/**
* Add sub filter.
*
* @param string $conjunction AND or OR.
* @param Filter $filter The sub filter.
*
* @return static
*/
private function sub($conjunction, Filter $filter)
{
if (!empty($this->expression)) {
$this->expression[] = $conjunction;
}
$expression = $filter->getExpression();
$variables = $filter->getVariables();
$index = count($this->variables);
$expression = str_replace('variables', 'variables[' . $index . ']', $expression);
$this->expression[] = '(' . $expression . ')';
$this->variables[] = $variables;
return $this;
}
