function Snowflake() {
  this.epoch = 1698796800000 // 自定义起始时间（毫秒）
  this.machineId = 1 // 固定 machineId
  this.sequence = 0
  this.lastTimestamp = -1

  this.sequenceBits = 12
  this.machineBits = 10
  this.maxSequence = 2 ** this.sequenceBits - 1 // 4095
}

// 获取当前时间（毫秒）
Snowflake.prototype.currentTime = function () {
  return Date.now()
}

// 等待下一毫秒
Snowflake.prototype.waitNextMillisecond = function (lastTimestamp) {
  let timestamp = this.currentTime()
  while (timestamp <= lastTimestamp) {
    timestamp = this.currentTime()
  }
  return timestamp
}

// 生成 BigInt ID
Snowflake.prototype.nextId = function () {
  let timestamp = this.currentTime()

  if (timestamp < this.lastTimestamp) {
    throw new Error('Clock moved backwards. Refusing to generate id.')
  }

  if (timestamp === this.lastTimestamp) {
    this.sequence = (this.sequence + 1) & this.maxSequence
    if (this.sequence === 0) {
      timestamp = this.waitNextMillisecond(this.lastTimestamp)
    }
  }
  else {
    this.sequence = 0
  }

  this.lastTimestamp = timestamp

  const id
    = (BigInt(timestamp - this.epoch) << BigInt(this.machineBits + this.sequenceBits))
      | (BigInt(this.machineId) << BigInt(this.sequenceBits))
      | BigInt(this.sequence)

  return id
}

// 返回固定长度字符串 ID（适合数据库存储）
Snowflake.prototype.nextIdStr = function (padLength = 20) {
  return this.nextId().toString().padStart(padLength, '0')
}