class ShanghaiTime {
  ShanghaiTime._();

  static const offset = Duration(hours: 8);

  /// A device-timezone-independent civil clock for Asia/Shanghai.
  static DateTime get now => toCivil(DateTime.now());

  static DateTime toCivil(DateTime instant) {
    final shifted = instant.toUtc().add(offset);
    return DateTime(
      shifted.year,
      shifted.month,
      shifted.day,
      shifted.hour,
      shifted.minute,
      shifted.second,
      shifted.millisecond,
      shifted.microsecond,
    );
  }

  static DateTime civilToUtc(DateTime civil) => DateTime.utc(
    civil.year,
    civil.month,
    civil.day,
    civil.hour,
    civil.minute,
    civil.second,
    civil.millisecond,
    civil.microsecond,
  ).subtract(offset);

  /// Parses API/cache instants. Legacy values without a zone were stored as UTC.
  static DateTime parseUtcInstant(String value) {
    final parsed = DateTime.parse(value);
    if (value.endsWith('Z') || RegExp(r'[+-]\d{2}:?\d{2}$').hasMatch(value)) {
      return parsed.toUtc();
    }
    return DateTime.utc(
      parsed.year,
      parsed.month,
      parsed.day,
      parsed.hour,
      parsed.minute,
      parsed.second,
      parsed.millisecond,
      parsed.microsecond,
    );
  }

  static DateTime parseCivil(String value) => toCivil(parseUtcInstant(value));

  static DateTime startOfDayUtc(DateTime civil) =>
      civilToUtc(DateTime(civil.year, civil.month, civil.day));

  static ({DateTime start, DateTime end}) monthRangeUtc(int year, int month) =>
      (
        start: civilToUtc(DateTime(year, month, 1)),
        end: civilToUtc(DateTime(year, month + 1, 1)),
      );

  static ({DateTime start, DateTime end}) yearRangeUtc(int year) => (
    start: civilToUtc(DateTime(year, 1, 1)),
    end: civilToUtc(DateTime(year + 1, 1, 1)),
  );

  static ({DateTime start, DateTime end}) weekRangeUtc(DateTime civil) {
    final date = DateTime(civil.year, civil.month, civil.day);
    final start = date.subtract(Duration(days: date.weekday - DateTime.monday));
    return (
      start: civilToUtc(start),
      end: civilToUtc(start.add(const Duration(days: 7))),
    );
  }

  static String formatCivil(DateTime value) =>
      '${value.year.toString().padLeft(4, '0')}-'
      '${value.month.toString().padLeft(2, '0')}-'
      '${value.day.toString().padLeft(2, '0')} '
      '${value.hour.toString().padLeft(2, '0')}:'
      '${value.minute.toString().padLeft(2, '0')}';

  static String formatDateTime(DateTime instant) {
    final value = toCivil(instant);
    return formatCivil(value);
  }
}