Rate Limiter - Token Bucket Algorithm

class TokenBucketRateLimiter {
    private int $capacity;
    private float $refillRate;
    private array $buckets = [];
    private $storage; // Redis or Cache

    public function __construct(int $capacity, float $refillRate, $storage = null) {
        $this->capacity = $capacity;
        $this->refillRate = $refillRate; // tokens per second
        $this->storage = $storage;
    }

    public function isAllowed(string $key, int $tokens = 1): bool {
        $bucket = $this->getBucket($key);
        $now = microtime(true);

        // Refill tokens
        $tokensToAdd = ($now - $bucket['lastRefill']) * $this->refillRate;
        $bucket['tokens'] = min(
            $this->capacity,
            $bucket['tokens'] + $tokensToAdd
        );
        $bucket['lastRefill'] = $now;

        // Check if enough tokens
        if ($bucket['tokens'] >= $tokens) {
            $bucket['tokens'] -= $tokens;
            $this->saveBucket($key, $bucket);
            return true;
        }

        $this->saveBucket($key, $bucket);
        return false;
    }

    private function getBucket(string $key): array {
        if ($this->storage) {
            $data = $this->storage->get("rate_limit:{$key}");
            return $data ? json_decode($data, true) : $this->newBucket();
        }

        return $this->buckets[$key] ?? $this->newBucket();
    }

    private function newBucket(): array {
        return [
            'tokens' => $this->capacity,
            'lastRefill' => microtime(true)
        ];
    }

    private function saveBucket(string $key, array $bucket): void {
        if ($this->storage) {
            $this->storage->setex(
                "rate_limit:{$key}",
                3600,
                json_encode($bucket)
            );
        } else {
            $this->buckets[$key] = $bucket;
        }
    }

    public function getRemaining(string $key): int {
        $bucket = $this->getBucket($key);
        return (int) floor($bucket['tokens']);
    }
}

// Usage
$limiter = new TokenBucketRateLimiter(100, 10); // 100 tokens, 10/sec refill
if ($limiter->isAllowed('user:123', 5)) {
    // Process request
} else {
    // Rate limit exceeded
}