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[mediawiki.git] / includes / tidy / Balancer.php

<?php
/**
 * An implementation of the tree building portion of the HTML5 parsing
 * spec.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Parser
 * @since 1.27
 * @author C. Scott Ananian, 2016
 */
namespace MediaWiki\Tidy;

use Wikimedia\Assert\Assert;
use Wikimedia\Assert\ParameterAssertionException;
use \ExplodeIterator;
use \IteratorAggregate;
use \ReverseArrayIterator;
use \Sanitizer;

// A note for future librarization[1] -- this file is a good candidate
// for splitting into an independent library, except that it is currently
// highly optimized for MediaWiki use.  It only implements the portions
// of the HTML5 tree builder used by tags supported by MediaWiki, and
// does not contain a true tokenizer pass, instead relying on
// comment stripping, attribute normalization, and escaping done by
// the MediaWiki Sanitizer.  It also deliberately avoids building
// a true DOM in memory, instead serializing elements to an output string
// as soon as possible (usually as soon as the tag is closed) to reduce
// its memory footprint.

// We've been gradually lifting some of these restrictions to handle
// non-sanitized output generated by extensions, but we shortcut the tokenizer
// for speed (primarily by splitting on `<`) and so rely on syntactic
// well-formedness.

// On the other hand, I've been pretty careful to note with comments in the
// code the places where this implementation omits features of the spec or
// depends on the MediaWiki Sanitizer.  Perhaps in the future we'll want to
// implement the missing pieces and make this a standalone PHP HTML5 parser.
// In order to do so, some sort of MediaWiki-specific API will need
// to be added to (a) allow the Balancer to bypass the tokenizer,
// and (b) support on-the-fly flattening instead of DOM node creation.

// [1]: https://www.mediawiki.org/wiki/Library_infrastructure_for_MediaWiki

/**
 * Utility constants and sets for the HTML5 tree building algorithm.
 * Sets are associative arrays indexed first by namespace and then by
 * lower-cased tag name.
 *
 * @ingroup Parser
 * @since 1.27
 */
class BalanceSets {
        const HTML_NAMESPACE = 'http://www.w3.org/1999/xhtml';
        const MATHML_NAMESPACE = 'http://www.w3.org/1998/Math/MathML';
        const SVG_NAMESPACE = 'http://www.w3.org/2000/svg';

        public static $unsupportedSet = [
                self::HTML_NAMESPACE => [
                        'html' => true, 'head' => true, 'body' => true, 'frameset' => true,
                        'frame' => true,
                        'plaintext' => true, 'isindex' => true,
                        'xmp' => true, 'iframe' => true, 'noembed' => true,
                        'noscript' => true, 'script' => true,
                        'title' => true
                ]
        ];

        public static $emptyElementSet = [
                self::HTML_NAMESPACE => [
                        'area' => true, 'base' => true, 'basefont' => true,
                        'bgsound' => true, 'br' => true, 'col' => true, 'command' => true,
                        'embed' => true, 'frame' => true, 'hr' => true, 'img' => true,
                        'input' => true, 'keygen' => true, 'link' => true, 'meta' => true,
                        'param' => true, 'source' => true, 'track' => true, 'wbr' => true
                ]
        ];

        public static $extraLinefeedSet = [
                self::HTML_NAMESPACE => [
                        'pre' => true, 'textarea' => true, 'listing' => true,
                ]
        ];

        public static $headingSet = [
                self::HTML_NAMESPACE => [
                        'h1' => true, 'h2' => true, 'h3' => true,
                        'h4' => true, 'h5' => true, 'h6' => true
                ]
        ];

        public static $specialSet = [
                self::HTML_NAMESPACE => [
                        'address' => true, 'applet' => true, 'area' => true,
                        'article' => true, 'aside' => true, 'base' => true,
                        'basefont' => true, 'bgsound' => true, 'blockquote' => true,
                        'body' => true, 'br' => true, 'button' => true, 'caption' => true,
                        'center' => true, 'col' => true, 'colgroup' => true, 'dd' => true,
                        'details' => true, 'dir' => true, 'div' => true, 'dl' => true,
                        'dt' => true, 'embed' => true, 'fieldset' => true,
                        'figcaption' => true, 'figure' => true, 'footer' => true,
                        'form' => true, 'frame' => true, 'frameset' => true, 'h1' => true,
                        'h2' => true, 'h3' => true, 'h4' => true, 'h5' => true,
                        'h6' => true, 'head' => true, 'header' => true, 'hgroup' => true,
                        'hr' => true, 'html' => true, 'iframe' => true, 'img' => true,
                        'input' => true, 'isindex' => true, 'li' => true, 'link' => true,
                        'listing' => true, 'main' => true, 'marquee' => true,
                        'menu' => true, 'menuitem' => true, 'meta' => true, 'nav' => true,
                        'noembed' => true, 'noframes' => true, 'noscript' => true,
                        'object' => true, 'ol' => true, 'p' => true, 'param' => true,
                        'plaintext' => true, 'pre' => true, 'script' => true,
                        'section' => true, 'select' => true, 'source' => true,
                        'style' => true, 'summary' => true, 'table' => true,
                        'tbody' => true, 'td' => true, 'template' => true,
                        'textarea' => true, 'tfoot' => true, 'th' => true, 'thead' => true,
                        'title' => true, 'tr' => true, 'track' => true, 'ul' => true,
                        'wbr' => true, 'xmp' => true
                ],
                self::SVG_NAMESPACE => [
                        'foreignobject' => true, 'desc' => true, 'title' => true
                ],
                self::MATHML_NAMESPACE => [
                        'mi' => true, 'mo' => true, 'mn' => true, 'ms' => true,
                        'mtext' => true, 'annotation-xml' => true
                ]
        ];

        public static $addressDivPSet = [
                self::HTML_NAMESPACE => [
                        'address' => true, 'div' => true, 'p' => true
                ]
        ];

        public static $tableSectionRowSet = [
                self::HTML_NAMESPACE => [
                        'table' => true, 'thead' => true, 'tbody' => true,
                        'tfoot' => true, 'tr' => true
                ]
        ];

        public static $impliedEndTagsSet = [
                self::HTML_NAMESPACE => [
                        'dd' => true, 'dt' => true, 'li' => true, 'optgroup' => true,
                        'option' => true, 'p' => true, 'rb' => true, 'rp' => true,
                        'rt' => true, 'rtc' => true
                ]
        ];

        public static $thoroughImpliedEndTagsSet = [
                self::HTML_NAMESPACE => [
                        'caption' => true, 'colgroup' => true, 'dd' => true, 'dt' => true,
                        'li' => true, 'optgroup' => true, 'option' => true, 'p' => true,
                        'rb' => true, 'rp' => true, 'rt' => true, 'rtc' => true,
                        'tbody' => true, 'td' => true, 'tfoot' => true, 'th' => true,
                        'thead' => true, 'tr' => true
                ]
        ];

        public static $tableCellSet = [
                self::HTML_NAMESPACE => [
                        'td' => true, 'th' => true
                ]
        ];
        public static $tableContextSet = [
                self::HTML_NAMESPACE => [
                        'table' => true, 'template' => true, 'html' => true
                ]
        ];

        public static $tableBodyContextSet = [
                self::HTML_NAMESPACE => [
                        'tbody' => true, 'tfoot' => true, 'thead' => true,
                        'template' => true, 'html' => true
                ]
        ];

        public static $tableRowContextSet = [
                self::HTML_NAMESPACE => [
                        'tr' => true, 'template' => true, 'html' => true
                ]
        ];

        // See https://html.spec.whatwg.org/multipage/forms.html#form-associated-element
        public static $formAssociatedSet = [
                self::HTML_NAMESPACE => [
                        'button' => true, 'fieldset' => true, 'input' => true,
                        'keygen' => true, 'object' => true, 'output' => true,
                        'select' => true, 'textarea' => true, 'img' => true
                ]
        ];

        public static $inScopeSet = [
                self::HTML_NAMESPACE => [
                        'applet' => true, 'caption' => true, 'html' => true,
                        'marquee' => true, 'object' => true,
                        'table' => true, 'td' => true, 'template' => true,
                        'th' => true
                ],
                self::SVG_NAMESPACE => [
                        'foreignobject' => true, 'desc' => true, 'title' => true
                ],
                self::MATHML_NAMESPACE => [
                        'mi' => true, 'mo' => true, 'mn' => true, 'ms' => true,
                        'mtext' => true, 'annotation-xml' => true
                ]
        ];

        private static $inListItemScopeSet = null;
        public static function inListItemScopeSet() {
                if ( self::$inListItemScopeSet === null ) {
                        self::$inListItemScopeSet = self::$inScopeSet;
                        self::$inListItemScopeSet[self::HTML_NAMESPACE]['ol'] = true;
                        self::$inListItemScopeSet[self::HTML_NAMESPACE]['ul'] = true;
                }
                return self::$inListItemScopeSet;
        }

        private static $inButtonScopeSet = null;
        public static function inButtonScopeSet() {
                if ( self::$inButtonScopeSet === null ) {
                        self::$inButtonScopeSet = self::$inScopeSet;
                        self::$inButtonScopeSet[self::HTML_NAMESPACE]['button'] = true;
                }
                return self::$inButtonScopeSet;
        }

        public static $inTableScopeSet = [
                self::HTML_NAMESPACE => [
                        'html' => true, 'table' => true, 'template' => true
                ]
        ];

        public static $inInvertedSelectScopeSet = [
                self::HTML_NAMESPACE => [
                        'option' => true, 'optgroup' => true
                ]
        ];

        public static $mathmlTextIntegrationPointSet = [
                self::MATHML_NAMESPACE => [
                        'mi' => true, 'mo' => true, 'mn' => true, 'ms' => true,
                        'mtext' => true
                ]
        ];

        public static $htmlIntegrationPointSet = [
                self::SVG_NAMESPACE => [
                        'foreignobject' => true,
                        'desc' => true,
                        'title' => true
                ]
        ];

        // For tidy compatibility.
        public static $tidyPWrapSet = [
                self::HTML_NAMESPACE => [
                        'body' => true, 'blockquote' => true,
                        // We parse with <body> as the fragment context, but the top-level
                        // element on the stack is actually <html>.  We could use the
                        // "adjusted current node" everywhere to work around this, but it's
                        // easier just to add <html> to the p-wrap set.
                        'html' => true,
                ],
        ];
        public static $tidyInlineSet = [
                self::HTML_NAMESPACE => [
                        'a' => true, 'abbr' => true, 'acronym' => true, 'applet' => true,
                        'b' => true, 'basefont' => true, 'bdo' => true, 'big' => true,
                        'br' => true, 'button' => true, 'cite' => true, 'code' => true,
                        'dfn' => true, 'em' => true, 'font' => true, 'i' => true,
                        'iframe' => true, 'img' => true, 'input' => true, 'kbd' => true,
                        'label' => true, 'legend' => true, 'map' => true, 'object' => true,
                        'param' => true, 'q' => true, 'rb' => true, 'rbc' => true,
                        'rp' => true, 'rt' => true, 'rtc' => true, 'ruby' => true,
                        's' => true, 'samp' => true, 'select' => true, 'small' => true,
                        'span' => true, 'strike' => true, 'strong' => true, 'sub' => true,
                        'sup' => true, 'textarea' => true, 'tt' => true, 'u' => true,
                        'var' => true,
                ],
        ];
}

/**
 * A BalanceElement is a simplified version of a DOM Node.  The main
 * difference is that we only keep BalanceElements around for nodes
 * currently on the BalanceStack of open elements.  As soon as an
 * element is closed, with some minor exceptions relating to the
 * tree builder "adoption agency algorithm", the element and all its
 * children are serialized to a string using the flatten() method.
 * This keeps our memory usage low.
 *
 * @ingroup Parser
 * @since 1.27
 */
class BalanceElement {
        /**
         * The namespace of the element.
         * @var string $namespaceURI
         */
        public $namespaceURI;
        /**
         * The lower-cased name of the element.
         * @var string $localName
         */
        public $localName;
        /**
         * Attributes for the element, in array form
         * @var array $attribs
         */
        public $attribs;

        /**
         * Parent of this element, or the string "flat" if this element has
         * already been flattened into its parent.
         * @var BalanceElement|string|null $parent
         */
        public $parent;

        /**
         * An array of children of this element.  Typically only the last
         * child will be an actual BalanceElement object; the rest will
         * be strings, representing either text nodes or flattened
         * BalanceElement objects.
         * @var BalanceElement[]|string[] $children
         */
        public $children;

        /**
         * A unique string identifier for Noah's Ark purposes, lazy initialized
         */
        private $noahKey;

        /**
         * The next active formatting element in the list, or null if this is the
         * end of the AFE list or if the element is not in the AFE list.
         */
        public $nextAFE;

        /**
         * The previous active formatting element in the list, or null if this is
         * the start of the list or if the element is not in the AFE list.
         */
        public $prevAFE;

        /**
         * The next element in the Noah's Ark species bucket.
         */
        public $nextNoah;

        /**
         * Make a new BalanceElement corresponding to the HTML DOM Element
         * with the given localname, namespace, and attributes.
         *
         * @param string $namespaceURI The namespace of the element.
         * @param string $localName The lowercased name of the tag.
         * @param array $attribs Attributes of the element
         */
        public function __construct( $namespaceURI, $localName, array $attribs ) {
                $this->localName = $localName;
                $this->namespaceURI = $namespaceURI;
                $this->attribs = $attribs;
                $this->contents = '';
                $this->parent = null;
                $this->children = [];
        }

        /**
         * Remove the given child from this element.
         * @param BalanceElement $elt
         */
        private function removeChild( BalanceElement $elt ) {
                Assert::precondition(
                        $this->parent !== 'flat', "Can't removeChild after flattening $this"
                );
                Assert::parameter(
                        $elt->parent === $this, 'elt', 'must have $this as a parent'
                );
                $idx = array_search( $elt, $this->children, true );
                Assert::parameter( $idx !== false, '$elt', 'must be a child of $this' );
                $elt->parent = null;
                array_splice( $this->children, $idx, 1 );
        }

        /**
         * Find $a in the list of children and insert $b before it.
         * @param BalanceElement $a
         * @param BalanceElement|string $b
         */
        public function insertBefore( BalanceElement $a, $b ) {
                Assert::precondition(
                        $this->parent !== 'flat', "Can't insertBefore after flattening."
                );
                $idx = array_search( $a, $this->children, true );
                Assert::parameter( $idx !== false, '$a', 'must be a child of $this' );
                if ( is_string( $b ) ) {
                        array_splice( $this->children, $idx, 0, [ $b ] );
                } else {
                        Assert::parameter( $b->parent !== 'flat', '$b', "Can't be flat" );
                        if ( $b->parent !== null ) {
                                $b->parent->removeChild( $b );
                        }
                        array_splice( $this->children, $idx, 0, [ $b ] );
                        $b->parent = $this;
                }
        }

        /**
         * Append $elt to the end of the list of children.
         * @param BalanceElement|string $elt
         */
        public function appendChild( $elt ) {
                Assert::precondition(
                        $this->parent !== 'flat', "Can't appendChild after flattening."
                );
                if ( is_string( $elt ) ) {
                        array_push( $this->children, $elt );
                        return;
                }
                // Remove $elt from parent, if it had one.
                if ( $elt->parent !== null ) {
                        $elt->parent->removeChild( $elt );
                }
                array_push( $this->children, $elt );
                $elt->parent = $this;
        }

        /**
         * Transfer all of the children of $elt to $this.
         * @param BalanceElement $elt
         */
        public function adoptChildren( BalanceElement $elt ) {
                Assert::precondition(
                        $elt->parent !== 'flat', "Can't adoptChildren after flattening."
                );
                foreach ( $elt->children as $child ) {
                        if ( !is_string( $child ) ) {
                                // This is an optimization which avoids an O(n^2) set of
                                // array_splice operations.
                                $child->parent = null;
                        }
                        $this->appendChild( $child );
                }
                $elt->children = [];
        }

        /**
         * Flatten this node and all of its children into a string, as specified
         * by the HTML serialization specification, and replace this node
         * in its parent by that string.
         *
         * @param array $config Balancer configuration; see Balancer::__construct().
         * @return string
         *
         * @see __toString()
         */
        public function flatten( array $config ) {
                Assert::parameter( $this->parent !== null, '$this', 'must be a child' );
                Assert::parameter( $this->parent !== 'flat', '$this', 'already flat' );
                $idx = array_search( $this, $this->parent->children, true );
                Assert::parameter(
                        $idx !== false, '$this', 'must be a child of its parent'
                );
                $tidyCompat = $config['tidyCompat'];
                if ( $tidyCompat ) {
                        $blank = true;
                        foreach ( $this->children as $elt ) {
                                if ( !is_string( $elt ) ) {
                                        $elt = $elt->flatten( $config );
                                }
                                if ( $blank && preg_match( '/[^\t\n\f\r ]/', $elt ) ) {
                                        $blank = false;
                                }
                        }
                        if ( $this->isHtmlNamed( 'mw:p-wrap' ) ) {
                                $this->localName = 'p';
                        } elseif ( $blank ) {
                                // Add 'mw-empty-elt' class so elements can be hidden via CSS
                                // for compatibility with legacy tidy.
                                if ( !count( $this->attribs ) &&
                                        ( $this->localName === 'tr' || $this->localName === 'li' )
                                ) {
                                        $this->attribs = [ 'class' => "mw-empty-elt" ];
                                }
                                $blank = false;
                        }
                        $flat = $blank ? '' : "{$this}";
                } else {
                        $flat = "{$this}";
                }
                $this->parent->children[$idx] = $flat;
                $this->parent = 'flat'; // for assertion checking
                return $flat;
        }

        /**
         * Serialize this node and all of its children to a string, as specified
         * by the HTML serialization specification.
         *
         * @return string The serialization of the BalanceElement
         * @see https://html.spec.whatwg.org/multipage/syntax.html#serialising-html-fragments
         */
        public function __toString() {
                $encAttribs = '';
                foreach ( $this->attribs as $name => $value ) {
                        $encValue = Sanitizer::encodeAttribute( $value );
                        $encAttribs .= " $name=\"$encValue\"";
                }
                if ( !$this->isA( BalanceSets::$emptyElementSet ) ) {
                        $out = "<{$this->localName}{$encAttribs}>";
                        $len = strlen( $out );
                        // flatten children
                        foreach ( $this->children as $elt ) {
                                $out .= "{$elt}";
                        }
                        $out .= "</{$this->localName}>";
                        if (
                                $this->isA( BalanceSets::$extraLinefeedSet ) &&
                                $out[$len] === "\n"
                        ) {
                                // Double the linefeed after pre/listing/textarea
                                // according to the HTML5 fragment serialization algorithm.
                                $out = substr( $out, 0, $len + 1 ) .
                                        substr( $out, $len );
                        }
                } else {
                        $out = "<{$this->localName}{$encAttribs} />";
                        Assert::invariant(
                                count( $this->children ) === 0,
                                "Empty elements shouldn't have children."
                        );
                }
                return $out;
        }

        // Utility functions on BalanceElements.

        /**
         * Determine if $this represents a specific HTML tag, is a member of
         * a tag set, or is equal to another BalanceElement.
         *
         * @param BalanceElement|array|string $set The target BalanceElement,
         *   set (from the BalanceSets class), or string (HTML tag name).
         * @return bool
         */
        public function isA( $set ) {
                if ( $set instanceof BalanceElement ) {
                        return $this === $set;
                } elseif ( is_array( $set ) ) {
                        return isset( $set[$this->namespaceURI] ) &&
                                isset( $set[$this->namespaceURI][$this->localName] );
                } else {
                        // assume this is an HTML element name.
                        return $this->isHtml() && $this->localName === $set;
                }
        }

        /**
         * Determine if this element is an HTML element with the specified name
         * @param string $tagName
         * @return bool
         */
        public function isHtmlNamed( $tagName ) {
                return $this->namespaceURI === BalanceSets::HTML_NAMESPACE
                        && $this->localName === $tagName;
        }

        /**
         * Determine if $this represents an element in the HTML namespace.
         *
         * @return bool
         */
        public function isHtml() {
                return $this->namespaceURI === BalanceSets::HTML_NAMESPACE;
        }

        /**
         * Determine if $this represents a MathML text integration point,
         * as defined in the HTML5 specification.
         *
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#mathml-text-integration-point
         */
        public function isMathmlTextIntegrationPoint() {
                return $this->isA( BalanceSets::$mathmlTextIntegrationPointSet );
        }

        /**
         * Determine if $this represents an HTML integration point,
         * as defined in the HTML5 specification.
         *
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#html-integration-point
         */
        public function isHtmlIntegrationPoint() {
                if ( $this->isA( BalanceSets::$htmlIntegrationPointSet ) ) {
                        return true;
                }
                if (
                        $this->namespaceURI === BalanceSets::MATHML_NAMESPACE &&
                        $this->localName === 'annotation-xml' &&
                        isset( $this->attribs['encoding'] ) &&
                        ( strcasecmp( $this->attribs['encoding'], 'text/html' ) == 0 ||
                        strcasecmp( $this->attribs['encoding'], 'application/xhtml+xml' ) == 0 )
                ) {
                        return true;
                }
                return false;
        }

        /**
         * Get a string key for the Noah's Ark algorithm
         */
        public function getNoahKey() {
                if ( $this->noahKey === null ) {
                        $attribs = $this->attribs;
                        ksort( $attribs );
                        $this->noahKey = serialize( [ $this->namespaceURI, $this->localName, $attribs ] );
                }
                return $this->noahKey;
        }
}

/**
 * The "stack of open elements" as defined in the HTML5 tree builder
 * spec.  This contains methods to ensure that content (start tags, text)
 * are inserted at the correct place in the output string, and to
 * flatten BalanceElements are they are closed to avoid holding onto
 * a complete DOM tree for the document in memory.
 *
 * The stack defines a PHP iterator to traverse it in "reverse order",
 * that is, the most-recently-added element is visited first in a
 * foreach loop.
 *
 * @ingroup Parser
 * @since 1.27
 * @see https://html.spec.whatwg.org/multipage/syntax.html#the-stack-of-open-elements
 */
class BalanceStack implements IteratorAggregate {
        /**
         * Backing storage for the stack.
         * @var BalanceElement[] $elements
         */
        private $elements = [];
        /**
         * Foster parent mode determines how nodes are inserted into the
         * stack.
         * @var bool $fosterParentMode
         * @see https://html.spec.whatwg.org/multipage/syntax.html#foster-parent
         */
        public $fosterParentMode = false;
        /**
         * Configuration options governing flattening.
         * @var array $config
         * @see Balancer::__construct()
         */
        private $config;
        /**
         * Reference to the current element
         */
        public $currentNode;

        /**
         * Create a new BalanceStack with a single BalanceElement on it,
         * representing the root &lt;html&gt; node.
         * @param array $config Balancer configuration; see Balancer::_construct().
         */
        public function __construct( array $config ) {
                // always a root <html> element on the stack
                array_push(
                        $this->elements,
                        new BalanceElement( BalanceSets::HTML_NAMESPACE, 'html', [] )
                );
                $this->currentNode = $this->elements[0];
                $this->config = $config;
        }

        /**
         * Return a string representing the output of the tree builder:
         * all the children of the root &lt;html&gt; node.
         * @return string
         */
        public function getOutput() {
                // Don't include the outer '<html>....</html>'
                $out = '';
                foreach ( $this->elements[0]->children as $elt ) {
                        $out .= is_string( $elt ) ? $elt :
                                $elt->flatten( $this->config );
                }
                return $out;
        }

        /**
         * Insert a comment at the appropriate place for inserting a node.
         * @param string $value Content of the comment.
         * @see https://html.spec.whatwg.org/multipage/syntax.html#insert-a-comment
         */
        public function insertComment( $value ) {
                // Just another type of text node, except for tidy p-wrapping.
                return $this->insertText( '<!--' . $value . '-->', true );
        }

        /**
         * Insert text at the appropriate place for inserting a node.
         * @param string $value
         * @param bool $isComment
         * @see https://html.spec.whatwg.org/multipage/syntax.html#appropriate-place-for-inserting-a-node
         */
        public function insertText( $value, $isComment = false ) {
                if (
                        $this->fosterParentMode &&
                        $this->currentNode->isA( BalanceSets::$tableSectionRowSet )
                ) {
                        $this->fosterParent( $value );
                } elseif (
                        $this->config['tidyCompat'] && !$isComment &&
                        $this->currentNode->isA( BalanceSets::$tidyPWrapSet )
                ) {
                        $this->insertHTMLElement( 'mw:p-wrap', [] );
                        return $this->insertText( $value );
                } else {
                        $this->currentNode->appendChild( $value );
                }
        }

        /**
         * Insert a BalanceElement at the appropriate place, pushing it
         * on to the open elements stack.
         * @param string $namespaceURI The element namespace
         * @param string $tag The tag name
         * @param string $attribs Normalized attributes, as a string.
         * @return BalanceElement
         * @see https://html.spec.whatwg.org/multipage/syntax.html#insert-a-foreign-element
         */
        public function insertForeignElement( $namespaceURI, $tag, $attribs ) {
                return $this->insertElement(
                        new BalanceElement( $namespaceURI, $tag, $attribs )
                );
        }

        /**
         * Insert an HTML element at the appropriate place, pushing it on to
         * the open elements stack.
         * @param string $tag The tag name
         * @param string $attribs Normalized attributes, as a string.
         * @return BalanceElement
         * @see https://html.spec.whatwg.org/multipage/syntax.html#insert-an-html-element
         */
        public function insertHTMLElement( $tag, $attribs ) {
                return $this->insertForeignElement(
                        BalanceSets::HTML_NAMESPACE, $tag, $attribs
                );
        }

        /**
         * Insert an element at the appropriate place and push it on to the
         * open elements stack.
         * @param BalanceElement $elt
         * @return BalanceElement
         * @see https://html.spec.whatwg.org/multipage/syntax.html#appropriate-place-for-inserting-a-node
         */
        public function insertElement( BalanceElement $elt ) {
                if (
                        $this->currentNode->isHtmlNamed( 'mw:p-wrap' ) &&
                        !$elt->isA( BalanceSets::$tidyInlineSet )
                ) {
                        // Tidy compatibility.
                        $this->pop();
                }
                if (
                        $this->fosterParentMode &&
                        $this->currentNode->isA( BalanceSets::$tableSectionRowSet )
                ) {
                        $elt = $this->fosterParent( $elt );
                } else {
                        $this->currentNode->appendChild( $elt );
                }
                Assert::invariant( $elt->parent !== null, "$elt must be in tree" );
                Assert::invariant( $elt->parent !== 'flat', "$elt must not have been previous flattened" );
                array_push( $this->elements, $elt );
                $this->currentNode = $elt;
                return $elt;
        }

        /**
         * Determine if the stack has $tag in scope.
         * @param BalanceElement|array|string $tag
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-scope
         */
        public function inScope( $tag ) {
                return $this->inSpecificScope( $tag, BalanceSets::$inScopeSet );
        }

        /**
         * Determine if the stack has $tag in button scope.
         * @param BalanceElement|array|string $tag
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-button-scope
         */
        public function inButtonScope( $tag ) {
                return $this->inSpecificScope( $tag, BalanceSets::inButtonScopeSet() );
        }

        /**
         * Determine if the stack has $tag in list item scope.
         * @param BalanceElement|array|string $tag
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-list-item-scope
         */
        public function inListItemScope( $tag ) {
                return $this->inSpecificScope( $tag, BalanceSets::inListItemScopeSet() );
        }

        /**
         * Determine if the stack has $tag in table scope.
         * @param BalanceElement|array|string $tag
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-table-scope
         */
        public function inTableScope( $tag ) {
                return $this->inSpecificScope( $tag, BalanceSets::$inTableScopeSet );
        }

        /**
         * Determine if the stack has $tag in select scope.
         * @param BalanceElement|array|string $tag
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-select-scope
         */
        public function inSelectScope( $tag ) {
                // Can't use inSpecificScope to implement this, since it involves
                // *inverting* a set of tags.  Implement manually.
                foreach ( $this as $elt ) {
                        if ( $elt->isA( $tag ) ) {
                                return true;
                        }
                        if ( !$elt->isA( BalanceSets::$inInvertedSelectScopeSet ) ) {
                                return false;
                        }
                }
                return false;
        }

        /**
         * Determine if the stack has $tag in a specific scope, $set.
         * @param BalanceElement|array|string $tag
         * @param BalanceElement|array|string $set
         * @return bool
         * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-the-specific-scope
         */
        public function inSpecificScope( $tag, $set ) {
                foreach ( $this as $elt ) {
                        if ( $elt->isA( $tag ) ) {
                                return true;
                        }
                        if ( $elt->isA( $set ) ) {
                                return false;
                        }
                }
                return false;
        }

        /**
         * Generate implied end tags.
         * @param string $butnot
         * @param bool $thorough True if we should generate end tags thoroughly.
         * @see https://html.spec.whatwg.org/multipage/syntax.html#generate-implied-end-tags
         */
        public function generateImpliedEndTags( $butnot = null, $thorough = false ) {
                $endTagSet = $thorough ?
                        BalanceSets::$thoroughImpliedEndTagsSet :
                        BalanceSets::$impliedEndTagsSet;
                while ( $this->currentNode ) {
                        if ( $butnot !== null && $this->currentNode->isHtmlNamed( $butnot ) ) {
                                break;
                        }
                        if ( !$this->currentNode->isA( $endTagSet ) ) {
                                break;
                        }
                        $this->pop();
                }
        }

        /**
         * Return the adjusted current node.
         */
        public function adjustedCurrentNode( $fragmentContext ) {
                return ( $fragmentContext && count( $this->elements ) === 1 ) ?
                        $fragmentContext : $this->currentNode;
        }

        /**
         * Return an iterator over this stack which visits the current node
         * first, and the root node last.
         * @return \Iterator
         */
        public function getIterator() {
                return new ReverseArrayIterator( $this->elements );
        }

        /**
         * Return the BalanceElement at the given position $idx, where
         * position 0 represents the root element.
         * @param int $idx
         * @return BalanceElement
         */
        public function node( $idx ) {
                return $this->elements[ $idx ];
        }

        /**
         * Replace the element at position $idx in the BalanceStack with $elt.
         * @param int $idx
         * @param BalanceElement $elt
         */
        public function replaceAt( $idx, BalanceElement $elt ) {
                Assert::precondition(
                        $this->elements[$idx]->parent !== 'flat',
                        'Replaced element should not have already been flattened.'
                );
                Assert::precondition(
                        $elt->parent !== 'flat',
                        'New element should not have already been flattened.'
                );
                $this->elements[$idx] = $elt;
                if ( $idx === count( $this->elements ) - 1 ) {
                        $this->currentNode = $elt;
                }
        }

        /**
         * Return the position of the given BalanceElement, set, or
         * HTML tag name string in the BalanceStack.
         * @param BalanceElement|array|string $tag
         * @return int
         */
        public function indexOf( $tag ) {
                for ( $i = count( $this->elements ) - 1; $i >= 0; $i-- ) {
                        if ( $this->elements[$i]->isA( $tag ) ) {
                                return $i;
                        }
                }
                return -1;
        }

        /**
         * Return the number of elements currently in the BalanceStack.
         * @return int
         */
        public function length() {
                return count( $this->elements );
        }

        /**
         * Remove the current node from the BalanceStack, flattening it
         * in the process.
         */
        public function pop() {
                $elt = array_pop( $this->elements );
                if ( count( $this->elements ) ) {
                        $this->currentNode = $this->elements[ count( $this->elements ) - 1 ];
                } else {
                        $this->currentNode = null;
                }
                if ( !$elt->isHtmlNamed( 'mw:p-wrap' ) ) {
                        $elt->flatten( $this->config );
                }
        }

        /**
         * Remove all nodes up to and including position $idx from the
         * BalanceStack, flattening them in the process.
         * @param int $idx
         */
        public function popTo( $idx ) {
                for ( $length = count( $this->elements ); $length > $idx; $length-- ) {
                        $this->pop();
                }
        }

        /**
         * Pop elements off the stack up to and including the first
         * element with the specified HTML tagname (or matching the given
         * set).
         * @param BalanceElement|array|string $tag
         */
        public function popTag( $tag ) {
                while ( $this->currentNode ) {
                        if ( $this->currentNode->isA( $tag ) ) {
                                $this->pop();
                                break;
                        }
                        $this->pop();
                }
        }

        /**
         * Pop elements off the stack *not including* the first element
         * in the specified set.
         * @param BalanceElement|array|string $set
         */
        public function clearToContext( $set ) {
                // Note that we don't loop to 0. Never pop the <html> elt off.
                for ( $length = count( $this->elements ); $length > 1; $length-- ) {
                        if ( $this->currentNode->isA( $set ) ) {
                                break;
                        }
                        $this->pop();
                }
        }

        /**
         * Remove the given $elt from the BalanceStack, optionally
         * flattening it in the process.
         * @param BalanceElement $elt The element to remove.
         * @param bool $flatten Whether to flatten the removed element.
         */
        public function removeElement( BalanceElement $elt, $flatten = true ) {
                Assert::parameter(
                        $elt->parent !== 'flat',
                        '$elt',
                        '$elt should not already have been flattened.'
                );
                Assert::parameter(
                        $elt->parent->parent !== 'flat',
                        '$elt',
                        'The parent of $elt should not already have been flattened.'
                );
                $idx = array_search( $elt, $this->elements, true );
                Assert::parameter( $idx !== false, '$elt', 'must be in stack' );
                array_splice( $this->elements, $idx, 1 );
                if ( $idx === count( $this->elements ) ) {
                        $this->currentNode = $this->elements[$idx - 1];
                }
                if ( $flatten ) {
                        // serialize $elt into its parent
                        // otherwise, it will eventually serialize when the parent
                        // is serialized, we just hold onto the memory for its
                        // tree of objects a little longer.
                        $elt->flatten( $this->config );
                }
                Assert::postcondition(
                        array_search( $elt, $this->elements, true ) === false,
                        '$elt should no longer be in open elements stack'
                );
        }

        /**
         * Find $a in the BalanceStack and insert $b after it.
         * @param BalanceElement $a
         * @param BalanceElement $b
         */
        public function insertAfter( BalanceElement $a, BalanceElement $b ) {
                $idx = $this->indexOf( $a );
                Assert::parameter( $idx !== false, '$a', 'must be in stack' );
                if ( $idx === count( $this->elements ) - 1 ) {
                        array_push( $this->elements, $b );
                        $this->currentNode = $b;
                } else {
                        array_splice( $this->elements, $idx + 1, 0, [ $b ] );
                }
        }

        // Fostering and adoption.

        /**
         * Foster parent the given $elt in the stack of open elements.
         * @param BalanceElement|string $elt
         * @return BalanceElement|string
         *
         * @see https://html.spec.whatwg.org/multipage/syntax.html#foster-parent
         */
        private function fosterParent( $elt ) {
                $lastTable = $this->indexOf( 'table' );
                $lastTemplate = $this->indexOf( 'template' );
                $parent = null;
                $before = null;

                if ( $lastTemplate >= 0 && ( $lastTable < 0 || $lastTemplate > $lastTable ) ) {
                        $parent = $this->elements[$lastTemplate];
                } elseif ( $lastTable >= 0 ) {
                        $parent = $this->elements[$lastTable]->parent;
                        // Assume all tables have parents, since we're not running scripts!
                        Assert::invariant(
                                $parent !== null, "All tables should have parents"
                        );
                        $before = $this->elements[$lastTable];
                } else {
                        $parent = $this->elements[0]; // the `html` element.
                }

                if ( $this->config['tidyCompat'] ) {
                        if ( is_string( $elt ) ) {
                                // We're fostering text: do we need a p-wrapper?
                                if ( $parent->isA( BalanceSets::$tidyPWrapSet ) ) {
                                        $this->insertHTMLElement( 'mw:p-wrap', [] );
                                        $this->insertText( $elt );
                                        return $elt;
                                }
                        } else {
                                // We're fostering an element; do we need to merge p-wrappers?
                                if ( $elt->isHtmlNamed( 'mw:p-wrap' ) ) {
                                        $idx = $before ?
                                                array_search( $before, $parent->children, true ) :
                                                count( $parent->children );
                                        $after = $idx > 0 ? $parent->children[$idx - 1] : '';
                                        if (
                                                $after instanceof BalanceElement &&
                                                $after->isHtmlNamed( 'mw:p-wrap' )
                                        ) {
                                                return $after; // Re-use existing p-wrapper.
                                        }
                                }
                        }
                }

                if ( $before ) {
                        $parent->insertBefore( $before, $elt );
                } else {
                        $parent->appendChild( $elt );
                }
                return $elt;
        }

        /**
         * Run the "adoption agency algoritm" (AAA) for the given subject
         * tag name.
         * @param string $tag The subject tag name.
         * @param BalanceActiveFormattingElements $afe The current
         *   active formatting elements list.
         * @return true if the adoption agency algorithm "did something", false
         *   if more processing is required by the caller.
         * @see https://html.spec.whatwg.org/multipage/syntax.html#adoption-agency-algorithm
         */
        public function adoptionAgency( $tag, $afe ) {
                // If the current node is an HTML element whose tag name is subject,
                // and the current node is not in the list of active formatting
                // elements, then pop the current node off the stack of open
                // elements and abort these steps.
                if (
                        $this->currentNode->isHtmlNamed( $tag ) &&
                        !$afe->isInList( $this->currentNode )
                ) {
                        $this->pop();
                        return true; // no more handling required
                }

                // Outer loop: If outer loop counter is greater than or
                // equal to eight, then abort these steps.
                for ( $outer = 0; $outer < 8; $outer++ ) {
                        // Let the formatting element be the last element in the list
                        // of active formatting elements that: is between the end of
                        // the list and the last scope marker in the list, if any, or
                        // the start of the list otherwise, and has the same tag name
                        // as the token.
                        $fmtElt = $afe->findElementByTag( $tag );

                        // If there is no such node, then abort these steps and instead
                        // act as described in the "any other end tag" entry below.
                        if ( !$fmtElt ) {
                                return false; // false means handle by the default case
                        }

                        // Otherwise, if there is such a node, but that node is not in
                        // the stack of open elements, then this is a parse error;
                        // remove the element from the list, and abort these steps.
                        $index = $this->indexOf( $fmtElt );
                        if ( $index < 0 ) {
                                $afe->remove( $fmtElt );
                                return true;   // true means no more handling required
                        }

                        // Otherwise, if there is such a node, and that node is also in
                        // the stack of open elements, but the element is not in scope,
                        // then this is a parse error; ignore the token, and abort
                        // these steps.
                        if ( !$this->inScope( $fmtElt ) ) {
                                return true;
                        }

                        // Let the furthest block be the topmost node in the stack of
                        // open elements that is lower in the stack than the formatting
                        // element, and is an element in the special category. There
                        // might not be one.
                        $furthestBlock = null;
                        $furthestBlockIndex = -1;
                        $stackLength = $this->length();
                        for ( $i = $index+1; $i < $stackLength; $i++ ) {
                                if ( $this->node( $i )->isA( BalanceSets::$specialSet ) ) {
                                        $furthestBlock = $this->node( $i );
                                        $furthestBlockIndex = $i;
                                        break;
                                }
                        }

                        // If there is no furthest block, then the UA must skip the
                        // subsequent steps and instead just pop all the nodes from the
                        // bottom of the stack of open elements, from the current node
                        // up to and including the formatting element, and remove the
                        // formatting element from the list of active formatting
                        // elements.
                        if ( !$furthestBlock ) {
                                $this->popTag( $fmtElt );
                                $afe->remove( $fmtElt );
                                return true;
                        }

                        // Let the common ancestor be the element immediately above
                        // the formatting element in the stack of open elements.
                        $ancestor = $this->node( $index-1 );

                        // Let a bookmark note the position of the formatting
                        // element in the list of active formatting elements
                        // relative to the elements on either side of it in the
                        // list.
                        $BOOKMARK = new BalanceElement( '[bookmark]', '[bookmark]', [] );
                        $afe->insertAfter( $fmtElt, $BOOKMARK );

                        // Let node and last node be the furthest block.
                        $node = $furthestBlock;
                        $lastNode = $furthestBlock;
                        $nodeIndex = $furthestBlockIndex;
                        $isAFE = false;

                        // Inner loop
                        for ( $inner = 1; true; $inner++ ) {
                                // Let node be the element immediately above node in
                                // the stack of open elements, or if node is no longer
                                // in the stack of open elements (e.g. because it got
                                // removed by this algorithm), the element that was
                                // immediately above node in the stack of open elements
                                // before node was removed.
                                $node = $this->node( --$nodeIndex );

                                // If node is the formatting element, then go
                                // to the next step in the overall algorithm.
                                if ( $node === $fmtElt ) break;

                                // If the inner loop counter is greater than three and node
                                // is in the list of active formatting elements, then remove
                                // node from the list of active formatting elements.
                                $isAFE = $afe->isInList( $node );
                                if ( $inner > 3 && $isAFE ) {
                                        $afe->remove( $node );
                                        $isAFE = false;
                                }

                                // If node is not in the list of active formatting
                                // elements, then remove node from the stack of open
                                // elements and then go back to the step labeled inner
                                // loop.
                                if ( !$isAFE ) {
                                        // Don't flatten here, since we're about to relocate
                                        // parts of this $node.
                                        $this->removeElement( $node, false );
                                        continue;
                                }

                                // Create an element for the token for which the
                                // element node was created with common ancestor as
                                // the intended parent, replace the entry for node
                                // in the list of active formatting elements with an
                                // entry for the new element, replace the entry for
                                // node in the stack of open elements with an entry for
                                // the new element, and let node be the new element.
                                $newElt = new BalanceElement(
                                        $node->namespaceURI, $node->localName, $node->attribs );
                                $afe->replace( $node, $newElt );
                                $this->replaceAt( $nodeIndex, $newElt );
                                $node = $newElt;

                                // If last node is the furthest block, then move the
                                // aforementioned bookmark to be immediately after the
                                // new node in the list of active formatting elements.
                                if ( $lastNode === $furthestBlock ) {
                                        $afe->remove( $BOOKMARK );
                                        $afe->insertAfter( $newElt, $BOOKMARK );
                                }

                                // Insert last node into node, first removing it from
                                // its previous parent node if any.
                                $node->appendChild( $lastNode );

                                // Let last node be node.
                                $lastNode = $node;
                        }

                        // If the common ancestor node is a table, tbody, tfoot,
                        // thead, or tr element, then, foster parent whatever last
                        // node ended up being in the previous step, first removing
                        // it from its previous parent node if any.
                        if (
                                $this->fosterParentMode &&
                                $ancestor->isA( BalanceSets::$tableSectionRowSet )
                        ) {
                                $this->fosterParent( $lastNode );
                        } else {
                                // Otherwise, append whatever last node ended up being in
                                // the previous step to the common ancestor node, first
                                // removing it from its previous parent node if any.
                                $ancestor->appendChild( $lastNode );
                        }

                        // Create an element for the token for which the
                        // formatting element was created, with furthest block
                        // as the intended parent.
                        $newElt2 = new BalanceElement(
                                $fmtElt->namespaceURI, $fmtElt->localName, $fmtElt->attribs );

                        // Take all of the child nodes of the furthest block and
                        // append them to the element created in the last step.
                        $newElt2->adoptChildren( $furthestBlock );

                        // Append that new element to the furthest block.
                        $furthestBlock->appendChild( $newElt2 );

                        // Remove the formatting element from the list of active
                        // formatting elements, and insert the new element into the
                        // list of active formatting elements at the position of
                        // the aforementioned bookmark.
                        $afe->remove( $fmtElt );
                        $afe->replace( $BOOKMARK, $newElt2 );

                        // Remove the formatting element from the stack of open
                        // elements, and insert the new element into the stack of
                        // open elements immediately below the position of the
                        // furthest block in that stack.
                        $this->removeElement( $fmtElt );
                        $this->insertAfter( $furthestBlock, $newElt2 );
                }

                return true;
        }

        /**
         * Return the contents of the open elements stack as a string for
         * debugging.
         * @return string
         */
        public function __toString() {
                $r = [];
                foreach ( $this->elements as $elt ) {
                        array_push( $r, $elt->localName );
                }
                return implode( $r, ' ' );
        }
}

/**
 * A pseudo-element used as a marker in the list of active formatting elements
 *
 * @ingroup Parser
 * @since 1.27
 */
class BalanceMarker {
        public $nextAFE;
        public $prevAFE;
}

/**
 * The list of active formatting elements, which is used to handle
 * mis-nested formatting element tags in the HTML5 tree builder
 * specification.
 *
 * @ingroup Parser
 * @since 1.27
 * @see https://html.spec.whatwg.org/multipage/syntax.html#list-of-active-formatting-elements
 */
class BalanceActiveFormattingElements {
        /** The last (most recent) element in the list */
        private $tail;

        /** The first (least recent) element in the list */
        private $head;

        /**
         * An array of arrays representing the population of elements in each bucket
         * according to the Noah's Ark clause. The outer array is stack-like, with each
         * integer-indexed element representing a segment of the list, bounded by
         * markers. The first element represents the segment of the list before the
         * first marker.
         *
         * The inner arrays are indexed by "Noah key", which is a string which uniquely
         * identifies each bucket according to the rules in the spec. The value in
         * the inner array is the first (least recently inserted) element in the bucket,
         * and subsequent members of the bucket can be found by iterating through the
         * singly-linked list via $node->nextNoah.
         *
         * This is optimised for the most common case of inserting into a bucket
         * with zero members, and deleting a bucket containing one member. In the
         * worst case, iteration through the list is still O(1) in the document
         * size, since each bucket can have at most 3 members.
         */
        private $noahTableStack = [ [] ];

        public function __destruct() {
                $next = null;
                for ( $node = $this->head; $node; $node = $next ) {
                        $next = $node->nextAFE;
                        $node->prevAFE = $node->nextAFE = $node->nextNoah = null;
                }
                $this->head = $this->tail = $this->noahTableStack = null;
        }

        public function insertMarker() {
                $elt = new BalanceMarker;
                if ( $this->tail ) {
                        $this->tail->nextAFE = $elt;
                        $elt->prevAFE = $this->tail;
                } else {
                        $this->head = $elt;
                }
                $this->tail = $elt;
                $this->noahTableStack[] = [];
        }

        /**
         * Follow the steps required when the spec requires us to "push onto the
         * list of active formatting elements".
         * @param BalanceElement $elt
         */
        public function push( BalanceElement $elt ) {
                // Must not be in the list already
                if ( $elt->prevAFE !== null || $this->head === $elt ) {
                        throw new ParameterAssertionException( '$elt',
                                'Cannot insert a node into the AFE list twice' );
                }

                // "Noah's Ark clause" -- if there are already three copies of
                // this element before we encounter a marker, then drop the last
                // one.
                $noahKey = $elt->getNoahKey();
                $table =& $this->noahTableStack[ count( $this->noahTableStack ) - 1 ];
                if ( !isset( $table[$noahKey] ) ) {
                        $table[$noahKey] = $elt;
                } else {
                        $count = 1;
                        $head = $tail = $table[$noahKey];
                        while ( $tail->nextNoah ) {
                                $tail = $tail->nextNoah;
                                $count++;
                        }
                        if ( $count >= 3 ) {
                                $this->remove( $head );
                        }
                        $tail->nextNoah = $elt;
                }
                // Add to the main AFE list
                if ( $this->tail ) {
                        $this->tail->nextAFE = $elt;
                        $elt->prevAFE = $this->tail;
                } else {
                        $this->head = $elt;
                }
                $this->tail = $elt;
        }

        /**
         * Follow the steps required when the spec asks us to "clear the list of
         * active formatting elements up to the last marker".
         */
        public function clearToMarker() {
                // Iterate back through the list starting from the tail
                $tail = $this->tail;
                while ( $tail && !( $tail instanceof BalanceMarker ) ) {
                        // Unlink the element
                        $prev = $tail->prevAFE;
                        $tail->prevAFE = null;
                        if ( $prev ) {
                                $prev->nextAFE = null;
                        }
                        $tail->nextNoah = null;
                        $tail = $prev;
                }
                // If we finished on a marker, unlink it and pop it off the Noah table stack
                if ( $tail ) {
                        $prev = $tail->prevAFE;
                        if ( $prev ) {
                                $prev->nextAFE = null;
                        }
                        $tail = $prev;
                        array_pop( $this->noahTableStack );
                } else {
                        // No marker: wipe the top-level Noah table (which is the only one)
                        $this->noahTableStack[0] = [];
                }
                // If we removed all the elements, clear the head pointer
                if ( !$tail ) {
                        $this->head = null;
                }
                $this->tail = $tail;
        }

        /**
         * Find and return the last element with the specified tag between the
         * end of the list and the last marker on the list.
         * Used when parsing &lt;a&gt; "in body mode".
         */
        public function findElementByTag( $tag ) {
                $elt = $this->tail;
                while ( $elt && !( $elt instanceof BalanceMarker ) ) {
                        if ( $elt->localName === $tag ) {
                                return $elt;
                        }
                        $elt = $elt->prevAFE;
                }
                return null;
        }

        /**
         * Determine whether an element is in the list of formatting elements.
         * @param BalanceElement $elt
         * @return boolean
         */
        public function isInList( BalanceElement $elt ) {
                return $this->head === $elt || $elt->prevAFE;
        }

        /**
         * Find the element $elt in the list and remove it.
         * Used when parsing &lt;a&gt; in body mode.
         *
         * @param BalanceElement $elt
         */
        public function remove( BalanceElement $elt ) {
                if ( $this->head !== $elt && !$elt->prevAFE ) {
                        throw new ParameterAssertionException( '$elt',
                                "Attempted to remove an element which is not in the AFE list" );
                }
                // Update head and tail pointers
                if ( $this->head === $elt ) {
                        $this->head = $elt->nextAFE;
                }
                if ( $this->tail === $elt ) {
                        $this->tail = $elt->prevAFE;
                }
                // Update previous element
                if ( $elt->prevAFE ) {
                        $elt->prevAFE->nextAFE = $elt->nextAFE;
                }
                // Update next element
                if ( $elt->nextAFE ) {
                        $elt->nextAFE->prevAFE = $elt->prevAFE;
                }
                // Clear pointers so that isInList() etc. will work
                $elt->prevAFE = $elt->nextAFE = null;
                // Update Noah list
                $this->removeFromNoahList( $elt );
        }

        private function addToNoahList( BalanceElement $elt ) {
                $noahKey = $elt->getNoahKey();
                $table =& $this->noahTableStack[ count( $this->noahTableStack ) - 1 ];
                if ( !isset( $table[$noahKey] ) ) {
                        $table[$noahKey] = $elt;
                } else {
                        $tail = $table[$noahKey];
                        while ( $tail->nextNoah ) {
                                $tail = $tail->nextNoah;
                        }
                        $tail->nextNoah = $elt;
                }
        }

        private function removeFromNoahList( BalanceElement $elt ) {
                $table =& $this->noahTableStack[ count( $this->noahTableStack ) - 1 ];
                $key = $elt->getNoahKey();
                $noahElt = $table[$key];
                if ( $noahElt === $elt ) {
                        if ( $noahElt->nextNoah ) {
                                $table[$key] = $noahElt->nextNoah;
                                $noahElt->nextNoah = null;
                        } else {
                                unset( $table[$key] );
                        }
                } else {
                        do {
                                $prevNoahElt = $noahElt;
                                $noahElt = $prevNoahElt->nextNoah;
                                if ( $noahElt === $elt ) {
                                        // Found it, unlink
                                        $prevNoahElt->nextNoah = $elt->nextNoah;
                                        $elt->nextNoah = null;
                                        break;
                                }
                        } while ( $noahElt );
                }
        }

        /**
         * Find element $a in the list and replace it with element $b
         *
         * @param BalanceElement $a
         * @param BalanceElement $b
         */
        public function replace( BalanceElement $a, BalanceElement $b ) {
                if ( $this->head !== $a && !$a->prevAFE ) {
                        throw new ParameterAssertionException( '$a',
                                "Attempted to replace an element which is not in the AFE list" );
                }
                // Update head and tail pointers
                if ( $this->head === $a ) {
                        $this->head = $b;
                }
                if ( $this->tail === $a ) {
                        $this->tail = $b;
                }
                // Update previous element
                if ( $a->prevAFE ) {
                        $a->prevAFE->nextAFE = $b;
                }
                // Update next element
                if ( $a->nextAFE ) {
                        $a->nextAFE->prevAFE = $b;
                }
                $b->prevAFE = $a->prevAFE;
                $b->nextAFE = $a->nextAFE;
                $a->nextAFE = $a->prevAFE = null;
                // Update Noah list
                $this->removeFromNoahList( $a );
                $this->addToNoahList( $b );
        }

        /**
         * Find $a in the list and insert $b after it.

         * @param BalanceElement $a
         * @param BalanceElement $b
         */
        public function insertAfter( BalanceElement $a, BalanceElement $b ) {
                if ( $this->head !== $a && !$a->prevAFE ) {
                        throw new ParameterAssertionException( '$a',
                                "Attempted to insert after an element which is not in the AFE list" );
                }
                if ( $this->tail === $a ) {
                        $this->tail = $b;
                }
                if ( $a->nextAFE ) {
                        $a->nextAFE->prevAFE = $b;
                }
                $b->nextAFE = $a->nextAFE;
                $b->prevAFE = $a;
                $a->nextAFE = $b;
                $this->addToNoahList( $b );
        }

        // @codingStandardsIgnoreStart Generic.Files.LineLength.TooLong
        /**
         * Reconstruct the active formatting elements.
         * @param BalanceStack $stack The open elements stack
         * @see https://html.spec.whatwg.org/multipage/syntax.html#reconstruct-the-active-formatting-elements
         */
        // @codingStandardsIgnoreEnd
        public function reconstruct( $stack ) {
                $entry = $this->tail;
                // If there are no entries in the list of active formatting elements,
                // then there is nothing to reconstruct
                if ( !$entry ) {
                        return;
                }
                // If the last is a marker, do nothing.
                if ( $entry instanceof BalanceMarker ) {
                        return;
                }
                // Or if it is an open element, do nothing.
                if ( $stack->indexOf( $entry ) >= 0 ) {
                        return;
                }

                // Loop backward through the list until we find a marker or an
                // open element
                $foundIt = false;
                while ( $entry->prevAFE ) {
                        $entry = $entry->prevAFE;
                        if ( $entry instanceof BalanceMarker || $stack->indexOf( $entry ) >= 0 ) {
                                $foundIt = true;
                                break;
                        }
                }

                // Now loop forward, starting from the element after the current one (or
                // the first element if we didn't find a marker or open element),
                // recreating formatting elements and pushing them back onto the list
                // of open elements.
                if ( $foundIt ) {
                        $entry = $entry->nextAFE;
                }
                do {
                        $newElement = $stack->insertHTMLElement(
                                $entry->localName,
                                $entry->attribs );
                        $this->replace( $entry, $newElement );
                        $entry = $newElement->nextAFE;
                } while ( $entry );
        }

        /**
         * Get a string representation of the AFE list, for debugging
         */
        public function __toString() {
                $prev = null;
                $s = '';
                for ( $node = $this->head; $node; $prev = $node, $node = $node->nextAFE ) {
                        if ( $node instanceof BalanceMarker ) {
                                $s .= "MARKER\n";
                                continue;
                        }
                        $s .= $node->localName . '#' . substr( md5( spl_object_hash( $node ) ), 0, 8 );
                        if ( $node->nextNoah ) {
                                $s .= " (noah sibling: {$node->nextNoah->localName}#" .
                                        substr( md5( spl_object_hash( $node->nextNoah ) ), 0, 8 ) .
                                        ')';
                        }
                        if ( $node->nextAFE && $node->nextAFE->prevAFE !== $node ) {
                                $s .= " (reverse link is wrong!)";
                        }
                        $s .= "\n";
                }
                if ( $prev !== $this->tail ) {
                        $s .= "(tail pointer is wrong!)\n";
                }
                return $s;
        }
}

/**
 * An implementation of the tree building portion of the HTML5 parsing
 * spec.
 *
 * This is used to balance and tidy output so that the result can
 * always be cleanly serialized/deserialized by an HTML5 parser.  It
 * does *not* guarantee "conforming" output -- the HTML5 spec contains
 * a number of constraints which are not enforced by the HTML5 parsing
 * process.  But the result will be free of gross errors: misnested or
 * unclosed tags, for example, and will be unchanged by spec-complient
 * parsing followed by serialization.
 *
 * The tree building stage is structured as a state machine.
 * When comparing the implementation to
 * https://www.w3.org/TR/html5/syntax.html#tree-construction
 * note that each state is implemented as a function with a
 * name ending in `Mode` (because the HTML spec refers to them
 * as insertion modes).  The current insertion mode is held by
 * the $parseMode property.
 *
 * The following simplifications have been made:
 * - We handle body content only (ie, we start `in body`.)
 * - The document is never in "quirks mode".
 * - All occurrences of < and > have been entity escaped, so we
 *   can parse tags by simply splitting on those two characters.
 *   (This also simplifies the handling of < inside <textarea>.)
 *   The character < must not appear inside comments.
 *   Similarly, all attributes have been "cleaned" and are double-quoted
 *   and escaped.
 * - All null characters are assumed to have been removed.
 * - The following elements are disallowed: <html>, <head>, <body>, <frameset>,
 *   <frame>, <plaintext>, <isindex>, <xmp>, <iframe>,
 *   <noembed>, <noscript>, <script>, <title>.  As a result,
 *   further simplifications can be made:
 *   - `frameset-ok` is not tracked.
 *   - `head element pointer` is not tracked (but presumed non-null)
 *   - Tokenizer has only a single mode. (<textarea> wants RCDATA and
 *     <style>/<noframes> want RAWTEXT modes which we only loosely emulate.)
 *
 *   We generally mark places where we omit cases from the spec due to
 *   disallowed elements with a comment: `// OMITTED: <element-name>`.
 *
 *   The HTML spec keeps a flag during the parsing process to track
 *   whether or not a "parse error" has been encountered.  We don't
 *   bother to track that flag, we just implement the error-handling
 *   process as specified.
 *
 * @ingroup Parser
 * @since 1.27
 * @see https://html.spec.whatwg.org/multipage/syntax.html#tree-construction
 */
class Balancer {
        private $parseMode;
        /** @var \Iterator */
        private $bitsIterator;
        private $allowedHtmlElements;
        /** @var BalanceActiveFormattingElements */
        private $afe;
        /** @var BalanceStack */
        private $stack;
        private $strict;
        private $allowComments;
        private $config;

        private $textIntegrationMode;
        private $pendingTableText;
        private $originalInsertionMode;
        private $fragmentContext;
        private $formElementPointer;
        private $ignoreLinefeed;
        private $inRCDATA;
        private $inRAWTEXT;

        /** @var callable|null */
        private $processingCallback;
        /** @var array */
        private $processingArgs;

        /**
         * Valid HTML5 comments.
         * Regex borrowed from Tim Starling's "remex-html" project.
         */
        const VALID_COMMENT_REGEX = "~ !--
                (                             # 1. Comment match detector
                        > | -> | # Invalid short close
                        (                         # 2. Comment contents
                                (?:
                                        (?! --> )
                                        (?! --!> )
                                        (?! --! \z )
                                        (?! -- \z )
                                        (?! - \z )
                                        .
                                )*+
                        )
                        (                         # 3. Comment close
                                --> |   # Normal close
                                --!> |  # Comment end bang
                                (                     # 4. Indicate matches requiring EOF
                                        --! |   # EOF in comment end bang state
                                        -- |    # EOF in comment end state
                                        -  |    # EOF in comment end dash state
                                                # EOF in comment state
                                )
                        )
                )
                ([^<]*) \z                    # 5. Non-tag text after the comment
                ~xs";

        /**
         * Create a new Balancer.
         * @param array $config Balancer configuration.  Includes:
         *     'strict' : boolean, defaults to false.
         *         When true, enforces syntactic constraints on input:
         *         all non-tag '<' must be escaped, all attributes must be
         *         separated by a single space and double-quoted.  This is
         *         consistent with the output of the Sanitizer.
         *     'allowedHtmlElements' : array, defaults to null.
         *         When present, the keys of this associative array give
         *         the acceptable HTML tag names.  When not present, no
         *         tag sanitization is done.
         *     'tidyCompat' : boolean, defaults to false.
         *         When true, the serialization algorithm is tweaked to
         *         provide historical compatibility with the old "tidy"
         *         program: <p>-wrapping is done to the children of
         *         <body> and <blockquote> elements, and empty elements
         *         are removed.
         *     'allowComments': boolean, defaults to true.
         *         When true, allows HTML comments in the input.
         *         The Sanitizer generally strips all comments, so if you
         *         are running on sanitized output you can set this to
         *         false to get a bit more performance.
         */
        public function __construct( array $config = [] ) {
                $this->config = $config = $config + [
                        'strict' => false,
                        'allowedHtmlElements' => null,
                        'tidyCompat' => false,
                        'allowComments' => true,
                ];
                $this->allowedHtmlElements = $config['allowedHtmlElements'];
                $this->strict = $config['strict'];
                $this->allowComments = $config['allowComments'];
                if ( $this->allowedHtmlElements !== null ) {
                        // Sanity check!
                        $bad = array_uintersect_assoc(
                                $this->allowedHtmlElements,
                                BalanceSets::$unsupportedSet[BalanceSets::HTML_NAMESPACE],
                                function( $a, $b ) {
                                        // Ignore the values (just intersect the keys) by saying
                                        // all values are equal to each other.
                                        return 0;
                                }
                        );
                        if ( count( $bad ) > 0 ) {
                                $badstr = implode( array_keys( $bad ), ',' );
                                throw new ParameterAssertionException(
                                        '$config',
                                        'Balance attempted with sanitization including ' .
                                        "unsupported elements: {$badstr}"
                                );
                        }
                }
        }

        /**
         * Return a balanced HTML string for the HTML fragment given by $text,
         * subject to the caveats listed in the class description.  The result
         * will typically be idempotent -- that is, rebalancing the output
         * would result in no change.
         *
         * @param string $text The markup to be balanced
         * @param callable $processingCallback Callback to do any variable or
         *   parameter replacements in HTML attributes values
         * @param array|bool $processingArgs Arguments for the processing callback
         * @return string The balanced markup
         */
        public function balance( $text, $processingCallback = null, $processingArgs = [] ) {
                $this->parseMode = 'inBodyMode';
                $this->bitsIterator = new ExplodeIterator( '<', $text );
                $this->afe = new BalanceActiveFormattingElements();
                $this->stack = new BalanceStack( $this->config );
                $this->processingCallback = $processingCallback;
                $this->processingArgs = $processingArgs;

                $this->textIntegrationMode =
                        $this->ignoreLinefeed =
                        $this->inRCDATA =
                        $this->inRAWTEXT = false;

                // The stack is constructed with an <html> element already on it.
                // Set this up as a fragment parsed with <body> as the context.
                $this->fragmentContext =
                        new BalanceElement( BalanceSets::HTML_NAMESPACE, 'body', [] );
                $this->resetInsertionMode();
                $this->formElementPointer = null;
                for ( $e = $this->fragmentContext; $e != null; $e = $e->parent ) {
                        if ( $e->isHtmlNamed( 'form' ) ) {
                                $this->formElementPointer = $e;
                                break;
                        }
                }

                // First element is text not tag
                $x = $this->bitsIterator->current();
                $this->bitsIterator->next();
                $this->insertToken( 'text', str_replace( '>', '&gt;', $x ) );
                // Now process each tag.
                while ( $this->bitsIterator->valid() ) {
                        $this->advance();
                }
                $this->insertToken( 'eof', null );
                $result = $this->stack->getOutput();
                // Free memory before returning.
                $this->bitsIterator = null;
                $this->afe = null;
                $this->stack = null;
                $this->fragmentContext = null;
                $this->formElementPointer = null;
                return $result;
        }

        /**
         * Pass a token to the tree builder.  The $token will be one of the
         * strings "tag", "endtag", or "text".
         */
        private function insertToken( $token, $value, $attribs = null, $selfClose = false ) {
                // validate tags against $unsupportedSet
                if ( $token === 'tag' || $token === 'endtag' ) {
                        if ( isset( BalanceSets::$unsupportedSet[BalanceSets::HTML_NAMESPACE][$value] ) ) {
                                // As described in "simplifications" above, these tags are
                                // not supported in the balancer.
                                Assert::invariant(
                                        !$this->strict,
                                        "Unsupported $token <$value> found."
                                );
                                return false;
                        }
                } elseif ( $token === 'text' && $value === '' ) {
                        // Don't actually inject the empty string as a text token.
                        return true;
                }
                // Support pre/listing/textarea by suppressing initial linefeed
                if ( $this->ignoreLinefeed ) {
                        $this->ignoreLinefeed = false;
                        if ( $token === 'text' ) {
                                if ( $value[0] === "\n" ) {
                                        if ( $value === "\n" ) {
                                                // Nothing would be left, don't inject the empty string.
                                                return true;
                                        }
                                        $value = substr( $value, 1 );
                                }
                        }
                }
                // Some hoops we have to jump through
                $adjusted = $this->stack->adjustedCurrentNode( $this->fragmentContext );

                $isForeign = true;
                if (
                        $this->stack->length() === 0 ||
                        $adjusted->isHtml() ||
                        $token === 'eof'
                ) {
                        $isForeign = false;
                } elseif ( $adjusted->isMathmlTextIntegrationPoint() ) {
                        if ( $token === 'text' ) {
                                $isForeign = false;
                        } elseif (
                                $token === 'tag' &&
                                $value !== 'mglyph' && $value !== 'malignmark'
                        ) {
                                $isForeign = false;
                        }
                } elseif (
                        $adjusted->namespaceURI === BalanceSets::MATHML_NAMESPACE &&
                        $adjusted->localName === 'annotation-xml' &&
                        $token === 'tag' && $value === 'svg'
                ) {
                        $isForeign = false;
                } elseif (
                        $adjusted->isHtmlIntegrationPoint() &&
                        ( $token === 'tag' || $token === 'text' )
                ) {
                        $isForeign = false;
                }
                if ( $isForeign ) {
                        return $this->insertForeignToken( $token, $value, $attribs, $selfClose );
                } else {
                        $func = $this->parseMode;
                        return $this->$func( $token, $value, $attribs, $selfClose );
                }
        }

        private function insertForeignToken( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        $this->stack->insertText( $value );
                        return true;
                } elseif ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'font':
                                if ( isset( $attribs['color'] )
                                        || isset( $attribs['face'] )
                                        || isset( $attribs['size'] )
                                ) {
                                        break;
                                }
                                // otherwise, fall through
                        case 'b':
                        case 'big':
                        case 'blockquote':
                        case 'body':
                        case 'br':
                        case 'center':
                        case 'code':
                        case 'dd':
                        case 'div':
                        case 'dl':
                        case 'dt':
                        case 'em':
                        case 'embed':
                        case 'h1':
                        case 'h2':
                        case 'h3':
                        case 'h4':
                        case 'h5':
                        case 'h6':
                        case 'head':
                        case 'hr':
                        case 'i':
                        case 'img':
                        case 'li':
                        case 'listing':
                        case 'menu':
                        case 'meta':
                        case 'nobr':
                        case 'ol':
                        case 'p':
                        case 'pre':
                        case 'ruby':
                        case 's':
                        case 'small':
                        case 'span':
                        case 'strong':
                        case 'strike':
                        case 'sub':
                        case 'sup':
                        case 'table':
                        case 'tt':
                        case 'u':
                        case 'ul':
                        case 'var':
                                if ( $this->fragmentContext ) {
                                        break;
                                }
                                while ( true ) {
                                        $this->stack->pop();
                                        $node = $this->stack->currentNode;
                                        if (
                                                $node->isMathmlTextIntegrationPoint() ||
                                                $node->isHtmlIntegrationPoint() ||
                                                $node->isHtml()
                                        ) {
                                                break;
                                        }
                                }
                                return $this->insertToken( $token, $value, $attribs, $selfClose );
                        }
                        // "Any other start tag"
                        $adjusted = ( $this->fragmentContext && $this->stack->length()===1 ) ?
                                $this->fragmentContext : $this->stack->currentNode;
                        $this->stack->insertForeignElement(
                                $adjusted->namespaceURI, $value, $attribs
                        );
                        if ( $selfClose ) {
                                $this->stack->pop();
                        }
                        return true;
                } elseif ( $token === 'endtag' ) {
                        $first = true;
                        foreach ( $this->stack as $i => $node ) {
                                if ( $node->isHtml() && !$first ) {
                                        // process the end tag as HTML
                                        $func = $this->parseMode;
                                        return $this->$func( $token, $value, $attribs, $selfClose );
                                } elseif ( $i === 0 ) {
                                        return true;
                                } elseif ( $node->localName === $value ) {
                                        $this->stack->popTag( $node );
                                        return true;
                                }
                                $first = false;
                        }
                }
        }

        /**
         * Grab the next "token" from $bitsIterator.  This is either a open/close
         * tag or text or a comment, depending on whether the Sanitizer approves.
         */
        private function advance() {
                $x = $this->bitsIterator->current();
                $this->bitsIterator->next();
                $regs = [];
                // Handle comments.  These won't be generated by mediawiki (they
                // are stripped in the Sanitizer) but may be generated by extensions.
                if (
                        $this->allowComments &&
                        !( $this->inRCDATA || $this->inRAWTEXT ) &&
                        preg_match( Balancer::VALID_COMMENT_REGEX, $x, $regs, PREG_OFFSET_CAPTURE ) &&
                        // verify EOF condition where necessary
                        ( $regs[4][1] < 0 || !$this->bitsIterator->valid() )
                ) {
                        $contents = $regs[2][0];
                        $rest = $regs[5][0];
                        $this->insertToken( 'comment', $contents );
                        $this->insertToken( 'text', str_replace( '>', '&gt;', $rest ) );
                        return;
                }
                // $slash: Does the current element start with a '/'?
                // $t: Current element name
                // $attribStr: String between element name and >
                // $brace: Ending '>' or '/>'
                // $rest: Everything until the next element from the $bitsIterator
                if ( preg_match( Sanitizer::ELEMENT_BITS_REGEX, $x, $regs ) ) {
                        list( /* $qbar */, $slash, $t, $attribStr, $brace, $rest ) = $regs;
                        $t = strtolower( $t );
                        if ( $this->strict ) {
                                // Verify that attributes are all properly double-quoted
                                Assert::invariant(
                                        preg_match(
                                                '/^( [:_A-Z0-9][-.:_A-Z0-9]*="[^"]*")*[ ]*$/i', $attribStr
                                        ),
                                        "Bad attribute string found"
                                );
                        }
                } else {
                        Assert::invariant(
                                !$this->strict, "< found which does not start a valid tag"
                        );
                        $slash = $t = $attribStr = $brace = $rest = null;
                }
                $goodTag = $t;
                if ( $this->inRCDATA ) {
                        if ( $slash && $t === $this->inRCDATA ) {
                                $this->inRCDATA = false;
                        } else {
                                // No tags allowed; this emulates the "rcdata" tokenizer mode.
                                $goodTag = false;
                        }
                }
                if ( $this->inRAWTEXT ) {
                        if ( $slash && $t === $this->inRAWTEXT ) {
                                $this->inRAWTEXT = false;
                        } else {
                                // No tags allowed, no entity-escaping done.
                                $goodTag = false;
                        }
                }
                $sanitize = $this->allowedHtmlElements !== null;
                if ( $sanitize ) {
                        $goodTag = $t && isset( $this->allowedHtmlElements[$t] );
                }
                if ( $goodTag ) {
                        if ( is_callable( $this->processingCallback ) ) {
                                call_user_func_array( $this->processingCallback, [ &$attribStr, $this->processingArgs ] );
                        }
                        if ( $sanitize ) {
                                $goodTag = Sanitizer::validateTag( $attribStr, $t );
                        }
                }
                if ( $goodTag ) {
                        if ( $sanitize ) {
                                $attribs = Sanitizer::decodeTagAttributes( $attribStr );
                                $attribs = Sanitizer::validateTagAttributes( $attribs, $t );
                        } else {
                                $attribs = Sanitizer::decodeTagAttributes( $attribStr );
                        }
                        $goodTag = $this->insertToken(
                                $slash ? 'endtag' : 'tag', $t, $attribs, $brace === '/>'
                        );
                }
                if ( $goodTag ) {
                        $rest = str_replace( '>', '&gt;', $rest );
                        $this->insertToken( 'text', str_replace( '>', '&gt;', $rest ) );
                } elseif ( $this->inRAWTEXT ) {
                        $this->insertToken( 'text', "<$x" );
                } else {
                        // bad tag; serialize entire thing as text.
                        $this->insertToken( 'text', '&lt;' . str_replace( '>', '&gt;', $x ) );
                }
        }

        private function switchMode( $mode ) {
                Assert::parameter(
                        substr( $mode, -4 )==='Mode', '$mode', 'should end in Mode'
                );
                $oldMode = $this->parseMode;
                $this->parseMode = $mode;
                return $oldMode;
        }

        private function switchModeAndReprocess( $mode, $token, $value, $attribs, $selfClose ) {
                $this->switchMode( $mode );
                return $this->insertToken( $token, $value, $attribs, $selfClose );
        }

        private function resetInsertionMode() {
                $last = false;
                foreach ( $this->stack as $i => $node ) {
                        if ( $i === 0 ) {
                                $last = true;
                                if ( $this->fragmentContext ) {
                                        $node = $this->fragmentContext;
                                }
                        }
                        if ( $node->isHtml() ) {
                                switch ( $node->localName ) {
                                case 'select':
                                        $stackLength = $this->stack->length();
                                        for ( $j = $i + 1; $j < $stackLength-1; $j++ ) {
                                                $ancestor = $this->stack->node( $stackLength-$j-1 );
                                                if ( $ancestor->isHtmlNamed( 'template' ) ) {
                                                        break;
                                                }
                                                if ( $ancestor->isHtmlNamed( 'table' ) ) {
                                                        $this->switchMode( 'inSelectInTableMode' );
                                                        return;
                                                }
                                        }
                                        $this->switchMode( 'inSelectMode' );
                                        return;
                                case 'tr':
                                        $this->switchMode( 'inRowMode' );
                                        return;
                                case 'tbody':
                                case 'tfoot':
                                case 'thead':
                                        $this->switchMode( 'inTableBodyMode' );
                                        return;
                                case 'caption':
                                        $this->switchMode( 'inCaptionMode' );
                                        return;
                                case 'colgroup':
                                        $this->switchMode( 'inColumnGroupMode' );
                                        return;
                                case 'table':
                                        $this->switchMode( 'inTableMode' );
                                        return;
                                case 'template':
                                        $this->switchMode(
                                                array_slice( $this->templateInsertionModes, -1 )[0]
                                        );
                                        return;
                                case 'body':
                                        $this->switchMode( 'inBodyMode' );
                                        return;
                                // OMITTED: <frameset>
                                // OMITTED: <html>
                                // OMITTED: <head>
                                default:
                                        if ( !$last ) {
                                                // OMITTED: <head>
                                                if ( $node->isA( BalanceSets::$tableCellSet ) ) {
                                                        $this->switchMode( 'inCellMode' );
                                                        return;
                                                }
                                        }
                                }
                        }
                        if ( $last ) {
                                $this->switchMode( 'inBodyMode' );
                                return;
                        }
                }
        }

        private function stopParsing() {
                // Most of the spec methods are inapplicable, other than step 2:
                // "pop all the nodes off the stack of open elements".
                // We're going to keep the top-most <html> element on the stack, though.

                // Clear the AFE list first, otherwise the element objects will stay live
                // during serialization, potentially using O(N^2) memory. Note that
                // popping the stack will never result in reconstructing the active
                // formatting elements.
                $this->afe = null;
                $this->stack->popTo( 1 );
        }

        private function parseRawText( $value, $attribs = null ) {
                $this->stack->insertHTMLElement( $value, $attribs );
                $this->inRAWTEXT = $value;
                $this->originalInsertionMode = $this->switchMode( 'inTextMode' );
                return true;
        }

        private function inTextMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        $this->stack->insertText( $value );
                        return true;
                } elseif ( $token === 'eof' ) {
                        $this->stack->pop();
                        return $this->switchModeAndReprocess(
                                $this->originalInsertionMode, $token, $value, $attribs, $selfClose
                        );
                } elseif ( $token === 'endtag' ) {
                        $this->stack->pop();
                        $this->switchMode( $this->originalInsertionMode );
                        return true;
                }
                return true;
        }

        private function inHeadMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        if ( preg_match( '/^[\x09\x0A\x0C\x0D\x20]+/', $value, $matches ) ) {
                                $this->stack->insertText( $matches[0] );
                                $value = substr( $value, strlen( $matches[0] ) );
                        }
                        if ( strlen( $value ) === 0 ) {
                                return true; // All text handled.
                        }
                        // Fall through to handle non-whitespace below.
                } elseif ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'meta':
                                // OMITTED: in a full HTML parser, this might change the encoding.
                                // falls through
                        // OMITTED: <html>
                        case 'base':
                        case 'basefont':
                        case 'bgsound':
                        case 'link':
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->pop();
                                return true;
                        // OMITTED: <title>
                        // OMITTED: <noscript>
                        case 'noframes':
                        case 'style':
                                return $this->parseRawText( $value, $attribs );
                        // OMITTED: <script>
                        case 'template':
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->afe->insertMarker();
                                // OMITTED: frameset_ok
                                $this->switchMode( 'inTemplateMode' );
                                $this->templateInsertionModes[] = $this->parseMode;
                                return true;
                        // OMITTED: <head>
                        }
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        // OMITTED: <head>
                        // OMITTED: <body>
                        // OMITTED: <html>
                        case 'br':
                                break; // handle at the bottom of the function
                        case 'template':
                                if ( $this->stack->indexOf( $value ) < 0 ) {
                                        return true; // Ignore the token.
                                }
                                $this->stack->generateImpliedEndTags( null, true /* thorough */ );
                                $this->stack->popTag( $value );
                                $this->afe->clearToMarker();
                                array_pop( $this->templateInsertionModes );
                                $this->resetInsertionMode();
                                return true;
                        default:
                                // ignore any other end tag
                                return true;
                        }
                } elseif ( $token === 'comment' ) {
                        $this->stack->insertComment( $value );
                        return true;
                }

                // If not handled above
                $this->inHeadMode( 'endtag', 'head' ); // synthetic </head>
                // Then redo this one
                return $this->insertToken( $token, $value, $attribs, $selfClose );
        }

        private function inBodyMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        $this->afe->reconstruct( $this->stack );
                        $this->stack->insertText( $value );
                        return true;
                } elseif ( $token === 'eof' ) {
                        if ( !empty( $this->templateInsertionModes ) ) {
                                return $this->inTemplateMode( $token, $value, $attribs, $selfClose );
                        }
                        $this->stopParsing();
                        return true;
                } elseif ( $token === 'tag' ) {
                        switch ( $value ) {
                        // OMITTED: <html>
                        case 'base':
                        case 'basefont':
                        case 'bgsound':
                        case 'link':
                        case 'meta':
                        case 'noframes':
                        // OMITTED: <script>
                        case 'style':
                        case 'template':
                        // OMITTED: <title>
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );
                        // OMITTED: <body>
                        // OMITTED: <frameset>

                        case 'address':
                        case 'article':
                        case 'aside':
                        case 'blockquote':
                        case 'center':
                        case 'details':
                        case 'dialog':
                        case 'dir':
                        case 'div':
                        case 'dl':
                        case 'fieldset':
                        case 'figcaption':
                        case 'figure':
                        case 'footer':
                        case 'header':
                        case 'hgroup':
                        case 'main':
                        case 'menu':
                        case 'nav':
                        case 'ol':
                        case 'p':
                        case 'section':
                        case 'summary':
                        case 'ul':
                                if ( $this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        case 'h1':
                        case 'h2':
                        case 'h3':
                        case 'h4':
                        case 'h5':
                        case 'h6':
                                if ( $this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                if ( $this->stack->currentNode->isA( BalanceSets::$headingSet ) ) {
                                        $this->stack->pop();
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        case 'pre':
                        case 'listing':
                                if ( $this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->ignoreLinefeed = true;
                                // OMITTED: frameset_ok
                                return true;

                        case 'form':
                                if (
                                        $this->formElementPointer &&
                                        $this->stack->indexOf( 'template' ) < 0
                                ) {
                                        return true; // in a form, not in a template.
                                }
                                if ( $this->stack->inButtonScope( "p" ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                $elt = $this->stack->insertHTMLElement( $value, $attribs );
                                if ( $this->stack->indexOf( 'template' ) < 0 ) {
                                        $this->formElementPointer = $elt;
                                }
                                return true;

                        case 'li':
                                // OMITTED: frameset_ok
                                foreach ( $this->stack as $node ) {
                                        if ( $node->isHtmlNamed( 'li' ) ) {
                                                $this->inBodyMode( 'endtag', 'li' );
                                                break;
                                        }
                                        if (
                                                $node->isA( BalanceSets::$specialSet ) &&
                                                !$node->isA( BalanceSets::$addressDivPSet )
                                        ) {
                                                break;
                                        }
                                }
                                if ( $this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        case 'dd':
                        case 'dt':
                                // OMITTED: frameset_ok
                                foreach ( $this->stack as $node ) {
                                        if ( $node->isHtmlNamed( 'dd' ) ) {
                                                $this->inBodyMode( 'endtag', 'dd' );
                                                break;
                                        }
                                        if ( $node->isHtmlNamed( 'dt' ) ) {
                                                $this->inBodyMode( 'endtag', 'dt' );
                                                break;
                                        }
                                        if (
                                                $node->isA( BalanceSets::$specialSet ) &&
                                                !$node->isA( BalanceSets::$addressDivPSet )
                                        ) {
                                                break;
                                        }
                                }
                                if ( $this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        // OMITTED: <plaintext>

                        case 'button':
                                if ( $this->stack->inScope( 'button' ) ) {
                                        $this->inBodyMode( 'endtag', 'button' );
                                        return $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                $this->afe->reconstruct( $this->stack );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        case 'a':
                                $activeElement = $this->afe->findElementByTag( 'a' );
                                if ( $activeElement ) {
                                        $this->inBodyMode( 'endtag', 'a' );
                                        if ( $this->afe->isInList( $activeElement ) ) {
                                                $this->afe->remove( $activeElement );
                                                // Don't flatten here, since when we fall
                                                // through below we might foster parent
                                                // the new <a> tag inside this one.
                                                $this->stack->removeElement( $activeElement, false );
                                        }
                                }
                                // Falls through
                        case 'b':
                        case 'big':
                        case 'code':
                        case 'em':
                        case 'font':
                        case 'i':
                        case 's':
                        case 'small':
                        case 'strike':
                        case 'strong':
                        case 'tt':
                        case 'u':
                                $this->afe->reconstruct( $this->stack );
                                $this->afe->push( $this->stack->insertHTMLElement( $value, $attribs ) );
                                return true;

                        case 'nobr':
                                $this->afe->reconstruct( $this->stack );
                                if ( $this->stack->inScope( 'nobr' ) ) {
                                        $this->inBodyMode( 'endtag', 'nobr' );
                                        $this->afe->reconstruct( $this->stack );
                                }
                                $this->afe->push( $this->stack->insertHTMLElement( $value, $attribs ) );
                                return true;

                        case 'applet':
                        case 'marquee':
                        case 'object':
                                $this->afe->reconstruct( $this->stack );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->afe->insertMarker();
                                // OMITTED: frameset_ok
                                return true;

                        case 'table':
                                // The document is never in "quirks mode"; see simplifications
                                // above.
                                if ( $this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                // OMITTED: frameset_ok
                                $this->switchMode( 'inTableMode' );
                                return true;

                        case 'area':
                        case 'br':
                        case 'embed':
                        case 'img':
                        case 'keygen':
                        case 'wbr':
                                $this->afe->reconstruct( $this->stack );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->pop();
                                // OMITTED: frameset_ok
                                return true;

                        case 'input':
                                $this->afe->reconstruct( $this->stack );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->pop();
                                // OMITTED: frameset_ok
                                // (hence we don't need to examine the tag's "type" attribute)
                                return true;

                        case 'menuitem':
                        case 'param':
                        case 'source':
                        case 'track':
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->pop();
                                return true;

                        case 'hr':
                                if ( $this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'endtag', 'p' );
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->pop();
                                return true;

                        case 'image':
                                // warts!
                                return $this->inBodyMode( $token, 'img', $attribs, $selfClose );

                        // OMITTED: <isindex>

                        case 'textarea':
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->ignoreLinefeed = true;
                                $this->inRCDATA = $value; // emulate rcdata tokenizer mode
                                // OMITTED: frameset_ok
                                return true;

                        // OMITTED: <xmp>
                        // OMITTED: <iframe>
                        // OMITTED: <noembed>
                        // OMITTED: <noscript>

                        case 'select':
                                $this->afe->reconstruct( $this->stack );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                switch ( $this->parseMode ) {
                                case 'inTableMode':
                                case 'inCaptionMode':
                                case 'inTableBodyMode':
                                case 'inRowMode':
                                case 'inCellMode':
                                        $this->switchMode( 'inSelectInTableMode' );
                                        return true;
                                default:
                                        $this->switchMode( 'inSelectMode' );
                                        return true;
                                }

                        case 'optgroup':
                        case 'option':
                                if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
                                        $this->inBodyMode( 'endtag', 'option' );
                                }
                                $this->afe->reconstruct( $this->stack );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        case 'rb':
                        case 'rtc':
                                if ( $this->stack->inScope( 'ruby' ) ) {
                                        $this->stack->generateImpliedEndTags();
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        case 'rp':
                        case 'rt':
                                if ( $this->stack->inScope( 'ruby' ) ) {
                                        $this->stack->generateImpliedEndTags( 'rtc' );
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;

                        case 'math':
                                $this->afe->reconstruct( $this->stack );
                                // We skip the spec's "adjust MathML attributes" and
                                // "adjust foreign attributes" steps, since the browser will
                                // do this later when it parses the output and it doesn't affect
                                // balancing.
                                $this->stack->insertForeignElement(
                                        BalanceSets::MATHML_NAMESPACE, $value, $attribs
                                );
                                if ( $selfClose ) {
                                        // emit explicit </math> tag.
                                        $this->stack->pop();
                                }
                                return true;

                        case 'svg':
                                $this->afe->reconstruct( $this->stack );
                                // We skip the spec's "adjust SVG attributes" and
                                // "adjust foreign attributes" steps, since the browser will
                                // do this later when it parses the output and it doesn't affect
                                // balancing.
                                $this->stack->insertForeignElement(
                                        BalanceSets::SVG_NAMESPACE, $value, $attribs
                                );
                                if ( $selfClose ) {
                                        // emit explicit </svg> tag.
                                        $this->stack->pop();
                                }
                                return true;

                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        // OMITTED: <frame>
                        case 'head':
                        case 'tbody':
                        case 'td':
                        case 'tfoot':
                        case 'th':
                        case 'thead':
                        case 'tr':
                                // Ignore table tags if we're not inTableMode
                                return true;
                        }

                        // Handle any other start tag here
                        $this->afe->reconstruct( $this->stack );
                        $this->stack->insertHTMLElement( $value, $attribs );
                        return true;
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        // </body>,</html> are unsupported.

                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );

                        case 'address':
                        case 'article':
                        case 'aside':
                        case 'blockquote':
                        case 'button':
                        case 'center':
                        case 'details':
                        case 'dialog':
                        case 'dir':
                        case 'div':
                        case 'dl':
                        case 'fieldset':
                        case 'figcaption':
                        case 'figure':
                        case 'footer':
                        case 'header':
                        case 'hgroup':
                        case 'listing':
                        case 'main':
                        case 'menu':
                        case 'nav':
                        case 'ol':
                        case 'pre':
                        case 'section':
                        case 'summary':
                        case 'ul':
                                // Ignore if there is not a matching open tag
                                if ( !$this->stack->inScope( $value ) ) {
                                        return true;
                                }
                                $this->stack->generateImpliedEndTags();
                                $this->stack->popTag( $value );
                                return true;

                        case 'form':
                                if ( $this->stack->indexOf( 'template' ) < 0 ) {
                                        $openform = $this->formElementPointer;
                                        $this->formElementPointer = null;
                                        if ( !$openform || !$this->stack->inScope( $openform ) ) {
                                                return true;
                                        }
                                        $this->stack->generateImpliedEndTags();
                                        // Don't flatten yet if we're removing a <form> element
                                        // out-of-order. (eg. `<form><div></form>`)
                                        $flatten = ( $this->stack->currentNode === $openform );
                                        $this->stack->removeElement( $openform, $flatten );
                                } else {
                                        if ( !$this->stack->inScope( 'form' ) ) {
                                                return true;
                                        }
                                        $this->stack->generateImpliedEndTags();
                                        $this->stack->popTag( 'form' );
                                }
                                return true;

                        case 'p':
                                if ( !$this->stack->inButtonScope( 'p' ) ) {
                                        $this->inBodyMode( 'tag', 'p', [] );
                                        return $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                $this->stack->generateImpliedEndTags( $value );
                                $this->stack->popTag( $value );
                                return true;

                        case 'li':
                                if ( !$this->stack->inListItemScope( $value ) ) {
                                        return true; // ignore
                                }
                                $this->stack->generateImpliedEndTags( $value );
                                $this->stack->popTag( $value );
                                return true;

                        case 'dd':
                        case 'dt':
                                if ( !$this->stack->inScope( $value ) ) {
                                        return true; // ignore
                                }
                                $this->stack->generateImpliedEndTags( $value );
                                $this->stack->popTag( $value );
                                return true;

                        case 'h1':
                        case 'h2':
                        case 'h3':
                        case 'h4':
                        case 'h5':
                        case 'h6':
                                if ( !$this->stack->inScope( BalanceSets::$headingSet ) ) {
                                        return true; // ignore
                                }
                                $this->stack->generateImpliedEndTags();
                                $this->stack->popTag( BalanceSets::$headingSet );
                                return true;

                        case 'sarcasm':
                                // Take a deep breath, then:
                                break;

                        case 'a':
                        case 'b':
                        case 'big':
                        case 'code':
                        case 'em':
                        case 'font':
                        case 'i':
                        case 'nobr':
                        case 's':
                        case 'small':
                        case 'strike':
                        case 'strong':
                        case 'tt':
                        case 'u':
                                if ( $this->stack->adoptionAgency( $value, $this->afe ) ) {
                                        return true; // If we did something, we're done.
                                }
                                break; // Go to the "any other end tag" case.

                        case 'applet':
                        case 'marquee':
                        case 'object':
                                if ( !$this->stack->inScope( $value ) ) {
                                        return true; // ignore
                                }
                                $this->stack->generateImpliedEndTags();
                                $this->stack->popTag( $value );
                                $this->afe->clearToMarker();
                                return true;

                        case 'br':
                                // Turn </br> into <br>
                                return $this->inBodyMode( 'tag', $value, [] );
                        }

                        // Any other end tag goes here
                        foreach ( $this->stack as $i => $node ) {
                                if ( $node->isHtmlNamed( $value ) ) {
                                        $this->stack->generateImpliedEndTags( $value );
                                        $this->stack->popTo( $i ); // including $i
                                        break;
                                } elseif ( $node->isA( BalanceSets::$specialSet ) ) {
                                        return true; // ignore this close token.
                                }
                        }
                        return true;
                } elseif ( $token === 'comment' ) {
                        $this->stack->insertComment( $value );
                        return true;
                } else {
                        Assert::invariant( false, "Bad token type: $token" );
                }
        }

        private function inTableMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        if ( $this->textIntegrationMode ) {
                                return $this->inBodyMode( $token, $value, $attribs, $selfClose );
                        } elseif ( $this->stack->currentNode->isA( BalanceSets::$tableSectionRowSet ) ) {
                                $this->pendingTableText = '';
                                $this->originalInsertionMode = $this->parseMode;
                                return $this->switchModeAndReprocess( 'inTableTextMode',
                                        $token, $value, $attribs, $selfClose );
                        }
                        // fall through to default case.
                } elseif ( $token === 'eof' ) {
                        $this->stopParsing();
                        return true;
                } elseif ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'caption':
                                $this->afe->insertMarker();
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->switchMode( 'inCaptionMode' );
                                return true;
                        case 'colgroup':
                                $this->stack->clearToContext( BalanceSets::$tableContextSet );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->switchMode( 'inColumnGroupMode' );
                                return true;
                        case 'col':
                                $this->inTableMode( 'tag', 'colgroup', [] );
                                return $this->insertToken( $token, $value, $attribs, $selfClose );
                        case 'tbody':
                        case 'tfoot':
                        case 'thead':
                                $this->stack->clearToContext( BalanceSets::$tableContextSet );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->switchMode( 'inTableBodyMode' );
                                return true;
                        case 'td':
                        case 'th':
                        case 'tr':
                                $this->inTableMode( 'tag', 'tbody', [] );
                                return $this->insertToken( $token, $value, $attribs, $selfClose );
                        case 'table':
                                if ( !$this->stack->inTableScope( $value ) ) {
                                        return true; // Ignore this tag.
                                }
                                $this->inTableMode( 'endtag', $value );
                                return $this->insertToken( $token, $value, $attribs, $selfClose );

                        case 'style':
                        // OMITTED: <script>
                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );

                        case 'input':
                                if ( !isset( $attribs['type'] ) || strcasecmp( $attribs['type'], 'hidden' ) !== 0 ) {
                                        break; // Handle this as "everything else"
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->pop();
                                return true;

                        case 'form':
                                if (
                                        $this->formElementPointer ||
                                        $this->stack->indexOf( 'template' ) >= 0
                                ) {
                                        return true; // ignore this token
                                }
                                $this->formElementPointer =
                                        $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->popTag( $this->formElementPointer );
                                return true;
                        }
                        // Fall through for "anything else" clause.
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'table':
                                if ( !$this->stack->inTableScope( $value ) ) {
                                        return true; // Ignore.
                                }
                                $this->stack->popTag( $value );
                                $this->resetInsertionMode();
                                return true;
                        // OMITTED: <body>
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        // OMITTED: <html>
                        case 'tbody':
                        case 'td':
                        case 'tfoot':
                        case 'th':
                        case 'thead':
                        case 'tr':
                                return true; // Ignore the token.
                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );
                        }
                        // Fall through for "anything else" clause.
                } elseif ( $token === 'comment' ) {
                        $this->stack->insertComment( $value );
                        return true;
                }
                // This is the "anything else" case:
                $this->stack->fosterParentMode = true;
                $this->inBodyMode( $token, $value, $attribs, $selfClose );
                $this->stack->fosterParentMode = false;
                return true;
        }

        private function inTableTextMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        $this->pendingTableText .= $value;
                        return true;
                }
                // Non-text token:
                $text = $this->pendingTableText;
                $this->pendingTableText = '';
                if ( preg_match( '/[^\x09\x0A\x0C\x0D\x20]/', $text ) ) {
                        // This should match the "anything else" case inTableMode
                        $this->stack->fosterParentMode = true;
                        $this->inBodyMode( 'text', $text );
                        $this->stack->fosterParentMode = false;
                } else {
                        // Pending text is just whitespace.
                        $this->stack->insertText( $text );
                }
                return $this->switchModeAndReprocess(
                        $this->originalInsertionMode, $token, $value, $attribs, $selfClose
                );
        }

        // helper for inCaptionMode
        private function endCaption() {
                if ( !$this->stack->inTableScope( 'caption' ) ) {
                        return false;
                }
                $this->stack->generateImpliedEndTags();
                $this->stack->popTag( 'caption' );
                $this->afe->clearToMarker();
                $this->switchMode( 'inTableMode' );
                return true;
        }

        private function inCaptionMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        case 'tbody':
                        case 'td':
                        case 'tfoot':
                        case 'th':
                        case 'thead':
                        case 'tr':
                                if ( $this->endCaption() ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        }
                        // Fall through to "anything else" case.
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'caption':
                                $this->endCaption();
                                return true;
                        case 'table':
                                if ( $this->endCaption() ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        case 'body':
                        case 'col':
                        case 'colgroup':
                        // OMITTED: <html>
                        case 'tbody':
                        case 'td':
                        case 'tfoot':
                        case 'th':
                        case 'thead':
                        case 'tr':
                                // Ignore the token
                                return true;
                        }
                        // Fall through to "anything else" case.
                }
                // The Anything Else case
                return $this->inBodyMode( $token, $value, $attribs, $selfClose );
        }

        private function inColumnGroupMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        if ( preg_match( '/^[\x09\x0A\x0C\x0D\x20]+/', $value, $matches ) ) {
                                $this->stack->insertText( $matches[0] );
                                $value = substr( $value, strlen( $matches[0] ) );
                        }
                        if ( strlen( $value ) === 0 ) {
                                return true; // All text handled.
                        }
                        // Fall through to handle non-whitespace below.
                } elseif ( $token === 'tag' ) {
                        switch ( $value ) {
                        // OMITTED: <html>
                        case 'col':
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->stack->pop();
                                return true;
                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );
                        }
                        // Fall through for "anything else".
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'colgroup':
                                if ( !$this->stack->currentNode->isHtmlNamed( 'colgroup' ) ) {
                                        return true; // Ignore the token.
                                }
                                $this->stack->pop();
                                $this->switchMode( 'inTableMode' );
                                return true;
                        case 'col':
                                return true; // Ignore the token.
                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );
                        }
                        // Fall through for "anything else".
                } elseif ( $token === 'eof' ) {
                        return $this->inBodyMode( $token, $value, $attribs, $selfClose );
                } elseif ( $token === 'comment' ) {
                        $this->stack->insertComment( $value );
                        return true;
                }

                // Anything else
                if ( !$this->stack->currentNode->isHtmlNamed( 'colgroup' ) ) {
                        return true; // Ignore the token.
                }
                $this->inColumnGroupMode( 'endtag', 'colgroup' );
                return $this->insertToken( $token, $value, $attribs, $selfClose );
        }

        // Helper function for inTableBodyMode
        private function endSection() {
                if ( !(
                        $this->stack->inTableScope( 'tbody' ) ||
                        $this->stack->inTableScope( 'thead' ) ||
                        $this->stack->inTableScope( 'tfoot' )
                ) ) {
                        return false;
                }
                $this->stack->clearToContext( BalanceSets::$tableBodyContextSet );
                $this->stack->pop();
                $this->switchMode( 'inTableMode' );
                return true;
        }
        private function inTableBodyMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'tr':
                                $this->stack->clearToContext( BalanceSets::$tableBodyContextSet );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->switchMode( 'inRowMode' );
                                return true;
                        case 'th':
                        case 'td':
                                $this->inTableBodyMode( 'tag', 'tr', [] );
                                $this->insertToken( $token, $value, $attribs, $selfClose );
                                return true;
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        case 'tbody':
                        case 'tfoot':
                        case 'thead':
                                if ( $this->endSection() ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        }
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'table':
                                if ( $this->endSection() ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        case 'tbody':
                        case 'tfoot':
                        case 'thead':
                                if ( $this->stack->inTableScope( $value ) ) {
                                        $this->endSection();
                                }
                                return true;
                        // OMITTED: <body>
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        // OMITTED: <html>
                        case 'td':
                        case 'th':
                        case 'tr':
                                return true; // Ignore the token.
                        }
                }
                // Anything else:
                return $this->inTableMode( $token, $value, $attribs, $selfClose );
        }

        // Helper function for inRowMode
        private function endRow() {
                if ( !$this->stack->inTableScope( 'tr' ) ) {
                        return false;
                }
                $this->stack->clearToContext( BalanceSets::$tableRowContextSet );
                $this->stack->pop();
                $this->switchMode( 'inTableBodyMode' );
                return true;
        }
        private function inRowMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'th':
                        case 'td':
                                $this->stack->clearToContext( BalanceSets::$tableRowContextSet );
                                $this->stack->insertHTMLElement( $value, $attribs );
                                $this->switchMode( 'inCellMode' );
                                $this->afe->insertMarker();
                                return true;
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        case 'tbody':
                        case 'tfoot':
                        case 'thead':
                        case 'tr':
                                if ( $this->endRow() ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        }
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'tr':
                                $this->endRow();
                                return true;
                        case 'table':
                                if ( $this->endRow() ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        case 'tbody':
                        case 'tfoot':
                        case 'thead':
                                if (
                                        $this->stack->inTableScope( $value ) &&
                                        $this->endRow()
                                ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        // OMITTED: <body>
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        // OMITTED: <html>
                        case 'td':
                        case 'th':
                                return true; // Ignore the token.
                        }
                }
                // Anything else:
                return $this->inTableMode( $token, $value, $attribs, $selfClose );
        }

        // Helper for inCellMode
        private function endCell() {
                if ( $this->stack->inTableScope( 'td' ) ) {
                        $this->inCellMode( 'endtag', 'td' );
                        return true;
                } elseif ( $this->stack->inTableScope( 'th' ) ) {
                        $this->inCellMode( 'endtag', 'th' );
                        return true;
                } else {
                        return false;
                }
        }
        private function inCellMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        case 'tbody':
                        case 'td':
                        case 'tfoot':
                        case 'th':
                        case 'thead':
                        case 'tr':
                                if ( $this->endCell() ) {
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        }
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'td':
                        case 'th':
                                if ( $this->stack->inTableScope( $value ) ) {
                                        $this->stack->generateImpliedEndTags();
                                        $this->stack->popTag( $value );
                                        $this->afe->clearToMarker();
                                        $this->switchMode( 'inRowMode' );
                                }
                                return true;
                        // OMITTED: <body>
                        case 'caption':
                        case 'col':
                        case 'colgroup':
                        // OMITTED: <html>
                                return true;

                        case 'table':
                        case 'tbody':
                        case 'tfoot':
                        case 'thead':
                        case 'tr':
                                if ( $this->stack->inTableScope( $value ) ) {
                                        $this->stack->generateImpliedEndTags();
                                        $this->stack->popTag( BalanceSets::$tableCellSet );
                                        $this->afe->clearToMarker();
                                        $this->switchMode( 'inRowMode' );
                                        $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        }
                }
                // Anything else:
                return $this->inBodyMode( $token, $value, $attribs, $selfClose );
        }

        private function inSelectMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' ) {
                        $this->stack->insertText( $value );
                        return true;
                } elseif ( $token === 'eof' ) {
                        return $this->inBodyMode( $token, $value, $attribs, $selfClose );
                } elseif ( $token === 'tag' ) {
                        switch ( $value ) {
                        // OMITTED: <html>
                        case 'option':
                                if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
                                        $this->stack->pop();
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;
                        case 'optgroup':
                                if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
                                        $this->stack->pop();
                                }
                                if ( $this->stack->currentNode->isHtmlNamed( 'optgroup' ) ) {
                                        $this->stack->pop();
                                }
                                $this->stack->insertHTMLElement( $value, $attribs );
                                return true;
                        case 'select':
                                $this->inSelectMode( 'endtag', $value ); // treat it like endtag
                                return true;
                        case 'input':
                        case 'keygen':
                        case 'textarea':
                                if ( !$this->stack->inSelectScope( 'select' ) ) {
                                        return true; // ignore token (fragment case)
                                }
                                $this->inSelectMode( 'endtag', 'select' );
                                return $this->insertToken( $token, $value, $attribs, $selfClose );
                        case 'script':
                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );
                        }
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'optgroup':
                                if (
                                        $this->stack->currentNode->isHtmlNamed( 'option' ) &&
                                        $this->stack->length() >= 2 &&
                                        $this->stack->node( $this->stack->length() - 2 )->isHtmlNamed( 'optgroup' )
                                ) {
                                        $this->stack->pop();
                                }
                                if ( $this->stack->currentNode->isHtmlNamed( 'optgroup' ) ) {
                                        $this->stack->pop();
                                }
                                return true;
                        case 'option':
                                if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
                                        $this->stack->pop();
                                }
                                return true;
                        case 'select':
                                if ( !$this->stack->inSelectScope( $value ) ) {
                                        return true; // fragment case
                                }
                                $this->stack->popTag( $value );
                                $this->resetInsertionMode();
                                return true;
                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );
                        }
                } elseif ( $token === 'comment' ) {
                        $this->stack->insertComment( $value );
                        return true;
                }
                // anything else: just ignore the token
                return true;
        }

        private function inSelectInTableMode( $token, $value, $attribs = null, $selfClose = false ) {
                switch ( $value ) {
                case 'caption':
                case 'table':
                case 'tbody':
                case 'tfoot':
                case 'thead':
                case 'tr':
                case 'td':
                case 'th':
                        if ( $token === 'tag' ) {
                                $this->inSelectInTableMode( 'endtag', 'select' );
                                return $this->insertToken( $token, $value, $attribs, $selfClose );
                        } elseif ( $token === 'endtag' ) {
                                if ( $this->stack->inTableScope( $value ) ) {
                                        $this->inSelectInTableMode( 'endtag', 'select' );
                                        return $this->insertToken( $token, $value, $attribs, $selfClose );
                                }
                                return true;
                        }
                }
                // anything else
                return $this->inSelectMode( $token, $value, $attribs, $selfClose );
        }

        private function inTemplateMode( $token, $value, $attribs = null, $selfClose = false ) {
                if ( $token === 'text' || $token === 'comment' ) {
                        return $this->inBodyMode( $token, $value, $attribs, $selfClose );
                } elseif ( $token === 'eof' ) {
                        if ( $this->stack->indexOf( 'template' ) < 0 ) {
                                $this->stopParsing();
                        } else {
                                $this->stack->popTag( 'template' );
                                $this->afe->clearToMarker();
                                array_pop( $this->templateInsertionModes );
                                $this->resetInsertionMode();
                                $this->insertToken( $token, $value, $attribs, $selfClose );
                        }
                        return true;
                } elseif ( $token === 'tag' ) {
                        switch ( $value ) {
                        case 'base':
                        case 'basefont':
                        case 'bgsound':
                        case 'link':
                        case 'meta':
                        case 'noframes':
                        // OMITTED: <script>
                        case 'style':
                        case 'template':
                        // OMITTED: <title>
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );

                        case 'caption':
                        case 'colgroup':
                        case 'tbody':
                        case 'tfoot':
                        case 'thead':
                                return $this->switchModeAndReprocess(
                                        'inTableMode', $token, $value, $attribs, $selfClose
                                );

                        case 'col':
                                return $this->switchModeAndReprocess(
                                        'inColumnGroupMode', $token, $value, $attribs, $selfClose
                                );

                        case 'tr':
                                return $this->switchModeAndReprocess(
                                        'inTableBodyMode', $token, $value, $attribs, $selfClose
                                );

                        case 'td':
                        case 'th':
                                return $this->switchModeAndReprocess(
                                        'inRowMode', $token, $value, $attribs, $selfClose
                                );
                        }
                        return $this->switchModeAndReprocess(
                                'inBodyMode', $token, $value, $attribs, $selfClose
                        );
                } elseif ( $token === 'endtag' ) {
                        switch ( $value ) {
                        case 'template':
                                return $this->inHeadMode( $token, $value, $attribs, $selfClose );
                        }
                        return true;
                } else {
                        Assert::invariant( false, "Bad token type: $token" );
                }
        }
}