flutter_lua_runtime/lib/runtime/game/runtime_locale.dart

import 'dart:ui' show Locale;

class RuntimeLocaleInfo {
  const RuntimeLocaleInfo({
    required this.requested,
    required this.resolved,
    required this.defaultLocale,
    required this.supportedLocales,
    required this.languageCode,
    this.scriptCode,
    this.countryCode,
  });

  final String requested;
  final String resolved;
  final String defaultLocale;
  final List<String> supportedLocales;
  final String languageCode;
  final String? scriptCode;
  final String? countryCode;

  Map<String, Object?> toMap() {
    return {
      'requested': requested,
      'resolved': resolved,
      'default': defaultLocale,
      'supported': supportedLocales,
      'languageCode': languageCode,
      if (scriptCode != null) 'scriptCode': scriptCode,
      if (countryCode != null) 'countryCode': countryCode,
    };
  }
}

class RuntimeLocaleResolver {
  const RuntimeLocaleResolver._();

  static RuntimeLocaleInfo resolve({
    required Locale requested,
    required String defaultLocale,
    required List<String> supportedLocales,
  }) {
    final requestedTag = normalizeTag(tagOf(requested));
    final fallback = normalizeTag(defaultLocale);
    final supported = supportedLocales.isEmpty
        ? [fallback]
        : supportedLocales.map(normalizeTag).toList(growable: false);
    final resolved = _resolveTag(
      requestedTag: requestedTag,
      fallback: fallback,
      supported: supported,
    );

    return RuntimeLocaleInfo(
      requested: requestedTag,
      resolved: resolved,
      defaultLocale: fallback,
      supportedLocales: supported,
      languageCode: requested.languageCode,
      scriptCode: requested.scriptCode,
      countryCode: requested.countryCode,
    );
  }

  static Locale localeFromTag(String tag) {
    final parts = normalizeTag(tag).split('-');
    if (parts.isEmpty || parts.first.isEmpty) {
      throw const FormatException('Locale tag must not be empty');
    }

    String? scriptCode;
    String? countryCode;
    for (final part in parts.skip(1)) {
      if (part.length == 4 && scriptCode == null) {
        scriptCode = part;
      } else {
        countryCode ??= part;
      }
    }

    return Locale.fromSubtags(
      languageCode: parts.first,
      scriptCode: scriptCode,
      countryCode: countryCode,
    );
  }

  static String tagOf(Locale locale) {
    final parts = [locale.languageCode];
    final scriptCode = locale.scriptCode;
    final countryCode = locale.countryCode;
    if (scriptCode != null && scriptCode.isNotEmpty) {
      parts.add(scriptCode);
    }
    if (countryCode != null && countryCode.isNotEmpty) {
      parts.add(countryCode);
    }
    return normalizeTag(parts.join('-'));
  }

  static String normalizeTag(String tag) {
    final normalized = tag.trim().replaceAll('_', '-');
    if (normalized.isEmpty) {
      throw const FormatException('Locale tag must not be empty');
    }

    final parts = normalized
        .split('-')
        .where((part) => part.isNotEmpty)
        .toList(growable: false);
    if (parts.isEmpty) {
      throw const FormatException('Locale tag must not be empty');
    }
    if (!_isLocalePart(parts.first)) {
      throw FormatException('Locale language code is invalid: ${parts.first}');
    }

    final result = <String>[parts.first.toLowerCase()];
    for (final part in parts.skip(1)) {
      if (!_isLocalePart(part)) {
        throw FormatException('Locale tag part is invalid: $part');
      }
      if (part.length == 4) {
        result.add(
          '${part[0].toUpperCase()}${part.substring(1).toLowerCase()}',
        );
      } else if (part.length == 2 || part.length == 3) {
        result.add(part.toUpperCase());
      } else {
        result.add(part.toLowerCase());
      }
    }
    return result.join('-');
  }

  static String _resolveTag({
    required String requestedTag,
    required String fallback,
    required List<String> supported,
  }) {
    final supportedSet = supported.toSet();
    if (supportedSet.contains(requestedTag)) {
      return requestedTag;
    }

    final requestedLanguage = requestedTag.split('-').first;
    for (final candidate in supported) {
      if (candidate.split('-').first == requestedLanguage) {
        return candidate;
      }
    }

    if (supportedSet.contains(fallback)) {
      return fallback;
    }
    return supported.first;
  }

  static bool _isLocalePart(String value) {
    return RegExp(r'^[A-Za-z0-9]{2,8}$').hasMatch(value);
  }
}