<?php
/**
 * 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.
 *
 * > Initially, the list of active formatting elements is empty.
 * > It is used to handle mis-nested formatting element tags.
 * >
 * > The list contains elements in the formatting category, and markers.
 * > The markers are inserted when entering applet, object, marquee,
 * > template, td, th, and caption elements, and are used to prevent
 * > formatting from "leaking" into applet, object, marquee, template,
 * > td, th, and caption elements.
 * >
 * > In addition, each element in the list of active formatting elements
 * > is associated with the token for which it was created, so that
 * > further elements can be created for that token if necessary.
 *
 * @since 6.4.0
 *
 * @access private
 * @ignore
 *
 * @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->walk_up() as $item ) {
            if ( $token->bookmark_name === $item->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->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->stack );

        return $current_node ? $current_node : null;
    }

    /**
     * Inserts a "marker" at the end of the list of active formatting elements.
     *
     * > The markers are inserted when entering applet, object, marquee,
     * > template, td, th, and caption elements, and are used to prevent
     * > formatting from "leaking" into applet, object, marquee, template,
     * > td, th, and caption elements.
     *
     * @see https://html.spec.whatwg.org/#concept-parser-marker
     *
     * @since 6.7.0
     */
    public function insert_marker(): void {
        $this->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 ) {
        /*
         * > If there are already three elements in the list of active formatting elements after the last marker,
         * > if any, or anywhere in the list if there are no markers, that have the same tag name, namespace, and
         * > attributes as element, then remove the earliest such element from the list of active formatting
         * > elements. For these purposes, the attributes must be compared as they were when the elements were
         * > created by the parser; two elements have the same attributes if all their parsed attributes can be
         * > paired such that the two attributes in each pair have identical names, namespaces, and values
         * > (the order of the attributes does not matter).
         *
         * @todo Implement the "Noah's Ark clause" to only add up to three of any given kind of formatting elements to the stack.
         */
        // > Add element to the list of active formatting elements.
        $this->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->walk_up() as $position_from_end => $item ) {
            if ( $token->bookmark_name !== $item->bookmark_name ) {
                continue;
            }

            $position_from_start = $this->count() - $position_from_end - 1;
            array_splice( $this->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 "foreach" loop.
     *
     * Example:
     *
     *     $html = '<em><strong><a>We are here';
     *     foreach ( $stack->walk_down() as $node ) {
     *         echo "{$node->node_name} -> ";
     *     }
     *     > EM -> STRONG -> A ->
     *
     * 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->stack );

        for ( $i = 0; $i < $count; $i++ ) {
            yield $this->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 "foreach" loop.
     *
     * Example:
     *
     *     $html = '<em><strong><a>We are here';
     *     foreach ( $stack->walk_up() as $node ) {
     *         echo "{$node->node_name} -> ";
     *     }
     *     > A -> STRONG -> EM ->
     *
     * 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->stack ) - 1; $i >= 0; $i-- ) {
            yield $this->stack[ $i ];
        }
    }

    /**
     * Clears the list of active formatting elements up to the last marker.
     *
     * > When the steps below require the UA to clear the list of active formatting elements up to
     * > the last marker, the UA must perform the following steps:
     * >
     * > 1. Let entry be the last (most recently added) entry in the list of active
     * >    formatting elements.
     * > 2. Remove entry from the list of active formatting elements.
     * > 3. If entry was a marker, then stop the algorithm at this point.
     * >    The list has been cleared up to the last marker.
     * > 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->walk_up() as $item ) {
            array_pop( $this->stack );
            if ( 'marker' === $item->node_name ) {
                break;
            }
        }
    }
}