class UserAgent

class UserAgent {
    
    /**
     * Identifies user agent language from the Accept-language HTTP header.
     *
     * The algorithm works as follows:
     * - map user agent language codes to available language codes.
     * - order all user agent language codes by qvalue from high to low.
     * - add generic user agent language codes if they aren't already specified
     *   but with a slightly lower qvalue.
     * - find the most specific available language code with the highest qvalue.
     * - if 2 or more languages are having the same qvalue, respect the order of
     *   them inside the $languages array.
     *
     * We perform user agent accept-language parsing only if page cache is
     * disabled, otherwise we would cache a user-specific preference.
     *
     * @param string $http_accept_language
     *   The value of the "Accept-Language" HTTP header.
     * @param array $langcodes
     *   An array of available language codes to pick from.
     * @param array $mappings
     *   (optional) Custom mappings to support user agents that are sending non
     *   standard language codes. No mapping is assumed by default.
     *
     * @return string
     *   The selected language code or FALSE if no valid language can be
     *   identified.
     */
    public static function getBestMatchingLangcode($http_accept_language, $langcodes, $mappings = []) {
        // The Accept-Language header contains information about the language
        // preferences configured in the user's user agent / operating system.
        // RFC 2616 (section 14.4) defines the Accept-Language header as follows:
        // @code
        //   Accept-Language = "Accept-Language" ":"
        //                  1#( language-range [ ";" "q" "=" qvalue ] )
        //   language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
        // @endcode
        // Samples: "hu, en-us;q=0.66, en;q=0.33", "hu,en-us;q=0.5"
        $ua_langcodes = [];
        if (preg_match_all('@(?<=[, ]|^)([a-zA-Z-]+|\\*)(?:;q=([0-9.]+))?(?:$|\\s*,\\s*)@', trim($http_accept_language), $matches, PREG_SET_ORDER)) {
            foreach ($matches as $match) {
                if ($mappings) {
                    $langcode = strtolower($match[1]);
                    foreach ($mappings as $ua_langcode => $standard_langcode) {
                        if ($langcode == $ua_langcode) {
                            $match[1] = $standard_langcode;
                        }
                    }
                }
                // We can safely use strtolower() here, tags are ASCII.
                // RFC2616 mandates that the decimal part is no more than three digits,
                // so we multiply the qvalue by 1000 to avoid floating point
                // comparisons.
                $langcode = strtolower($match[1]);
                $qvalue = isset($match[2]) ? (double) $match[2] : 1;
                // Take the highest qvalue for this langcode. Although the request
                // supposedly contains unique langcodes, our mapping possibly resolves
                // to the same langcode for different qvalues. Keep the highest.
                $ua_langcodes[$langcode] = max((int) ($qvalue * 1000), $ua_langcodes[$langcode] ?? 0);
            }
        }
        // We should take pristine values from the HTTP headers, but Internet
        // Explorer from version 7 sends only specific language tags (eg. fr-CA)
        // without the corresponding generic tag (fr) unless explicitly configured.
        // In that case, we assume that the lowest value of the specific tags is the
        // value of the generic language to be as close to the HTTP 1.1 spec as
        // possible.
        // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4 and
        // http://blogs.msdn.com/b/ie/archive/2006/10/17/accept-language-header-for-internet-explorer-7.aspx
        asort($ua_langcodes);
        foreach ($ua_langcodes as $langcode => $qvalue) {
            // For Chinese languages the generic tag is either zh-hans or zh-hant, so
            // we need to handle this separately, we can not split $langcode on the
            // first occurrence of '-' otherwise we get a non-existing language zh.
            // All other languages use a langcode without a '-', so we can safely
            // split on the first occurrence of it.
            if (strlen($langcode) > 7 && (substr($langcode, 0, 7) == 'zh-hant' || substr($langcode, 0, 7) == 'zh-hans')) {
                $generic_tag = substr($langcode, 0, 7);
            }
            else {
                $generic_tag = strtok($langcode, '-');
            }
            if (!empty($generic_tag) && !isset($ua_langcodes[$generic_tag])) {
                // Add the generic langcode, but make sure it has a lower qvalue as the
                // more specific one, so the more specific one gets selected if it's
                // defined by both the user agent and us.
                $ua_langcodes[$generic_tag] = $qvalue - 0.1;
            }
        }
        // Find the added language with the greatest qvalue, following the rules
        // of RFC 2616 (section 14.4). If several languages have the same qvalue,
        // prefer the one with the greatest weight.
        $best_match_langcode = FALSE;
        $max_qvalue = 0;
        foreach ($langcodes as $langcode_case_sensitive) {
            // Language tags are case insensitive (RFC2616, sec 3.10).
            $langcode = strtolower($langcode_case_sensitive);
            // If nothing matches below, the default qvalue is the one of the wildcard
            // language, if set, or is 0 (which will never match).
            $qvalue = $ua_langcodes['*'] ?? 0;
            // Find the longest possible prefix of the user agent supplied language
            // ('the language-range') that matches this site language ('the language
            // tag').
            $prefix = $langcode;
            do {
                if (isset($ua_langcodes[$prefix])) {
                    $qvalue = $ua_langcodes[$prefix];
                    break;
                }
            } while ($prefix = substr($prefix, 0, strrpos($prefix, '-')));
            // Find the best match.
            if ($qvalue > $max_qvalue) {
                $best_match_langcode = $langcode_case_sensitive;
                $max_qvalue = $qvalue;
            }
        }
        return $best_match_langcode;
    }

}