xref: /dokuwiki/vendor/splitbrain/lesserphp/src/Parser.php (revision a646a37b3a8a80e780de8ad5cacd58b7f3248acf)

<?php
/**
 * http://leafo.net/lessphp
 *
 * LESS CSS compiler, adapted from http://lesscss.org
 *
 * Copyright 2013, Leaf Corcoran <leafot@gmail.com>
 * Copyright 2016, Marcus Schwarz <github@maswaba.de>
 * Licensed under MIT or GPLv3, see LICENSE
 */

namespace LesserPHP;

use Exception;
use LesserPHP\Utils\Util;
use stdClass;

/**
 * responsible for taking a string of LESS code and converting it into a syntax tree
 */
class Parser
{

    public $eatWhiteDefault;
    public $sourceName;
    public $writeComments;
    public $count;
    public $line;
    public $env;
    public $buffer;
    public $seenComments;
    public $inExp;

    protected static $nextBlockId = 0; // used to uniquely identify blocks

    protected static $precedence = [
        '=<' => 0,
        '>=' => 0,
        '=' => 0,
        '<' => 0,
        '>' => 0,

        '+' => 1,
        '-' => 1,
        '*' => 2,
        '/' => 2,
        '%' => 2,
    ];

    protected static $whitePattern;
    protected static $commentMulti;

    protected static $commentSingle = '//';
    protected static $commentMultiLeft = '/*';
    protected static $commentMultiRight = '*/';

    // regex string to match any of the operators
    protected static $operatorString;

    // these properties will supress division unless it's inside parenthases
    protected static $supressDivisionProps =
        ['/border-radius$/i', '/^font$/i'];

    protected $blockDirectives = [
        'font-face',
        'keyframes',
        'page',
        '-moz-document',
        'viewport',
        '-moz-viewport',
        '-o-viewport',
        '-ms-viewport'
    ];
    protected $lineDirectives = ['charset'];

    /**
     * if we are in parens we can be more liberal with whitespace around
     * operators because it must evaluate to a single value and thus is less
     * ambiguous.
     *
     * Consider:
     *     property1: 10 -5; // is two numbers, 10 and -5
     *     property2: (10 -5); // should evaluate to 5
     */
    protected $inParens = false;

    // caches preg escaped literals
    protected static $literalCache = [];

    protected $currentProperty;

    /**
     * @param string|null $sourceName name used for error messages
     */
    public function __construct(?string $sourceName = null)
    {
        $this->eatWhiteDefault = true;
        $this->sourceName = $sourceName; // name used for error messages

        $this->writeComments = false;

        if (!self::$operatorString) {
            self::$operatorString =
                '(' . implode('|', array_map(
                    [Util::class, 'pregQuote'],
                    array_keys(self::$precedence)
                )) . ')';

            $commentSingle = Util::pregQuote(self::$commentSingle);
            $commentMultiLeft = Util::pregQuote(self::$commentMultiLeft);
            $commentMultiRight = Util::pregQuote(self::$commentMultiRight);

            self::$commentMulti = $commentMultiLeft . '.*?' . $commentMultiRight;
            self::$whitePattern = '/' . $commentSingle . '[^\n]*\s*|(' . self::$commentMulti . ')\s*|\s+/Ais';
        }
    }

    /**
     * @throws Exception
     */
    public function parse($buffer)
    {
        $this->count = 0;
        $this->line = 1;

        $this->env = null; // block stack
        $this->buffer = $this->writeComments ? $buffer : $this->removeComments($buffer);
        $this->pushSpecialBlock('root');
        $this->eatWhiteDefault = true;
        $this->seenComments = [];

        // trim whitespace on head
        // if (preg_match('/^\s+/', $this->buffer, $m)) {
        //  $this->line += substr_count($m[0], "\n");
        //  $this->buffer = ltrim($this->buffer);
        // }
        $this->whitespace();

        // parse the entire file
        while (false !== $this->parseChunk()) {
            // no-op
        }

        if ($this->count != strlen($this->buffer)) {
            $this->throwError(sprintf(
                "parse error: count mismatches buffer length %d != %d",
                $this->count,
                strlen($this->buffer)
            ));
        }

        // TODO report where the block was opened
        if (!property_exists($this->env, 'parent') || !is_null($this->env->parent)) {
            $this->throwError('parse error: unclosed block');
        }

        return $this->env;
    }

    /**
     * Parse a single chunk off the head of the buffer and append it to the
     * current parse environment.
     * Returns false when the buffer is empty, or when there is an error.
     *
     * This function is called repeatedly until the entire document is
     * parsed.
     *
     * This parser is most similar to a recursive descent parser. Single
     * functions represent discrete grammatical rules for the language, and
     * they are able to capture the text that represents those rules.
     *
     * Consider the function Lessc::keyword(). (all parse functions are
     * structured the same)
     *
     * The function takes a single reference argument. When calling the
     * function it will attempt to match a keyword on the head of the buffer.
     * If it is successful, it will place the keyword in the referenced
     * argument, advance the position in the buffer, and return true. If it
     * fails then it won't advance the buffer and it will return false.
     *
     * All of these parse functions are powered by Lessc::match(), which behaves
     * the same way, but takes a literal regular expression. Sometimes it is
     * more convenient to use match instead of creating a new function.
     *
     * Because of the format of the functions, to parse an entire string of
     * grammatical rules, you can chain them together using &&.
     *
     * But, if some of the rules in the chain succeed before one fails, then
     * the buffer position will be left at an invalid state. In order to
     * avoid this, Lessc::seek() is used to remember and set buffer positions.
     *
     * Before parsing a chain, use $s = $this->seek() to remember the current
     * position into $s. Then if a chain fails, use $this->seek($s) to
     * go back where we started.
     *
     * @throws Exception
     */
    protected function parseChunk()
    {
        if (empty($this->buffer)) return false;
        $s = $this->seek();

        if ($this->whitespace()) {
            return true;
        }

        // setting a property
        if ($this->keyword($key) && $this->assign() &&
            $this->propertyValue($value, $key) && $this->end()) {
            $this->append(['assign', $key, $value], $s);
            return true;
        } else {
            $this->seek($s);
        }


        // look for special css blocks
        if ($this->literal('@', false)) {
            $this->count--;

            // media
            if ($this->literal('@media')) {
                if (($this->mediaQueryList($mediaQueries) || true)
                    && $this->literal('{')) {
                    $media = $this->pushSpecialBlock('media');
                    $media->queries = is_null($mediaQueries) ? [] : $mediaQueries;
                    return true;
                } else {
                    $this->seek($s);
                    return false;
                }
            }

            if ($this->literal('@', false) && $this->keyword($dirName)) {
                if ($this->isDirective($dirName, $this->blockDirectives)) {
                    if (($this->openString('{', $dirValue, null, [';']) || true) &&
                        $this->literal('{')) {
                        $dir = $this->pushSpecialBlock('directive');
                        $dir->name = $dirName;
                        if (isset($dirValue)) $dir->value = $dirValue;
                        return true;
                    }
                } elseif ($this->isDirective($dirName, $this->lineDirectives)) {
                    if ($this->propertyValue($dirValue) && $this->end()) {
                        $this->append(['directive', $dirName, $dirValue]);
                        return true;
                    }
                } elseif ($this->literal(':', true)) {
                    //Ruleset Definition
                    if (($this->openString('{', $dirValue, null, [';']) || true) &&
                        $this->literal('{')) {
                        $dir = $this->pushBlock($this->fixTags(['@' . $dirName]));
                        $dir->name = $dirName;
                        if (isset($dirValue)) $dir->value = $dirValue;
                        return true;
                    }
                }
            }

            $this->seek($s);
        }

        // setting a variable
        if ($this->variable($var) && $this->assign() &&
            $this->propertyValue($value) && $this->end()) {
            $this->append(['assign', $var, $value], $s);
            return true;
        } else {
            $this->seek($s);
        }

        if ($this->import($importValue)) {
            $this->append($importValue, $s);
            return true;
        }

        // opening parametric mixin
        if ($this->tag($tag, true) && $this->argumentDef($args, $isVararg) &&
            ($this->guards($guards) || true) &&
            $this->literal('{')) {
            $block = $this->pushBlock($this->fixTags([$tag]));
            $block->args = $args;
            $block->isVararg = $isVararg;
            if (!empty($guards)) $block->guards = $guards;
            return true;
        } else {
            $this->seek($s);
        }

        // opening a simple block
        if ($this->tags($tags) && $this->literal('{', false)) {
            $tags = $this->fixTags($tags);
            $this->pushBlock($tags);
            return true;
        } else {
            $this->seek($s);
        }

        // closing a block
        if ($this->literal('}', false)) {
            try {
                $block = $this->pop();
            } catch (Exception $e) {
                $this->seek($s);
                $this->throwError($e->getMessage());
            }

            $hidden = false;
            if (is_null($block->type)) {
                $hidden = true;
                if (!isset($block->args)) {
                    foreach ($block->tags as $tag) {
                        if (!is_string($tag) || $tag[0] != Constants::MPREFIX) {
                            $hidden = false;
                            break;
                        }
                    }
                }

                foreach ($block->tags as $tag) {
                    if (is_string($tag)) {
                        $this->env->children[$tag][] = $block;
                    }
                }
            }

            if (!$hidden) {
                $this->append(['block', $block], $s);
            }

            // this is done here so comments aren't bundled into he block that
            // was just closed
            $this->whitespace();
            return true;
        }

        // mixin
        if ($this->mixinTags($tags) &&
            ($this->argumentDef($argv, $isVararg) || true) &&
            ($this->keyword($suffix) || true) && $this->end()) {
            $tags = $this->fixTags($tags);
            $this->append(['mixin', $tags, $argv, $suffix], $s);
            return true;
        } else {
            $this->seek($s);
        }

        // spare ;
        if ($this->literal(';')) return true;

        return false; // got nothing, throw error
    }

    protected function isDirective($dirname, $directives)
    {
        // TODO: cache pattern in parser
        $pattern = implode(
            '|',
            array_map([Util::class, 'pregQuote'], $directives)
        );
        $pattern = '/^(-[a-z-]+-)?(' . $pattern . ')$/i';

        return preg_match($pattern, $dirname);
    }

    protected function fixTags($tags)
    {
        // move @ tags out of variable namespace
        foreach ($tags as &$tag) {
            if ($tag[0] == Constants::VPREFIX)
                $tag[0] = Constants::MPREFIX;
        }
        return $tags;
    }

    // a list of expressions
    protected function expressionList(&$exps)
    {
        $values = [];

        while ($this->expression($exp)) {
            $values[] = $exp;
        }

        if (count($values) == 0) return false;

        $exps = Lessc::compressList($values, ' ');
        return true;
    }

    /**
     * Attempt to consume an expression.
     * @link http://en.wikipedia.org/wiki/Operator-precedence_parser#Pseudo-code
     */
    protected function expression(&$out)
    {
        if ($this->value($lhs)) {
            $out = $this->expHelper($lhs, 0);

            // look for / shorthand
            if (!empty($this->env->supressedDivision)) {
                unset($this->env->supressedDivision);
                $s = $this->seek();
                if ($this->literal('/') && $this->value($rhs)) {
                    $out = ['list', '', [$out, ['keyword', '/'], $rhs]];
                } else {
                    $this->seek($s);
                }
            }

            return true;
        }
        return false;
    }

    /**
     * recursively parse infix equation with $lhs at precedence $minP
     */
    protected function expHelper($lhs, $minP)
    {
        $this->inExp = true;
        $ss = $this->seek();

        while (true) {
            $whiteBefore = isset($this->buffer[$this->count - 1]) &&
                ctype_space($this->buffer[$this->count - 1]);

            // If there is whitespace before the operator, then we require
            // whitespace after the operator for it to be an expression
            $needWhite = $whiteBefore && !$this->inParens;

            if (
                $this->match(self::$operatorString . ($needWhite ? '\s' : ''), $m) &&
                self::$precedence[$m[1]] >= $minP
            ) {
                if (
                    !$this->inParens && isset($this->env->currentProperty) && $m[1] == '/' &&
                    empty($this->env->supressedDivision)
                ) {
                    foreach (self::$supressDivisionProps as $pattern) {
                        if (preg_match($pattern, $this->env->currentProperty)) {
                            $this->env->supressedDivision = true;
                            break 2;
                        }
                    }
                }


                $whiteAfter = isset($this->buffer[$this->count - 1]) &&
                    ctype_space($this->buffer[$this->count - 1]);

                if (!$this->value($rhs)) break;

                // peek for next operator to see what to do with rhs
                if (
                    $this->peek(self::$operatorString, $next) &&
                    self::$precedence[$next[1]] > self::$precedence[$m[1]]
                ) {
                    $rhs = $this->expHelper($rhs, self::$precedence[$next[1]]);
                }

                $lhs = ['expression', $m[1], $lhs, $rhs, $whiteBefore, $whiteAfter];
                $ss = $this->seek();

                continue;
            }

            break;
        }

        $this->seek($ss);

        return $lhs;
    }

    // consume a list of values for a property
    public function propertyValue(&$value, $keyName = null)
    {
        $values = [];

        if ($keyName !== null) $this->env->currentProperty = $keyName;

        $s = null;
        while ($this->expressionList($v)) {
            $values[] = $v;
            $s = $this->seek();
            if (!$this->literal(',')) break;
        }

        if ($s) $this->seek($s);

        if ($keyName !== null) unset($this->env->currentProperty);

        if (count($values) == 0) return false;

        $value = Lessc::compressList($values, ', ');
        return true;
    }

    protected function parenValue(&$out)
    {
        $s = $this->seek();

        // speed shortcut
        if (isset($this->buffer[$this->count]) && $this->buffer[$this->count] != '(') {
            return false;
        }

        $inParens = $this->inParens;
        if ($this->literal('(') &&
            ($this->inParens = true) && $this->expression($exp) &&
            $this->literal(')')) {
            $out = $exp;
            $this->inParens = $inParens;
            return true;
        } else {
            $this->inParens = $inParens;
            $this->seek($s);
        }

        return false;
    }

    // a single value
    protected function value(&$value)
    {
        $s = $this->seek();

        // speed shortcut
        if (isset($this->buffer[$this->count]) && $this->buffer[$this->count] == '-') {
            // negation
            if ($this->literal('-', false) &&
                (($this->variable($inner) && $inner = ['variable', $inner]) ||
                    $this->unit($inner) ||
                    $this->parenValue($inner))) {
                $value = ['unary', '-', $inner];
                return true;
            } else {
                $this->seek($s);
            }
        }

        if ($this->parenValue($value)) return true;
        if ($this->unit($value)) return true;
        if ($this->color($value)) return true;
        if ($this->func($value)) return true;
        if ($this->stringValue($value)) return true;

        if ($this->keyword($word)) {
            $value = ['keyword', $word];
            return true;
        }

        // try a variable
        if ($this->variable($var)) {
            $value = ['variable', $var];
            return true;
        }

        // unquote string (should this work on any type?
        if ($this->literal('~') && $this->stringValue($str)) {
            $value = ['escape', $str];
            return true;
        } else {
            $this->seek($s);
        }

        // css hack: \0
        if ($this->literal('\\') && $this->match('([0-9]+)', $m)) {
            $value = ['keyword', '\\' . $m[1]];
            return true;
        } else {
            $this->seek($s);
        }

        return false;
    }

    // an import statement
    protected function import(&$out)
    {
        if (!$this->literal('@import')) return false;

        // @import "something.css" media;
        // @import url("something.css") media;
        // @import url(something.css) media;

        if ($this->propertyValue($value)) {
            $out = ['import', $value];
            return true;
        }
        return false;
    }

    protected function mediaQueryList(&$out)
    {
        if ($this->genericList($list, 'mediaQuery', ',', false)) {
            $out = $list[2];
            return true;
        }
        return false;
    }

    protected function mediaQuery(&$out)
    {
        $s = $this->seek();

        $expressions = null;
        $parts = [];

        if (
            (
                $this->literal('only') && ($only = true) ||
                $this->literal('not') && ($not = true) ||
                true
            ) &&
            $this->keyword($mediaType)
        ) {
            $prop = ['mediaType'];
            if (isset($only)) $prop[] = 'only';
            if (isset($not)) $prop[] = 'not';
            $prop[] = $mediaType;
            $parts[] = $prop;
        } else {
            $this->seek($s);
        }


        if (!empty($mediaType) && !$this->literal('and')) {
            // ~
        } else {
            $this->genericList($expressions, 'mediaExpression', 'and', false);
            if (is_array($expressions)) $parts = array_merge($parts, $expressions[2]);
        }

        if (count($parts) == 0) {
            $this->seek($s);
            return false;
        }

        $out = $parts;
        return true;
    }

    protected function mediaExpression(&$out)
    {
        $s = $this->seek();
        $value = null;
        if ($this->literal('(') &&
            $this->keyword($feature) &&
            ($this->literal(':') && $this->expression($value) || true) &&
            $this->literal(')')) {
            $out = ['mediaExp', $feature];
            if ($value) $out[] = $value;
            return true;
        } elseif ($this->variable($variable)) {
            $out = ['variable', $variable];
            return true;
        }

        $this->seek($s);
        return false;
    }

    // an unbounded string stopped by $end
    protected function openString($end, &$out, $nestingOpen = null, $rejectStrs = null)
    {
        $oldWhite = $this->eatWhiteDefault;
        $this->eatWhiteDefault = false;

        $stop = ["'", '"', '@{', $end];
        $stop = array_map([Util::class, 'pregQuote'], $stop);
        // $stop[] = self::$commentMulti;

        if (!is_null($rejectStrs)) {
            $stop = array_merge($stop, $rejectStrs);
        }

        $patt = '(.*?)(' . implode('|', $stop) . ')';

        $nestingLevel = 0;

        $content = [];
        while ($this->match($patt, $m, false)) {
            if (!empty($m[1])) {
                $content[] = $m[1];
                if ($nestingOpen) {
                    $nestingLevel += substr_count($m[1], $nestingOpen);
                }
            }

            $tok = $m[2];

            $this->count -= strlen($tok);
            if ($tok == $end) {
                if ($nestingLevel == 0) {
                    break;
                } else {
                    $nestingLevel--;
                }
            }

            if (($tok == "'" || $tok == '"') && $this->stringValue($str)) {
                $content[] = $str;
                continue;
            }

            if ($tok == '@{' && $this->interpolation($inter)) {
                $content[] = $inter;
                continue;
            }

            if (!empty($rejectStrs) && in_array($tok, $rejectStrs)) {
                break;
            }

            $content[] = $tok;
            $this->count += strlen($tok);
        }

        $this->eatWhiteDefault = $oldWhite;

        if (count($content) == 0) return false;

        // trim the end
        if (is_string(end($content))) {
            $content[count($content) - 1] = rtrim(end($content));
        }

        $out = ['string', '', $content];
        return true;
    }

    protected function stringValue(&$out)
    {
        $s = $this->seek();
        if ($this->literal('"', false)) {
            $delim = '"';
        } elseif ($this->literal("'", false)) {
            $delim = "'";
        } else {
            return false;
        }

        $content = [];

        // look for either ending delim , escape, or string interpolation
        $patt = '([^\n]*?)(@\{|\\\\|' . Util::pregQuote($delim) . ')';

        $oldWhite = $this->eatWhiteDefault;
        $this->eatWhiteDefault = false;

        while ($this->match($patt, $m, false)) {
            $content[] = $m[1];
            if ($m[2] == '@{') {
                $this->count -= strlen($m[2]);
                if ($this->interpolation($inter)) {
                    $content[] = $inter;
                } else {
                    $this->count += strlen($m[2]);
                    $content[] = '@{'; // ignore it
                }
            } elseif ($m[2] == '\\') {
                $content[] = $m[2];
                if ($this->literal($delim, false)) {
                    $content[] = $delim;
                }
            } else {
                $this->count -= strlen($delim);
                break; // delim
            }
        }

        $this->eatWhiteDefault = $oldWhite;

        if ($this->literal($delim)) {
            $out = ['string', $delim, $content];
            return true;
        }

        $this->seek($s);
        return false;
    }

    protected function interpolation(&$out)
    {
        $oldWhite = $this->eatWhiteDefault;
        $this->eatWhiteDefault = true;

        $s = $this->seek();
        if ($this->literal('@{') &&
            $this->openString('}', $interp, null, ["'", '"', ';']) &&
            $this->literal('}', false)) {
            $out = ['interpolate', $interp];
            $this->eatWhiteDefault = $oldWhite;
            if ($this->eatWhiteDefault) $this->whitespace();
            return true;
        }

        $this->eatWhiteDefault = $oldWhite;
        $this->seek($s);
        return false;
    }

    protected function unit(&$unit)
    {
        // speed shortcut
        if (isset($this->buffer[$this->count])) {
            $char = $this->buffer[$this->count];
            if (!ctype_digit($char) && $char != '.') return false;
        }

        if ($this->match('([0-9]+(?:\.[0-9]*)?|\.[0-9]+)([%a-zA-Z]+)?', $m)) {
            $unit = ['number', $m[1], empty($m[2]) ? '' : $m[2]];
            return true;
        }
        return false;
    }

    // a # color
    protected function color(&$out)
    {
        if ($this->match('(#(?:[0-9a-f]{8}|[0-9a-f]{6}|[0-9a-f]{3}))', $m)) {
            if (strlen($m[1]) > 7) {
                $out = ['string', '', [$m[1]]];
            } else {
                $out = ['raw_color', $m[1]];
            }
            return true;
        }

        return false;
    }

    /**
     * consume an argument definition list surrounded by ()
     * each argument is a variable name with optional value
     * or at the end a ... or a variable named followed by ...
     * arguments are separated by , unless a ; is in the list, then ; is the
     * delimiter.
     *
     * @throws Exception
     */
    protected function argumentDef(&$args, &$isVararg)
    {
        $s = $this->seek();
        if (!$this->literal('(')) return false;

        $values = [];
        $delim = ',';
        $method = 'expressionList';

        $value = null;
        $rhs = null;

        $isVararg = false;
        while (true) {
            if ($this->literal('...')) {
                $isVararg = true;
                break;
            }

            if ($this->$method($value)) {
                if ($value[0] == 'variable') {
                    $arg = ['arg', $value[1]];
                    $ss = $this->seek();

                    if ($this->assign() && $this->$method($rhs)) {
                        $arg[] = $rhs;
                    } else {
                        $this->seek($ss);
                        if ($this->literal('...')) {
                            $arg[0] = 'rest';
                            $isVararg = true;
                        }
                    }

                    $values[] = $arg;
                    if ($isVararg) break;
                    continue;
                } else {
                    $values[] = ['lit', $value];
                }
            }


            if (!$this->literal($delim)) {
                if ($delim == ',' && $this->literal(';')) {
                    // found new delim, convert existing args
                    $delim = ';';
                    $method = 'propertyValue';

                    // transform arg list
                    if (isset($values[1])) { // 2 items
                        $newList = [];
                        foreach ($values as $i => $arg) {
                            switch ($arg[0]) {
                                case 'arg':
                                    if ($i) {
                                        $this->throwError('Cannot mix ; and , as delimiter types');
                                    }
                                    $newList[] = $arg[2];
                                    break;
                                case 'lit':
                                    $newList[] = $arg[1];
                                    break;
                                case 'rest':
                                    $this->throwError('Unexpected rest before semicolon');
                            }
                        }

                        $newList = ['list', ', ', $newList];

                        switch ($values[0][0]) {
                            case 'arg':
                                $newArg = ['arg', $values[0][1], $newList];
                                break;
                            case 'lit':
                                $newArg = ['lit', $newList];
                                break;
                        }
                    } elseif ($values) { // 1 item
                        $newArg = $values[0];
                    }

                    if ($newArg) {
                        $values = [$newArg];
                    }
                } else {
                    break;
                }
            }
        }

        if (!$this->literal(')')) {
            $this->seek($s);
            return false;
        }

        $args = $values;

        return true;
    }

    // consume a list of tags
    // this accepts a hanging delimiter
    protected function tags(&$tags, $simple = false, $delim = ',')
    {
        $tags = [];
        while ($this->tag($tt, $simple)) {
            $tags[] = $tt;
            if (!$this->literal($delim)) break;
        }
        if (count($tags) == 0) return false;

        return true;
    }

    // list of tags of specifying mixin path
    // optionally separated by > (lazy, accepts extra >)
    protected function mixinTags(&$tags)
    {
        $tags = [];
        while ($this->tag($tt, true)) {
            $tags[] = $tt;
            $this->literal('>');
        }

        if (count($tags) == 0) return false;

        return true;
    }

    // a bracketed value (contained within in a tag definition)
    protected function tagBracket(&$parts, &$hasExpression)
    {
        // speed shortcut
        if (isset($this->buffer[$this->count]) && $this->buffer[$this->count] != '[') {
            return false;
        }

        $s = $this->seek();

        $hasInterpolation = false;

        if ($this->literal('[', false)) {
            $attrParts = ['['];
            // keyword, string, operator
            while (true) {
                if ($this->literal(']', false)) {
                    $this->count--;
                    break; // get out early
                }

                if ($this->match('\s+', $m)) {
                    $attrParts[] = ' ';
                    continue;
                }
                if ($this->stringValue($str)) {
                    // escape parent selector, (yuck)
                    foreach ($str[2] as &$chunk) {
                        if (is_string($chunk)) {
                            $chunk = str_replace(Constants::PARENT_SELECTOR, "$&$", $chunk);
                        }
                    }

                    $attrParts[] = $str;
                    $hasInterpolation = true;
                    continue;
                }

                if ($this->keyword($word)) {
                    $attrParts[] = $word;
                    continue;
                }

                if ($this->interpolation($inter)) {
                    $attrParts[] = $inter;
                    $hasInterpolation = true;
                    continue;
                }

                // operator, handles attr namespace too
                if ($this->match('[|-~\$\*\^=]+', $m)) {
                    $attrParts[] = $m[0];
                    continue;
                }

                break;
            }

            if ($this->literal(']', false)) {
                $attrParts[] = ']';
                foreach ($attrParts as $part) {
                    $parts[] = $part;
                }
                $hasExpression = $hasExpression || $hasInterpolation;
                return true;
            }
            $this->seek($s);
        }

        $this->seek($s);
        return false;
    }

    // a space separated list of selectors
    protected function tag(&$tag, $simple = false)
    {
        if ($simple)
            $chars = '^@,:;{}\][>\(\) "\'';
        else $chars = '^@,;{}["\'';

        $s = $this->seek();

        $hasExpression = false;
        $parts = [];
        while ($this->tagBracket($parts, $hasExpression)) {
            // no-op
        }

        $oldWhite = $this->eatWhiteDefault;
        $this->eatWhiteDefault = false;

        while (true) {
            if ($this->match('([' . $chars . '0-9][' . $chars . ']*)', $m)) {
                $parts[] = $m[1];
                if ($simple) break;

                while ($this->tagBracket($parts, $hasExpression)) {
                    // no-op
                }
                continue;
            }

            if (isset($this->buffer[$this->count]) && $this->buffer[$this->count] == '@') {
                if ($this->interpolation($interp)) {
                    $hasExpression = true;
                    $interp[2] = true; // don't unescape
                    $parts[] = $interp;
                    continue;
                }

                if ($this->literal('@')) {
                    $parts[] = '@';
                    continue;
                }
            }

            if ($this->unit($unit)) { // for keyframes
                $parts[] = $unit[1];
                $parts[] = $unit[2];
                continue;
            }

            break;
        }

        $this->eatWhiteDefault = $oldWhite;
        if (!$parts) {
            $this->seek($s);
            return false;
        }

        if ($hasExpression) {
            $tag = ['exp', ['string', '', $parts]];
        } else {
            $tag = trim(implode($parts));
        }

        $this->whitespace();
        return true;
    }

    // a css function
    protected function func(&$func)
    {
        $s = $this->seek();

        if ($this->match('(%|[\w\-_][\w\-_:\.]+|[\w_])', $m) && $this->literal('(')) {
            $fname = $m[1];

            $sPreArgs = $this->seek();

            $args = [];
            while (true) {
                $ss = $this->seek();
                // this ugly nonsense is for ie filter properties
                if ($this->keyword($name) && $this->literal('=') && $this->expressionList($value)) {
                    $args[] = ['string', '', [$name, '=', $value]];
                } else {
                    $this->seek($ss);
                    if ($this->expressionList($value)) {
                        $args[] = $value;
                    }
                }

                if (!$this->literal(',')) break;
            }
            $args = ['list', ',', $args];

            if ($this->literal(')')) {
                $func = ['function', $fname, $args];
                return true;
            } elseif ($fname == 'url') {
                // couldn't parse and in url? treat as string
                $this->seek($sPreArgs);
                if ($this->openString(')', $string) && $this->literal(')')) {
                    $func = ['function', $fname, $string];
                    return true;
                }
            }
        }

        $this->seek($s);
        return false;
    }

    // consume a less variable
    protected function variable(&$name)
    {
        $s = $this->seek();
        if ($this->literal(Constants::VPREFIX, false) &&
            ($this->variable($sub) || $this->keyword($name))) {
            if (!empty($sub)) {
                $name = ['variable', $sub];
            } else {
                $name = Constants::VPREFIX . $name;
            }
            return true;
        }

        $name = null;
        $this->seek($s);
        return false;
    }

    /**
     * Consume an assignment operator
     * Can optionally take a name that will be set to the current property name
     */
    protected function assign($name = null)
    {
        if ($name) $this->currentProperty = $name;
        return $this->literal(':') || $this->literal('=');
    }

    // consume a keyword
    protected function keyword(&$word)
    {
        if ($this->match('([\w_\-\*!"][\w\-_"]*)', $m)) {
            $word = $m[1];
            return true;
        }
        return false;
    }

    // consume an end of statement delimiter
    protected function end()
    {
        if ($this->literal(';', false)) {
            return true;
        } elseif ($this->count == strlen($this->buffer) || $this->buffer[$this->count] == '}') {
            // if there is end of file or a closing block next then we don't need a ;
            return true;
        }
        return false;
    }

    protected function guards(&$guards)
    {
        $s = $this->seek();

        if (!$this->literal('when')) {
            $this->seek($s);
            return false;
        }

        $guards = [];

        while ($this->guardGroup($g)) {
            $guards[] = $g;
            if (!$this->literal(',')) break;
        }

        if (count($guards) == 0) {
            $guards = null;
            $this->seek($s);
            return false;
        }

        return true;
    }

    // a bunch of guards that are and'd together
    // TODO rename to guardGroup
    protected function guardGroup(&$guardGroup)
    {
        $s = $this->seek();
        $guardGroup = [];
        while ($this->guard($guard)) {
            $guardGroup[] = $guard;
            if (!$this->literal('and')) break;
        }

        if (count($guardGroup) == 0) {
            $guardGroup = null;
            $this->seek($s);
            return false;
        }

        return true;
    }

    protected function guard(&$guard)
    {
        $s = $this->seek();
        $negate = $this->literal('not');

        if ($this->literal('(') && $this->expression($exp) && $this->literal(')')) {
            $guard = $exp;
            if ($negate) $guard = ['negate', $guard];
            return true;
        }

        $this->seek($s);
        return false;
    }

    /* raw parsing functions */

    protected function literal($what, $eatWhitespace = null)
    {
        if ($eatWhitespace === null) $eatWhitespace = $this->eatWhiteDefault;

        // shortcut on single letter
        if (!isset($what[1]) && isset($this->buffer[$this->count])) {
            if ($this->buffer[$this->count] == $what) {
                if (!$eatWhitespace) {
                    $this->count++;
                    return true;
                }
                // goes below...
            } else {
                return false;
            }
        }

        if (!isset(self::$literalCache[$what])) {
            self::$literalCache[$what] = Util::pregQuote($what);
        }

        return $this->match(self::$literalCache[$what], $m, $eatWhitespace);
    }

    protected function genericList(&$out, $parseItem, $delim = '', $flatten = true)
    {
        $s = $this->seek();
        $items = [];
        $value = null;
        while ($this->$parseItem($value)) {
            $items[] = $value;
            if ($delim) {
                if (!$this->literal($delim)) break;
            }
        }

        if (count($items) == 0) {
            $this->seek($s);
            return false;
        }

        if ($flatten && count($items) == 1) {
            $out = $items[0];
        } else {
            $out = ['list', $delim, $items];
        }

        return true;
    }


    // advance counter to next occurrence of $what
    // $until - don't include $what in advance
    // $allowNewline, if string, will be used as valid char set
    protected function to($what, &$out, $until = false, $allowNewline = false)
    {
        if (is_string($allowNewline)) {
            $validChars = $allowNewline;
        } else {
            $validChars = $allowNewline ? '.' : "[^\n]";
        }
        if (!$this->match('(' . $validChars . '*?)' . Lessc::preg_quote($what), $m, !$until)) return false;
        if ($until) $this->count -= strlen($what); // give back $what
        $out = $m[1];
        return true;
    }

    // try to match something on head of buffer
    protected function match($regex, &$out, $eatWhitespace = null)
    {
        if ($eatWhitespace === null) $eatWhitespace = $this->eatWhiteDefault;

        $r = '/' . $regex . ($eatWhitespace && !$this->writeComments ? '\s*' : '') . '/Ais';
        if (preg_match($r, $this->buffer, $out, 0, $this->count)) {
            $this->count += strlen($out[0]);
            if ($eatWhitespace && $this->writeComments) $this->whitespace();
            return true;
        }
        return false;
    }

    // match some whitespace
    protected function whitespace()
    {
        if ($this->writeComments) {
            $gotWhite = false;
            while (preg_match(self::$whitePattern, $this->buffer, $m, 0, $this->count)) {
                if (isset($m[1]) && empty($this->seenComments[$this->count])) {
                    $this->append(['comment', $m[1]]);
                    $this->seenComments[$this->count] = true;
                }
                $this->count += strlen($m[0]);
                $gotWhite = true;
            }
            return $gotWhite;
        } else {
            $this->match('', $m);
            return strlen($m[0]) > 0;
        }
    }

    // match something without consuming it
    protected function peek($regex, &$out = null, $from = null)
    {
        if (is_null($from)) $from = $this->count;
        $r = '/' . $regex . '/Ais';
        return preg_match($r, $this->buffer, $out, 0, $from);
    }

    // seek to a spot in the buffer or return where we are on no argument
    protected function seek($where = null)
    {
        if ($where === null) return $this->count;
        else $this->count = $where;
        return true;
    }

    /* misc functions */

    /**
     * Throw a parser exception
     *
     * This function tries to use the current parsing context to provide
     * additional info on where/why the error occurred.
     *
     * @param string $msg The error message to throw
     * @param int|null $count A line number counter to use instead of the current count
     * @param \Throwable|null $previous A previous exception to chain
     * @throws ParserException
     */
    public function throwError(string $msg = 'parse error', ?int $count = null, ?\Throwable $previous = null)
    {
        $count = is_null($count) ? $this->count : $count;

        $line = $this->line + substr_count(substr($this->buffer, 0, $count), "\n");

        if ($this->peek("(.*?)(\n|$)", $m, $count)) {
            $culprit = $m[1];
        }

        throw new ParserException(
            $msg,
            $culprit,
            $this->sourceName,
            $line,
            $previous
        );
    }

    protected function pushBlock($selectors = null, $type = null)
    {
        $b = new stdClass();
        $b->parent = $this->env;

        $b->type = $type;
        $b->id = self::$nextBlockId++;

        $b->isVararg = false; // TODO: kill me from here
        $b->tags = $selectors;

        $b->props = [];
        $b->children = [];

        // add a reference to the parser so
        // we can access the parser to throw errors
        // or retrieve the sourceName of this block.
        $b->parser = $this;

        // so we know the position of this block
        $b->count = $this->count;

        $this->env = $b;
        return $b;
    }

    // push a block that doesn't multiply tags
    protected function pushSpecialBlock($type)
    {
        return $this->pushBlock(null, $type);
    }

    // append a property to the current block
    protected function append($prop, $pos = null)
    {
        if ($pos !== null) $prop[-1] = $pos;
        $this->env->props[] = $prop;
    }

    // pop something off the stack
    protected function pop()
    {
        $old = $this->env;
        $this->env = $this->env->parent;
        return $old;
    }

    // remove comments from $text
    // todo: make it work for all functions, not just url
    protected function removeComments($text)
    {
        $look = ['url(', '//', '/*', '"', "'"];

        $out = '';
        $min = null;
        while (true) {
            // find the next item
            foreach ($look as $token) {
                $pos = strpos($text, $token);
                if ($pos !== false) {
                    if (!isset($min) || $pos < $min[1]) $min = [$token, $pos];
                }
            }

            if (is_null($min)) break;

            $count = $min[1];
            $skip = 0;
            $newlines = 0;
            switch ($min[0]) {
                case 'url(':
                    if (preg_match('/url\(.*?\)/', $text, $m, 0, $count))
                        $count += strlen($m[0]) - strlen($min[0]);
                    break;
                case '"':
                case "'":
                    if (preg_match('/' . $min[0] . '.*?(?<!\\\\)' . $min[0] . '/', $text, $m, 0, $count))
                        $count += strlen($m[0]) - 1;
                    break;
                case '//':
                    $skip = strpos($text, "\n", $count);
                    if ($skip === false) $skip = strlen($text) - $count;
                    else $skip -= $count;
                    break;
                case '/*':
                    if (preg_match('/\/\*.*?\*\//s', $text, $m, 0, $count)) {
                        $skip = strlen($m[0]);
                        $newlines = substr_count($m[0], "\n");
                    }
                    break;
            }

            if ($skip == 0) $count += strlen($min[0]);

            $out .= substr($text, 0, $count) . str_repeat("\n", $newlines);
            $text = substr($text, $count + $skip);

            $min = null;
        }

        return $out . $text;
    }
}