Add TableGenerator Implementation

class TableGenerator extends AbstractIdGenerator
{
    private $_tableName;
    private $_sequenceName;
    private $_allocationSize;
    private $_nextValue;
    private $_maxValue;

    public function __construct($tableName, $sequenceName = 'default', $allocationSize = 10)
    {
        $this->_tableName = $tableName;
        $this->_sequenceName = $sequenceName;
        $this->_allocationSize = $allocationSize;
    }

    public function generate(EntityManager $em, $entity)
    {
        if ($this->_maxValue === null || $this->_nextValue == $this->_maxValue) {
            // Allocate new values
            $conn = $em->getConnection();
            if ($conn->getTransactionNestingLevel() == 0) {

                // use select for update
                $sql = $conn->getDatabasePlatform()->getTableHiLoCurrentValSql($this->_tableName, $this->_sequenceName);
                $currentLevel = $conn->fetchColumn($sql);
                if ($currentLevel != null) {
                    $this->_nextValue = $currentLevel;
                    $this->_maxValue = $this->_nextValue + $this->_allocationSize;

                    $updateSql = $conn->getDatabasePlatform()->getTableHiLoUpdateNextValSql(
                        $this->_tableName, $this->_sequenceName, $this->_allocationSize
                    );
                    
                    if ($conn->executeUpdate($updateSql, array(1 => $currentLevel, 2 => $currentLevel+1)) !== 1) {
                        // no affected rows, concurrency issue, throw exception
                    }
                } else {
                    // no current level returned, TableGenerator seems to be broken, throw exception
                }
            } else {
                // only table locks help here, implement this or throw exception?
                // or do we want to work with table locks exclusively?
            }
        }
        return $this->_nextValue++;
    }
}