LanguageCode.php

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<?php
class LanguageCode {
    private static $deprecatedLanguageCodeMapping = [
        // Note that als is actually a valid ISO 639 code (Tosk Albanian), but it
        // was previously used in MediaWiki for Alsatian, which comes under gsw
        'als' => 'gsw', // T25215
        'bat-smg' => 'sgs', // T27522
        'be-x-old' => 'be-tarask', // T11823
        'fiu-vro' => 'vro', // T31186
        'roa-rup' => 'rup', // T17988
        'zh-classical' => 'lzh', // T30443
        'zh-min-nan' => 'nan', // T30442
        'zh-yue' => 'yue', // T30441
    ];

    private static $nonstandardLanguageCodeMapping = [
        // All codes returned by Language::fetchLanguageNames() validated
        // against IANA registry at
        //   https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry
        // with help of validator at
        //   http://schneegans.de/lv/
        'cbk-zam' => 'cbk', // T124657
        'de-formal' => 'de-x-formal',
        'eml' => 'egl', // T36217
        'en-rtl' => 'en-x-rtl',
        'es-formal' => 'es-x-formal',
        'hu-formal' => 'hu-x-formal',
        'map-bms' => 'jv-x-bms', // [[en:Banyumasan_dialect]] T125073
        'mo' => 'ro-Cyrl-MD', // T125073
        'nrm' => 'nrf', // [[en:Norman_language]] T25216
        'nl-informal' => 'nl-x-informal',
        'roa-tara' => 'nap-x-tara', // [[en:Tarantino_dialect]]
        'simple' => 'en-simple',
        'sr-ec' => 'sr-Cyrl', // T117845
        'sr-el' => 'sr-Latn', // T117845

        // Although these next codes aren't *wrong* per se, including
        // both the script and the country code helps compatibility with
        // other BCP 47 users. Note that MW also uses `zh-Hans`/`zh-Hant`,
        // without a country code, and those should be left alone.
        // (See $variantfallbacks in LanguageZh.php for Hans/Hant id.)
        'zh-cn' => 'zh-Hans-CN',
        'zh-sg' => 'zh-Hans-SG',
        'zh-my' => 'zh-Hans-MY',
        'zh-tw' => 'zh-Hant-TW',
        'zh-hk' => 'zh-Hant-HK',
        'zh-mo' => 'zh-Hant-MO',
    ];

    public static function getDeprecatedCodeMapping() {
        return self::$deprecatedLanguageCodeMapping;
    }

    public static function getNonstandardLanguageCodeMapping() {
        $result = [];
        foreach ( self::$deprecatedLanguageCodeMapping as $code => $ignore ) {
            $result[$code] = self::bcp47( $code );
        }
        foreach ( self::$nonstandardLanguageCodeMapping as $code => $ignore ) {
            $result[$code] = self::bcp47( $code );
        }
        return $result;
    }

    public static function replaceDeprecatedCodes( $code ) {
        return self::$deprecatedLanguageCodeMapping[$code] ?? $code;
    }

    public static function bcp47( $code ) {
        $code = self::replaceDeprecatedCodes( strtolower( $code ) );
        if ( isset( self::$nonstandardLanguageCodeMapping[$code] ) ) {
            $code = self::$nonstandardLanguageCodeMapping[$code];
        }
        $codeSegment = explode( '-', $code );
        $codeBCP = [];
        foreach ( $codeSegment as $segNo => $seg ) {
            // when previous segment is x, it is a private segment and should be lc
            if ( $segNo > 0 && strtolower( $codeSegment[( $segNo - 1 )] ) == 'x' ) {
                $codeBCP[$segNo] = strtolower( $seg );
            // ISO 3166 country code
            } elseif ( ( strlen( $seg ) == 2 ) && ( $segNo > 0 ) ) {
                $codeBCP[$segNo] = strtoupper( $seg );
            // ISO 15924 script code
            } elseif ( ( strlen( $seg ) == 4 ) && ( $segNo > 0 ) ) {
                $codeBCP[$segNo] = ucfirst( strtolower( $seg ) );
            // Use lowercase for other cases
            } else {
                $codeBCP[$segNo] = strtolower( $seg );
            }
        }
        $langCode = implode( '-', $codeBCP );
        return $langCode;
    }
}