golang-uuidv1

源码解读 github.com/satori/go.uuid

// NewV1 returns UUID based on current timestamp and MAC address.
func (g *generator) NewV1() UUID {
	u := UUID{}

	timeNow, clockSeq, hardwareAddr := g.getStorage()
	//前32bits time-low
	binary.BigEndian.PutUint32(u[0:], uint32(timeNow))
	// 16bits time-mid
	binary.BigEndian.PutUint16(u[4:], uint16(timeNow>>32))
	// 	o  Set the 12 least significant bits (bits zero through 11) of the
	//       time_hi_and_version field equal to bits 48 through 59 from the
	//       timestamp in the same order of significance.

	//    o  Set the four most significant bits (bits 12 through 15) of the
	//       time_hi_and_version field to the 4-bit version number
	//       corresponding to the UUID version being created, as shown in the
	//       table above.
	// 4bits(version) + 12bits(time_hi)
	binary.BigEndian.PutUint16(u[6:], uint16(timeNow>>48))
	// 8bits clock-seq-and-reserved + 8bits clock-seq-low
	binary.BigEndian.PutUint16(u[8:], clockSeq)
	//48bits node
	copy(u[10:], hardwareAddr)
    // 设置版本
    u.SetVersion(V1)
    // 设置变体
	u.SetVariant(VariantRFC4122)

	return u
}

BigEndian 这里使用大端序,可能会出问题

获取时钟,当期时间 和 mac 地址

// Returns UUID v1/v2 storage state.
// Returns epoch timestamp, clock sequence, and hardware address.
func (g *generator) getStorage() (uint64, uint16, []byte) {
	//sync.Once确保mac地址,和clockSequence只会初始化一次
	g.storageOnce.Do(g.initStorage)

	g.storageMutex.Lock()
	defer g.storageMutex.Unlock()

	timeNow := epochFunc()
	// Clock changed backwards since last UUID generation.
	// Should increase clock sequence.
	// 如果系统时间出现回退或者,在100ns内多次产生uuid.高并发的时候需要clockSequence区分周期
	if timeNow <= g.lastTime {
		g.clockSequence++
	}
	g.lastTime = timeNow

	return timeNow, g.clockSequence, g.hardwareAddr[:]
}

初始化时钟和 mac 地址

func (g *generator) initStorage() {
	g.initClockSequence()
	g.initHardwareAddr()
}

func (g *generator) initClockSequence() {
	buf := make([]byte, 2)
	g.safeRandom(buf)
	// 获取随机数作为初始 clockSequence
	g.clockSequence = binary.BigEndian.Uint16(buf)
}

func (g *generator) initHardwareAddr() {
	interfaces, err := net.Interfaces()
	if err == nil {
		for _, iface := range interfaces {
			fmt.Printf("iface.HardwareAddr %v \n", iface)
			if len(iface.HardwareAddr) >= 6 {
				// 找到mac地址
				copy(g.hardwareAddr[:], iface.HardwareAddr)
				return
			}
		}
	}

	// Initialize hardwareAddr randomly in case
	// of real network interfaces absence
	// 没mac地址就先随机
	g.safeRandom(g.hardwareAddr[:])

	// Set multicast bit as recommended in RFC 4122
	// 	For systems with no IEEE address, a randomly or pseudo-randomly
	//  generated value may be used; see Section 4.5.  The multicast bit must
	//  be set in such addresses, in order that they will never conflict with
	//  addresses obtained from network cards.

	g.hardwareAddr[0] |= 0x01
}

当前时间.

• 初始时间- UUID epoch (October 15, 1582) and Unix epoch (January 1, 1970).
• 当期时间戳(100ns 为单位)

const epochStart = 122192928000000000)

// Returns difference in 100-nanosecond intervals between
// UUID epoch (October 15, 1582) and current time.
// This is default epoch calculation function.
func unixTimeFunc() uint64 {
	return epochStart + uint64(time.Now().UnixNano()/100)
}

总结

v1 生成 uuid 的基础数据是 100ns 基本的时间戳 + mac 地址(没有的情况为随机)
一般加入 timestamp 1.是为了时间维度 2.是为了便于 sort
但是 uuid 规范的的顺序是 time-low - time-mid - 4bits(version) + 12bits(time_hi)
所以是不能根据 uuid 来 sort 的