<?php
class SnowFlake
{
    const TWEPOCH = 1288834974657; // 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 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));*/

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

    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));
    }
}