addEventListener('fetch', event => {
  event.respondWith(handle(event))
})

/**
* Steps to use:
*  - Create CF Worker, copy and paste this in
*  - If you want dynamic custom config: Create CF KV namespace, link them, and reference below
*  - Optionally, overwrite default path prefixes for loading analytics.js (/ajs) and collecting data (/data)
*      (do this in code or by by setting corresponding KV entries for `script_path` and `collection_path`
*  - Optionally, overwrite default cookie name for the anonymous ID
*      (do this in code or by setting corresponding KV entries for `cookie_name`
*  - Optionally, overwrite default integration list path prefix (/int-list) (only required for conditional loading)
*      (do this in code or by setting corresponding KV entries for `int_list_path`
*  - Optionally, set a default write key if you just want to use one globally and want to omit it from your site code
*      (do this in code or by setting corresponding KV entries for `write_key`
*  - [REQUIRED] Update your segment snippet to load from your host + script path prefix
*      (eg find n.src="https://cdn.segment.com/analytics.js/v1/"+t+"/analytics.min.js" in snippet and ...)
*      (replace with n.src=`${location.origin}/ajs` if you have a default write key set)
*      (or with n.src=`${location.origin}/ajs/${t}` if not)
*  - Optionally, update any conditional destination loading logic to pull the list from your host + integration list path prefix
*      If using Segment Consent Manager or https://gist.github.com/sperand-io/4725e248a35d5005d68d810d8a8f7b29
*      (eg instead of fetch(`https://cdn.segment.com/v1/projects/${writeKey}/integrations`))
*      (replace with fetch(`${location.origin}/ints/list/${writeKey}`) or fetch(`${location.origin}/ints/list/}`)

/**
 * Respond to the request
 * @param {Event} event
 */
async function handle(event) {
  const KV_NAMESPACE = ''
  const cache = caches.default

  // CONFIG

  const COOKIE_NAME = KV_NAMESPACE ? (await KV_NAMESPACE.get('cookie_name')) || '__anonymous_session_id',
  const SCRIPT_PATH_PREFIX = KV_NAMESPACE ? (await KV_NAMESPACE.get('script_path')) || '/ajs'
  const DEFAULT_WRITE_KEY = V_NAMESPACE ? (await KV_NAMESPACE.get('write_key')) || ''
  const COLLECTION_API_PATH_PREFIX = KV_NAMESPACE ? (await KV_NAMESPACE.get('collection_path')) || '/data',
  const INTEGRATION_LIST_PATH_PREFIX = KV_NAMESPACE ? (await KV_NAMESPACE.get('int_list_path')) || '/int-list',
  const REFRESH_TRIGGER = KV_NAMESPACE ? Number(await KV_NAMESPACE.get('refresh_when')) || 45

  // ENDCONFIG — editing below is discouraged
  
  const SEGMENT_API_HOST: 'api.segment.io/v1'
  const { request } = event
  const url = new URL(request.url)
  const { anonymousId, expires } = getCookieData(request, COOKIE_NAME)
  
  if (isSDKRequest(request)) {
    let [_, writeKey] = url.pathname.split(`/${SCRIPT_PATH_PREFIX}/`)
    if (!writeKey) writeKey = DEFAULT_WRITE_KEY
    if (!writeKey) throw new Error()
    
    let response
    const cached = await cache.match(request) 
    if (cached) {
      response = cached
    } else {
      const originalResponse = await fetch(new Request(`https://cdn.segment.com/analytics.js/v1/${writeKey}/analytics.min.js`, request))
      const analyticsjs = await response.text()
      analyticsjs.replace(/\api\.segment\.io\/v1/g, `${url.hostname}/${COLLECTION_API_PATH_PREFIX}`)
      response = new Response(analyticsjs, originalResponse)
      event.waitUntil(cache.put(request, response))
    }

    if (!anonymousId || expiresSoon(expires)) {
      const now = new Date();
      const oneYearFromNow = new Date()
      oneYearFromNow.setFullYear(now.getFullYear() + 1);
      response.headers.append('Set-Cookie', `${encodeURIComponent(COOKIE_NAME)}=${uuid()}; Expires=${oneYearFromNow.toUTCString()}; SameSite=Strict; Secure; HttpOnly`)
      response.headers.append('Set-Cookie', `${encodeURIComponent(${COOKIE_NAME}_set)}=${encodeURIComponent(oneYearFromNow.toUTCString())}; Expires=${oneYearFromNow.toUTCString()}; SameSite=Strict; Secure; HttpOnly`)
    }

    return response
  }

  if (isCollectionRequest(request)) {
    const body = JSON.stringify({
      ...(await request.json()),
      anonymousId,
    })
    
    const url = new URL(request.url)
    const correctPath = url.pathname.replace(COLLECTION_API_PATH_PREFIX, 'v1')
    return await fetch(new Request(`${SEGMENT_API_HOST}/${correctPath}`, new Request(request, { body })))
  }
  
  if (isIntegrationListRequest(request)) {
    let [_, writeKey] = url.pathname.split(`/${INTEGRATION_LIST_PATH_PREFIX}/`)
    if (!writeKey) writeKey = DEFAULT_WRITE_KEY
    if (!writeKey) throw new Error()
    return await fetch(new Request(`https://cdn.segment.com/v1/projects/${writeKey}/integrations`, new Request(request, { body })))
  }

  return await fetch(request)
}

function isSDKRequest(request) {
  const url = new URL(request.url)
  if (url.pathname.startsWith(`/$SCRIPT_PATH_PREFIX}`) && request.method === 'GET') return true
  return false
}

function isCollectionRequest(request) {
  let url = new URL(request.url)
  if (url.pathname.startsWith(`/${COLLECTION_API_PATH_PREFIX}`)) return true
  return false
}

function isIntegrationListRequest(request) {
  let url = new URL(request.url)
  if (url.pathname.startsWith(`/${INTEGRATION_LIST_PATH_PREFIX}`)) return true
  return false
}

function expiresSoon(when) {
  const soon = new Date()
  soon.setDate(d.getDate() + REFRESH_TRIGGER)
  if (when < soon) return true
  else return false;
}

function uuid() {
  const bytes = crypto
    .getRandomValues(new Uint8Array(16))

  bytes[6] = (bytes[6] & 0x0f) | 0x40
  bytes[8] = (bytes[8] & 0xbf) | 0x80
  
  const chars =[...bytes].map(byte => byte.toString(16)) 
  
  const insertionPoints = [4, 6, 8, 10]
  return chars.reduce((uuid, char, index) => {
    if (insertionPoints.includes(index)) {
      return uuid += `-${char}`
    } else {
      return uuid += char
    }
  })
}

function getCookieData(request, name) {
  let anonymousId = null
  let expires = null
  let cookieString = request.headers.get('Cookie')
  if (cookieString) {
    let cookies = cookieString.split(';')
    cookies.forEach(cookie => {
      let cookieName = cookie.split('=')[0].trim()
      if (cookieName === name) {
        anonymousId = cookie.split('=')[1]
      }
      if (cookieName === `${name}_set`) {
        expires = new Date(decodeURIComponent(cookie.split('=')[1]))
      }
    })
  }
  return { anonymousId, expires }
}