<?php

/**
 * Einfacher DB-Cache für teure Aggregations-Queries (Dashboard, Reports).
 *
 * Verwendung:
 *   $kpis = ReportCache::remember('dashboard.kpis', 300, fn() => $reportRepo->kpis());
 *
 * Garbage-Collection: läuft mit kleiner Wahrscheinlichkeit (1 %) bei jedem
 * Aufruf — kein eigener Cron nötig.
 */
class ReportCache
{
    private PDO $pdo;
    private static ?ReportCache $instance = null;

    public function __construct(Database $db)
    {
        $this->pdo = $db->getConnection();
    }

    public static function bind(ReportCache $instance): void
    {
        self::$instance = $instance;
    }

    /**
     * Liefert den gecachten Wert oder ruft $producer() auf und speichert das Ergebnis.
     * Bei Cache-Fehlern wird $producer() ausgeführt — die Anwendung bricht nie ab.
     */
    public static function remember(string $key, int $ttlSeconds, callable $producer)
    {
        if (self::$instance === null) {
            return $producer();
        }
        return self::$instance->doRemember($key, $ttlSeconds, $producer);
    }

    public static function forget(string $key): void
    {
        self::$instance?->doForget($key);
    }

    public static function flush(): void
    {
        self::$instance?->doFlush();
    }

    private function doRemember(string $key, int $ttlSeconds, callable $producer)
    {
        $this->gcMaybe();
        try {
            $stmt = $this->pdo->prepare(
                "SELECT cache_value FROM report_cache
                  WHERE cache_key = :k AND expires_at > NOW() LIMIT 1"
            );
            $stmt->execute(['k' => $key]);
            $val = $stmt->fetchColumn();
            if ($val !== false) {
                $decoded = json_decode((string)$val, true);
                if ($decoded !== null || $val === 'null') return $decoded;
            }
        } catch (Throwable) {
            return $producer();
        }

        $value = $producer();
        try {
            $stmt = $this->pdo->prepare(
                "INSERT INTO report_cache (cache_key, cache_value, expires_at)
                 VALUES (:k, :v, DATE_ADD(NOW(), INTERVAL :ttl SECOND))
                 ON DUPLICATE KEY UPDATE
                     cache_value = VALUES(cache_value),
                     expires_at  = VALUES(expires_at)"
            );
            $stmt->execute([
                'k'   => $key,
                'v'   => json_encode($value, JSON_UNESCAPED_UNICODE),
                'ttl' => max(1, $ttlSeconds),
            ]);
        } catch (Throwable $e) {
            error_log('ReportCache write ' . $key . ': ' . $e->getMessage());
        }
        return $value;
    }

    private function doForget(string $key): void
    {
        try {
            $stmt = $this->pdo->prepare("DELETE FROM report_cache WHERE cache_key = :k");
            $stmt->execute(['k' => $key]);
        } catch (Throwable) { /* selbstheilend */ }
    }

    private function doFlush(): void
    {
        try { $this->pdo->exec("DELETE FROM report_cache"); }
        catch (Throwable) { /* selbstheilend */ }
    }

    private function gcMaybe(): void
    {
        // Mit 1 % Wahrscheinlichkeit abgelaufene Einträge wegräumen.
        if (random_int(0, 99) !== 0) return;
        try { $this->pdo->exec("DELETE FROM report_cache WHERE expires_at <= NOW()"); }
        catch (Throwable) { /* selbstheilend */ }
    }
}