Work on complex selector handling

src/wp-includes/html-api/class-wp-css-selectors.php

@@ -1033,11 +1033,47 @@ private static function parse_subclass_selector( string $input, int &$offset ) {
  */
 final class WP_CSS_Complex_Selector extends WP_CSS_Selector_Parser {
 	public function matches( WP_HTML_Processor $processor ): bool {
-		// @todo this can throw on parse.
-		if ( count( $this->selectors ) > 1 ) {
-			throw new Exception( 'Combined complex selectors are not supported yet.' );
+		if ( count( $this->selectors ) === 1 ) {
+			return $this->selectors[0]->matches( $processor );
+		}
+
+		// First selector must match this location.
+		if ( ! $this->selectors[0]->matches( $processor ) ) {
+			return false;
+		}
+
+		$breadcrumbs = array_slice( array_reverse( $processor->get_breadcrumbs() ), 1 );
+		$selectors   = array_slice( $this->selectors, 1 );
+		return $this->explore_matches( $selectors, $breadcrumbs );
+	}
+
+	/**
+	 * This only looks at breadcrumbs and can therefore only support type selectors.
+	 *
+	 * @param array<WP_CSS_Selector> $selectors
+	 */
+	private function explore_matches( array $selectors, array $breadcrumbs ): bool {
+		if ( array() === $selectors ) {
+			return true;
+		}
+		if ( array() === $breadcrumbs ) {
+			return false;
+		}
+
+		$combinator = $selectors[0];
+		$selector   = $selectors[1];
+
+		switch ( $combinator ) {
+			case self::COMBINATOR_CHILD:
+				if ( '*' === $selector->type_selector->ident || strcasecmp( $breadcrumbs[0], $selector->type_selector->ident ) === 0 ) {
+					return $this->explore_matches( array_slice( $selectors, 2 ), array_slice( $breadcrumbs, 1 ) );
+				}
+				return $this->explore_matches( $selectors, array_slice( $breadcrumbs, 1 ) );
+
+			case self::COMBINATOR_DESCENDANT:
+			default:
+				throw new Exception( "Combinator '{$combinator}' is not supported yet." );
 		}
-		return $this->selectors[0]->matches( $processor );
 	}
 
 	const COMBINATOR_CHILD              = '>';
@@ -1047,12 +1083,15 @@ public function matches( WP_HTML_Processor $processor ): bool {
 
 	/**
 	 * even indexes are WP_CSS_Selector, odd indexes are string combinators.
+	 * In reverse order to match the current element and then work up the tree.
+	 * Any non-final selector is a type selector.
+	 *
 	 * @var array<WP_CSS_Selector>
 	 */
 	public $selectors = array();
 
 	private function __construct( array $selectors ) {
-		$this->selectors = $selectors;
+		$this->selectors = array_reverse( $selectors );
 	}
 
 	public static function parse( string $input, int &$offset ): ?self {