function isNumber(l) {
  return /^[0-9]$/.test(l)
}

function isSpace(l) {
  return l === ' '
}

function isDot(l) {
  return l === '.'
}

function isValidLetter(l){
 return isNumber(l) || isSpace(l) || isDot(l)
}

function ipv4ToInt(input) {
  const ip = '.'+input
  const length = ip.length
  let order = 0
  let result = 0
  let current = ""
  let spaceAfterNumberOccur = false
  for (let i = 0; i < length; i++){
    const position = length-i-1
    const letter = ip[position]
    if (!isValidLetter(letter)) throw new Error(`read invalid letter ${letter} at ${position}`)

    if (isNumber(letter) && spaceAfterNumberOccur) throw new Error(`space between number at ${position}`)

    if (isNumber(letter)) {
      current = letter + current
    } else if (isSpace(letter)){
      if (isNumber(current[0])) spaceAfterNumberOccur = true
    } else if (isDot(letter)){
      result += parseInt(current, 10) * Math.pow(256, order)
      order += 1
      current = ""
      spaceAfterNumberOccur = false
    } else {
      throw new Error(`invalid input, this should not occur unless you removed the guard code.`)
    }
  }
  return result
}

function assert(i, j) {
  if (i !== j) throw new Error(`${i} not equal to ${j}`)
}

function shouldThrow(fn) {
  try{
    fn()
  } catch(e){
    return
  }
 throw new Error('should throw error, but it did not.')
}

function test() {
  assert(ipv4ToInt("172.168.5.1"), 2896692481)
  assert(ipv4ToInt("172 .168.5.1"), 2896692481)
  assert(ipv4ToInt("172. 168.5.1"), 2896692481)
  assert(ipv4ToInt(" 172.168.5.1"), 2896692481)
  shouldThrow(function() { ipv4ToInt("172.16 8.5.1") })
  shouldThrow(function() { ipv4ToInt("172.168.a.1") })
  console.log('all pass')
}