let scheduleNext

if (typeof setImmediate != 'undefined') {
  if (!process.nextTick) {
    let npending = 0
    const reset = () => { npending = 0 }
    scheduleNext = function scheduleNext(callback) {
      if (npending == 0) {
        process.nextTick(() => {
          callback()
          process.nextTick(reset)
        })
        ++npending
      } else if (npending < 4) {
        process.nextTick(callback)
        ++npending
      } else {
        setImmediate(callback)
      }
    }
  } else {
    scheduleNext = setImmediate
  }
} else {
  scheduleNext = process.nextTick.bind(process)
}


const NS_PER_SEC = 1e9
const NS_PER_MS = 1e6

function sched(label /*, ...cbs :()=>void[] */) {
  const cbs = Array.prototype.slice.call(arguments, 1)
  console.log(`scheduleNext "${label}"`)
  const startTime = process.hrtime()
  scheduleNext(() => {
    const diff = process.hrtime(startTime)
    const duration = ((diff[0] * 1000) + diff[1]) / NS_PER_MS
    console.log(`callback "${label}": ${duration.toFixed(3)} ms`)
    if (cbs) {
      for (const cb of cbs) { cb() }
    }
  })
}

sched('1')
sched('2',
  sched.bind(null, '2.1',
    sched.bind(null, '2.1.1'),
    sched.bind(null, '2.1.2'),
    sched.bind(null, '2.1.3')))
sched('3',
  sched.bind(null, '3.1'),
  sched.bind(null, '3.2',
    sched.bind(null, '3.2.1'),
    sched.bind(null, '3.2.2'),
    sched.bind(null, '3.2.3')))
sched('4')
sched('5')