全局ID生成器

        snowflake的結構如下(每部分用-分開): 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000 第一位爲未使用，接下來的41位爲毫秒級時間(41位的長度可以使用69年)，然後是5位datacenterId和5位workerId(10位的長度最多支持部署1024個節點） ，最後12位是毫秒內的計數（12位的計數順序號支持每個節點每毫秒產生4096個ID序號）

一共加起來剛好64位，爲一個Long型。(轉換成字符串後長度最多19)

例子：

currWoker := &idworker.IdWorker{} 
currWoker.InitIdWorker(1000, 1)
newId , newIdErr:= currWoker.NexiId()

源代碼如下：

package idworker

import (
	"errors"
	"fmt"
	"sync"
	"time"
)

type IdWorker struct {
	startTime             int64
	workerIdBits          uint
	datacenterIdBits      uint
	maxWorkerId           int64
	maxDatacenterId       int64
	sequenceBits          uint
	workerIdLeftShift     uint
	datacenterIdLeftShift uint
	timestampLeftShift    uint
	sequenceMask          int64
	workerId              int64
	datacenterId          int64
	sequence              int64
	lastTimestamp         int64
	signMask              int64
	idLock                *sync.Mutex
}

func (this *IdWorker) InitIdWorker(workerId, datacenterId int64) error {

	var baseValue int64 = -1
	this.startTime = 1463834116272
	this.workerIdBits = 5
	this.datacenterIdBits = 5
	this.maxWorkerId = baseValue ^ (baseValue << this.workerIdBits)
	this.maxDatacenterId = baseValue ^ (baseValue << this.datacenterIdBits)
	this.sequenceBits = 12
	this.workerIdLeftShift = this.sequenceBits
	this.datacenterIdLeftShift = this.workerIdBits + this.workerIdLeftShift
	this.timestampLeftShift = this.datacenterIdBits + this.datacenterIdLeftShift
	this.sequenceMask = baseValue ^ (baseValue << this.sequenceBits)
	this.sequence = 0
	this.lastTimestamp = -1
	this.signMask = ^baseValue + 1

	this.idLock = &sync.Mutex{}

	if this.workerId < 0 || this.workerId > this.maxWorkerId {
		return errors.New(fmt.Sprintf("workerId[%v] is less than 0 or greater than maxWorkerId[%v].", workerId, datacenterId))
	}
	if this.datacenterId < 0 || this.datacenterId > this.maxDatacenterId {
		return errors.New(fmt.Sprintf("datacenterId[%d] is less than 0 or greater than maxDatacenterId[%d].", workerId, datacenterId))
	}
	this.workerId = workerId
	this.datacenterId = datacenterId
	return nil
}

func (this *IdWorker) NextId() (int64, error) {
	this.idLock.Lock()
	timestamp := time.Now().UnixNano()
	if timestamp < this.lastTimestamp {
		return -1, errors.New(fmt.Sprintf("Clock moved backwards.  Refusing to generate id for %d milliseconds", this.lastTimestamp-timestamp))
	}

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

	this.lastTimestamp = timestamp

	this.idLock.Unlock()

	id := ((timestamp - this.startTime) << this.timestampLeftShift) |
		(this.datacenterId << this.datacenterIdLeftShift) |
		(this.workerId << this.workerIdLeftShift) |
		this.sequence

	if id < 0 {
		id = -id
	}

	return id, nil
}

func (this *IdWorker) tilNextMillis() int64 {
	timestamp := time.Now().UnixNano()
	if timestamp <= this.lastTimestamp {
		timestamp = time.Now().UnixNano() / int64(time.Millisecond)
	}
	return timestamp
}