<?php

class Snowflake
{
  const EPOCH = 1543223810238;  // 起始時間戳，毫秒

  const SEQUENCE_BITS = 12;  //序號部分12位
  const SEQUENCE_MAX = -1 ^ (-1 << self::SEQUENCE_BITS); // 序號最大值

  const WORKER_BITS = 10; // 節(jié)點部分10位
  const WORKER_MAX = -1 ^ (-1 << self::WORKER_BITS); // 節(jié)點最大數值

  const TIME_SHIFT = self::WORKER_BITS + self::SEQUENCE_BITS; // 時間戳部分左偏移量
  const WORKER_SHIFT = self::SEQUENCE_BITS;  // 節(jié)點部分左偏移量

  protected $timestamp;  // 上次ID生成時間戳
  protected $workerId;  // 節(jié)點ID
  protected $sequence;  // 序號
  protected $lock;    // Swoole 互斥鎖

  public function __construct($workerId)
  {
    if ($workerId < 0 || $workerId > self::WORKER_MAX) {
      trigger_error("Worker ID 超出范圍");
      exit(0);
    }

    $this->timestamp = 0;
    $this->workerId = $workerId;
    $this->sequence = 0;
    $this->lock = new swoole_lock(SWOOLE_MUTEX);
  }

  /**
   * 生成ID
   * @return int
   */
  public function getId()
  {
    $this->lock->lock();  // 這里一定要記得加鎖
    $now = $this->now();
    if ($this->timestamp == $now) {
      $this->sequence++;

      if ($this->sequence > self::SEQUENCE_MAX) {
        // 當前毫秒內生成的序號已經超出最大范圍，等待下一毫秒重新生成
        while ($now <= $this->timestamp) {
          $now = $this->now();
        }
      }
    } else {
      $this->sequence = 0;
    }

    $this->timestamp = $now;  // 更新ID生時間戳

    $id = (($now - self::EPOCH) << self::TIME_SHIFT) | ($this->workerId << self::WORKER_SHIFT) | $this->sequence;
    $this->lock->unlock(); //解鎖

    return $id;
  }

  /**
   * 獲取當前毫秒
   * @return string
   */
  public function now()
  {
    return sprintf("%.0f", microtime(true) * 1000);
  }

}

-1 ^ (-1 << self::SEQUENCE_BITS)
就是-1的二進制表示為1的補碼,其實等同于 :
2**self::SEQUENCE_BITS - 1

(($now - self::EPOCH) << self::TIME_SHIFT) | ($this->workerId << self::WORKER_SHIFT) | $this->sequence;