雪花演算法的實現

最近看了下雪花演算法，自己試著寫了一下

<?php
class SnowFlake
{
    const TWEPOCH = 0; // 時間起始標記點，作為基準，一般取系統的最近時間（一旦確定不能變動）

    const WORKER_ID_BITS     = 5; // 機器標識位數
    const DATACENTER_ID_BITS = 5; // 資料中心標識位數
    const SEQUENCE_BITS      = 12; // 毫秒內自增位

    private $workerId; // 工作機器ID
    private $datacenterId; // 資料中心ID
    private $sequence; // 毫秒內序列

    private $maxWorkerId     = -1 ^ (-1 << self::WORKER_ID_BITS); // 機器ID最大值
    private $maxDatacenterId = -1 ^ (-1 << self::DATACENTER_ID_BITS); // 資料中心ID最大值

    private $workerIdShift      = self::SEQUENCE_BITS; // 機器ID偏左移位數
    private $datacenterIdShift  = self::SEQUENCE_BITS + self::WORKER_ID_BITS; // 資料中心ID左移位數
    private $timestampLeftShift = self::SEQUENCE_BITS + self::WORKER_ID_BITS + self::DATACENTER_ID_BITS; // 時間毫秒左移位數
    private $sequenceMask       = -1 ^ (-1 << self::SEQUENCE_BITS); // 生成序列的掩碼

    private $lastTimestamp = -1; // 上次生產id時間戳

    public function __construct($workerId, $datacenterId, $sequence = 0)
    {
        if ($workerId > $this->maxWorkerId || $workerId < 0) {
            throw new Exception("worker Id can't be greater than {$this->maxWorkerId} or less than 0");
        }

        if ($datacenterId > $this->maxDatacenterId || $datacenterId < 0) {
            throw new Exception("datacenter Id can't be greater than {$this->maxDatacenterId} or less than 0");
        }

        $this->workerId     = $workerId;
        $this->datacenterId = $datacenterId;
        $this->sequence     = $sequence;
    }

    public function createId()
    {
        $timestamp = $this->createTimestamp();

        if ($timestamp < $this->lastTimestamp) {//當產生的時間戳小於上次的生成的時間戳時，報錯
            $diffTimestamp = bcsub($this->lastTimestamp, $timestamp);
            throw new Exception("Clock moved backwards.  Refusing to generate id for {$diffTimestamp} milliseconds");
        }

        if ($this->lastTimestamp == $timestamp) {//當生成的時間戳等於上次生成的時間戳的時候
            $this->sequence = ($this->sequence + 1) & $this->sequenceMask;//序列自增一次

            if (0 == $this->sequence) {//當序列為0時，重新生成最新的時間戳
                $timestamp = $this->createNextTimestamp($this->lastTimestamp);
            }
        } else {//當生成的時間戳不等於上次的生成的時間戳的時候，序列歸0
            $this->sequence = 0;
        }

        $this->lastTimestamp = $timestamp;

        return (($timestamp - self::TWEPOCH) << $this->timestampLeftShift) |
            ($this->datacenterId << $this->datacenterIdShift) |
            ($this->workerId << $this->workerIdShift) |
            $this->sequence;
    }

    protected function createNextTimestamp($lastTimestamp) //生成一個大於等於 上次生成的時間戳 的時間戳
    {
        $timestamp = $this->createTimestamp();
        while ($timestamp <= $lastTimestamp) {
            $timestamp = $this->createTimestamp();
        }

        return $timestamp;
    }

    protected function createTimestamp()//生成毫秒級別的時間戳
    {
        return floor(microtime(true) * 1000);
    }
}
?>

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