全局ID生成器

        snowflake的结构如下(每部分用-分开): 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000 第一位为未使用，接下来的41位为毫秒级时间(41位的长度可以使用69年)，然后是5位datacenterId和5位workerId(10位的长度最多支持部署1024个节点） ，最后12位是毫秒内的计数（12位的计数顺序号支持每个节点每毫秒产生4096个ID序号）

一共加起来刚好64位，为一个Long型。(转换成字符串后长度最多19)

例子：

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

源代码如下：

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
}