k( $comments, 'comment_ID' ),
		);

		$key          = md5( serialize( wp_array_slice_assoc( $this->query_vars, array_keys( $this->query_var_defaults ) ) ) );
		$last_changed = wp_cache_get_last_changed( 'comment' );

		// Fetch an entire level of the descendant tree at a time.
		$level        = 0;
		$exclude_keys = array( 'parent', 'parent__in', 'parent__not_in' );
		do {
			// Parent-child relationships may be cached. Only query for those that are not.
			$child_ids           = array();
			$uncached_parent_ids = array();
			$_parent_ids         = $levels[ $level ];
			if ( $_parent_ids ) {
				$cache_keys = array();
				foreach ( $_parent_ids as $parent_id ) {
					$cache_keys[ $parent_id ] = "get_comment_child_ids:$parent_id:$key:$last_changed";
				}
				$cache_data = wp_cache_get_multiple( array_values( $cache_keys ), 'comment-queries' );
				foreach ( $_parent_ids as $parent_id ) {
					$parent_child_ids = $cache_data[ $cache_keys[ $parent_id ] ];
					if ( false !== $parent_child_ids ) {
						$child_ids = array_merge( $child_ids, $parent_child_ids );
					} else {
						$uncached_parent_ids[] = $parent_id;
					}
				}
			}

			if ( $uncached_parent_ids ) {
				// Fetch this level of comments.
				$parent_query_args = $this->query_vars;
				foreach ( $exclude_keys as $exclude_key ) {
					$parent_query_args[ $exclude_key ] = '';
				}
				$parent_query_args['parent__in']    = $uncached_parent_ids;
				$parent_query_args['no_found_rows'] = true;
				$parent_query_args['hierarchical']  = false;
				$parent_query_args['offset']        = 0;
				$parent_query_args['number']        = 0;

				$level_comments = get_comments( $parent_query_args );

				// Cache parent-child relationships.
				$parent_map = array_fill_keys( $uncached_parent_ids, array() );
				foreach ( $level_comments as $level_comment ) {
					$parent_map[ $level_comment->comment_parent ][] = $level_comment->comment_ID;
					$child_ids[]                                    = $level_comment->comment_ID;
				}

				$data = array();
				foreach ( $parent_map as $parent_id => $children ) {
					$cache_key          = "get_comment_child_ids:$parent_id:$key:$last_changed";
					$data[ $cache_key ] = $children;
				}
				wp_cache_set_multiple( $data, 'comment-queries' );
			}

			++$level;
			$levels[ $level ] = $child_ids;
		} while ( $child_ids );

		// Prime comment caches for non-top-level comments.
		$descendant_ids = array();
		for ( $i = 1, $c = count( $levels ); $i < $c; $i++ ) {
			$descendant_ids = array_merge( $descendant_ids, $levels[ $i ] );
		}

		_prime_comment_caches( $descendant_ids, $this->query_vars['update_comment_meta_cache'] );

		// Assemble a flat array of all comments + descendants.
		$all_comments = $comments;
		foreach ( $descendant_ids as $descendant_id ) {
			$all_comments[] = get_comment( $descendant_id );
		}

		// If a threaded representation was requested, build the tree.
		if ( 'threaded' === $this->query_vars['hierarchical'] ) {
			$threaded_comments = array();
			$ref               = array();
			foreach ( $all_comments as $k => $c ) {
				$_c = get_comment( $c->comment_ID );

				// If the comment isn't in the reference array, it goes in the top level of the thread.
				if ( ! isset( $ref[ $c->comment_parent ] ) ) {
					$threaded_comments[ $_c->comment_ID ] = $_c;
					$ref[ $_c->comment_ID ]               = $threaded_comments[ $_c->comment_ID ];

					// Otherwise, set it as a child of its parent.
				} else {

					$ref[ $_c->comment_parent ]->add_child( $_c );
					$ref[ $_c->comment_ID ] = $ref[ $_c->comment_parent ]->get_child( $_c->comment_ID );
				}
			}

			// Set the 'populated_children' flag, to ensure additional database queries aren't run.
			foreach ( $ref as $_ref ) {
				$_ref->populated_children( true );
			}

			$comments = $threaded_comments;
		} else {
			$comments = $all_comments;
		}

		return $comments;
	}

	/**
	 * Used internally to generate an SQL string for searching across multiple columns.
	 *
	 * @since 3.1.0
	 *
	 * @global wpdb $wpdb WordPress database abstraction object.
	 *
	 * @param string   $search  Search string.
	 * @param string[] $columns Array of columns to search.
	 * @return string Search SQL.
	 */
	protected function get_search_sql( $search, $columns ) {
		global $wpdb;

		$like = '%' . $wpdb->esc_like( $search ) . '%';

		$searches = array();
		foreach ( $columns as $column ) {
			$searches[] = $wpdb->prepare( "$column LIKE %s", $like );
		}

		return ' AND (' . implode( ' OR ', $searches ) . ')';
	}

	/**
	 * Parse and sanitize 'orderby' keys passed to the comment query.
	 *
	 * @since 4.2.0
	 *
	 * @global wpdb $wpdb WordPress database abstraction object.
	 *
	 * @param string $orderby Alias for the field to order by.
	 * @return string|false Value to used in the ORDER clause. False otherwise.
	 */
	protected function parse_orderby( $orderby ) {
		global $wpdb;

		$allowed_keys = array(
			'comment_agent',
			'comment_approved',
			'comment_author',
			'comment_author_email',
			'comment_author_IP',
			'comment_author_url',
			'comment_content',
			'comment_date',
			'comment_date_gmt',
			'comment_ID',
			'comment_karma',
			'comment_parent',
			'comment_post_ID',
			'comment_type',
			'user_id',
		);

		if ( ! empty( $this->query_vars['meta_key'] ) ) {
			$allowed_keys[] = $this->query_vars['meta_key'];
			$allowed_keys[] = 'meta_value';
			$allowed_keys[] = 'meta_value_num';
		}

		$meta_query_clauses = $this->meta_query->get_clauses();
		if ( $meta_query_clauses ) {
			$allowed_keys = array_merge( $allowed_keys, array_keys( $meta_query_clauses ) );
		}

		$parsed = false;
		if ( $this->query_vars['meta_key'] === $orderby || 'meta_value' === $orderby ) {
			$parsed = "$wpdb->commentmeta.meta_value";
		} elseif ( 'meta_value_num' === $orderby ) {
			$parsed = "$wpdb->commentmeta.meta_value+0";
		} elseif ( 'comment__in' === $orderby ) {
			$comment__in = implode( ',', array_map( 'absint', $this->query_vars['comment__in'] ) );
			$parsed      = "FIELD( {$wpdb->comments}.comment_ID, $comment__in )";
		} elseif ( in_array( $orderby, $allowed_keys, true ) ) {

			if ( isset( $meta_query_clauses[ $orderby ] ) ) {
				$meta_clause = $meta_query_clauses[ $orderby ];
				$parsed      = sprintf( 'CAST(%s.meta_value AS %s)', esc_sql( $meta_clause['alias'] ), esc_sql( $meta_clause['cast'] ) );
			} else {
				$parsed = "$wpdb->comments.$orderby";
			}
		}

		return $parsed;
	}

	/**
	 * Parse an 'order' query variable and cast it to ASC or DESC as necessary.
	 *
	 * @since 4.2.0
	 *
	 * @param string $order The 'order' query variable.
	 * @return string The sanitized 'order' query variable.
	 */
	protected function parse_order( $order ) {
		if ( ! is_string( $order ) || empty( $order ) ) {
			return 'DESC';
		}

		if ( 'ASC' === strtoupper( $order ) ) {
			return 'ASC';
		} else {
			return 'DESC';
		}
	}
}