package com.webmanage.util;
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import java.util.concurrent.locks.ReentrantLock;
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/**
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* Snowflake 雪花算法ID生成器（64位）
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* 结构：1位符号位 + 41位时间戳 + 5位数据中心 + 5位工作机器 + 12位序列
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*/
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public class SnowflakeIdWorker {
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private final long twepoch;
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private final long workerIdBits = 5L;
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private final long datacenterIdBits = 5L;
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private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
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private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
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private final long sequenceBits = 12L;
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private final long workerIdShift = sequenceBits;
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private final long datacenterIdShift = sequenceBits + workerIdBits;
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private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
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private final long sequenceMask = -1L ^ (-1L << sequenceBits);
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private final long workerId;
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private final long datacenterId;
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private long sequence = 0L;
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private long lastTimestamp = -1L;
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private final ReentrantLock lock = new ReentrantLock(true);
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/**
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* @param workerId 工作机器ID (0~31)
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* @param datacenterId 数据中心ID (0~31)
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* @param twepochMs 自定义纪元时间（毫秒），建议为系统上线前固定时间戳，默认 2020-01-01
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*/
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public SnowflakeIdWorker(long workerId, long datacenterId, long twepochMs) {
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if (workerId > maxWorkerId || workerId < 0) {
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throw new IllegalArgumentException("worker Id can't be greater than " + maxWorkerId + " or less than 0");
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}
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if (datacenterId > maxDatacenterId || datacenterId < 0) {
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throw new IllegalArgumentException("datacenter Id can't be greater than " + maxDatacenterId + " or less than 0");
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}
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this.workerId = workerId;
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this.datacenterId = datacenterId;
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this.twepoch = twepochMs;
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}
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/**
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* 线程安全生成下一个ID
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*/
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public long nextId() {
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lock.lock();
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try {
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long timestamp = currentTime();
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if (timestamp < lastTimestamp) {
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// 时钟回拨处理：等待到 lastTimestamp
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long offset = lastTimestamp - timestamp;
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try {
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Thread.sleep(offset);
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} catch (InterruptedException ignored) { }
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timestamp = currentTime();
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if (timestamp < lastTimestamp) {
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// 如果仍小于，强制使用 lastTimestamp
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timestamp = lastTimestamp;
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}
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}
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if (lastTimestamp == timestamp) {
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sequence = (sequence + 1) & sequenceMask;
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if (sequence == 0) {
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// 同毫秒内序列溢出，等待下一毫秒
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timestamp = tilNextMillis(lastTimestamp);
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}
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} else {
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sequence = 0L;
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}
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lastTimestamp = timestamp;
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return ((timestamp - twepoch) << timestampLeftShift)
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| (datacenterId << datacenterIdShift)
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| (workerId << workerIdShift)
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| sequence;
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} finally {
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lock.unlock();
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}
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}
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private long tilNextMillis(long lastTimestamp) {
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long timestamp = currentTime();
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while (timestamp <= lastTimestamp) {
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timestamp = currentTime();
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}
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return timestamp;
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}
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private long currentTime() {
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return System.currentTimeMillis();
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}
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}
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