code: function Id (_id) {
    var dec
    var hex
  
    if (typeof _id === 'number') {
      dec = _id
      var tmp = ('00000000000000' + _id.toString(16))
      hex = tmp.substring(tmp.length - 12, tmp.length)
    }
    if (typeof _id === 'string') {
      dec = parseInt(_id, 16)
      hex = _id
    }
    if (typeof _id === 'undefined') {
      hex = sha1((~~(Math.random() * 1e9)).toString(36) + Date.now())
        .substring(0, 12)
      dec = parseInt(hex, 16)
    }
  
    this.toHex = function () {
      return hex
    }
  
    this.toDec = function () {
      return dec
    }
  
    this.next = function () {
      if (hex === maxHex) {
        return '000000000000'
      } else {
        var a = ('000000000000' + ((dec + 1).toString(16)))
        return a.substring(a.length - 12, a.length)
      }
    }
  
    return this
  }
docs: var maxDec = parseInt(maxHex, 16)