<?php
/**
 * Serves cached pages before WordPress loads (used by advanced-cache.php).
 *
 * @package Akantio\Cache
 */

declare(strict_types=1);

namespace Akantio\Cache\Application\PageCache;

use Akantio\Cache\Domain\PageCache\CacheabilityPolicy;
use Akantio\Cache\Domain\PageCache\CacheKey;
use Akantio\Cache\Domain\PageCache\CacheRules;
use Akantio\Cache\Domain\PageCache\RequestContext;

/**
 * Hit: send headers and the stored file, then terminate. Miss or bypass: annotate and return so
 * WordPress renders the page.
 */
final class CacheServer {

	public const PRELOAD_HEADER = 'x-akantio-preload';

	private readonly CacheRules $rules;

	/**
	 * @param array<string, mixed> $config  The "page_cache" section of runtime.php.
	 * @param PageStore            $store   Page store.
	 * @param ResponseEmitter      $emitter Output.
	 * @param HitCounter|null      $counter Hit/miss counter.
	 */
	public function __construct(
		private readonly array $config,
		private readonly PageStore $store,
		private readonly ResponseEmitter $emitter,
		private readonly ?HitCounter $counter = null,
	) {
		$this->rules = CacheRules::fromArray( is_array( $config['rules'] ?? null ) ? $config['rules'] : [] );
	}

	/**
	 * Handles a request.
	 *
	 * @param RequestContext        $request Request.
	 * @param array<string, string> $headers Request headers keyed by lowercase name.
	 */
	public function handle( RequestContext $request, array $headers ): void {
		if ( true !== ( $this->config['enabled'] ?? false ) || $this->isPreload( $headers ) ) {
			return;
		}

		$decision = ( new CacheabilityPolicy( $this->rules ) )->forRequest( $request );
		if ( ! $decision->isCacheable() ) {
			$this->emitter->header( 'X-Akantio-Cache', 'BYPASS' );
			$this->emitter->header( 'X-Akantio-Cache-Reason', (string) $decision->reason()?->value );
			return;
		}

		$key  = CacheKey::fromRequest( $request, $this->rules );
		$page = $this->store->fetch( $key, $headers['accept-encoding'] ?? '' );
		if ( null === $page ) {
			$this->emitter->header( 'X-Akantio-Cache', 'MISS' );
			$this->counter?->record( 'miss' );
			return;
		}

		$this->counter?->record( 'hit' );

		$this->sendHit( $request, $key, $page, $headers );
	}

	/**
	 * Sends a cache hit.
	 *
	 * @param RequestContext        $request Request.
	 * @param CacheKey              $key     Key.
	 * @param CachedPage            $page    Page.
	 * @param array<string, string> $headers Request headers.
	 */
	private function sendHit( RequestContext $request, CacheKey $key, CachedPage $page, array $headers ): void {
		$this->emitter->header( 'Content-Type', 'text/html; charset=UTF-8' );
		$this->emitter->header( 'Vary', 'Accept-Encoding' );
		$this->emitter->header( 'Last-Modified', gmdate( 'D, d M Y H:i:s', $page->modified ) . ' GMT' );
		$this->emitter->header( 'Cache-Control', 'no-cache' );
		$this->emitter->header( 'CDN-Cache-Control', 'max-age=' . (int) ( $this->config['cdn_max_age'] ?? 0 ) );
		$this->emitter->header( 'Cache-Tag', $key->host() );
		$this->emitter->header( 'X-Akantio-Cache', 'HIT' );

		$since = isset( $headers['if-modified-since'] ) ? strtotime( $headers['if-modified-since'] ) : false;
		if ( false !== $since && $since >= $page->modified ) {
			$this->emitter->status( 304 );
			$this->emitter->terminate();
			return;
		}

		if ( null !== $page->encoding ) {
			$this->emitter->header( 'Content-Encoding', $page->encoding );
		}
		if ( 'HEAD' !== strtoupper( $request->method ) ) {
			$this->emitter->sendFile( $page->path );
		}
		$this->emitter->terminate();
	}

	/**
	 * Authentic preload requests must regenerate the page.
	 *
	 * @param array<string, string> $headers Request headers.
	 */
	private function isPreload( array $headers ): bool {
		$token = (string) ( $this->config['preload_token'] ?? '' );

		return '' !== $token && isset( $headers[ self::PRELOAD_HEADER ] ) && hash_equals( $token, $headers[ self::PRELOAD_HEADER ] );
	}
}