/**
* Twitter的SnowFlake演算法,使用SnowFlake演算法生成一個整數，然後轉化為62進制變成一個短地址URL
*
* https://github.com/beyondfengyu/SnowFlake
*/
public class SnowFlakeShortUrl {

/**
* 起始的時間戳
*/
private final static long START_TIMESTAMP = 1480166465631L;

/**
* 每一部分佔用的位數
*/
private final static long SEQUENCE_BIT = 12; //序列號佔用的位數
private final static long MACHINE_BIT = 5; //機器標識佔用的位數
private final static long DATA_CENTER_BIT = 5; //資料中心佔用的位數

/**
* 每一部分的最大值
*/
private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);
private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
private final static long MAX_DATA_CENTER_NUM = -1L ^ (-1L << DATA_CENTER_BIT);

/**
* 每一部分向左的位移
*/
private final static long MACHINE_LEFT = SEQUENCE_BIT;
private final static long DATA_CENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
private final static long TIMESTAMP_LEFT = DATA_CENTER_LEFT + DATA_CENTER_BIT;

private long dataCenterId; //資料中心
private long machineId; //機器標識
private long sequence = 0L; //序列號
private long lastTimeStamp = -1L; //上一次時間戳

private long getNextMill() {
long mill = getNewTimeStamp();
while (mill <= lastTimeStamp) {
mill = getNewTimeStamp();
}
return mill;
}

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

/**
* 根據指定的資料中心ID和機器標誌ID生成指定的序列號
*
* @param dataCenterId 資料中心ID
* @param machineId 機器標誌ID
*/
public SnowFlakeShortUrl(long dataCenterId, long machineId) {
if (dataCenterId > MAX_DATA_CENTER_NUM || dataCenterId < 0) {
throw new IllegalArgumentException("DtaCenterId can't be greater than MAX_DATA_CENTER_NUM or less than 0！");
}
if (machineId > MAX_MACHINE_NUM || machineId < 0) {
throw new IllegalArgumentException("MachineId can't be greater than MAX_MACHINE_NUM or less than 0！");
}
this.dataCenterId = dataCenterId;
this.machineId = machineId;
}

/**
* 產生下一個ID
*
* @return
*/
public synchronized long nextId() {
long currTimeStamp = getNewTimeStamp();
if (currTimeStamp < lastTimeStamp) {
throw new RuntimeException("Clock moved backwards. Refusing to generate id");
}

if (currTimeStamp == lastTimeStamp) {
//相同毫秒內，序列號自增
sequence = (sequence + 1) & MAX_SEQUENCE;
//同一毫秒的序列數已經達到最大
if (sequence == 0L) {
currTimeStamp = getNextMill();
}
} else {
//不同毫秒內，序列號置為0
sequence = 0L;
}

lastTimeStamp = currTimeStamp;

return (currTimeStamp - START_TIMESTAMP) << TIMESTAMP_LEFT //時間戳部分
| dataCenterId << DATA_CENTER_LEFT //資料中心部分
| machineId << MACHINE_LEFT //機器標識部分
| sequence; //序列號部分
}

public static void main(String[] args) {
SnowFlakeShortUrl snowFlake = new SnowFlakeShortUrl(2, 3);

for (int i = 0; i < (1 << 4); i++) {
//10進位制
System.out.println(snowFlake.nextId());
}
}
}