Magento - How do I programmatically add tags to an existing product?

As the title says: How do I programatically add one or more tags to an existing product? I couldn't find anything useful about this topic on the internet, so every help, link or knowledge is welcome.

Thanks in advance!

Ok I found it by myself. I've copied and customized the saveAction() of the app/code/core/Mage/Tag/Controllers/IndexController.php and also some additional functions to get this thing to work.

require_once $_SERVER['DOCUMENT_ROOT'] . "/app/Mage.php";
    umask(0);
    Mage::app();
    Mage::app()->getTranslator()->init('frontend');
    Mage::getSingleton('core/session', array('name' => 'frontend'));


$customerSession = Mage::getSingleton('customer/session');

$tagName    = 'Urban Extraloud';//(string) $this->getRequest()->getQuery('productTagName');
$productId  = 257;//(int)$this->getRequest()->getParam('product');

if(strlen($tagName) && $productId) {
    $session = Mage::getSingleton('catalog/session');
    $product = Mage::getModel('catalog/product')
        ->load($productId);
    if(!$product->getId()){
        //$session->addError($this->__('Unable to save tag(s).'));
    } else {
        try {
            $customerId = 58; //$customerSession->getCustomerId();
            $storeId = Mage::app()->getStore()->getId();

            $tagNamesArr = _cleanTags(_extractTags($tagName));

            $counter = new Varien_Object(array(
                "new" => 0,
                "exist" => array(),
                "success" => array(),
                "recurrence" => array())
            );

            $tagModel = Mage::getModel('tag/tag');
            $tagRelationModel = Mage::getModel('tag/tag_relation');

            foreach ($tagNamesArr as $tagName) {
                $tagModel->unsetData()
                    ->loadByName($tagName)
                    ->setStoreId($storeId)
                    ->setName($tagName);

                $tagRelationModel->unsetData()
                    ->setStoreId($storeId)
                    ->setProductId($productId)
                    ->setCustomerId($customerId)
                    ->setActive(Mage_Tag_Model_Tag_Relation::STATUS_ACTIVE)
                    ->setCreatedAt( $tagRelationModel->getResource()->formatDate(time()) );

                if (!$tagModel->getId()) {
                    $tagModel->setFirstCustomerId($customerId)
                        ->setFirstStoreId($storeId)
                        ->setStatus($tagModel->getPendingStatus())
                        ->save();

                    $tagRelationModel->setTagId($tagModel->getId())->save();
                    $counter->setNew($counter->getNew() + 1);
                } else {
                    $tagStatus = $tagModel->getStatus();
                    $tagRelationModel->setTagId($tagModel->getId());

                    switch($tagStatus) {
                        case $tagModel->getApprovedStatus():
                            if(_checkLinkBetweenTagProduct($tagRelationModel)) {
                                $relation = _getLinkBetweenTagCustomerProduct($tagRelationModel, $tagModel);
                                if ($relation->getId()) {
                                    if (!$relation->getActive()) {
                                        $tagRelationModel
                                            ->setId($relation->getId())
                                            ->save();
                                    }
                                } else {
                                    $tagRelationModel->save();
                                }
                                $counter->setExist(array_merge($counter->getExist(), array($tagName)));
                            } else {
                                $tagRelationModel->save();
                                $counter->setSuccess(array_merge($counter->getSuccess(), array($tagName)));
                            }
                            break;
                        case $tagModel->getPendingStatus():
                            $relation = _getLinkBetweenTagCustomerProduct($tagRelationModel, $tagModel);
                            if ($relation->getId()) {
                                if (!$relation->getActive()) {
                                    $tagRelationModel
                                        ->setId($relation->getId())
                                        ->save();
                                }
                            } else {
                                $tagRelationModel->save();
                            }
                            $counter->setNew($counter->getNew() + 1);
                            break;
                        case $tagModel->getDisabledStatus():
                            if(_checkLinkBetweenTagCustomerProduct($tagRelationModel, $tagModel)) {
                                $counter->setRecurrence(array_merge($counter->getRecurrence(), array($tagName)));
                            } else {
                                $tagModel->setStatus($tagModel->getPendingStatus())->save();
                                $tagRelationModel->save();
                                $counter->setNew($counter->getNew() + 1);
                            }
                            break;
                    }
                }
            }

        } catch (Exception $e) {

            print 'Unable to save tag(s)';
        }
    }
}






function _extractTags($tagNamesInString)
{
    return explode("\n", preg_replace("/(\'(.*?)\')|(\s+)/i", "$1\n", $tagNamesInString));
}

/**
 * Clears the tag from the separating characters.
 *
 * @param array $tagNamesArr
 * @return array
 */
function _cleanTags(array $tagNamesArr)
{
    foreach( $tagNamesArr as $key => $tagName ) {
        $tagNamesArr[$key] = trim($tagNamesArr[$key], '\'');
        $tagNamesArr[$key] = trim($tagNamesArr[$key]);
        if( $tagNamesArr[$key] == '' ) {
            unset($tagNamesArr[$key]);
        }
    }
    return $tagNamesArr;
}


 /**
 * Checks whether the already marked this product in this store by this tag.
 *
 * @param Mage_Tag_Model_Tag_Relation $tagRelationModel
 * @return boolean
 */
function _checkLinkBetweenTagProduct($tagRelationModel)
{
    $customerId = $tagRelationModel->getCustomerId();
    $tagRelationModel->setCustomerId(null);
    $res = in_array($tagRelationModel->getProductId(), $tagRelationModel->getProductIds());
    $tagRelationModel->setCustomerId($customerId);
    return $res;
}


/**
 * Checks whether the already marked this product in this store by this tag and by this customer.
 *
 * @param Mage_Tag_Model_Tag_Relation $tagRelationModel
 * @param Mage_Tag_Model_Tag $tagModel
 * @return boolean
 */
function _checkLinkBetweenTagCustomerProduct($tagRelationModel, $tagModel)
{
    return (count(_getLinkBetweenTagCustomerProduct($tagRelationModel, $tagModel)
        ->getProductIds()) > 0);
}

/**
 * Get relation model for marked product in this store by this tag and by this customer.
 *
 * @param Mage_Tag_Model_Tag_Relation $tagRelationModel
 * @param Mage_Tag_Model_Tag $tagModel
 * @return Mage_Tag_Model_Tag_Relation
 */
function _getLinkBetweenTagCustomerProduct($tagRelationModel, $tagModel)
{
    return Mage::getModel('tag/tag_relation')->loadByTagCustomer(
        $tagRelationModel->getProductId(),
        $tagModel->getId(),
        $tagRelationModel->getCustomerId(),
        $tagRelationModel->getStoreId()
    );
}