private const INSTRUMENT_NAME = 'mobile-app-redirect';

public function __construct() {
    parent::__construct( 'MobileAppRedirect' );
}

public function execute( $subPage ) {
    $out = $this->getOutput();
    $config = $this->getConfig();
    $request = $this->getRequest();

    $ua = $request->getHeader( 'User-Agent' ) ?: '';
    $campaign = $request->getVal( 'campaign' );

    $iosUrl = $this->buildIOSUrl(
        $config->get( 'MobileAppAppleStoreUrl' ),
        $campaign
    );
    $androidUrl = $this->buildAndroidUrl(
        $config->get( 'MobileAppGooglePlayUrl' ),
        $campaign
    );

    if ( $this->isIOS( $ua ) ) {
        $this->logVisit( 'ios', $campaign, $iosUrl );
        $out->redirect( $iosUrl );
        return;
    }

    if ( $this->isAndroid( $ua ) ) {
        $this->logVisit( 'android', $campaign, $androidUrl );
        $out->redirect( $androidUrl );
        return;
    }

    // Fallback: show both links
    $this->logVisit( 'other', $campaign, null );
    $this->setHeaders();
    $out->addHTML(
        Html::element( 'a', [ 'href' => $iosUrl ], 'Download on the App Store' )
        . Html::element( 'br' )
        . Html::element( 'a', [ 'href' => $androidUrl ], 'Get it on Google Play' )
    );
}

/**
 * Log a visit event to product_metrics.web_base stream
 * via the Test Kitchen PHP SDK.
 *
 * @param string $platform 'ios', 'android', or 'other'
 * @param string|null $campaign Campaign ID from URL parameter
 * @param string|null $redirectUrl The store URL being redirected to, or null for fallback
 */
private function logVisit(
    string $platform,
    ?string $campaign,
    ?string $redirectUrl
): void {
    $action = $redirectUrl ? 'redirect' : 'fallback';

    $interactionData = [
        'action_source' => 'MobileAppRedirect',
        'action_context' => json_encode( [
            'platform' => $platform,
            'campaign' => $campaign ?? '',
            'redirect_url' => $redirectUrl ?? '',
        ] ),
    ];

    $instrumentManager = MediaWikiServices::getInstance()
        ->getService( 'TestKitchen.Sdk.InstrumentManager' );
    $instrument = $instrumentManager->getInstrument( self::INSTRUMENT_NAME );
    $instrument->send( $action, $interactionData );
}

/**
 * @param string $ua User-Agent header value
 * @return bool
 */
private function isIOS( string $ua ): bool {
    if ( preg_match( '/iPhone|iPad|iPod/i', $ua ) ) {
        return true;
    }

    // iPadOS 13+ reports as desktop Safari on macOS.
    // Match Macintosh + Safari without Chrome/Firefox as a heuristic.
    // Trade-off: real desktop Safari users will get the iOS redirect.
    if ( preg_match( '/Macintosh.*Mac OS X/i', $ua )
        && preg_match( '/Safari/i', $ua )
        && !preg_match( '/Chrome|Chromium|Firefox/i', $ua )
    ) {
        return true;
    }

    return false;
}

/**
 * @param string $ua User-Agent header value
 * @return bool
 */
private function isAndroid( string $ua ): bool {
    return (bool)preg_match( '/Android/i', $ua );
}

/**
 * @param string $baseUrl App Store base URL
 * @param string|null $campaign Campaign ID
 * @return string
 */
private function buildIOSUrl( string $baseUrl, ?string $campaign ): string {
    if ( $campaign !== null && $campaign !== '' ) {
        $baseUrl = wfAppendQuery( $baseUrl, [
            'ct' => $campaign,
        ] );
    }
    return $baseUrl;
}

/**
 * @param string $baseUrl Google Play base URL
 * @param string|null $campaign Campaign ID
 * @return string
 */
private function buildAndroidUrl( string $baseUrl, ?string $campaign ): string {
    if ( $campaign !== null && $campaign !== '' ) {
        $baseUrl = wfAppendQuery( $baseUrl, [
            'referrer' => 'utm_campaign=' . urlencode( $campaign ),
        ] );
    }
    return $baseUrl;
}

"SpecialPages": {
    "MobileAppRedirect": "SpecialMobileAppRedirect"
},
"AutoloadClasses": {
    "SpecialMobileAppRedirect": "includes/specials/SpecialMobileAppRedirect.php"
},
"config": {
    "MobileAppAppleStoreUrl": {
        "value": "https://apps.apple.com/app/your-app-id",
        "description": "URL for the iOS App Store listing"
    },
    "MobileAppGooglePlayUrl": {
        "value": "https://play.google.com/store/apps/details?id=your.package.name",
        "description": "URL for the Google Play Store listing"
    }
}

"mobileappredirect": "Mobile App Redirect"