etrieves the current comment author for use in the feeds.
 *
 * @since 2.0.0
 *
 * @return string Comment Author.
 */
function get_comment_author_rss() {
	/**
	 * Filters the current comment author for use in a feed.
	 *
	 * @since 1.5.0
	 *
	 * @see get_comment_author()
	 *
	 * @param string $comment_author The current comment author.
	 */
	return apply_filters( 'comment_author_rss', get_comment_author() );
}

/**
 * Displays the current comment author in the feed.
 *
 * @since 1.0.0
 */
function comment_author_rss() {
	echo get_comment_author_rss();
}

/**
 * Displays the current comment content for use in the feeds.
 *
 * @since 1.0.0
 */
function comment_text_rss() {
	$comment_text = get_comment_text();
	/**
	 * Filters the current comment content for use in a feed.
	 *
	 * @since 1.5.0
	 *
	 * @param string $comment_text The content of the current comment.
	 */
	$comment_text = apply_filters( 'comment_text_rss', $comment_text );
	echo $comment_text;
}

/**
 * Retrieves all of the post categories, formatted for use in feeds.
 *
 * All of the categories for the current post in the feed loop, will be
 * retrieved and have feed markup added, so that they can easily be added to the
 * RSS2, Atom, or RSS1 and RSS0.91 RDF feeds.
 *
 * @since 2.1.0
 *
 * @param string $type Optional, default is the type returned by get_default_feed().
 * @return string All of the post categories for displaying in the feed.
 */
function get_the_category_rss( $type = null ) {
	if ( empty( $type ) ) {
		$type = get_default_feed();
	}
	$categories = get_the_category();
	$tags       = get_the_tags();
	$the_list   = '';
	$cat_names  = array();

	$filter = 'rss';
	if ( 'atom' === $type ) {
		$filter = 'raw';
	}

	if ( ! empty( $categories ) ) {
		foreach ( (array) $categories as $category ) {
			$cat_names[] = sanitize_term_field( 'name', $category->name, $category->term_id, 'category', $filter );
		}
	}

	if ( ! empty( $tags ) ) {
		foreach ( (array) $tags as $tag ) {
			$cat_names[] = sanitize_term_field( 'name', $tag->name, $tag->term_id, 'post_tag', $filter );
		}
	}

	$cat_names = array_unique( $cat_names );

	foreach ( $cat_names as $cat_name ) {
		if ( 'rdf' === $type ) {
			$the_list .= "\t\t<dc:subject><![CDATA[$cat_name]]></dc:subject>\n";
		} elseif ( 'atom' === $type ) {
			$the_list .= sprintf( '<category scheme="%1$s" term="%2$s" />', esc_attr( get_bloginfo_rss( 'url' ) ), esc_attr( $cat_name ) );
		} else {
			$the_list .= "\t\t<category><![CDATA[" . html_entity_decode( $cat_name, ENT_COMPAT, get_option( 'blog_charset' ) ) . "]]></category>\n";
		}
	}

	/**
	 * Filters all of the post categories for display in a feed.
	 *
	 * @since 1.2.0
	 *
	 * @param string $the_list All of the RSS post categories.
	 * @param string $type     Type of feed. Possible values include 'rss2', 'atom'.
	 *                         Default 'rss2'.
	 */
	return apply_filters( 'the_category_rss', $the_list, $type );
}

/**
 * Displays the post categories in the feed.
 *
 * @since 0.71
 *
 * @see get_the_category_rss() For better explanation.
 *
 * @param string $type Optional, default is the type returned by get_default_feed().
 */
function the_category_rss( $type = null ) {
	echo get_the_category_rss( $type );
}

/**
 * Displays the HTML type based on the blog setting.
 *
 * The two possible values are either 'xhtml' or 'html'.
 *
 * @since 2.2.0
 */
function html_type_rss() {
	$type = get_bloginfo( 'html_type' );
	if ( strpos( $type, 'xhtml' ) !== false ) {
		$type = 'xhtml';
	} else {
		$type = 'html';
	}
	echo $type;
}

/**
 * Displays the rss enclosure for the current post.
 *
 * Uses the global $post to check whether the post requires a password and if
 * the user has the password for the post. If not then it will return before
 * displaying.
 *
 * Also uses the function get_post_custom() to get the post's 'enclosure'
 * metadata field and parses the value to display the enclosure(s). The
 * enclosure(s) consist of enclosure HTML tag(s) with a URI and other
 * attributes.
 *
 * @since 1.5.0
 */
function rss_enclosure() {
	if ( post_password_required() ) {
		return;
	}

	foreach ( (array) get_post_custom() as $key => $val ) {
		if ( 'enclosure' === $key ) {
			foreach ( (array) $val as $enc ) {
				$enclosure = explode( "\n", $enc );

				// Only get the first element, e.g. 'audio/mpeg' from 'audio/mpeg mpga mp2 mp3'.
				$t    = preg_split( '/[ \t]/', trim( $enclosure[2] ) );
				$type = $t[0];

				/**
				 * Filters the RSS enclosure HTML link tag for the current post.
				 *
				 * @since 2.2.0
				 *
				 * @param string $html_link_tag The HTML link tag with a URI and other attributes.
				 */
				echo apply_filters( 'rss_enclosure', '<enclosure url="' . esc_url( trim( $enclosure[0] ) ) . '" length="' . absint( trim( $enclosure[1] ) ) . '" type="' . esc_attr( $type ) . '" />' . "\n" );
			}
		}
	}
}

/**
 * Displays the atom enclosure for the current post.
 *
 * Uses the global $post to check whether the post requires a password and if
 * the user has the password for the post. If not then it will return before
 * displaying.
 *
 * Also uses the function get_post_custom() to get the post's 'enclosure'
 * metadata field and parses the value to display the enclosure(s). The
 * enclosure(s) consist of link HTML tag(s) with a URI and other attributes.
 *
 * @since 2.2.0
 */
function atom_enclosure() {
	if ( post_password_required() ) {
		return;
	}

	foreach ( (array) get_post_custom() as $key => $val ) {
		if ( 'enclosure' === $key ) {
			foreach ( (array) $val as $enc ) {
				$enclosure = explode( "\n", $enc );

				$url    = '';
				$type   = '';
				$length = 0;

				$mimes = get_allowed_mime_types();

				// Parse URL.
				if ( isset( $enclosure[0] ) && is_string( $enclosure[0] ) ) {
					$url = trim( $enclosure[0] );
				}

				// Parse length and type.
				for ( $i = 1; $i <= 2; $i++ ) {
					if ( isset( $enclosure[ $i ] ) ) {
						if ( is_numeric( $enclosure[ $i ] ) ) {
							$length = trim( $enclosure[ $i ] );
						} elseif ( in_array( $enclosure[ $i ], $mimes, true ) ) {
							$type = trim( $enclosure[ $i ] );
						}
					}
				}

				$html_link_tag = sprintf(
					"<link href=\"%s\" rel=\"enclosure\" length=\"%d\" type=\"%s\" />\n",
					esc_url( $url ),
					esc_attr( $length ),
					esc_attr( $type )
				);

				/**
				 * Filters the atom enclosure HTML link tag for the current post.
				 *
				 * @since 2.2.0
				 *
				 * @param string $html_link_tag The HTML link tag with a URI and other attributes.
				 */
				echo apply_filters( 'atom_enclosure', $html_link_tag );
			}
		}
	}
}

/**
 * Determines the type of a string of data with the data formatted.
 *
 * Tell whether the type is text, HTML, or XHTML, per RFC 4287 section 3.1.
 *
 * In the case of WordPress, text is defined as containing no markup,
 * XHTML is defined as "well formed", and HTML as tag soup (i.e., the rest).
 *
 * Container div tags are added to XHTML values, per section 3.1.1.3.
 *
 * @link http://www.atomenabled.org/developers/syndication/atom-format-spec.php#rfc.section.3.1
 *
 * @since 2.5.0
 *
 * @param string $data Input string.
 * @return array array(type, value)
 */
function prep_atom_text_construct( $data ) {
	if ( strpos( $data, '<' ) === false && strpos( $data, '&' ) === false ) {
		return array( 'text', $data );
	}

	if ( ! function_exists( 'xml_parser_create' ) ) {
		trigger_error( __( "PHP's XML extension is not available. Please contact your hosting provider to enable PHP's XML extension." ) );

		return array( 'html', "<![CDATA[$data]]>" );
	}

	$parser = xml_parser_create();
	xml_parse( $parser, '<div>' . $data . '</div>', true );
	$code = xml_get_error_code( $parser );
	xml_parser_free( $parser );
	unset( $parser );

	if ( ! $code ) {
		if ( strpos( $data, '<' ) === false ) {
			return array( 'text', $data );
		} else {
			$data = "<div xmlns='http://www.w3.org/1999/xhtml'>$data</div>";
			return array( 'xhtml', $data );
		}
	}

	if ( strpos( $data, ']]>' ) === false ) {
		return array( 'html', "<![CDATA[$data]]>" );
	} else {
		return array( 'html', htmlspecialchars( $data ) );
	}
}

/**
 * Displays Site Icon in atom feeds.
 *
 * @since 4.3.0
 *
 * @see get_site_icon_url()
 */
function atom_site_icon() {
	$url = get_site_icon_url( 32 );
	if ( $url ) {
		echo '<icon>' . convert_chars( $url ) . "</icon>\n";
	}
}

/**
 * Displays Site Icon in RSS2.
 *
 * @since 4.3.0
 */
function rss2_site_icon() {
	$rss_title = get_wp_title_rss();
	if ( empty( $rss_title ) ) {
		$rss_title = get_bloginfo_rss( 'name' );
	}

	$url = get_site_icon_url( 32 );
	if ( $url ) {
		echo '
<image>
	<url>' . convert_chars( $url ) . '</url>
	<title>' . $rss_title . '</title>
	<link>' . get_bloginfo_rss( 'url' ) . '</link>
	<width>32</width>
	<height>32</height>
</image> ' . "\n";
	}
}

/**
 * Returns the link for the currently displayed feed.
 *
 * @since 5.3.0
 *
 * @return string Correct link for the atom:self element.
 */
function get_self_link() {
	$host = parse_url( home_url() );
	return set_url_scheme( 'http://' . $host['host'] . wp_unslash( $_SERVER['REQUEST_URI'] ) );
}

/**
 * Displays the link for the currently displayed feed in a XSS safe way.
 *
 * Generate a correct link for the atom:self element.
 *
 * @since 2.5.0
 */
function self_link() {
	/**
	 * Filters the current feed URL.
	 *
	 * @since 3.6.0
	 *
	 * @see set_url_scheme()
	 * @see wp_unslash()
	 *
	 * @param string $feed_link The link for the feed with set URL scheme.
	 */
	echo esc_url( apply_filters( 'self_link', get_self_link() ) );
}

/**
 * Gets the UTC time of the most recently modified post from WP_Query.
 *
 * If viewing a comment feed, the time of the most recently modified
 * comment will be returned.
 *
 * @global WP_Query $wp_query WordPress Query object.
 *
 * @since 5.2.0
 *
 * @param string $format Date format string to return the time in.
 * @return string|false The time in requested format, or false on failure.
 */
function get_feed_build_date( $format ) {
	global $wp_query;

	$datetime          = false;
	$max_modified_time = false;
	$utc               = new DateTimeZone( 'UTC' );

	if ( ! empty( $wp_query ) && $wp_query->have_posts() ) {
		// Extract the post modified times from the posts.
		$modified_times = wp_list_pluck( $wp_query->posts, 'post_modified_gmt' );

		// If this is a comment feed, check those objects too.
		if ( $wp_query->is_comment_feed() && $wp_query->comment_count ) {
			// Extract the comment modified times from the comments.
			$comment_times = wp_list_pluck( $wp_query->comments, 'comment_date_gmt' );

			// Add the comment times to the post times for comparison.
			$modified_times = array_merge( $modified_times, $comment_times );
		}

		// Determine the maximum modified time.
		$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', max( $modified_times ), $utc );
	}

	if ( false === $datetime ) {
		// Fall back to last time any post was modified or published.
		$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', get_lastpostmodified( 'GMT' ), $utc );
	}

	if ( false !== $datetime ) {
		$max_modified_time = $datetime->format( $format );
	}

	/**
	 * Filters the date the last post or comment in the query was modified.
	 *
	 * @since 5.2.0
	 *
	 * @param string|false $max_modified_time Date the last post or comment was modified in the query, in UTC.
	 *                                        False on failure.
	 * @param string       $format            The date format requested in get_feed_build_date().
	 */
	return apply_filters( 'get_feed_build_date', $max_modified_time, $format );
}

/**
 * Returns the content type for specified feed type.
 *
 * @since 2.8.0
 *
 * @param string $type Type of feed. Possible values include 'rss', rss2', 'atom', and 'rdf'.
 */
function feed_content_type( $type = '' ) {
	if ( empty( $type ) ) {
		$type = get_default_feed();
	}

	$types = array(
		'rss'      => 'application/rss+xml',
		'rss2'     => 'application/rss+xml',
		'rss-http' => 'text/xml',
		'atom'     => 'application/atom+xml',
		'rdf'      => 'application/rdf+xml',
	);

	$content_type = ( ! empty( $types[ $type ] ) ) ? $types[ $type ] : 'application/octet-stream';

	/**
	 * Filters the content type for a specific feed type.
	 *
	 * @since 2.8.0
	 *
	 * @param string $content_type Content type indicating the type of data that a feed contains.
	 * @param string $type         Type of feed. Possible values include 'rss', rss2', 'atom', and 'rdf'.
	 */
	return apply_filters( 'feed_content_type', $content_type, $type );
}

/**
 * Builds SimplePie object based on RSS or Atom feed from URL.
 *
 * @since 2.8.0
 *
 * @param string|string[] $url URL of feed to retrieve. If an array of URLs, the feeds are merged
 *                             using SimplePie's multifeed feature.
 *                             See also {@link http://simplepie.org/wiki/faq/typical_multifeed_gotchas}
 * @return SimplePie|WP_Error SimplePie object on success or WP_Error object on failure.
 */
function fetch_feed( $url ) {
	if ( ! class_exists( 'SimplePie', false ) ) {
		require_once ABSPATH . WPINC . '/class-simplepie.php';
	}

	require_once ABSPATH . WPINC . '/class-wp-feed-cache-transient.php';
	require_once ABSPATH . WPINC . '/class-wp-simplepie-file.php';
	require_once ABSPATH . WPINC . '/class-wp-simplepie-sanitize-kses.php';

	$feed = new SimplePie();

	$feed->set_sanitize_class( 'WP_SimplePie_Sanitize_KSES' );
	// We must manually overwrite $feed->sanitize because SimplePie's constructor
	// sets it before we have a chance to set the sanitization class.
	$feed->sanitize = new WP_SimplePie_Sanitize_KSES();

	// Register the cache handler using the recommended method for SimplePie 1.3 or later.
	if ( method_exists( 'SimplePie_Cache', 'register' ) ) {
		SimplePie_Cache::register( 'wp_transient', 'WP_Feed_Cache_Transient' );
		$feed->set_cache_location( 'wp_transient' );
	} else {
		// Back-compat for SimplePie 1.2.x.
		require_once ABSPATH . WPINC . '/class-wp-feed-cache.php';
		$feed->set_cache_class( 'WP_Feed_Cache' );
	}

	$feed->set_file_class( 'WP_SimplePie_File' );

	$feed->set_feed_url( $url );
	/** This filter is documented in wp-includes/class-wp-feed-cache-transient.php */
	$feed->set_cache_duration( apply_filters( 'wp_feed_cache_transient_lifetime', 12 * HOUR_IN_SECONDS, $url ) );

	/**
	 * Fires just before processing the SimplePie feed object.
	 *
	 * @since 3.0.0
	 *
	 * @param SimplePie       $feed SimplePie feed object (passed by reference).
	 * @param string|string[] $url  URL of feed or array of URLs of feeds to retrieve.
	 */
	do_action_ref_array( 'wp_feed_options', array( &$feed, $url ) );

	$feed->init();
	$feed->set_output_encoding( get_option( 'blog_charset' ) );

	if ( $feed->error() ) {
		return new WP_Error( 'simplepie-error', $feed->error() );
	}

	return $feed;
}