const taskPool = [] // 请求池
let numRequest = 0 // 正在执行数量
const maxRequest = 4 // 可配置
let taskTimer // 轮询的定时器
const cachedTask = {} // 存放的任务数据

// 预加载池的添加
function addTaskIntoPool(task) {
  return new Promise((resolve, reject) => {
    const cache = cachedTask[task.key]
    const subscribe = (executeRes) => {
      if (executeRes.success) {
        resolve(executeRes.res)
      } else {
        reject(executeRes.err)
      }
    }
    const priority = (task.priority || task.priority === 0) || 999
    if (cache) {
      cache.priority = priority
      const callbacks = cache.callbacks || []
      callbacks.push(subscribe)
    } else {
      task.callbacks = [subscribe]
      cachedTask[task.key] = task
      taskPool.push(task)
    }
  })
}
// 执行下载
function executeTask() {
  if (taskPool.length > 0) {
    sortTaskPool()
    const executeRequest = maxRequest - numRequest
    if (executeRequest > 0) {
      for (let i = 0; i < executeRequest; i++) {
        const task = taskPool.shift()
        if (!task) {
          return
        }
        numRequest++
        task.execute().then((res) => {
          numRequest--
          task.callbacks && task.callbacks.map(callback => {
            callback({ success: true, res })
          })
          executeTask()
        }, (err) => {
          numRequest--
          task.callbacks && task.callbacks.map(callback => {
            callback({ success: false, err })
          })
          delete cachedTask[task.key]
          executeTask()
        })
      }
    }
  } else {
    startTaskTimer()
  }
}
function sortTaskPool() {
  if (taskPool.length > 0) {
    taskPool.sort((a, b) => a - b)
  }
}
// 轮询检查请求池中是否有请求需要执行
function startTaskTimer() {
  taskTimer = setInterval(() => {
    if (taskPool.length > 0) {
      stopTaskTimer()
      executeTask()
    }
  }, 500)
}
// 停止轮询
function stopTaskTimer() {
  clearInterval(taskTimer)
  taskTimer = null
}

export default {
  addTaskIntoPool,
  executeTask,
  startTaskTimer
}