package com.example.test.Controllers;

/**
 * Twitter_Snowflake<br>
 * SnowFlake的結(jié)構(gòu)如下(每部分用-分開(kāi)):<br>
 * 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000 <br>
 * 1位標(biāo)識(shí)馒铃，由于long基本類型在Java中是帶符號(hào)的您宪，最高位是符號(hào)位冲茸，正數(shù)是0，負(fù)數(shù)是1固灵，所以id一般是正數(shù)捅伤，最高位是0<br>
 * 41位時(shí)間截(毫秒級(jí))，注意巫玻，41位時(shí)間截不是存儲(chǔ)當(dāng)前時(shí)間的時(shí)間截丛忆，而是存儲(chǔ)時(shí)間截的差值（當(dāng)前時(shí)間截 - 開(kāi)始時(shí)間截)
 * 得到的值），這里的的開(kāi)始時(shí)間截仍秤，一般是我們的id生成器開(kāi)始使用的時(shí)間熄诡，由我們程序來(lái)指定的（如下下面程序IdWorker類的startTime屬性）。41位的時(shí)間截诗力，可以使用69年凰浮，年T = (1L << 41) / (1000L * 60 * 60 * 24 * 365) = 69<br>
 * 10位的數(shù)據(jù)機(jī)器位，可以部署在1024個(gè)節(jié)點(diǎn)苇本，包括5位datacenterId和5位workerId<br>
 * 12位序列袜茧，毫秒內(nèi)的計(jì)數(shù)，12位的計(jì)數(shù)順序號(hào)支持每個(gè)節(jié)點(diǎn)每毫秒(同一機(jī)器瓣窄，同一時(shí)間截)產(chǎn)生4096個(gè)ID序號(hào)<br>
 * 加起來(lái)剛好64位惫周，為一個(gè)Long型。<br>
 * SnowFlake的優(yōu)點(diǎn)是康栈，整體上按照時(shí)間自增排序，并且整個(gè)分布式系統(tǒng)內(nèi)不會(huì)產(chǎn)生ID碰撞(由數(shù)據(jù)中心ID和機(jī)器ID作區(qū)分)喷橙，并且效率較高啥么，經(jīng)測(cè)試，SnowFlake每秒能夠產(chǎn)生26萬(wàn)ID左右贰逾。
 */
public class SnowflakeIdWorker {

    // ==============================Fields===========================================
    /** 開(kāi)始時(shí)間截 (2022-01-01) */
    private final long twepoch = 1640966400000L;

    /** 機(jī)器id所占的位數(shù) */
    private final long workerIdBits = 5L;

    /** 數(shù)據(jù)標(biāo)識(shí)id所占的位數(shù) */
    private final long datacenterIdBits = 5L;

    /** 支持的最大機(jī)器id悬荣，結(jié)果是31 (這個(gè)移位算法可以很快的計(jì)算出幾位二進(jìn)制數(shù)所能表示的最大十進(jìn)制數(shù)) */
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);

    /** 支持的最大數(shù)據(jù)標(biāo)識(shí)id，結(jié)果是31 */
    private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);

    /** 序列在id中占的位數(shù) */
    private final long sequenceBits = 12L;

    /** 機(jī)器ID向左移12位 */
    private final long workerIdShift = sequenceBits;

    /** 數(shù)據(jù)標(biāo)識(shí)id向左移17位(12+5) */
    private final long datacenterIdShift = sequenceBits + workerIdBits;

    /** 時(shí)間截向左移22位(5+5+12) */
    private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;

    /** 生成序列的掩碼疙剑，這里為4095 (0b111111111111=0xfff=4095) */
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);

    /** 工作機(jī)器ID(0~31) */
    private long workerId;

    /** 數(shù)據(jù)中心ID(0~31) */
    private long datacenterId;

    /** 毫秒內(nèi)序列(0~4095) */
    private long sequence = 0L;

    /** 上次生成ID的時(shí)間截 */
    private long lastTimestamp = -1L;

    //==============================Constructors=====================================
    /**
     * 構(gòu)造函數(shù)
     * @param workerId 工作ID (0~31)
     * @param datacenterId 數(shù)據(jù)中心ID (0~31)
     */
    public SnowflakeIdWorker(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    // ==============================Methods==========================================
    /**
     * 獲得下一個(gè)ID (該方法是線程安全的)
     * @return SnowflakeId
     */
    public synchronized long nextId() {
        long timestamp = timeGen();

        //如果當(dāng)前時(shí)間小于上一次ID生成的時(shí)間戳氯迂，說(shuō)明系統(tǒng)時(shí)鐘回退過(guò)這個(gè)時(shí)候應(yīng)當(dāng)拋出異常
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(
                    String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

        //如果是同一時(shí)間生成的践叠，則進(jìn)行毫秒內(nèi)序列
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            //毫秒內(nèi)序列溢出
            if (sequence == 0) {
                //阻塞到下一個(gè)毫秒,獲得新的時(shí)間戳
                timestamp = tilNextMillis(lastTimestamp);
            }
        }
        //時(shí)間戳改變，毫秒內(nèi)序列重置
        else {
            sequence = 0L;
        }

        //上次生成ID的時(shí)間截
        lastTimestamp = timestamp;

        //移位并通過(guò)或運(yùn)算拼到一起組成64位的ID
        return ((timestamp - twepoch) << timestampLeftShift) //
                | (datacenterId << datacenterIdShift) //
                | (workerId << workerIdShift) //
                | sequence;
    }

    /**
     * 阻塞到下一個(gè)毫秒嚼蚀，直到獲得新的時(shí)間戳
     * @param lastTimestamp 上次生成ID的時(shí)間截
     * @return 當(dāng)前時(shí)間戳
     */
    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    /**
     * 返回以毫秒為單位的當(dāng)前時(shí)間
     * @return 當(dāng)前時(shí)間(毫秒)
     */
    protected long timeGen() {
        return System.currentTimeMillis();
    }

    // ==============================靜態(tài)方法調(diào)用==========================================
    /**
     * 靜態(tài)方法獲得下一個(gè)ID
     * @return SnowflakeId
     */
    static long id() {
        // 入?yún)⒏鶕?jù)實(shí)際場(chǎng)景優(yōu)化
        SnowflakeIdWorker idWorker = new SnowflakeIdWorker(0, 0);
        return idWorker.nextId();
    }
}