Cache.php

<?php

class Cache
{
    public function has(string $id): bool
    {
    }

    public function get(string $id): array
    {
    }

    public function set(string $id, array $values): bool
    {
    }
}

Client.php

<?php

// Libarary Code
class Client
{
    public function __construct(WebSocket $webSocket)
    {
    }

    public function on(string $event, Closure $closure): void
    {
    }

    public function send(string $message): void
    {
    }

    public function getId(): string
    {
    }
}

Controller.php

<?php

class Controller
{
    private $cache;

    /** @var CrawlRequest[] */
    private $queue = [];

    public function __construct()
    {
        $this->cache = new Cache();
        $this->crawler = new Crawler();
    }

    public function connect(WebSocket $webSocket)
    {
        $client = new Client($webSocket);
        $client->on('message', $this->getOnMessageClosure($client));
        $client->on('disconnect', $this->getOnDisconnectClosure($client));
    }

    /**
     * If a client closes the window remove all waiting crawling requests because the client
     * will probably not come back
     *
     * @param Client $client
     * @return Closure
     */
    private function getOnDisconnectClosure(Client $client): Closure
    {
        return function () use ($client) {
            foreach ($this->queue as $index => $crawlRequest) {
                if ($crawlRequest->getClient() === $client) {
                    unset($this->queue[$index]);
                }
            }
        };
    }

    private function getOnMessageClosure(Client $client): Closure
    {
        return function (string $message) use ($client) {
            $url = $message['url'];
            $forceRefresh = $message['force_refresh'] ?? false;

            $crawlRequest = new CrawlRequest($client, $url);

            // TODO: Check if the given URL is either 1. already cache or 2. already queued and act accordingly

            if (isset($this->queue[$crawlRequest->getId()])) {
                // TODO: figure out what to do if a client tries to request a crawl a second time.
                // Probably send back a "you're already enqueued, please be patient" message
            }

            if (!$this->cache->has($url) || $forceRefresh) {

                $this->queue[$crawlRequest->getId()] = $crawlRequest;

                while (true) {
                    $queuePosition = $this->getQueuePosition($crawlRequest);

                    if ($queuePosition > 1) {
                        // Tell client: You are on position X
                        $client->send(json_encode(['pos' => $queuePosition]));
                        sleep(5);
                    } else {
                        $url = $this->queue->dequeue();

                        // Crawl pages and send state to client over websocket
                        $results = $this->crawler->crawl($url, $client);

                        $this->cache->set($url, $results);
                    }
                }
            }
            $result = $this->cache->get($url);
            $client->send(json_encode($result));
        };
    }

    private function getQueuePosition(CrawlRequest $crawlRequest)
    {
        $count = 0;

        $searched = $crawlRequest->getId();
        foreach (array_keys($this->queue) as $id) {
            if ($id === $searched) {
                return $count;
            }
            $count++;
        }

        throw new \Exception('not found');
    }
}

CrawlRequest.php

<?php

class CrawlRequest
{
    private Client $client;

    private string $url;

    private string $id;

    public function __construct(Client $client, string $url)
    {
        $this->client = $client;
        $this->url = $url;

        $this->id = sha1(
            json_encode(
                [
                    $client->getId(),
                    $url,
                ]
            )
        );
    }

    // Returns a unique id for client/url combination
    public function getId(): string
    {
        return $this->id;
    }

    public function getClient(): Client
    {
        return $this->client;
    }

    public function getUrl(): string
    {
        return $this->url;
    }
}

Server.php

<?php

class Server
{
    private Controller $controller;

    private SocketIO $socketIO;

    public function __construct(SocketIO $socketIO)
    {
        $this->controller = new Controller();
        $this->socketIO = $socketIO;
    }

    public function run()
    {
        $this->socketIO->on(
            'connect',
            function (WebSocket $webSocket) {
                $this->controller->connect($webSocket);
            }
        );
    }
}

SocketIO.php

<?php

// Libarary Code
interface SocketIO
{
    public function on(string $event, Closure $closure): void;
}

WebSocket.php

<?php

// Libarary Code
interface WebSocket
{
}