雪花演算法簡單描述：
+ 最高位是符號位，始終為0，不可用。
+ 41位的時間序列，精確到毫秒級，41位的長度可以使用69年。時間位還有一個很重要的作用是可以根據時間進行排序。
+ 10位的機器標識，10位的長度最多支援部署1024個節點。
+ 12位的計數序列號，序列號即一系列的自增id，可以支援同一節點同一毫秒生成多個ID序號，12位的計數序列號支援每個節點每毫秒產生4096個ID序號。

看的出來，這個演算法很簡潔也很簡單，但依舊是一個很好的ID生成策略。其中，10位器識別符號一般是5位IDC+5位machine編號，唯一確定一臺機器。

我不知道Twitter是如何確定10位機器標識和12位序列號的，不過，很明顯，這是可以改變的。

public class SnowFlakeGenerator {

    public static class Factory {
        /**
         * 每一部分佔用位數的預設值
         */
        private final static int DEFAULT_MACHINE_BIT_NUM = 5;   //機器標識佔用的位數
        private final static int DEFAULT_IDC_BIT_NUM = 5;//資料中心佔用的位數

        private int machineBitNum;
        private
 
 int idcBitNum;

        public Factory() {
            this.idcBitNum = DEFAULT_IDC_BIT_NUM;
            this.machineBitNum = DEFAULT_MACHINE_BIT_NUM;
        }

        public Factory(int machineBitNum, int idcBitNum) {
            this.idcBitNum = idcBitNum;
            this.machineBitNum = machineBitNum;
        }

        public
 
 SnowFlakeGenerator create(long idcId, long machineId) {
            return new SnowFlakeGenerator(this.idcBitNum, this.machineBitNum, idcId, machineId);
        }
    }

    /**
     * 起始的時間戳
     * 作者寫程式碼時的時間戳
     */
    private final static long START_STAMP = 1508143349995L;

    /**
     * 可分配的位數
     */
    private final static int REMAIN_BIT_NUM = 22;

    /**
     * idc編號
     */
    private long idcId;

    /**
     * 機器編號
     */
    private long machineId;

    /**
     * 當前序列號
     */
    private long sequence = 0L;

    /**
     * 上次最新時間戳
     */
    private long lastStamp = -1L;

    /**
     * idc偏移量：一次計算出，避免重複計算
     */
    private int idcBitLeftOffset;

    /**
     * 機器id偏移量：一次計算出，避免重複計算
     */
    private int machineBitLeftOffset;

    /**
     * 時間戳偏移量：一次計算出，避免重複計算
     */
    private int timestampBitLeftOffset;

    /**
     * 最大序列值：一次計算出，避免重複計算
     */
    private int maxSequenceValue;

    private SnowFlakeGenerator(int idcBitNum, int machineBitNum, long idcId, long machineId) {
        int sequenceBitNum = REMAIN_BIT_NUM - idcBitNum - machineBitNum;

        if (idcBitNum <= 0 || machineBitNum <= 0 || sequenceBitNum <= 0) {
            throw new IllegalArgumentException("error bit number");
        }

        this.maxSequenceValue = ~(-1 << sequenceBitNum);

        machineBitLeftOffset = sequenceBitNum;
        idcBitLeftOffset = idcBitNum + sequenceBitNum;
        timestampBitLeftOffset = idcBitNum + machineBitNum + sequenceBitNum;

        this.idcId = idcId;
        this.machineId = machineId;
    }

    /**
     * 產生下一個ID
     */
    public synchronized long nextId() {
        long currentStamp = getTimeMill();
        if (currentStamp < lastStamp) {
            throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastStamp - currentStamp));
        }

        //新的毫秒，序列從0開始，否則序列自增
        if (currentStamp == lastStamp) {
            sequence = (sequence + 1) & this.maxSequenceValue;
            if (sequence == 0L) {
                //Twitter原始碼中的邏輯是迴圈，直到下一個毫秒
                lastStamp = tilNextMillis();
//                throw new IllegalStateException("sequence over flow");
            }
        } else {
            sequence = 0L;
        }

        lastStamp = currentStamp;

        return (currentStamp - START_STAMP) << timestampBitLeftOffset | idcId << idcBitLeftOffset | machineId << machineBitLeftOffset | sequence;
    }

    private long getTimeMill() {
        return System.currentTimeMillis();
    }

    private long tilNextMillis() {
        long timestamp = getTimeMill();
        while (timestamp <= lastStamp) {
            timestamp = getTimeMill();
        }
        return timestamp;
    }
}

也附上Twitter的scala版：

/** Copyright 2010-2012 Twitter, Inc.*/
package com.twitter.service.snowflake

import com.twitter.ostrich.stats.Stats
import com.twitter.service.snowflake.gen._
import java.util.Random
import com.twitter.logging.Logger

/**
 * An object that generates IDs.
 * This is broken into a separate class in case
 * we ever want to support multiple worker threads
 * per process
 */
class IdWorker(val workerId: Long, val datacenterId: Long, private val reporter: Reporter, var sequence: Long = 0L)
extends Snowflake.Iface {
  private[this] def genCounter(agent: String) = {
    Stats.incr("ids_generated")
    Stats.incr("ids_generated_%s".format(agent))
  }
  private[this] val exceptionCounter = Stats.getCounter("exceptions")
  private[this] val log = Logger.get
  private[this] val rand = new Random

  val twepoch = 1288834974657L

  private[this] val workerIdBits = 5L
  private[this] val datacenterIdBits = 5L
  private[this] val maxWorkerId = -1L ^ (-1L << workerIdBits)
  private[this] val maxDatacenterId = -1L ^ (-1L << datacenterIdBits)
  private[this] val sequenceBits = 12L

  private[this] val workerIdShift = sequenceBits
  private[this] val datacenterIdShift = sequenceBits + workerIdBits
  private[this] val timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits
  private[this] val sequenceMask = -1L ^ (-1L << sequenceBits)

  private[this] var lastTimestamp = -1L

  // sanity check for workerId
  if (workerId > maxWorkerId || workerId < 0) {
    exceptionCounter.incr(1)
    throw new IllegalArgumentException("worker Id can't be greater than %d or less than 0".format(maxWorkerId))
  }

  if (datacenterId > maxDatacenterId || datacenterId < 0) {
    exceptionCounter.incr(1)
    throw new IllegalArgumentException("datacenter Id can't be greater than %d or less than 0".format(maxDatacenterId))
  }

  log.info("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, workerid %d",
    timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, workerId)

  def get_id(useragent: String): Long = {
    if (!validUseragent(useragent)) {
      exceptionCounter.incr(1)
      throw new InvalidUserAgentError
    }

    val id = nextId()
    genCounter(useragent)

    reporter.report(new AuditLogEntry(id, useragent, rand.nextLong))
    id
  }

  def get_worker_id(): Long = workerId
  def get_datacenter_id(): Long = datacenterId
  def get_timestamp() = System.currentTimeMillis

  protected[snowflake] def nextId(): Long = synchronized {
    var timestamp = timeGen()

    if (timestamp < lastTimestamp) {
      exceptionCounter.incr(1)
      log.error("clock is moving backwards.  Rejecting requests until %d.", lastTimestamp);
      throw new InvalidSystemClock("Clock moved backwards.  Refusing to generate id for %d milliseconds".format(
        lastTimestamp - timestamp))
    }

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

    lastTimestamp = timestamp
    ((timestamp - twepoch) << timestampLeftShift) |
      (datacenterId << datacenterIdShift) |
      (workerId << workerIdShift) | 
      sequence
  }

  protected def tilNextMillis(lastTimestamp: Long): Long = {
    var timestamp = timeGen()
    while (timestamp <= lastTimestamp) {
      timestamp = timeGen()
    }
    timestamp
  }

  protected def timeGen(): Long = System.currentTimeMillis()

  val AgentParser = """([a-zA-Z][a-zA-Z\-0-9]*)""".r

  def validUseragent(useragent: String): Boolean = useragent match {
    case AgentParser(_) => true
    case _ => false
  }
}