function siftUp(heap, node, i) {
  var index = i;

  while (index > 0) {
    var parentIndex = index - 1 >>> 1;
    var parent = heap[parentIndex];

    if (compare(parent, node) > 0) {
      // The parent is larger. Swap positions.
      heap[parentIndex] = node;
      heap[index] = parent;
      index = parentIndex;
    } else {
      // The parent is smaller. Exit.
      return;
    }
  }
}

function push(heap, node) {
  var index = heap.length;
  heap.push(node);
  siftUp(heap, node, index);
}

function siftDown(heap, node, i) {
  var index = i;
  var length = heap.length;
  var halfLength = length >>> 1;

  while (index < halfLength) {
    var leftIndex = (index + 1) * 2 - 1;
    var left = heap[leftIndex];
    var rightIndex = leftIndex + 1;
    // If the left or right node is smaller, swap with the smaller of those.
    var right = heap[rightIndex]; 

    if (compare(left, node) < 0) {
      if (rightIndex < length && compare(right, left) < 0) {
        heap[index] = right;
        heap[rightIndex] = node;
        index = rightIndex;
      } else {
        heap[index] = left;
        heap[leftIndex] = node;
        index = leftIndex;
      }
    } else if (rightIndex < length && compare(right, node) < 0) {
      heap[index] = right;
      heap[rightIndex] = node;
      index = rightIndex;
    } else {
      // Neither child is smaller. Exit.
      return;
    }
  }
}

function pop(heap) {
  if (heap.length === 0) {
    return null;
  }

  var first = heap[0];
  var last = heap.pop();

  if (last !== first) {
    heap[0] = last;
    siftDown(heap, last, 0);
  }

  return first;
}

function peek(heap) {
  return heap.length === 0 ? null : heap[0];
}

function compare(a, b) {
  // Compare sort index first, then task id.
  var diff = a.sortIndex - b.sortIndex;
  return diff !== 0 ? diff : a.id - b.id;
}

function unstable_scheduleCallback(priorityLevel, callback, options) {
  ...
}

var newTask = {
  id: taskIdCounter++,
  callback: callback,
  priorityLevel: priorityLevel,
  startTime: startTime,
  expirationTime: expirationTime,
  sortIndex: -1
};

if (startTime > currentTime) {
  // This is a delayed task.
  newTask.sortIndex = startTime;
  push(timerQueue, newTask);
  ...
} else {
  newTask.sortIndex = expirationTime;
  push(taskQueue, newTask);
  // wait until the next time we yield.
  ...
}

var currentTime = getCurrentTime();

/* eslint-disable no-var */
var getCurrentTime;
var hasPerformanceNow = typeof performance === 'object' && typeof performance.now === 'function';

if (hasPerformanceNow) {
  var localPerformance = performance;

  getCurrentTime = function () {
    return localPerformance.now();
  };
} else {
  var localDate = Date;
  var initialTime = localDate.now();

  getCurrentTime = function () {
    return localDate.now() - initialTime;
  };

var startTime;

if (typeof options === 'object' && options !== null) {
  var delay = options.delay;

  if (typeof delay === 'number' && delay > 0) {
    startTime = currentTime + delay;
  } else {
    startTime = currentTime;
  }
} else {
  startTime = currentTime;
}

unstable_scheduleCallback(NormalPriority, cb, { delay: 2000 });

var timeout;

switch (priorityLevel) {
  case ImmediatePriority:
    timeout = IMMEDIATE_PRIORITY_TIMEOUT;
    break;

  case UserBlockingPriority:
    timeout = USER_BLOCKING_PRIORITY_TIMEOUT;
    break;

  case IdlePriority:
    timeout = IDLE_PRIORITY_TIMEOUT;
    break;

  case LowPriority:
    timeout = LOW_PRIORITY_TIMEOUT;
    break;

  case NormalPriority:
  default:
    timeout = NORMAL_PRIORITY_TIMEOUT;
    break;
}

var expirationTime = startTime + timeout;

// 表示已經(jīng)到期，立即執(zhí)行
var IMMEDIATE_PRIORITY_TIMEOUT = -1;

var USER_BLOCKING_PRIORITY_TIMEOUT = 250;
var NORMAL_PRIORITY_TIMEOUT = 5000;

// 設(shè)置一個大值，表示永不過期
var LOW_PRIORITY_TIMEOUT = 10000;

// Tasks are stored on a min heap
var IDLE_PRIORITY_TIMEOUT = maxSigned31BitInt;

if (startTime > currentTime) {
  // This is a delayed task.
  newTask.sortIndex = startTime;
  push(timerQueue, newTask);
  ...
} else {
  newTask.sortIndex = expirationTime;
  push(taskQueue, newTask);
  // wait until the next time we yield.
  ...
}

var taskQueue = [];
// Incrementing id counter. Used to maintain insertion order.
var timerQueue = [];