<?php
/**
 * Serves the markdown variant of singular content and advertises it.
 *
 * @package AEL42A
 */

namespace AEL42A;

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Markdown controller.
 *
 * Access paths (decision D1 in the plan):
 *   1. `?format=markdown` - primary, works with every permalink structure.
 *   2. `<permalink>.md`   - rewrite-based, best effort.
 *   3. `Accept: text/markdown` preferred over text/html - content negotiation.
 *
 * Every HTML singular view advertises the markdown variant via a
 * `<link rel="alternate">` element and a `Link:` HTTP header.
 */
class Markdown_Controller {

	const QUERY_VAR = 'ael42a_md';

	/**
	 * Telemetry counters.
	 *
	 * @var Telemetry
	 */
	private $telemetry;

	/**
	 * Converter instance.
	 *
	 * @var Markdown_Converter
	 */
	private $converter;

	/**
	 * Constructor.
	 *
	 * @param Telemetry $telemetry Telemetry counters.
	 */
	public function __construct( $telemetry ) {
		$this->telemetry = $telemetry;
		$this->converter = new Markdown_Converter();
	}

	/**
	 * Hook everything up.
	 */
	public function register_hooks() {
		add_action( 'init', array( __CLASS__, 'add_rewrite_rules' ) );
		add_filter( 'query_vars', array( $this, 'register_query_vars' ) );
		add_action( 'template_redirect', array( $this, 'maybe_serve_markdown' ), 1 );
		add_action( 'template_redirect', array( $this, 'send_alternate_link_header' ), 20 );
		add_action( 'wp_head', array( $this, 'print_alternate_link' ) );
	}

	/**
	 * `.md` suffix rewrite rules. Static so activation can call it before
	 * the component graph exists.
	 */
	public static function add_rewrite_rules() {
		// Posts and flat slugs (single path segment).
		add_rewrite_rule( '([^/]+)\.md$', 'index.php?name=$matches[1]&' . self::QUERY_VAR . '=1', 'top' );
		// Hierarchical pages (nested path).
		add_rewrite_rule( '(.+?/[^/]+)\.md$', 'index.php?pagename=$matches[1]&' . self::QUERY_VAR . '=1', 'top' );
	}

	/**
	 * Register our query var.
	 *
	 * @param string[] $vars Public query vars.
	 * @return string[]
	 */
	public function register_query_vars( $vars ) {
		$vars[] = self::QUERY_VAR;
		return $vars;
	}

	/**
	 * Serve the markdown variant when requested, then exit.
	 */
	public function maybe_serve_markdown() {
		if ( ! is_singular() ) {
			return;
		}

		$post = get_queried_object();
		if ( ! $post instanceof \WP_Post || ! $this->is_servable( $post ) ) {
			return;
		}

		if ( ! $this->wants_markdown() ) {
			return;
		}

		$markdown = $this->converter->document( $post );

		$this->telemetry->increment( 'markdown_hits' );
		$this->telemetry->record_bot_hit_from_request();

		status_header( 200 );
		header( 'Content-Type: text/markdown; charset=utf-8' );
		header( 'Vary: Accept' );
		header( 'X-42A-AEL: ' . AEL42A_VERSION );

		echo $markdown; // phpcs:ignore WordPress.Security.EscapeOutput -- plain text/markdown body, not HTML.
		exit;
	}

	/**
	 * Whether the current request asked for markdown.
	 *
	 * @return bool
	 */
	private function wants_markdown() {
		// 1. ?format=markdown.
		if ( isset( $_GET['format'] ) && 'markdown' === sanitize_key( wp_unslash( $_GET['format'] ) ) ) { // phpcs:ignore WordPress.Security.NonceVerification.Recommended
			return true;
		}

		// 2. `.md` rewrite matched.
		if ( '1' === get_query_var( self::QUERY_VAR ) || 1 === get_query_var( self::QUERY_VAR ) ) {
			return true;
		}

		// 3. Accept header prefers text/markdown over text/html.
		$accept = isset( $_SERVER['HTTP_ACCEPT'] ) ? sanitize_text_field( wp_unslash( $_SERVER['HTTP_ACCEPT'] ) ) : '';
		if ( '' !== $accept && false !== stripos( $accept, 'text/markdown' ) ) {
			$md_q   = $this->accept_quality( $accept, 'text/markdown' );
			$html_q = max(
				$this->accept_quality( $accept, 'text/html' ),
				$this->accept_quality( $accept, 'application/xhtml+xml' )
			);
			if ( $md_q > 0 && $md_q >= $html_q ) {
				return true;
			}
		}

		return false;
	}

	/**
	 * Extract the q-value for a media type from an Accept header.
	 *
	 * @param string $accept Accept header.
	 * @param string $type   Media type to look for.
	 * @return float Quality (0 when absent).
	 */
	private function accept_quality( $accept, $type ) {
		foreach ( explode( ',', $accept ) as $part ) {
			$pieces = array_map( 'trim', explode( ';', $part ) );
			$media  = strtolower( array_shift( $pieces ) );
			if ( $media !== strtolower( $type ) ) {
				continue;
			}
			$q = 1.0;
			foreach ( $pieces as $param ) {
				if ( 0 === stripos( $param, 'q=' ) ) {
					$q = (float) substr( $param, 2 );
				}
			}
			return $q;
		}
		return 0.0;
	}

	/**
	 * Whether a post may be served as markdown.
	 *
	 * @param \WP_Post $post Post.
	 * @return bool
	 */
	private function is_servable( $post ) {
		if ( 'publish' !== $post->post_status ) {
			return false;
		}
		if ( post_password_required( $post ) ) {
			return false;
		}
		if ( ! is_post_publicly_viewable( $post ) ) {
			return false;
		}
		return true;
	}

	/**
	 * `Link:` header advertising the markdown alternate on HTML responses.
	 */
	public function send_alternate_link_header() {
		if ( headers_sent() || ! is_singular() ) {
			return;
		}
		$post = get_queried_object();
		if ( ! $post instanceof \WP_Post || ! $this->is_servable( $post ) ) {
			return;
		}
		$url = self::markdown_url( $post );
		header( 'Link: <' . esc_url_raw( $url ) . '>; rel="alternate"; type="text/markdown"', false );
	}

	/**
	 * `<link rel="alternate">` element on singular HTML views.
	 */
	public function print_alternate_link() {
		if ( ! is_singular() ) {
			return;
		}
		$post = get_queried_object();
		if ( ! $post instanceof \WP_Post || ! $this->is_servable( $post ) ) {
			return;
		}
		printf(
			'<link rel="alternate" type="text/markdown" href="%s" />' . "\n",
			esc_url( self::markdown_url( $post ) )
		);
	}

	/**
	 * Canonical markdown URL for a post.
	 *
	 * @param \WP_Post|int $post Post or ID.
	 * @return string
	 */
	public static function markdown_url( $post ) {
		return add_query_arg( 'format', 'markdown', get_permalink( $post ) );
	}
}