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Filename : Minifier.php
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<?php
namespace W3TCL\YuiCssMin;

/*!
 * CssMin
 * Author: Tubal Martin - http://tubalmartin.me/
 * Repo: https://github.com/tubalmartin/YUI-CSS-compressor-PHP-port
 *
 * This is a PHP port of the CSS minification tool distributed with YUICompressor,
 * itself a port of the cssmin utility by Isaac Schlueter - http://foohack.com/
 * Permission is hereby granted to use the PHP version under the same
 * conditions as the YUICompressor.
 */

/*!
 * YUI Compressor
 * http://developer.yahoo.com/yui/compressor/
 * Author: Julien Lecomte - http://www.julienlecomte.net/
 * Copyright (c) 2013 Yahoo! Inc. All rights reserved.
 * The copyrights embodied in the content of this file are licensed
 * by Yahoo! Inc. under the BSD (revised) open source license.
 */

class Minifier
{
	const QUERY_FRACTION = '_CSSMIN_QF_';
	const COMMENT_TOKEN = '_CSSMIN_CMT_%d_';
	const COMMENT_TOKEN_START = '_CSSMIN_CMT_';
	const RULE_BODY_TOKEN = '_CSSMIN_RBT_%d_';
	const PRESERVED_TOKEN = '_CSSMIN_PTK_%d_';

	// Token lists
	private $comments = array();
	private $ruleBodies = array();
	private $preservedTokens = array();

	// Output options
	private $keepImportantComments = true;
	private $keepSourceMapComment = false;
	private $linebreakPosition = 0;

	// PHP ini limits
	private $raisePhpLimits;
	private $memoryLimit;
	private $maxExecutionTime = 60; // 1 min
	private $pcreBacktrackLimit;
	private $pcreRecursionLimit;

	// Color maps
	private $hexToNamedColorsMap;
	private $namedToHexColorsMap;

	// Regexes
	private $numRegex;
	private $charsetRegex = '/@charset [^;]+;/Si';
	private $importRegex = '/@import [^;]+;/Si';
	private $namespaceRegex = '/@namespace [^;]+;/Si';
	private $namedToHexColorsRegex;
	private $shortenOneZeroesRegex;
	private $shortenTwoZeroesRegex;
	private $shortenThreeZeroesRegex;
	private $shortenFourZeroesRegex;
	private $unitsGroupRegex = '(?:ch|cm|em|ex|gd|in|mm|px|pt|pc|q|rem|vh|vmax|vmin|vw|%)';

	public static function minify_static($css, $options = array()) {
		$m = new Minifier();

		$css = \W3TCL\Minify\Minify_CSS_UriRewriter::rewrite($css, $options);

		return $m->run($css);
	}

	/**
	 * @param bool|int $raisePhpLimits If true, PHP settings will be raised if needed
	 */
	public function __construct($raisePhpLimits = true)
	{
		$this->raisePhpLimits = (bool) $raisePhpLimits;
		$this->memoryLimit = 128 * 1048576; // 128MB in bytes
		$this->pcreBacktrackLimit = 1000 * 1000;
		$this->pcreRecursionLimit = 500 * 1000;
		$this->hexToNamedColorsMap = Colors::getHexToNamedMap();
		$this->namedToHexColorsMap = Colors::getNamedToHexMap();
		$this->namedToHexColorsRegex = sprintf(
			'/([:,( ])(%s)( |,|\)|;|$)/Si',
			implode('|', array_keys($this->namedToHexColorsMap))
		);
		$this->numRegex = sprintf('-?\d*\.?\d+%s?', $this->unitsGroupRegex);
		$this->setShortenZeroValuesRegexes();
	}

	/**
	 * Parses & minifies the given input CSS string
	 * @param string $css
	 * @return string
	 */
	public function run($css = '')
	{
		if (empty($css) || !is_string($css)) {
			return '';
		}

		$this->resetRunProperties();

		if ($this->raisePhpLimits) {
			$this->doRaisePhpLimits();
		}

		return $this->minify($css);
	}

	/**
	 * Sets whether to keep or remove sourcemap special comment.
	 * Sourcemap comments are removed by default.
	 * @param bool $keepSourceMapComment
	 */
	public function keepSourceMapComment($keepSourceMapComment = true)
	{
		$this->keepSourceMapComment = (bool) $keepSourceMapComment;
	}

	/**
	 * Sets whether to keep or remove important comments.
	 * Important comments outside of a declaration block are kept by default.
	 * @param bool $removeImportantComments
	 */
	public function removeImportantComments($removeImportantComments = true)
	{
		$this->keepImportantComments = !(bool) $removeImportantComments;
	}

	/**
	 * Sets the approximate column after which long lines will be splitted in the output
	 * with a linebreak.
	 * @param int $position
	 */
	public function setLineBreakPosition($position)
	{
		$this->linebreakPosition = (int) $position;
	}

	/**
	 * Sets the memory limit for this script
	 * @param int|string $limit
	 */
	public function setMemoryLimit($limit)
	{
		$this->memoryLimit = Utils::normalizeInt($limit);
	}

	/**
	 * Sets the maximum execution time for this script
	 * @param int|string $seconds
	 */
	public function setMaxExecutionTime($seconds)
	{
		$this->maxExecutionTime = (int) $seconds;
	}

	/**
	 * Sets the PCRE backtrack limit for this script
	 * @param int $limit
	 */
	public function setPcreBacktrackLimit($limit)
	{
		$this->pcreBacktrackLimit = (int) $limit;
	}

	/**
	 * Sets the PCRE recursion limit for this script
	 * @param int $limit
	 */
	public function setPcreRecursionLimit($limit)
	{
		$this->pcreRecursionLimit = (int) $limit;
	}

	/**
	 * Builds regular expressions needed for shortening zero values
	 */
	private function setShortenZeroValuesRegexes()
	{
		$zeroRegex = '0'. $this->unitsGroupRegex;
		$numOrPosRegex = '('. $this->numRegex .'|top|left|bottom|right|center) ';
		$oneZeroSafeProperties = array(
			'(?:line-)?height',
			'(?:(?:min|max)-)?width',
			'top',
			'left',
			'background-position',
			'bottom',
			'right',
			'border(?:-(?:top|left|bottom|right))?(?:-width)?',
			'border-(?:(?:top|bottom)-(?:left|right)-)?radius',
			'column-(?:gap|width)',
			'margin(?:-(?:top|left|bottom|right))?',
			'outline-width',
			'padding(?:-(?:top|left|bottom|right))?'
		);

		// First zero regex
		$regex = '/(^|;)('. implode('|', $oneZeroSafeProperties) .'):%s/Si';
		$this->shortenOneZeroesRegex = sprintf($regex, $zeroRegex);

		// Multiple zeroes regexes
		$regex = '/(^|;)(margin|padding|border-(?:width|radius)|background-position):%s/Si';
		$this->shortenTwoZeroesRegex = sprintf($regex, $numOrPosRegex . $zeroRegex);
		$this->shortenThreeZeroesRegex = sprintf($regex, $numOrPosRegex . $numOrPosRegex . $zeroRegex);
		$this->shortenFourZeroesRegex = sprintf($regex, $numOrPosRegex . $numOrPosRegex . $numOrPosRegex . $zeroRegex);
	}

	/**
	 * Resets properties whose value may change between runs
	 */
	private function resetRunProperties()
	{
		$this->comments = array();
		$this->ruleBodies = array();
		$this->preservedTokens = array();
	}

	/**
	 * Tries to configure PHP to use at least the suggested minimum settings
	 * @return void
	 */
	private function doRaisePhpLimits()
	{
		$phpLimits = array(
			'memory_limit' => $this->memoryLimit,
			'max_execution_time' => $this->maxExecutionTime,
			'pcre.backtrack_limit' => $this->pcreBacktrackLimit,
			'pcre.recursion_limit' =>  $this->pcreRecursionLimit
		);

		// If current settings are higher respect them.
		foreach ($phpLimits as $name => $suggested) {
			$current = Utils::normalizeInt(ini_get($name));

			if ($current >= $suggested) {
				continue;
			}

			// memoryLimit exception: allow -1 for "no memory limit".
			if ($name === 'memory_limit' && $current === -1) {
				continue;
			}

			// maxExecutionTime exception: allow 0 for "no memory limit".
			if ($name === 'max_execution_time' && $current === 0) {
				continue;
			}

			ini_set($name, $suggested);
		}
	}

	/**
	 * Registers a preserved token
	 * @param string $token
	 * @return string The token ID string
	 */
	private function registerPreservedToken($token)
	{
		$tokenId = sprintf(self::PRESERVED_TOKEN, count($this->preservedTokens));
		$this->preservedTokens[$tokenId] = $token;
		return $tokenId;
	}

	/**
	 * Registers a candidate comment token
	 * @param string $comment
	 * @return string The comment token ID string
	 */
	private function registerCommentToken($comment)
	{
		$tokenId = sprintf(self::COMMENT_TOKEN, count($this->comments));
		$this->comments[$tokenId] = $comment;
		return $tokenId;
	}

	/**
	 * Registers a rule body token
	 * @param string $body the minified rule body
	 * @return string The rule body token ID string
	 */
	private function registerRuleBodyToken($body)
	{
		if (empty($body)) {
			return '';
		}

		$tokenId = sprintf(self::RULE_BODY_TOKEN, count($this->ruleBodies));
		$this->ruleBodies[$tokenId] = $body;
		return $tokenId;
	}

	/**
	 * Parses & minifies the given input CSS string
	 * @param string $css
	 * @return string
	 */
	private function minify($css)
	{
		// Process data urls
		$css = $this->processDataUrls($css);

		// Process comments
		$css = preg_replace_callback(
			'/(?<!\\\\)\/\*(.*?)\*(?<!\\\\)\//Ss',
			array($this, 'processCommentsCallback'),
			$css
		);

		// IE7: Process Microsoft matrix filters (whitespaces between Matrix parameters). Can contain strings inside.
		$css = preg_replace_callback(
			'/filter:\s*progid:DXImageTransform\.Microsoft\.Matrix\(([^)]+)\)/Ss',
			array($this, 'processOldIeSpecificMatrixDefinitionCallback'),
			$css
		);

		// Process quoted unquotable attribute selectors to unquote them. Covers most common cases.
		// Likelyhood of a quoted attribute selector being a substring in a string: Very very low.
		$css = preg_replace(
			'/\[\s*([a-z][a-z-]+)\s*([\*\|\^\$~]?=)\s*[\'"](-?[a-z_][a-z0-9-_]+)[\'"]\s*\]/Ssi',
			'[$1$2$3]',
			$css
		);

		// Process strings so their content doesn't get accidentally minified
		$css = preg_replace_callback(
			'/(?:"(?:[^\\\\"]|\\\\.|\\\\)*")|'."(?:'(?:[^\\\\']|\\\\.|\\\\)*')/S",
			array($this, 'processStringsCallback'),
			$css
		);

		// Normalize all whitespace strings to single spaces. Easier to work with that way.
		$css = preg_replace('/\s+/S', ' ', $css);

		// Process comments
		$css = $this->processComments($css);

		// Process rule bodies
		$css = $this->processRuleBodies($css);

		// Process at-rules and selectors
		$css = $this->processAtRulesAndSelectors($css);

		// Restore preserved rule bodies before splitting
		$css = strtr($css, $this->ruleBodies);

		// Some source control tools don't like it when files containing lines longer
		// than, say 8000 characters, are checked in. The linebreak option is used in
		// that case to split long lines after a specific column.
		if ($this->linebreakPosition > 0) {
			$l = strlen($css);
			$offset = $this->linebreakPosition;
			while (preg_match('/(?<!\\\\)\}(?!\n)/S', $css, $matches, PREG_OFFSET_CAPTURE, $offset)) {
				$matchIndex = $matches[0][1];
				$css = substr_replace($css, "\n", $matchIndex + 1, 0);
				$offset = $matchIndex + 2 + $this->linebreakPosition;
				$l += 1;
				if ($offset > $l) {
					break;
				}
			}
		}

		// Restore preserved comments and strings
		$css = strtr($css, $this->preservedTokens);

		return trim($css);
	}

	/**
	 * Searches & replaces all data urls with tokens before we start compressing,
	 * to avoid performance issues running some of the subsequent regexes against large string chunks.
	 * @param string $css
	 * @return string
	 */
	private function processDataUrls($css)
	{
		$ret = '';
		$searchOffset = $substrOffset = 0;

		// Since we need to account for non-base64 data urls, we need to handle
		// ' and ) being part of the data string.
		while (preg_match('/url\(\s*(["\']?)data:/Si', $css, $m, PREG_OFFSET_CAPTURE, $searchOffset)) {
			$matchStartIndex = $m[0][1];
			$dataStartIndex = $matchStartIndex + 4; // url( length
			$searchOffset = $matchStartIndex + strlen($m[0][0]);
			$terminator = $m[1][0]; // ', " or empty (not quoted)
			$terminatorRegex = '/(?<!\\\\)'. (strlen($terminator) === 0 ? '' : $terminator.'\s*') .'(\))/S';

			$ret .= substr($css, $substrOffset, $matchStartIndex - $substrOffset);

			// Terminator found
			if (preg_match($terminatorRegex, $css, $matches, PREG_OFFSET_CAPTURE, $searchOffset)) {
				$matchEndIndex = $matches[1][1];
				$searchOffset = $matchEndIndex + 1;
				$token = substr($css, $dataStartIndex, $matchEndIndex - $dataStartIndex);

				// Remove all spaces only for base64 encoded URLs.
				if (stripos($token, 'base64,') !== false) {
					$token = preg_replace('/\s+/S', '', $token);
				}

				$ret .= 'url('. $this->registerPreservedToken(trim($token)) .')';
			// No end terminator found, re-add the whole match. Should we throw/warn here?
			} else {
				$ret .= substr($css, $matchStartIndex, $searchOffset - $matchStartIndex);
			}

			$substrOffset = $searchOffset;
		}

		$ret .= substr($css, $substrOffset);

		return $ret;
	}

	/**
	 * Registers all comments found as candidates to be preserved.
	 * @param array $matches
	 * @return string
	 */
	private function processCommentsCallback($matches)
	{
		return '/*'. $this->registerCommentToken($matches[1]) .'*/';
	}

	/**
	 * Preserves old IE Matrix string definition
	 * @param array $matches
	 * @return string
	 */
	private function processOldIeSpecificMatrixDefinitionCallback($matches)
	{
		return 'filter:progid:DXImageTransform.Microsoft.Matrix('. $this->registerPreservedToken($matches[1]) .')';
	}

	/**
	 * Preserves strings found
	 * @param array $matches
	 * @return string
	 */
	private function processStringsCallback($matches)
	{
		$match = $matches[0];
		$quote = substr($match, 0, 1);
		$match = substr($match, 1, -1);

		// maybe the string contains a comment-like substring?
		// one, maybe more? put'em back then
		if (strpos($match, self::COMMENT_TOKEN_START) !== false) {
			$match = strtr($match, $this->comments);
		}

		// minify alpha opacity in filter strings
		$match = str_ireplace('progid:DXImageTransform.Microsoft.Alpha(Opacity=', 'alpha(opacity=', $match);

		return $quote . $this->registerPreservedToken($match) . $quote;
	}

	/**
	 * Preserves or removes comments found.
	 * @param string $css
	 * @return string
	 */
	private function processComments($css)
	{
		foreach ($this->comments as $commentId => $comment) {
			$commentIdString = '/*'. $commentId .'*/';

			// ! in the first position of the comment means preserve
			// so push to the preserved tokens keeping the !
			if ($this->keepImportantComments && strpos($comment, '!') === 0) {
				$preservedTokenId = $this->registerPreservedToken($comment);
				// Put new lines before and after /*! important comments
				$css = str_replace($commentIdString, "\n/*$preservedTokenId*/\n", $css);
				continue;
			}

			// # sourceMappingURL= in the first position of the comment means sourcemap
			// so push to the preserved tokens if {$this->keepSourceMapComment} is truthy.
			if ($this->keepSourceMapComment && strpos($comment, '# sourceMappingURL=') === 0) {
				$preservedTokenId = $this->registerPreservedToken($comment);
				// Add new line before the sourcemap comment
				$css = str_replace($commentIdString, "\n/*$preservedTokenId*/", $css);
				continue;
			}

			// Keep empty comments after child selectors (IE7 hack)
			// e.g. html >/**/ body
			if (strlen($comment) === 0 && strpos($css, '>/*'.$commentId) !== false) {
				$css = str_replace($commentId, $this->registerPreservedToken(''), $css);
				continue;
			}

			// in all other cases kill the comment
			$css = str_replace($commentIdString, '', $css);
		}

		// Normalize whitespace again
		$css = preg_replace('/ +/S', ' ', $css);

		return $css;
	}

	/**
	 * Finds, minifies & preserves all rule bodies.
	 * @param string $css the whole stylesheet.
	 * @return string
	 */
	private function processRuleBodies($css)
	{
		$ret = '';
		$searchOffset = $substrOffset = 0;

		while (($blockStartPos = strpos($css, '{', $searchOffset)) !== false) {
			$blockEndPos = strpos($css, '}', $blockStartPos);
			$nextBlockStartPos = strpos($css, '{', $blockStartPos + 1);
			$ret .= substr($css, $substrOffset, $blockStartPos - $substrOffset);

			if ($nextBlockStartPos !== false && $nextBlockStartPos < $blockEndPos) {
				$ret .= substr($css, $blockStartPos, $nextBlockStartPos - $blockStartPos);
				$searchOffset = $nextBlockStartPos;
			} else {
				$ruleBody = substr($css, $blockStartPos + 1, $blockEndPos - $blockStartPos - 1);
				$ruleBodyToken = $this->registerRuleBodyToken($this->processRuleBody($ruleBody));
				$ret .= '{'. $ruleBodyToken .'}';
				$searchOffset = $blockEndPos + 1;
			}

			$substrOffset = $searchOffset;
		}

		$ret .= substr($css, $substrOffset);

		return $ret;
	}

	/**
	 * Compresses non-group rule bodies.
	 * @param string $body The rule body without curly braces
	 * @return string
	 */
	private function processRuleBody($body)
	{
		$body = trim($body);

		// Remove spaces before the things that should not have spaces before them.
		$body = preg_replace('/ ([:=,)*\/;\n])/S', '$1', $body);

		// Remove the spaces after the things that should not have spaces after them.
		$body = preg_replace('/([:=,(*\/!;\n]) /S', '$1', $body);

		// Replace multiple semi-colons in a row by a single one
		$body = preg_replace('/;;+/S', ';', $body);

		// Remove semicolon before closing brace except when:
		// - The last property is prefixed with a `*` (lte IE7 hack) to avoid issues on Symbian S60 3.x browsers.
		if (!preg_match('/\*[a-z0-9-]+:[^;]+;$/Si', $body)) {
			$body = rtrim($body, ';');
		}

		// Remove important comments inside a rule body (because they make no sense here).
		if (strpos($body, '/*') !== false) {
			$body = preg_replace('/\n?\/\*[A-Z0-9_]+\*\/\n?/S', '', $body);
		}

		// Empty rule body? Exit :)
		if (empty($body)) {
			return '';
		}

		// Shorten font-weight values
		$body = preg_replace(
			array('/(font-weight:)bold\b/Si', '/(font-weight:)normal\b/Si'),
			array('${1}700', '${1}400'),
			$body
		);

		// Shorten background property
		$body = preg_replace('/(background:)(?:none|transparent)( !|;|$)/Si', '${1}0 0$2', $body);

		// Shorten opacity IE filter
		$body = str_ireplace('progid:DXImageTransform.Microsoft.Alpha(Opacity=', 'alpha(opacity=', $body);

		// Shorten colors from rgb(51,102,153) to #336699, rgb(100%,0%,0%) to #ff0000 (sRGB color space)
		// Shorten colors from hsl(0, 100%, 50%) to #ff0000 (sRGB color space)
		// This makes it more likely that it'll get further compressed in the next step.
		$body = preg_replace_callback(
			'/(rgb|hsl)\(([0-9,.% -]+)\)(.|$)/Si',
			array($this, 'shortenHslAndRgbToHexCallback'),
			$body
		);

		// Shorten colors from #AABBCC to #ABC or shorter color name:
		// - Look for hex colors which don't have a "=" in front of them (to avoid MSIE filters)
		$body = preg_replace_callback(
			'/(?<!=)#([0-9a-f]{3,6})( |,|\)|;|$)/Si',
			array($this, 'shortenHexColorsCallback'),
			$body
		);

		// Shorten long named colors with a shorter HEX counterpart: white -> #fff.
		// Run at least 2 times to cover most cases
		$body = preg_replace_callback(
			array($this->namedToHexColorsRegex, $this->namedToHexColorsRegex),
			array($this, 'shortenNamedColorsCallback'),
			$body
		);

		// Replace positive sign from numbers before the leading space is removed.
		// +1.2em to 1.2em, +.8px to .8px, +2% to 2%
		$body = preg_replace('/([ :,(])\+(\.?\d+)/S', '$1$2', $body);

		// shorten ms to s
		$body = preg_replace_callback('/([ :,(])(-?)(\d{3,})ms/Si', function ($matches) {
			return $matches[1] . $matches[2] . ((int) $matches[3] / 1000) .'s';
		}, $body);

		// Remove leading zeros from integer and float numbers.
		// 000.6 to .6, -0.8 to -.8, 0050 to 50, -01.05 to -1.05
		$body = preg_replace('/([ :,(])(-?)0+([1-9]?\.?\d+)/S', '$1$2$3', $body);

		// Remove trailing zeros from float numbers.
		// -6.0100em to -6.01em, .0100 to .01, 1.200px to 1.2px
		$body = preg_replace('/([ :,(])(-?\d?\.\d+?)0+([^\d])/S', '$1$2$3', $body);

		// Remove trailing .0 -> -9.0 to -9
		$body = preg_replace('/([ :,(])(-?\d+)\.0([^\d])/S', '$1$2$3', $body);

		// Replace 0 length numbers with 0
		$body = preg_replace('/([ :,(])-?\.?0+([^\d])/S', '${1}0$2', $body);

		// Shorten zero values for safe properties only
		$body = preg_replace(
			array(
				$this->shortenOneZeroesRegex,
				$this->shortenTwoZeroesRegex,
				$this->shortenThreeZeroesRegex,
				$this->shortenFourZeroesRegex
			),
			array(
				'$1$2:0',
				'$1$2:$3 0',
				'$1$2:$3 $4 0',
				'$1$2:$3 $4 $5 0'
			),
			$body
		);

		// Replace 0 0 0; or 0 0 0 0; with 0 0 for background-position property.
		$body = preg_replace('/(background-position):0(?: 0){2,3}( !|;|$)/Si', '$1:0 0$2', $body);

		// Shorten suitable shorthand properties with repeated values
		$body = preg_replace(
			array(
				'/(margin|padding|border-(?:width|radius)):('.$this->numRegex.')(?: \2)+( !|;|$)/Si',
				'/(border-(?:style|color)):([#a-z0-9]+)(?: \2)+( !|;|$)/Si'
			),
			'$1:$2$3',
			$body
		);
		$body = preg_replace(
			array(
				'/(margin|padding|border-(?:width|radius)):'.
				'('.$this->numRegex.') ('.$this->numRegex.') \2 \3( !|;|$)/Si',
				'/(border-(?:style|color)):([#a-z0-9]+) ([#a-z0-9]+) \2 \3( !|;|$)/Si'
			),
			'$1:$2 $3$4',
			$body
		);
		$body = preg_replace(
			array(
				'/(margin|padding|border-(?:width|radius)):'.
				'('.$this->numRegex.') ('.$this->numRegex.') ('.$this->numRegex.') \3( !|;|$)/Si',
				'/(border-(?:style|color)):([#a-z0-9]+) ([#a-z0-9]+) ([#a-z0-9]+) \3( !|;|$)/Si'
			),
			'$1:$2 $3 $4$5',
			$body
		);

		// Lowercase some common functions that can be values
		$body = preg_replace_callback(
			'/(?:attr|blur|brightness|circle|contrast|cubic-bezier|drop-shadow|ellipse|from|grayscale|'.
			'hsla?|hue-rotate|inset|invert|local|minmax|opacity|perspective|polygon|rgba?|rect|repeat|saturate|sepia|'.
			'steps|to|url|var|-webkit-gradient|'.
			'(?:-(?:atsc|khtml|moz|ms|o|wap|webkit)-)?(?:calc|(?:repeating-)?(?:linear|radial)-gradient))\(/Si',
			array($this, 'strtolowerCallback'),
			$body
		);

		// Lowercase all uppercase properties
		$body = preg_replace_callback('/(?:^|;)[A-Z-]+:/S', array($this, 'strtolowerCallback'), $body);

		return $body;
	}

	/**
	 * Compresses At-rules and selectors.
	 * @param string $css the whole stylesheet with rule bodies tokenized.
	 * @return string
	 */
	private function processAtRulesAndSelectors($css)
	{
		$charset = '';
		$imports = '';
		$namespaces = '';

		// Remove spaces before the things that should not have spaces before them.
		$css = preg_replace('/ ([@{};>+)\]~=,\/\n])/S', '$1', $css);

		// Remove the spaces after the things that should not have spaces after them.
		$css = preg_replace('/([{}:;>+(\[~=,\/\n]) /S', '$1', $css);

		// Shorten shortable double colon (CSS3) pseudo-elements to single colon (CSS2)
		$css = preg_replace('/::(before|after|first-(?:line|letter))(\{|,)/Si', ':$1$2', $css);

		// Retain space for special IE6 cases
		$css = preg_replace_callback('/:first-(line|letter)(\{|,)/Si', function ($matches) {
			return ':first-'. strtolower($matches[1]) .' '. $matches[2];
		}, $css);

		// Find a fraction that may used in some @media queries such as: (min-aspect-ratio: 1/1)
		// Add token to add the "/" back in later
		$css = preg_replace('/\(([a-z-]+):([0-9]+)\/([0-9]+)\)/Si', '($1:$2'. self::QUERY_FRACTION .'$3)', $css);

		// Remove empty rule blocks up to 2 levels deep.
		$css = preg_replace(array_fill(0, 2, '/(\{)[^{};\/\n]+\{\}/S'), '$1', $css);
		$css = preg_replace('/[^{};\/\n]+\{\}/S', '', $css);

		// Two important comments next to each other? Remove extra newline.
		if ($this->keepImportantComments) {
			$css = str_replace("\n\n", "\n", $css);
		}

		// Restore fraction
		$css = str_replace(self::QUERY_FRACTION, '/', $css);

		// Lowercase some popular @directives
		$css = preg_replace_callback(
			'/(?<!\\\\)@(?:charset|document|font-face|import|(?:-(?:atsc|khtml|moz|ms|o|wap|webkit)-)?keyframes|media|'.
			'namespace|page|supports|viewport)/Si',
			array($this, 'strtolowerCallback'),
			$css
		);

		// Lowercase some popular media types
		$css = preg_replace_callback(
			'/[ ,](?:all|aural|braille|handheld|print|projection|screen|tty|tv|embossed|speech)[ ,;{]/Si',
			array($this, 'strtolowerCallback'),
			$css
		);

		// Lowercase some common pseudo-classes & pseudo-elements
		$css = preg_replace_callback(
			'/(?<!\\\\):(?:active|after|before|checked|default|disabled|empty|enabled|first-(?:child|of-type)|'.
			'focus(?:-within)?|hover|indeterminate|in-range|invalid|lang\(|last-(?:child|of-type)|left|link|not\(|'.
			'nth-(?:child|of-type)\(|nth-last-(?:child|of-type)\(|only-(?:child|of-type)|optional|out-of-range|'.
			'read-(?:only|write)|required|right|root|:selection|target|valid|visited)/Si',
			array($this, 'strtolowerCallback'),
			$css
		);

		// @charset handling
		if (preg_match($this->charsetRegex, $css, $matches)) {
			// Keep the first @charset at-rule found
			$charset = $matches[0];
			// Delete all @charset at-rules
			$css = preg_replace($this->charsetRegex, '', $css);
		}

		// @import handling
		$css = preg_replace_callback($this->importRegex, function ($matches) use (&$imports) {
			// Keep all @import at-rules found for later
			$imports .= $matches[0];
			// Delete all @import at-rules
			return '';
		}, $css);

		// @namespace handling
		$css = preg_replace_callback($this->namespaceRegex, function ($matches) use (&$namespaces) {
			// Keep all @namespace at-rules found for later
			$namespaces .= $matches[0];
			// Delete all @namespace at-rules
			return '';
		}, $css);

		// Order critical at-rules:
		// 1. @charset first
		// 2. @imports below @charset
		// 3. @namespaces below @imports
		$css = $charset . $imports . $namespaces . $css;

		return $css;
	}

	/**
	 * Converts hsl() & rgb() colors to HEX format.
	 * @param $matches
	 * @return string
	 */
	private function shortenHslAndRgbToHexCallback($matches)
	{
		$type = $matches[1];
		$values = explode(',', $matches[2]);
		$terminator = $matches[3];

		if ($type === 'hsl') {
			$values = Utils::hslToRgb($values);
		}

		$hexColors = Utils::rgbToHex($values);

		// Restore space after rgb() or hsl() function in some cases such as:
		// background-image: linear-gradient(to bottom, rgb(210,180,140) 10%, rgb(255,0,0) 90%);
		if (!empty($terminator) && !preg_match('/[ ,);]/S', $terminator)) {
			$terminator = ' '. $terminator;
		}

		return '#'. implode('', $hexColors) . $terminator;
	}

	/**
	 * Compresses HEX color values of the form #AABBCC to #ABC or short color name.
	 * @param $matches
	 * @return string
	 */
	private function shortenHexColorsCallback($matches)
	{
		$hex = $matches[1];

		// Shorten suitable 6 chars HEX colors
		if (strlen($hex) === 6 && preg_match('/^([0-9a-f])\1([0-9a-f])\2([0-9a-f])\3$/Si', $hex, $m)) {
			$hex = $m[1] . $m[2] . $m[3];
		}

		// Lowercase
		$hex = '#'. strtolower($hex);

		// Replace Hex colors with shorter color names
		$color = array_key_exists($hex, $this->hexToNamedColorsMap) ? $this->hexToNamedColorsMap[$hex] : $hex;

		return $color . $matches[2];
	}

	/**
	 * Shortens all named colors with a shorter HEX counterpart for a set of safe properties
	 * e.g. white -> #fff
	 * @param array $matches
	 * @return string
	 */
	private function shortenNamedColorsCallback($matches)
	{
		return $matches[1] . $this->namedToHexColorsMap[strtolower($matches[2])] . $matches[3];
	}

	/**
	 * Makes a string lowercase
	 * @param array $matches
	 * @return string
	 */
	private function strtolowerCallback($matches)
	{
		return strtolower($matches[0]);
	}
}