/* @flow */

import type Watcher from './watcher'
import config from '../config'
import { callHook } from '../instance/lifecycle'

import {
 warn,
 nextTick,
 devtools
} from '../util/index'

const queue: Array = []
let has: { [key: number]: ?true } = {}
let circular: { [key: number]: number } = {}
let waiting = false
let flushing = false
let index = 0

/**
 * Reset the scheduler's state.
 */
function resetSchedulerState () {
 queue.length = 0
 has = {}
 if (process.env.NODE_ENV !== 'production') {
 circular = {}
 }
 waiting = flushing = false
}

/**
 * Flush both queues and run the watchers.
 */
function flushSchedulerQueue () {
 flushing = true
 let watcher, id, vm

 // Sort queue before flush.
 // This ensures that:
 // 1. Components are updated from parent to child. (because parent is always
 // created before the child)
 // 2. A component's user watchers are run before its render watcher (because
 // user watchers are created before the render watcher)
 // 3. If a component is destroyed during a parent component's watcher run,
 // its watchers can be skipped.
 queue.sort((a, b) => a.id - b.id)

 // do not cache length because more watchers might be pushed
 // as we run existing watchers
 for (index = 0; index < queue.length; index++) {
 watcher = queue[index]
 id = watcher.id
 has[id] = null
 watcher.run()
 // in dev build, check and stop circular updates.
 if (process.env.NODE_ENV !== 'production' && has[id] != null) {
  circular[id] = (circular[id] || 0) + 1
  if (circular[id] > config._maxUpdateCount) {
  warn(
   'You may have an infinite update loop ' + (
   watcher.user
    ? `in watcher with expression "${watcher.expression}"`
    : `in a component render function.`
   ),
   watcher.vm
  )
  break
  }
 }
 }

 // reset scheduler before updated hook called
 const oldQueue = queue.slice()
 resetSchedulerState()

 // call updated hooks
 index = oldQueue.length
 while (index--) {
 watcher = oldQueue[index]
 vm = watcher.vm
 if (vm._watcher === watcher && vm._isMounted) {
  callHook(vm, 'updated')
 }
 }

 // devtool hook
 /* istanbul ignore if */
 if (devtools && config.devtools) {
 devtools.emit('flush')
 }
}

/**
 * Push a watcher into the watcher queue.
 * Jobs with duplicate IDs will be skipped unless it's
 * pushed when the queue is being flushed.
 */
export function queueWatcher (watcher: Watcher) {
 const id = watcher.id
 if (has[id] == null) {
 has[id] = true
 if (!flushing) {
  queue.push(watcher)
 } else {
  // if already flushing, splice the watcher based on its id
  // if already past its id, it will be run next immediately.
  let i = queue.length - 1
  while (i >= 0 && queue[i].id > watcher.id) {
  i--
  }
  queue.splice(Math.max(i, index) + 1, 0, watcher)
 }
 // queue the flush
 if (!waiting) {
  waiting = true
  nextTick(flushSchedulerQueue)
 }
 }
}