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/**
 * HTML API: WP_HTML_Active_Formatting_Elements class
 *
 * @package WordPress
 * @subpackage HTML-API
 * @since 6.4.0
 */

/**
 * Core class used by the HTML processor during HTML parsing
 * for managing the stack of active formatting elements.
 *
 * This class is designed for internal use by the HTML processor.
 *
 * &gt; Initially, the list of active formatting elements is empty.
 * &gt; It is used to handle mis-nested formatting element tags.
 * &gt;
 * &gt; The list contains elements in the formatting category, and markers.
 * &gt; The markers are inserted when entering applet, object, marquee,
 * &gt; template, td, th, and caption elements, and are used to prevent
 * &gt; formatting from &quot;leaking&quot; into applet, object, marquee, template,
 * &gt; td, th, and caption elements.
 * &gt;
 * &gt; In addition, each element in the list of active formatting elements
 * &gt; is associated with the token for which it was created, so that
 * &gt; further elements can be created for that token if necessary.
 *
 * @since 6.4.0
 *
 * @access private
 *
 * @see https://html.spec.whatwg.org/#list-of-active-formatting-elements
 * @see WP_HTML_Processor
 */
class WP_HTML_Active_Formatting_Elements {
	/**
	 * Holds the stack of active formatting element references.
	 *
	 * @since 6.4.0
	 *
	 * @var WP_HTML_Token[]
	 */
	private $stack = array();

	/**
	 * Reports if a specific node is in the stack of active formatting elements.
	 *
	 * @since 6.4.0
	 *
	 * @param WP_HTML_Token $token Look for this node in the stack.
	 * @return bool Whether the referenced node is in the stack of active formatting elements.
	 */
	public function contains_node( WP_HTML_Token $token ) {
		foreach ( $this-&gt;walk_up() as $item ) {
			if ( $token-&gt;bookmark_name === $item-&gt;bookmark_name ) {
				return true;
			}
		}

		return false;
	}

	/**
	 * Returns how many nodes are currently in the stack of active formatting elements.
	 *
	 * @since 6.4.0
	 *
	 * @return int How many node are in the stack of active formatting elements.
	 */
	public function count() {
		return count( $this-&gt;stack );
	}

	/**
	 * Returns the node at the end of the stack of active formatting elements,
	 * if one exists. If the stack is empty, returns null.
	 *
	 * @since 6.4.0
	 *
	 * @return WP_HTML_Token|null Last node in the stack of active formatting elements, if one exists, otherwise null.
	 */
	public function current_node() {
		$current_node = end( $this-&gt;stack );

		return $current_node ? $current_node : null;
	}

	/**
	 * Inserts a &quot;marker&quot; at the end of the list of active formatting elements.
	 *
	 * &gt; The markers are inserted when entering applet, object, marquee,
	 * &gt; template, td, th, and caption elements, and are used to prevent
	 * &gt; formatting from &quot;leaking&quot; into applet, object, marquee, template,
	 * &gt; td, th, and caption elements.
	 *
	 * @see https://html.spec.whatwg.org/#concept-parser-marker
	 *
	 * @since 6.7.0
	 */
	public function insert_marker(): void {
		$this-&gt;push( new WP_HTML_Token( null, 'marker', false ) );
	}

	/**
	 * Pushes a node onto the stack of active formatting elements.
	 *
	 * @since 6.4.0
	 *
	 * @see https://html.spec.whatwg.org/#push-onto-the-list-of-active-formatting-elements
	 *
	 * @param WP_HTML_Token $token Push this node onto the stack.
	 */
	public function push( WP_HTML_Token $token ) {
		/*
		 * &gt; If there are already three elements in the list of active formatting elements after the last marker,
		 * &gt; if any, or anywhere in the list if there are no markers, that have the same tag name, namespace, and
		 * &gt; attributes as element, then remove the earliest such element from the list of active formatting
		 * &gt; elements. For these purposes, the attributes must be compared as they were when the elements were
		 * &gt; created by the parser; two elements have the same attributes if all their parsed attributes can be
		 * &gt; paired such that the two attributes in each pair have identical names, namespaces, and values
		 * &gt; (the order of the attributes does not matter).
		 *
		 * @todo Implement the &quot;Noah's Ark clause&quot; to only add up to three of any given kind of formatting elements to the stack.
		 */
		// &gt; Add element to the list of active formatting elements.
		$this-&gt;stack[] = $token;
	}

	/**
	 * Removes a node from the stack of active formatting elements.
	 *
	 * @since 6.4.0
	 *
	 * @param WP_HTML_Token $token Remove this node from the stack, if it's there already.
	 * @return bool Whether the node was found and removed from the stack of active formatting elements.
	 */
	public function remove_node( WP_HTML_Token $token ) {
		foreach ( $this-&gt;walk_up() as $position_from_end =&gt; $item ) {
			if ( $token-&gt;bookmark_name !== $item-&gt;bookmark_name ) {
				continue;
			}

			$position_from_start = $this-&gt;count() - $position_from_end - 1;
			array_splice( $this-&gt;stack, $position_from_start, 1 );
			return true;
		}

		return false;
	}

	/**
	 * Steps through the stack of active formatting elements, starting with the
	 * top element (added first) and walking downwards to the one added last.
	 *
	 * This generator function is designed to be used inside a &quot;foreach&quot; loop.
	 *
	 * Example:
	 *
	 *     $html = '&lt;em&gt;&lt;strong&gt;&lt;a&gt;We are here';
	 *     foreach ( $stack-&gt;walk_down() as $node ) {
	 *         echo &quot;{$node-&gt;node_name} -&gt; &quot;;
	 *     }
	 *     &gt; EM -&gt; STRONG -&gt; A -&gt;
	 *
	 * To start with the most-recently added element and walk towards the top,
	 * see WP_HTML_Active_Formatting_Elements::walk_up().
	 *
	 * @since 6.4.0
	 */
	public function walk_down() {
		$count = count( $this-&gt;stack );

		for ( $i = 0; $i &lt; $count; $i++ ) {
			yield $this-&gt;stack[ $i ];
		}
	}

	/**
	 * Steps through the stack of active formatting elements, starting with the
	 * bottom element (added last) and walking upwards to the one added first.
	 *
	 * This generator function is designed to be used inside a &quot;foreach&quot; loop.
	 *
	 * Example:
	 *
	 *     $html = '&lt;em&gt;&lt;strong&gt;&lt;a&gt;We are here';
	 *     foreach ( $stack-&gt;walk_up() as $node ) {
	 *         echo &quot;{$node-&gt;node_name} -&gt; &quot;;
	 *     }
	 *     &gt; A -&gt; STRONG -&gt; EM -&gt;
	 *
	 * To start with the first added element and walk towards the bottom,
	 * see WP_HTML_Active_Formatting_Elements::walk_down().
	 *
	 * @since 6.4.0
	 */
	public function walk_up() {
		for ( $i = count( $this-&gt;stack ) - 1; $i &gt;= 0; $i-- ) {
			yield $this-&gt;stack[ $i ];
		}
	}

	/**
	 * Clears the list of active formatting elements up to the last marker.
	 *
	 * &gt; When the steps below require the UA to clear the list of active formatting elements up to
	 * &gt; the last marker, the UA must perform the following steps:
	 * &gt;
	 * &gt; 1. Let entry be the last (most recently added) entry in the list of active
	 * &gt;    formatting elements.
	 * &gt; 2. Remove entry from the list of active formatting elements.
	 * &gt; 3. If entry was a marker, then stop the algorithm at this point.
	 * &gt;    The list has been cleared up to the last marker.
	 * &gt; 4. Go to step 1.
	 *
	 * @see https://html.spec.whatwg.org/multipage/parsing.html#clear-the-list-of-active-formatting-elements-up-to-the-last-marker
	 *
	 * @since 6.7.0
	 */
	public function clear_up_to_last_marker(): void {
		foreach ( $this-&gt;walk_up() as $item ) {
			array_pop( $this-&gt;stack );
			if ( 'marker' === $item-&gt;node_name ) {
				break;
			}
		}
	}
}
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