body>`)
  }
})

export function vitals (opts) {
  return { 
    indexHtmlTransforms: [transformer(opts)]
  }
}