php實現雪花算法（ID遞增）

雪花算法簡單描述：

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位的長度最多支持部署1024個節點。
12位的計數序列號，序列號即一系列的自增id，可以支持同一節點同一毫秒生成多個ID序號，12位的計數序列號支持每個節點每毫秒產生4096個ID序號。
看的出來，這個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位器標識符一般是5位IDC+5位machine編號，唯一確定一台機器。

<?php
class lib_snowflake
{
    const TWEPOCH = 1488834974657; // 時間起始標記點，作為基准，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; // 數據中心標識位數
    const SEQUENCE_BITS      = 11; // 毫秒內自增位

    private $workerId; // 工作機器ID(0~31)
    private $datacenterId; // 數據中心ID(0~31)
    private $sequence; // 毫秒內序列(0~4095)

    private $maxWorkerId     = -1 ^ (-1 << self::WORKER_ID_BITS); // 機器ID最大值31
    private $maxDatacenterId = -1 ^ (-1 << self::DATACENTER_ID_BITS); // 數據中心ID最大值31

    private $workerIdShift      = self::SEQUENCE_BITS; // 機器ID偏左移11位
    private $datacenterIdShift  = self::SEQUENCE_BITS + self::WORKER_ID_BITS; // 數據中心ID左移16位
    private $timestampLeftShift = self::SEQUENCE_BITS + self::WORKER_ID_BITS + self::DATACENTER_ID_BITS; // 時間毫秒左移21位
    private $sequenceMask       = -1 ^ (-1 << self::SEQUENCE_BITS); // 生成序列的掩碼4095

    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 nextId()
    {
        $timestamp = $this->timeGen();

        if ($timestamp < $this->lastTimestamp) {
            $diffTimestamp = bcsub($this->lastTimestamp, $timestamp);
            throw new Exception("Clock moved backwards.  Refusing to generate id for {$diffTimestamp} milliseconds");
        }

        if ($this->lastTimestamp == $timestamp) {
            $this->sequence = ($this->sequence + 1) & $this->sequenceMask;

            if (0 == $this->sequence) {
                $timestamp = $this->tilNextMillis($this->lastTimestamp);
            }
        } else {
            $this->sequence = 0;
        }

        $this->lastTimestamp = $timestamp;

        $gmpTimestamp    = gmp_init($this->leftShift(bcsub($timestamp, self::TWEPOCH), $this->timestampLeftShift));
        $gmpDatacenterId = gmp_init($this->leftShift($this->datacenterId, $this->datacenterIdShift));
        $gmpWorkerId     = gmp_init($this->leftShift($this->workerId, $this->workerIdShift));
        $gmpSequence     = gmp_init($this->sequence);
        return gmp_strval(gmp_or(gmp_or(gmp_or($gmpTimestamp, $gmpDatacenterId), $gmpWorkerId), $gmpSequence));
    }

    protected function tilNextMillis($lastTimestamp)
    {
        $timestamp = $this->timeGen();
        while ($timestamp <= $lastTimestamp) {
            $timestamp = $this->timeGen();
        }

        return $timestamp;
    }

    protected function timeGen()
    {
        return floor(microtime(true) * 1000);
    }

    // 左移 <<
    protected function leftShift($a, $b)
    {
        return bcmul($a, bcpow(2, $b));
    }
}