* @since 6.0.0
	 *
	 * @uses $registered_webfonts To access and update the registered webfonts registry (passed by reference).
	 * @uses $fn_get_webfonts_from_theme_json To run the function that gets the webfonts from theme.json.
	 * @uses $fn_convert_keys_to_kebab_case To run the function that converts keys into kebab-case.
	 * @uses $fn_validate_webfont To run the function that validates each font-face (webfont) from theme.json.
	 */
	$fn_register_webfonts = static function() use ( &$registered_webfonts, $fn_get_webfonts_from_theme_json, $fn_convert_keys_to_kebab_case, $fn_validate_webfont, $fn_transform_src_into_uri ) {
		$registered_webfonts = array();

		foreach ( $fn_get_webfonts_from_theme_json() as $webfont ) {
			if ( ! is_array( $webfont ) ) {
				continue;
			}

			$webfont = $fn_convert_keys_to_kebab_case( $webfont );

			$webfont = $fn_validate_webfont( $webfont );

			$webfont['src'] = $fn_transform_src_into_uri( (array) $webfont['src'] );

			// Skip if not valid.
			if ( empty( $webfont ) ) {
				continue;
			}

			$registered_webfonts[] = $webfont;
		}
	};

	/**
	 * Orders 'src' items to optimize for browser support.
	 *
	 * @since 6.0.0
	 *
	 * @param array $webfont Webfont to process.
	 * @return array Ordered `src` items.
	 */
	$fn_order_src = static function( array $webfont ) {
		$src         = array();
		$src_ordered = array();

		foreach ( $webfont['src'] as $url ) {
			// Add data URIs first.
			if ( str_starts_with( trim( $url ), 'data:' ) ) {
				$src_ordered[] = array(
					'url'    => $url,
					'format' => 'data',
				);
				continue;
			}
			$format         = pathinfo( $url, PATHINFO_EXTENSION );
			$src[ $format ] = $url;
		}

		// Add woff2.
		if ( ! empty( $src['woff2'] ) ) {
			$src_ordered[] = array(
				'url'    => sanitize_url( $src['woff2'] ),
				'format' => 'woff2',
			);
		}

		// Add woff.
		if ( ! empty( $src['woff'] ) ) {
			$src_ordered[] = array(
				'url'    => sanitize_url( $src['woff'] ),
				'format' => 'woff',
			);
		}

		// Add ttf.
		if ( ! empty( $src['ttf'] ) ) {
			$src_ordered[] = array(
				'url'    => sanitize_url( $src['ttf'] ),
				'format' => 'truetype',
			);
		}

		// Add eot.
		if ( ! empty( $src['eot'] ) ) {
			$src_ordered[] = array(
				'url'    => sanitize_url( $src['eot'] ),
				'format' => 'embedded-opentype',
			);
		}

		// Add otf.
		if ( ! empty( $src['otf'] ) ) {
			$src_ordered[] = array(
				'url'    => sanitize_url( $src['otf'] ),
				'format' => 'opentype',
			);
		}
		$webfont['src'] = $src_ordered;

		return $webfont;
	};

	/**
	 * Compiles the 'src' into valid CSS.
	 *
	 * @since 6.0.0
	 *
	 * @param string $font_family Font family.
	 * @param array  $value       Value to process.
	 * @return string The CSS.
	 */
	$fn_compile_src = static function( $font_family, array $value ) {
		$src = "local($font_family)";

		foreach ( $value as $item ) {

			if (
				str_starts_with( $item['url'], site_url() ) ||
				str_starts_with( $item['url'], home_url() )
			) {
				$item['url'] = wp_make_link_relative( $item['url'] );
			}

			$src .= ( 'data' === $item['format'] )
				? ", url({$item['url']})"
				: ", url('{$item['url']}') format('{$item['format']}')";
		}

		return $src;
	};

	/**
	 * Compiles the font variation settings.
	 *
	 * @since 6.0.0
	 *
	 * @param array $font_variation_settings Array of font variation settings.
	 * @return string The CSS.
	 */
	$fn_compile_variations = static function( array $font_variation_settings ) {
		$variations = '';

		foreach ( $font_variation_settings as $key => $value ) {
			$variations .= "$key $value";
		}

		return $variations;
	};

	/**
	 * Builds the font-family's CSS.
	 *
	 * @since 6.0.0
	 *
	 * @uses $fn_compile_src To run the function that compiles the src.
	 * @uses $fn_compile_variations To run the function that compiles the variations.
	 *
	 * @param array $webfont Webfont to process.
	 * @return string This font-family's CSS.
	 */
	$fn_build_font_face_css = static function( array $webfont ) use ( $fn_compile_src, $fn_compile_variations ) {
		$css = '';

		// Wrap font-family in quotes if it contains spaces.
		if (
			str_contains( $webfont['font-family'], ' ' ) &&
			! str_contains( $webfont['font-family'], '"' ) &&
			! str_contains( $webfont['font-family'], "'" )
		) {
			$webfont['font-family'] = '"' . $webfont['font-family'] . '"';
		}

		foreach ( $webfont as $key => $value ) {
			/*
			 * Skip "provider", since it's for internal API use,
			 * and not a valid CSS property.
			 */
			if ( 'provider' === $key ) {
				continue;
			}

			// Compile the "src" parameter.
			if ( 'src' === $key ) {
				$value = $fn_compile_src( $webfont['font-family'], $value );
			}

			// If font-variation-settings is an array, convert it to a string.
			if ( 'font-variation-settings' === $key && is_array( $value ) ) {
				$value = $fn_compile_variations( $value );
			}

			if ( ! empty( $value ) ) {
				$css .= "$key:$value;";
			}
		}

		return $css;
	};

	/**
	 * Gets the '@font-face' CSS styles for locally-hosted font files.
	 *
	 * @since 6.0.0
	 *
	 * @uses $registered_webfonts To access and update the registered webfonts registry (passed by reference).
	 * @uses $fn_order_src To run the function that orders the src.
	 * @uses $fn_build_font_face_css To run the function that builds the font-face CSS.
	 *
	 * @return string The `@font-face` CSS.
	 */
	$fn_get_css = static function() use ( &$registered_webfonts, $fn_order_src, $fn_build_font_face_css ) {
		$css = '';

		foreach ( $registered_webfonts as $webfont ) {
			// Order the webfont's `src` items to optimize for browser support.
			$webfont = $fn_order_src( $webfont );

			// Build the @font-face CSS for this webfont.
			$css .= '@font-face{' . $fn_build_font_face_css( $webfont ) . '}';
		}

		return $css;
	};

	/**
	 * Generates and enqueues webfonts styles.
	 *
	 * @since 6.0.0
	 *
	 * @uses $fn_get_css To run the function that gets the CSS.
	 */
	$fn_generate_and_enqueue_styles = static function() use ( $fn_get_css ) {
		// Generate the styles.
		$styles = $fn_get_css();

		// Bail out if there are no styles to enqueue.
		if ( '' === $styles ) {
			return;
		}

		// Enqueue the stylesheet.
		wp_register_style( 'wp-webfonts', '' );
		wp_enqueue_style( 'wp-webfonts' );

		// Add the styles to the stylesheet.
		wp_add_inline_style( 'wp-webfonts', $styles );
	};

	/**
	 * Generates and enqueues editor styles.
	 *
	 * @since 6.0.0
	 *
	 * @uses $fn_get_css To run the function that gets the CSS.
	 */
	$fn_generate_and_enqueue_editor_styles = static function() use ( $fn_get_css ) {
		// Generate the styles.
		$styles = $fn_get_css();

		// Bail out if there are no styles to enqueue.
		if ( '' === $styles ) {
			return;
		}

		wp_add_inline_style( 'wp-block-library', $styles );
	};

	add_action( 'wp_loaded', $fn_register_webfonts );
	add_action( 'wp_enqueue_scripts', $fn_generate_and_enqueue_styles );
	add_action( 'admin_init', $fn_generate_and_enqueue_editor_styles );
}