import { Task, TaskCallback } from './type'

const queue: Task[] = []
const threshold: number = 5
const transitions: (() => void)[] = []
let deadline: number = 0

export const startTransition = (cb: () => void) => {
  transitions.push(cb) && translate()
}

export const schedule = (callback: TaskCallback) => {
  queue.push({ callback })
  startTransition(flush)
}

const task = (pending: boolean) => {
  const cb = () => transitions.splice(0, 1).forEach((c) => c())
  if (!pending && typeof queueMicrotask !== 'undefined') {
    return () => queueMicrotask(cb)
  }
  if (typeof MessageChannel !== 'undefined') {
    const { port1, port2 } = new MessageChannel()
    port1.onmessage = cb
    return () => port2.postMessage(null)
  }
  return () => setTimeout(cb)
}

let translate = task(false)

const flush = () => {
  deadline = getTime() + threshold
  let job = peek(queue)
  while (job && !shouldYield()) {
    const { callback } = job
    job.callback = null
    const next = callback()
    if (next) {
      job.callback = next
    } else {
      queue.shift()
    }
    job = peek(queue)
  }
  job && (translate = task(shouldYield())) && startTransition(flush)
}

export const shouldYield = () => {
  return getTime() >= deadline
}

export const getTime = () => performance.now()

const peek = (queue: Task[]) => queue[0]