<?php
namespace TDL_GMC_Lite;

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

class Feed_Cache {
    private static $instance = null;
    private const TTL = 1800;
    private const LOCK_TTL = 300;

    public static function instance(): self {
        if ( self::$instance === null ) self::$instance = new self();
        return self::$instance;
    }

    public static function init(): void {
        add_action( 'admin_init', [ __CLASS__, 'handle_admin_actions' ] );
        add_action( 'save_post_product', [ __CLASS__, 'invalidate_product' ], 20, 3 );
        add_action( 'woocommerce_save_product_variation', [ __CLASS__, 'invalidate_variation' ], 20, 2 );
        add_action( 'updated_option', [ __CLASS__, 'invalidate_option' ], 20, 3 );
    }

    public static function invalidate_product( int $post_id, $post, bool $update ): void {
        if ( wp_is_post_revision( $post_id ) || wp_is_post_autosave( $post_id ) ) return;
        self::instance()->clear_all();
    }

    public static function invalidate_variation( int $variation_id, int $index ): void {
        self::instance()->clear_all();
    }

    public static function invalidate_option( string $option, $old_value, $value ): void {
        if ( strpos( $option, 'tdl_gmc_' ) === 0 ) self::instance()->clear_all();
    }

    public static function handle_admin_actions(): void {
        if ( ! is_admin() || ! current_user_can( THREEDL_GMC_CAP ) ) return;
        if ( empty( $_GET['tdl_feed_action'] ) || empty( $_GET['_wpnonce'] ) ) return;
        $action = sanitize_key( wp_unslash( $_GET['tdl_feed_action'] ) );
        if ( ! wp_verify_nonce( sanitize_text_field( wp_unslash( $_GET['_wpnonce'] ) ), 'tdl_feed_admin_action' ) ) return;

        if ( $action === 'clear_cache' ) {
            self::instance()->clear_all();
            wp_safe_redirect( add_query_arg( 'tdl_feed_notice', 'cache_cleared', remove_query_arg( [ 'tdl_feed_action', '_wpnonce' ] ) ) );
            exit;
        }

        if ( $action === 'rebuild_cache' ) {
            $notice = 'rebuilt';
            try {
                self::instance()->rebuild( 'xml' );
            } catch ( \Throwable $error ) {
                $notice = 'rebuild_failed';
            }
            wp_safe_redirect( add_query_arg( 'tdl_feed_notice', $notice, remove_query_arg( [ 'tdl_feed_action', '_wpnonce' ] ) ) );
            exit;
        }
    }

    public function serve( string $mode ): void {
        $path = $this->cache_file( $mode );
        $lock = $this->lock_file( $mode );
        if ( $this->is_fresh( $path ) ) $this->send_file( $path );

        if ( $this->has_valid_lock( $lock ) ) {
            if ( is_readable( $path ) ) $this->send_file( $path, true );
            status_header( 503 );
            header( 'Content-Type: text/plain; charset=utf-8' );
            header( 'Retry-After: 30' );
            echo esc_html__( 'Feed is being generated. Try again in a moment.', '3dl-product-feed-for-google' );
            exit;
        }

        try {
            $this->rebuild( $mode );
            $this->send_file( $path );
        } catch ( \Throwable $error ) {
            if ( is_readable( $path ) ) $this->send_file( $path, true );
            status_header( 500 );
            header( 'Content-Type: text/plain; charset=utf-8' );
            echo esc_html__( 'Feed generation failed.', '3dl-product-feed-for-google' );
            exit;
        }
    }

    public function rebuild( string $mode ): void {
        $path = $this->cache_file( $mode );
        $lock = $this->lock_file( $mode );
        if ( $this->has_valid_lock( $lock ) ) {
            throw new \RuntimeException( 'Feed generation is locked.' );
        }

        $this->acquire_lock( $lock );
        try {
            $xml = $this->generate( $mode );
            if ( trim( $xml ) === '' ) throw new \RuntimeException( 'Empty feed output.' );
            $this->write_atomic( $path, $xml );

            // Diagnostics are useful, but a stats-file write problem must never break the public feed.
            try {
                $stats = Feed::instance()->get_stats();
                $stats['generated_at'] = time();
                $stats['version'] = defined( 'TDL_GMC_LITE_VER' ) ? TDL_GMC_LITE_VER : '';
                $this->write_atomic( $this->stats_file( $mode ), (string) wp_json_encode( $stats, JSON_PRETTY_PRINT ) );
            } catch ( \Throwable $stats_error ) {
                // Keep serving the successfully generated XML feed without diagnostics.
            }
        } finally {
            $this->release_lock( $lock );
        }
    }

    public function clear_all(): void {
        foreach ( [ 'xml' ] as $mode ) {
            foreach ( [ $this->cache_file( $mode ), $this->lock_file( $mode ), $this->stats_file( $mode ) ] as $file ) {
                if ( file_exists( $file ) ) @unlink( $file );
            }
        }
    }

    public function cache_info( string $mode ): array {
        $path = $this->cache_file( $mode );
        return [
            'path'   => $path,
            'exists' => is_readable( $path ),
            'mtime'  => is_readable( $path ) ? (int) filemtime( $path ) : 0,
            'size'   => is_readable( $path ) ? (int) filesize( $path ) : 0,
            'fresh'  => $this->is_fresh( $path ),
        ];
    }

    public function stats_info( string $mode ): array {
        $path = $this->stats_file( $mode );
        if ( ! is_readable( $path ) ) return [];
        $decoded = json_decode( (string) file_get_contents( $path ), true );
        return is_array( $decoded ) ? $decoded : [];
    }

    private function generate( string $mode ): string {
        if ( ! class_exists( '\\WooCommerce' ) ) throw new \RuntimeException( 'WooCommerce missing.' );
        if ( $mode !== 'xml' ) throw new \InvalidArgumentException( 'Unknown feed mode.' );
        $renderer = Feed::instance();
        if ( function_exists( 'wp_raise_memory_limit' ) ) wp_raise_memory_limit( 'admin' );
        if ( function_exists( 'set_time_limit' ) ) @set_time_limit( 120 );
        ob_start();
        $renderer->output();
        return (string) ob_get_clean();
    }

    private function send_file( string $path, bool $stale = false ): void {
        if ( ! is_readable( $path ) ) {
            status_header( 404 );
            header( 'Content-Type: text/plain; charset=utf-8' );
            echo esc_html__( 'Feed cache file not found.', '3dl-product-feed-for-google' );
            exit;
        }
        if ( ! headers_sent() ) {
            header( 'Content-Type: application/xml; charset=utf-8' );
            header( 'X-TDL-Feed-Cache: ' . ( $stale ? 'STALE' : 'HIT' ) );
            header( 'Cache-Control: public, max-age=300, s-maxage=300' );
            header( 'Last-Modified: ' . gmdate( 'D, d M Y H:i:s', (int) filemtime( $path ) ) . ' GMT' );
        }
        readfile( $path );
        exit;
    }

    private function is_fresh( string $path ): bool {
        return is_readable( $path ) && ( time() - (int) filemtime( $path ) ) < self::TTL;
    }

    private function has_valid_lock( string $lock ): bool {
        return is_readable( $lock ) && ( time() - (int) filemtime( $lock ) ) < self::LOCK_TTL;
    }

    private function acquire_lock( string $lock ): void {
        $dir = dirname( $lock );
        if ( ! is_dir( $dir ) && ! wp_mkdir_p( $dir ) ) throw new \RuntimeException( 'Cannot create feed cache directory.' );
        if ( file_put_contents( $lock, (string) time(), LOCK_EX ) === false ) throw new \RuntimeException( 'Cannot create feed lock.' );
    }

    private function release_lock( string $lock ): void {
        if ( file_exists( $lock ) ) @unlink( $lock );
    }

    private function write_atomic( string $path, string $contents ): void {
        $dir = dirname( $path );
        if ( ! is_dir( $dir ) && ! wp_mkdir_p( $dir ) ) throw new \RuntimeException( 'Cannot create feed cache directory.' );
        $tmp = $path . '.tmp-' . wp_generate_password( 8, false, false );
        if ( file_put_contents( $tmp, $contents, LOCK_EX ) === false ) throw new \RuntimeException( 'Cannot write feed cache.' );
        @chmod( $tmp, 0644 );
        if ( ! @rename( $tmp, $path ) ) {
            @unlink( $tmp );
            throw new \RuntimeException( 'Cannot publish feed cache.' );
        }
    }

    private function cache_file( string $mode ): string {
        $upload = wp_get_upload_dir();
        $base = trailingslashit( $upload['basedir'] ) . 'tdl-feed-cache';
        return trailingslashit( $base ) . $this->file_name( $mode );
    }

    private function stats_file( string $mode ): string {
        return $this->cache_file( $mode ) . '.stats.json';
    }

    private function lock_file( string $mode ): string {
        return $this->cache_file( $mode ) . '.lock';
    }

    private function file_name( string $mode ): string {
        return $mode === 'xml' ? '3dl-gmc-feed.xml' : 'feed.xml';
    }
}