<?php

/**
 * Forgivingly lexes HTML (SGML-style) markup into tokens.
 *
 * A lexer parses a string of SGML-style markup and converts them into
 * corresponding tokens.  It doesn't check for well-formedness, although its
 * internal mechanism may make this automatic (such as the case of
 * HTMLPurifier_Lexer_DOMLex).  There are several implementations to choose
 * from.
 *
 * A lexer is HTML-oriented: it might work with XML, but it's not
 * recommended, as we adhere to a subset of the specification for optimization
 * reasons. This might change in the future. Also, most tokenizers are not
 * expected to handle DTDs or PIs.
 *
 * This class should not be directly instantiated, but you may use create() to
 * retrieve a default copy of the lexer.  Being a supertype, this class
 * does not actually define any implementation, but offers commonly used
 * convenience functions for subclasses.
 *
 * @note The unit tests will instantiate this class for testing purposes, as
 *       many of the utility functions require a class to be instantiated.
 *       This means that, even though this class is not runnable, it will
 *       not be declared abstract.
 *
 * @par
 *
 * @note
 * We use tokens rather than create a DOM representation because DOM would:
 *
 * @par
 *  -# Require more processing and memory to create,
 *  -# Is not streamable, and
 *  -# Has the entire document structure (html and body not needed).
 *
 * @par
 * However, DOM is helpful in that it makes it easy to move around nodes
 * without a lot of lookaheads to see when a tag is closed. This is a
 * limitation of the token system and some workarounds would be nice.
 */
class HTMLPurifier_Lexer
{

    /**
     * Whether or not this lexer implements line-number/column-number tracking.
     * If it does, set to true.
     */
    public $tracksLineNumbers = false;

    // -- STATIC ----------------------------------------------------------

    /**
     * Retrieves or sets the default Lexer as a Prototype Factory.
     *
     * By default HTMLPurifier_Lexer_DOMLex will be returned. There are
     * a few exceptions involving special features that only DirectLex
     * implements.
     *
     * @note The behavior of this class has changed, rather than accepting
     *       a prototype object, it now accepts a configuration object.
     *       To specify your own prototype, set %Core.LexerImpl to it.
     *       This change in behavior de-singletonizes the lexer object.
     *
     * @param $config Instance of HTMLPurifier_Config
     * @return Concrete lexer.
     */
    public static function create($config) {

        if (!($config instanceof HTMLPurifier_Config)) {
            $lexer = $config;
            trigger_error("Passing a prototype to
              HTMLPurifier_Lexer::create() is deprecated, please instead
              use %Core.LexerImpl", E_USER_WARNING);
        } else {
            $lexer = $config->get('Core.LexerImpl');
        }

        $needs_tracking =
            $config->get('Core.MaintainLineNumbers') ||
            $config->get('Core.CollectErrors');

        $inst = null;
        if (is_object($lexer)) {
            $inst = $lexer;
        } else {

            if (is_null($lexer)) { do {
                // auto-detection algorithm

                if ($needs_tracking) {
                    $lexer = 'DirectLex';
                    break;
                }

                if (
                    class_exists('DOMDocument') &&
                    method_exists('DOMDocument', 'loadHTML') &&
                    !extension_loaded('domxml')
                ) {
                    // check for DOM support, because while it's part of the
                    // core, it can be disabled compile time. Also, the PECL
                    // domxml extension overrides the default DOM, and is evil
                    // and nasty and we shan't bother to support it
                    $lexer = 'DOMLex';
                } else {
                    $lexer = 'DirectLex';
                }

            } while(0); } // do..while so we can break

            // instantiate recognized string names
            switch ($lexer) {
                case 'DOMLex':
                    $inst = new HTMLPurifier_Lexer_DOMLex();
                    break;
                case 'DirectLex':
                    $inst = new HTMLPurifier_Lexer_DirectLex();
                    break;
                case 'PH5P':
                    $inst = new HTMLPurifier_Lexer_PH5P();
                    break;
                default:
                    throw new HTMLPurifier_Exception("Cannot instantiate unrecognized Lexer type " . htmlspecialchars($lexer));
            }
        }

        if (!$inst) throw new HTMLPurifier_Exception('No lexer was instantiated');

        // once PHP DOM implements native line numbers, or we
        // hack out something using XSLT, remove this stipulation
        if ($needs_tracking && !$inst->tracksLineNumbers) {
            throw new HTMLPurifier_Exception('Cannot use lexer that does not support line numbers with Core.MaintainLineNumbers or Core.CollectErrors (use DirectLex instead)');
        }

        return $inst;

    }

    // -- CONVENIENCE MEMBERS ---------------------------------------------

    public function __construct() {
        $this->_entity_parser = new HTMLPurifier_EntityParser();
    }

    /**
     * Most common entity to raw value conversion table for special entities.
     */
    protected $_special_entity2str =
            array(
                    '&quot;' => '"',
                    '&amp;'  => '&',
                    '&lt;'   => '<',
                    '&gt;'   => '>',
                    '&#39;'  => "'",
                    '&#039;' => "'",
                    '&#x27;' => "'"
            );

    /**
     * Parses special entities into the proper characters.
     *
     * This string will translate escaped versions of the special characters
     * into the correct ones.
     *
     * @warning
     * You should be able to treat the output of this function as
     * completely parsed, but that's only because all other entities should
     * have been handled previously in substituteNonSpecialEntities()
     *
     * @param $string String character data to be parsed.
     * @returns Parsed character data.
     */
    public function parseData($string) {

        // following functions require at least one character
        if ($string === '') return '';

        // subtracts amps that cannot possibly be escaped
        $num_amp = substr_count($string, '&') - substr_count($string, '& ') -
            ($string[strlen($string)-1] === '&' ? 1 : 0);

        if (!$num_amp) return $string; // abort if no entities
        $num_esc_amp = substr_count($string, '&amp;');
        $string = strtr($string, $this->_special_entity2str);

        // code duplication for sake of optimization, see above
        $num_amp_2 = substr_count($string, '&') - substr_count($string, '& ') -
            ($string[strlen($string)-1] === '&' ? 1 : 0);

        if ($num_amp_2 <= $num_esc_amp) return $string;

        // hmm... now we have some uncommon entities. Use the callback.
        $string = $this->_entity_parser->substituteSpecialEntities($string);
        return $string;
    }

    /**
     * Lexes an HTML string into tokens.
     *
     * @param $string String HTML.
     * @return HTMLPurifier_Token array representation of HTML.
     */
    public function tokenizeHTML($string, $config, $context) {
        trigger_error('Call to abstract class', E_USER_ERROR);
    }

    /**
     * Translates CDATA sections into regular sections (through escaping).
     *
     * @param $string HTML string to process.
     * @returns HTML with CDATA sections escaped.
     */
    protected static function escapeCDATA($string) {
        return preg_replace_callback(
            '/<!\[CDATA\[(.+?)\]\]>/s',
            array('HTMLPurifier_Lexer', 'CDATACallback'),
            $string
        );
    }

    /**
     * Special CDATA case that is especially convoluted for <script>
     */
    protected static function escapeCommentedCDATA($string) {
        return preg_replace_callback(
            '#<!--//--><!\[CDATA\[//><!--(.+?)//--><!\]\]>#s',
            array('HTMLPurifier_Lexer', 'CDATACallback'),
            $string
        );
    }

    /**
     * Callback function for escapeCDATA() that does the work.
     *
     * @warning Though this is public in order to let the callback happen,
     *          calling it directly is not recommended.
     * @params $matches PCRE matches array, with index 0 the entire match
     *                  and 1 the inside of the CDATA section.
     * @returns Escaped internals of the CDATA section.
     */
    protected static function CDATACallback($matches) {
        // not exactly sure why the character set is needed, but whatever
        return htmlspecialchars($matches[1], ENT_COMPAT, 'UTF-8');
    }

    /**
     * Takes a piece of HTML and normalizes it by converting entities, fixing
     * encoding, extracting bits, and other good stuff.
     * @todo Consider making protected
     */
    public function normalize($html, $config, $context) {

        // normalize newlines to \n
        $html = str_replace("\r\n", "\n", $html);
        $html = str_replace("\r", "\n", $html);

        if ($config->get('HTML.Trusted')) {
            // escape convoluted CDATA
            $html = $this->escapeCommentedCDATA($html);
        }

        // escape CDATA
        $html = $this->escapeCDATA($html);

        // extract body from document if applicable
        if ($config->get('Core.ConvertDocumentToFragment')) {
            $html = $this->extractBody($html);
        }

        // expand entities that aren't the big five
        $html = $this->_entity_parser->substituteNonSpecialEntities($html);

        // clean into wellformed UTF-8 string for an SGML context: this has
        // to be done after entity expansion because the entities sometimes
        // represent non-SGML characters (horror, horror!)
        $html = HTMLPurifier_Encoder::cleanUTF8($html);

        return $html;
    }

    /**
     * Takes a string of HTML (fragment or document) and returns the content
     * @todo Consider making protected
     */
    public function extractBody($html) {
        $matches = array();
        $result = preg_match('!<body[^>]*>(.*)</body>!is', $html, $matches);
        if ($result) {
            return $matches[1];
        } else {
            return $html;
        }
    }

}

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