public function getCalculationFields(array $fieldData)
    {
        $output = array();

        // Perform automatic calculation
        if (isset($this->_fieldDefinition['gtf_calculate_using'])) {
            $sources = explode(self::FIELD_SEP, $this->_fieldDefinition['gtf_calculate_using']);

            foreach ($sources as $source) {
                if (isset($fieldData[$source]) && $fieldData[$source]) {
                    $output[$source] = $fieldData[$source];
                } else {
                    $output[$source] = null;
                }
            }
        }
        return array_reverse($output, true);
    }