<?php
/**
 * Public feed request handling (transport independent).
 *
 * @package FeedWeaver
 */

namespace FeedWeaver\Http;

use FeedWeaver\Access\Access_Policy;
use FeedWeaver\Config\Feed_Config;
use FeedWeaver\Config\Feed_Repository;
use FeedWeaver\Feed\Feed_Builder;

defined( 'ABSPATH' ) || exit;

/**
 * Maps a requested slug to a response description. Does not send anything,
 * which keeps it testable.
 */
class Feed_Controller {

	/** @var Feed_Repository */
	private $repository;

	/** @var Feed_Builder */
	private $builder;

	/** @var Access_Policy */
	private $access;

	/**
	 * @param Feed_Repository    $repository Repository.
	 * @param Feed_Builder       $builder     Builder.
	 * @param Access_Policy|null $access     Access policy.
	 */
	public function __construct( Feed_Repository $repository, Feed_Builder $builder, ?Access_Policy $access = null ) {
		$this->repository = $repository;
		$this->builder    = $builder;
		$this->access     = $access ? $access : new Access_Policy( $repository );
	}

	/**
	 * Produces the response for a slug.
	 *
	 * @param string $slug          Requested slug.
	 * @param string $if_none_match Value of the If-None-Match request header.
	 * @param array  $request       Credentials and connection facts for protected feeds (see Access_Policy::authorize()).
	 * @return array{status:int,headers:array<string,string>,body:string,nocache:bool}
	 */
	public function respond( string $slug, string $if_none_match = '', array $request = array() ): array {
		try {
			return $this->build_response( $slug, $if_none_match, $request );
		} catch ( \Throwable $e ) {
			return $this->unavailable();
		}
	}

	/**
	 * Builds the response; exceptions are handled by respond().
	 *
	 * @param string $slug          Requested slug.
	 * @param string $if_none_match If-None-Match header.
	 * @param array  $request       Request facts.
	 * @return array{status:int,headers:array<string,string>,body:string,nocache:bool}
	 */
	private function build_response( string $slug, string $if_none_match, array $request ): array {
		if ( 1 !== preg_match( '/^[a-z0-9]+(-[a-z0-9]+)*$/D', $slug ) || strlen( $slug ) > 60 ) {
			return $this->not_found();
		}
		$feed_id = $this->repository->id_for_slug( $slug );
		$config  = $feed_id ? $this->repository->find( $feed_id ) : null;
		if ( ! $config || ! $config->enabled ) {
			return $this->not_found();
		}

		// Authorization comes first: nothing below (the cache lookup, the query, the ETag
		// comparison, the 304) runs for a request that has not proven it may read this feed.
		$protected = Feed_Config::ACCESS_PROTECTED === $config->access;
		if ( $protected ) {
			$decision = $this->access->authorize( $config, $feed_id, $request );
			if ( ! $decision->allowed ) {
				return array(
					'status'  => $decision->status,
					'headers' => $decision->headers,
					'body'    => $decision->body,
					'nocache' => true,
				);
			}
		}

		$result = $this->builder->build( $config, true, false );
		if ( ! $result->ok ) {
			return $this->unavailable();
		}

		$etag    = '"' . md5( $result->body ) . '"';
		$max_age = max( 0, (int) apply_filters( 'feedweaver_http_max_age', 300, $config ) );
		$headers = array(
			'Content-Type'           => $result->content_type,
			'X-Content-Type-Options' => 'nosniff',
			'X-Robots-Tag'           => 'noindex',
			'Cache-Control'          => 'public, max-age=' . $max_age,
			'ETag'                   => $etag,
		);
		if ( $protected ) {
			// Never shareable: not by CDNs, proxies or page caches. Browsers may not store it either.
			$headers['Cache-Control']   = 'private, no-store, max-age=0';
			$headers['Pragma']          = 'no-cache';
			$headers['Expires']         = 'Wed, 11 Jan 1984 05:00:00 GMT';
			$headers['Vary']            = 'Authorization';
			$headers['Referrer-Policy'] = 'no-referrer';
		}
		if ( $this->etag_matches( $if_none_match, $etag ) ) {
			return array(
				'status'  => 304,
				'headers' => $headers,
				'body'    => '',
				'nocache' => $protected,
			);
		}
		return array(
			'status'  => 200,
			'headers' => $headers,
			'body'    => $result->body,
			'nocache' => $protected,
		);
	}

	/**
	 * Whether an If-None-Match header (list, weak validators, or *) matches the ETag.
	 *
	 * @param string $header If-None-Match value.
	 * @param string $etag   Current ETag, quoted.
	 * @return bool
	 */
	private function etag_matches( string $header, string $etag ): bool {
		if ( '' === trim( $header ) ) {
			return false;
		}
		if ( '*' === trim( $header ) ) {
			return true;
		}
		foreach ( explode( ',', $header ) as $candidate ) {
			$candidate = trim( $candidate );
			if ( 0 === strncmp( $candidate, 'W/', 2 ) ) {
				$candidate = substr( $candidate, 2 );
			}
			if ( $candidate === $etag ) {
				return true;
			}
		}
		return false;
	}

	/**
	 * A generic failure response that reveals nothing about the cause.
	 *
	 * @return array{status:int,headers:array<string,string>,body:string,nocache:bool}
	 */
	private function unavailable(): array {
		return array(
			'status'  => 500,
			'headers' => array(
				'Content-Type'           => 'text/plain; charset=UTF-8',
				'Cache-Control'          => 'no-store',
				'X-Content-Type-Options' => 'nosniff',
			),
			'body'    => 'Feed unavailable.',
			'nocache' => true,
		);
	}

	/**
	 * A 404 response description.
	 *
	 * @return array{status:int,headers:array<string,string>,body:string}
	 */
	private function not_found(): array {
		return array(
			'status'  => 404,
			'headers' => array( 'Cache-Control' => 'no-store' ),
			'body'    => '',
			'nocache' => true,
		);
	}
}