public static function get_activity_comments( $activity_id, $left, $right, $spam = 'ham_only', $top_level_parent_id = 0 ) {
		global $wpdb;

		$function_args = func_get_args();

		if ( empty( $top_level_parent_id ) ) {
			$top_level_parent_id = $activity_id;
		}

		$comments = wp_cache_get( $activity_id, 'bp_activity_comments' );

		// We store the string 'none' to cache the fact that the
		// activity item has no comments.
		if ( 'none' === $comments ) {
			$comments = false;

			// A true cache miss.
		} elseif ( empty( $comments ) ) {

			$bp = buddypress();

			// Select the user's fullname with the query.
			if ( bp_is_active( 'xprofile' ) ) {
				$fullname_select = ", pd.value as user_fullname";
				$fullname_from = ", {$bp->profile->table_name_data} pd ";
				$fullname_where = "AND pd.user_id = a.user_id AND pd.field_id = 1";

				// Prevent debug errors.
			} else {
				$fullname_select = $fullname_from = $fullname_where = '';
			}

			// Don't retrieve activity comments marked as spam.
			if ( 'ham_only' == $spam ) {
				$spam_sql = 'AND a.is_spam = 0';
			} elseif ( 'spam_only' == $spam ) {
				$spam_sql = 'AND a.is_spam = 1';
			} else {
				$spam_sql = '';
			}

			// Legacy query - not recommended.

			/**
			 * Filters if BuddyPress should use the legacy activity query.
			 *
			 * @since 2.0.0
			 *
			 * @param bool                 $value     Whether or not to use the legacy query.
			 * @param BP_Activity_Activity $value     Magic method referring to currently called method.
			 * @param array                $func_args Array of the method's argument list.
			 */
			if ( apply_filters( 'bp_use_legacy_activity_query', false, __METHOD__, $function_args ) ) {

				/**
				 * Filters the MySQL prepared statement for the legacy activity query.
				 *
				 * @since 1.5.0
				 *
				 * @param string $value       Prepared statement for the activity query.
				 * @param int    $activity_id Activity ID to fetch comments for.
				 * @param int    $left        Left-most node boundary.
				 * @param int    $right       Right-most node boundary.
				 * @param string $spam_sql    SQL Statement portion to differentiate between ham or spam.
				 */
				$sql = apply_filters( 'bp_activity_comments_user_join_filter', $wpdb->prepare( "SELECT a.*, u.user_email, u.user_nicename, u.user_login, u.display_name{$fullname_select} FROM {$bp->activity->table_name} a, {$wpdb->users} u{$fullname_from} WHERE u.ID = a.user_id {$fullname_where} AND a.type = 'activity_comment' {$spam_sql} AND a.item_id = %d AND a.mptt_left > %d AND a.mptt_left < %d ORDER BY a.date_recorded ASC", $top_level_parent_id, $left, $right ), $activity_id, $left, $right, $spam_sql );

				$descendants = $wpdb->get_results( $sql );

				// We use the mptt BETWEEN clause to limit returned
				// descendants to the correct part of the tree.
			} else {
				$sql = $wpdb->prepare( "SELECT id FROM {$bp->activity->table_name} a WHERE a.type = 'activity_comment' {$spam_sql} AND a.item_id = %d and a.mptt_left > %d AND a.mptt_left < %d ORDER BY a.date_recorded ASC", $top_level_parent_id, $left, $right );

				$descendant_ids = $wpdb->get_col( $sql );
				$descendants    = self::get_activity_data( $descendant_ids );
				$descendants    = self::append_user_fullnames( $descendants );
				$descendants    = self::generate_action_strings( $descendants );
			}

			$ref = array();

			// Loop descendants and build an assoc array.
			foreach ( (array) $descendants as $d ) {
				$d->children = array();

				// If we have a reference on the parent.
				if ( isset( $ref[ $d->secondary_item_id ] ) ) {
					$ref[ $d->secondary_item_id ]->children[ $d->id ] = $d;
					$ref[ $d->id ] =& $ref[ $d->secondary_item_id ]->children[ $d->id ];

					// If we don't have a reference on the parent, put in the root level.
				} else {
					$comments[ $d->id ] = $d;
					$ref[ $d->id ] =& $comments[ $d->id ];
				}
			}

			// Calculate depth for each item.
			foreach ( $ref as &$r ) {
				$depth = 1;
				$parent_id = $r->secondary_item_id;

				while ( $parent_id !== $r->item_id ) {
					$depth++;

					// When display_comments=stream, the parent comment may not be part of the
					// returned results, so we manually fetch it.
					if ( empty( $ref[ $parent_id ] ) ) {
						$direct_parent = new BP_Activity_Activity( $parent_id );
						if ( isset( $direct_parent->secondary_item_id ) ) {
							// If the direct parent is not an activity update, that means we've reached
							// the parent activity item (eg. new_blog_post).
							if ( 'activity_update' !== $direct_parent->type ) {
								$parent_id = $r->item_id;

							} else {
								$parent_id = $direct_parent->secondary_item_id;
							}

						} else {
							// Something went wrong.  Short-circuit the depth calculation.
							$parent_id = $r->item_id;
						}
					} else {
						$parent_id = $ref[ $parent_id ]->secondary_item_id;
					}
				}
				$r->depth = $depth;
			}

			// If we cache a value of false, it'll count as a cache
			// miss the next time the activity comments are fetched.
			// Storing the string 'none' is a hack workaround to
			// avoid unnecessary queries.
			if ( false === $comments ) {
				$cache_value = 'none';
			} else {
				$cache_value = $comments;
			}

			wp_cache_set( $activity_id, $cache_value, 'bp_activity_comments' );
		}

		return $comments;
	}