#php #mist

Mist.php

<?php

/**
 * 薄雾算法
 *
 * 1      2                                                     48         56       64
 * +------+-----------------------------------------------------+----------+----------+
 * retain | increas                                             | salt     | sequence |
 * +------+-----------------------------------------------------+----------+----------+
 * 0      | 0000000000 0000000000 0000000000 0000000000 0000000 | 00000000 | 00000000 |
 * +------+-----------------------------------------------------+------------+--------+
 *
 * 0. 最高位，占 1 位，保持为 0，使得值永远为正数；
 * 1. 自增数，占 47 位，自增数在高位能保证结果值呈递增态势，遂低位可以为所欲为；
 * 2. 随机因子一，占 8 位，上限数值 255，使结果值不可预测；
 * 3. 随机因子二，占 8 位，上限数值 255，使结果值不可预测；
 *
 * 编号上限为百万亿级，上限值计算为 140737488355327 即 int64(1 << 47 - 1)，假设每天取值 10 亿，能使用 385+ 年
 */
class Mist
{
    /**
     * 随机因子二进制位数
     */
    public const SALT_BIT = 8;

    /**
     * 随机因子移位数
     */
    public const SALT_SHIFT = 8;

    /**
     * 自增数移位数
     */
    public const INCREASE_SHIFT = self::SALT_BIT + self::SALT_SHIFT;

    /**
     * @var int 自增数
     */
    public $increase = 0;

    /**
     * @var int 随机因子一
     */
    public $saltA = 0;

    /**
     * @var int 随机因子二
     */
    public $saltB = 0;

    public function __construct(int $increase = 1)
    {
        $this->increase = $increase;
    }

    public function generate(): int
    {
        $this->increase = ++$this->increase;

        // 获取随机因子数值
        try {
            $this->saltA = random_int($this->saltA, 255);
        } catch (\Throwable $e) {
            $this->saltA = $this->saltA++;
        }

        try {
            $this->saltB = random_int($this->saltB, 255);
        } catch (\Throwable $e) {
            $this->saltB = $this->saltB++;
        }

        return ($this->increase << self::INCREASE_SHIFT) | ($this->saltA << self::SALT_SHIFT) | $this->saltB;
    }
}