private static function getFeedItemIdsForCampaignFeed($campaignFeed)
    {
        $feedItemIds = [];

        if ($campaignFeed->getMatchingFunction()->getOperator() === FunctionOperator::IN) {
            // Check if matchingFunction is of the form IN(FEED_ITEM_ID,{xxx,xxx}).
            // Extract feed items if applicable.
            $feedItemIds = array_merge(
                $feedItemIds,
                self::getFeedItemIdsFromArgument($campaignFeed->getMatchingFunction())
            );
        } elseif ($campaignFeed->getMatchingFunction()->getOperator() === FunctionOperator::AND_VALUE) {
            foreach ($campaignFeed->getMatchingFunction()->getLhsOperand() as $argument) {
                // Check if matchingFunction is of the form IN(FEED_ITEM_ID,{xxx,xxx}).
                // Extract feed items if applicable.
                if ($argument instanceof FunctionOperand) {
                    if ($argument->getValue()->getOperator() === FunctionOperator::IN) {
                        $feedItemIds = array_merge(
                            $feedItemIds,
                            self::getFeedItemIdsFromArgument($argument->getValue())
                        );
                    }
                }
            }
        }

        return $feedItemIds;
    }