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Firebase Push ID, in PHP
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| <?php | |
| /** | |
| * Fancy ID generator that creates 20-character string identifiers with the following properties: | |
| * | |
| * 1. They're based on timestamp so that they sort *after* any existing ids. | |
| * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs. | |
| * 3. They sort *lexicographically* (so the timestamp is converted to characters that will sort properly). | |
| * 4. They're monotonically increasing. Even if you generate more than one in the same timestamp, the | |
| * latter ones will sort after the former ones. We do this by using the previous random bits | |
| * but "incrementing" them by 1 (only in the case of a timestamp collision). | |
| */ | |
| class PushId | |
| { | |
| /** | |
| * Modeled after base64 web-safe chars, but ordered by ASCII. | |
| * | |
| * @var string | |
| */ | |
| const PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz'; | |
| /** | |
| * Timestamp of last push, used to prevent local collisions if you push twice in one ms. | |
| * | |
| * @var int | |
| */ | |
| private $lastPushTime = 0; | |
| /** | |
| * We generate 72-bits of randomness which get turned into 12 characters and appended to the | |
| * timestamp to prevent collisions with other clients. We store the last characters we | |
| * generated because in the event of a collision, we'll use those same characters except | |
| * "incremented" by one. | |
| * | |
| * @var array | |
| */ | |
| private $lastRandChars = []; | |
| public function __construct() { | |
| $this->lastRandChars = []; | |
| $this->lastPushTime = 0; | |
| } | |
| public function generate(): string | |
| { | |
| $now = microtime(true) * 1000; | |
| $isDuplicateTime = ($now === $this->$lastPushTime); | |
| $this->$lastPushTime = $now; | |
| $timeStampChars = new SplFixedArray(8); | |
| for ($i = 7; $i >= 0; $i--) { | |
| $timeStampChars[$i] = substr(self::PUSH_CHARS, (int) $now % 64, 1); | |
| $now = floor($now / 64); | |
| } | |
| static::assert((int) $now === 0, 'We should have converted the entire timestamp.'); | |
| $id = implode('', $timeStampChars->toArray()); | |
| if (!$isDuplicateTime) { | |
| for ($i = 0; $i < 12; $i++) { | |
| $this->$lastRandChars[$i] = random_int(0, 63); | |
| } | |
| } else { | |
| // If the timestamp hasn't changed since last push, use the same random number, except incremented by 1. | |
| for ($i = 11; $i >= 0 && $this->$lastRandChars[$i] === 63; $i--) { | |
| $this->$lastRandChars[$i] = 0; | |
| } | |
| $this->$lastRandChars[$i]++; | |
| } | |
| for ($i = 0; strlen($id) < 20; $i++) { | |
| $id .= substr(self::PUSH_CHARS, $this->$lastRandChars[$i], 1); | |
| } | |
| static::assert(strlen($id) === 20, 'Length should be 20.'); | |
| return $id; | |
| } | |
| /** | |
| * @param bool $condition | |
| * @param string $message | |
| */ | |
| private static function assert($condition, $message = '') | |
| { | |
| if ($condition !== true) { | |
| throw new RuntimeException($message); | |
| } | |
| } | |
| } |
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