ray_unique( $extensions );
	}

	/**
	 * Get the names of the HTML attributes where WP Rocket must search for image files.
	 *
	 * @since 3.4
	 *
	 * @return array
	 */
	private function get_attribute_names() {
		$attributes = [ 'href', 'src', 'srcset', 'data-large_image', 'data-thumb' ];

		/**
		 * Filter the names of the HTML attributes where WP Rocket must search for image files.
		 * Don't prepend new names with `data-`, WPR will do it. For example if you want to add `data-foo-bar`, you only need to add `foo-bar` or `bar` to the list.
		 *
		 * @since 3.4
		 *
		 * @param array $attributes An array of HTML attribute names.
		 */
		$attributes = apply_filters( 'rocket_attributes_for_webp', $attributes );
		$attributes = array_filter(
			(array) $attributes,
			function( $attributes ) {
				return $attributes && is_string( $attributes );
			}
		);

		return array_unique( $attributes );
	}

	/**
	 * Convert a URL to an absolute path.
	 *
	 * @since 3.4
	 *
	 * @param  string $url URL to convert.
	 * @return string|bool
	 */
	private function url_to_path( $url ) {
		static $hosts, $site_host, $subdir_levels;

		$url_host = wp_parse_url( $url, PHP_URL_HOST );

		// Relative path.
		if ( null === $url_host ) {
			if ( ! isset( $subdir_levels ) ) {
				$subdir_levels = substr_count( preg_replace( '@^https?://@', '', site_url() ), '/' );
			}

			if ( $subdir_levels ) {
				$url = ltrim( $url, '/' );
				$url = explode( '/', $url );
				array_splice( $url, 0, $subdir_levels );
				$url = implode( '/', $url );
			}

			$url = site_url( $url );
		}

		// CDN.
		if ( ! isset( $hosts ) ) {
			$hosts = $this->cdn_subscriber->get_cdn_hosts( [], [ 'all', 'images' ] );
			$hosts = array_flip( $hosts );
		}

		if ( isset( $hosts[ $url_host ] ) ) {
			if ( ! isset( $site_host ) ) {
				$site_host = wp_parse_url( site_url( '/' ), PHP_URL_HOST );
			}
			if ( $site_host ) {
				$url = preg_replace( '@^(https?://)' . $url_host . '/@', '$1' . $site_host . '/', $url );
			}
		}

		// URL to path.
		$url   = preg_replace( '@^https?:@', '', $url );
		$paths = $this->get_url_to_path_associations();

		if ( ! $paths ) {
			// Uh?
			return false;
		}

		foreach ( $paths as $asso_url => $asso_path ) {
			if ( 0 === strpos( $url, $asso_url ) ) {
				$file = str_replace( $asso_url, $asso_path, $url );
				break;
			}
		}

		if ( empty( $file ) ) {
			return false;
		}

		/** This filter is documented in inc/functions/formatting.php. */
		return (string) apply_filters( 'rocket_url_to_path', $file, $url );
	}

	/**
	 * Add a webp extension to a URL.
	 *
	 * @since 3.4
	 *
	 * @param  string $url        A URL (I see you're very surprised).
	 * @param  string $extensions Allowed image extensions.
	 * @return string|bool        The same URL with a webp extension if the file exists. False if the webp image doesn't exist.
	 */
	private function url_to_webp( $url, $extensions ) {
		if ( ! preg_match( '@^(?<src>.+\.(?<extension>' . $extensions . '))(?<query>(?:\?.*)?)$@i', $url, $src_url ) ) {
			// Probably something like "image.jpg.webp".
			return false;
		}

		$src_path = $this->url_to_path( $src_url['src'] );

		if ( empty( $src_path ) ) {
			return false;
		}

		$src_path_webp = preg_replace( '@\.' . $src_url['extension'] . '$@', '.webp', $src_path );

		if ( $this->filesystem->exists( $src_path_webp ) ) {
			// File name: image.jpg => image.webp.
			return preg_replace( '@\.' . $src_url['extension'] . '$@', '.webp', $src_url['src'] ) . $src_url['query'];
		}

		if ( $this->filesystem->exists( $src_path . '.webp' ) ) {
			// File name: image.jpg => image.jpg.webp.
			return $src_url['src'] . '.webp' . $src_url['query'];
		}

		return false;
	}

	/**
	 * Add webp extension to URLs in a srcset attribute.
	 *
	 * @since 3.4
	 *
	 * @param  array|string $srcset_values Value of a srcset attribute.
	 * @param  string       $extensions    Allowed image extensions.
	 * @return string|bool                 An array similar to $srcset_values, with webp extensions when the files exist. False if no images have webp versions.
	 */
	private function srcset_to_webp( $srcset_values, $extensions ) {
		if ( ! $srcset_values ) {
			return false;
		}

		if ( ! is_array( $srcset_values ) ) {
			$srcset_values = explode( ',', $srcset_values );
		}

		$has_webp = false;

		foreach ( $srcset_values as $i => $srcset_value ) {
			$srcset_value = preg_split( '/\s+/', trim( $srcset_value ) );

			if ( count( $srcset_value ) > 2 ) {
				// Not a good idea to have space characters in file name.
				$descriptor   = array_pop( $srcset_value );
				$srcset_value = [
					'url'        => implode( ' ', $srcset_value ),
					'descriptor' => $descriptor,
				];
			} else {
				$srcset_value = [
					'url'        => $srcset_value[0],
					'descriptor' => ! empty( $srcset_value[1] ) ? $srcset_value[1] : '1x',
				];
			}

			$url_webp = $this->url_to_webp( $srcset_value['url'], $extensions );

			if ( ! $url_webp ) {
				$srcset_values[ $i ] = implode( ' ', $srcset_value );
				continue;
			}

			$srcset_values[ $i ] = $url_webp . ' ' . $srcset_value['descriptor'];
			$has_webp            = true;
		}

		if ( ! $has_webp ) {
			return false;
		}

		return implode( ',', $srcset_values );
	}

	/**
	 * Get a list of URL/path associations.
	 * URLs are schema-less, starting by a double slash.
	 *
	 * @since 3.4
	 *
	 * @return array A list of URLs as keys and paths as values.
	 */
	private function get_url_to_path_associations() {
		static $list;

		if ( isset( $list ) ) {
			return $list;
		}

		$content_url = preg_replace( '@^https?:@', '', content_url( '/' ) );
		$content_dir = trailingslashit( rocket_get_constant( 'WP_CONTENT_DIR' ) );
		$list        = [ $content_url => $content_dir ];

		$upload     = wp_upload_dir();
		$upload_dir = trailingslashit( $upload['basedir'] );

		if ( strpos( $upload_dir, $content_dir ) === false ) {
			$upload_url          = preg_replace( '@^https?:@', '', trailingslashit( $upload['baseurl'] ) );
			$list[ $upload_url ] = $upload_dir;
		}

		/**
		 * Filter the list of URL/path associations.
		 * The URLs with the most levels must come first.
		 *
		 * @since 3.4
		 *
		 * @param array $list The list of URL/path associations. URLs are schema-less, starting by a double slash.
		 */
		$list = apply_filters( 'rocket_url_to_path_associations', $list );
		$list = array_filter(
			$list,
			function( $path, $url ) {
				return $path && $url && is_string( $path ) && is_string( $url );
			},
			ARRAY_FILTER_USE_BOTH
		);

		if ( $list ) {
			$list = array_unique( $list );
		}

		return $list;
	}

	/**
	 * Get a list of plugins that serve webp images on frontend.
	 * If the CDN is used, this won't list plugins that use a technique not compatible with CDN.
	 *
	 * @since 3.4
	 *
	 * @return array The WebP plugin names.
	 */
	private function get_plugins_serving_webp() {
		$webp_plugins = $this->get_webp_plugins();

		if ( ! $webp_plugins ) {
			// Somebody probably messed up.
			return [];
		}

		$checks       = [];
		$is_using_cdn = $this->is_using_cdn();

		foreach ( $webp_plugins as $plugin ) {
			if ( $is_using_cdn && $plugin->is_serving_webp_compatible_with_cdn() ) {
				$checks[ $plugin->get_id() ] = $plugin->get_name();
			} elseif ( ! $is_using_cdn && $plugin->is_serving_webp() ) {
				$checks[ $plugin->get_id() ] = $plugin->get_name();
			}
		}

		return $checks;
	}

	/**
	 * Change Youtube thumbnail extension.
	 *
	 * @param string $extension extension from the thumbnail.
	 *
	 * @return string
	 */
	public function change_youtube_thumbnail( $extension ) {
		if ( ! $this->options_data->get( 'cache_webp', 0 ) ) {
			return $extension;
		}
		return 'webp';
	}

}