import 'package:flutter_test/flutter_test.dart';
import 'package:nearle_pos/core/utils/formatters.dart';

/// The fault these cover was found in live data, not in a test.
///
/// Bill INV-2608-T5EDD-00116 carried `billedat 2026-08-05T12:49:28.245Z` beside
/// `receivedat 2026-08-05T07:19:28.586Z` — the bill appearing to have been rung
/// five and a half hours *after* the back office received it. Exactly the IST
/// offset, every time.
///
/// Nothing was lying. `DateTime.toIso8601String()` on a local time emits no
/// zone marker at all, Go's `time.Parse` fills that silence with UTC, and the
/// till's wall clock was recorded as though it had been read in London.
void main() {
  group('a timestamp says which zone it was read in', () {
    test('carries an explicit offset', () {
      final iso = Formatters.isoWithOffset(DateTime(2026, 8, 5, 12, 49, 28));

      expect(
        iso,
        matches(r'^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.*[+-]\d{2}:\d{2}$'),
        reason: 'without an offset the receiver has to guess, and guesses UTC',
      );
    });

    test('round-trips back to the same instant', () {
      // The whole point. The old form parsed to a different moment than the one
      // the cashier rang the bill at.
      final rung = DateTime(2026, 8, 5, 12, 49, 28, 245);

      final parsed = DateTime.parse(Formatters.isoWithOffset(rung));

      expect(
        parsed.toUtc(),
        rung.toUtc(),
        reason: 'the instant must survive the trip to the back office',
      );
    });

    test('keeps the wall clock the till actually showed', () {
      // Load-bearing for businessdate. The back office derives a day's takings
      // from the wall clock, so a bill rung at 12:49 must still read 12:49 —
      // converting to UTC before sending would have moved it to 07:19 and put
      // late-evening sales on the previous day.
      final iso = Formatters.isoWithOffset(DateTime(2026, 8, 5, 12, 49, 28));

      expect(iso, startsWith('2026-08-05T12:49:28'));
    });

    test('does not drop a half-hour offset', () {
      // India is +05:30. An implementation that formatted only whole hours
      // would produce +05:00 here and be wrong by thirty minutes — the kind of
      // error that survives review because it looks almost right.
      final offset = DateTime.now().timeZoneOffset;
      final iso = Formatters.isoWithOffset(DateTime(2026, 8, 5, 12, 0));

      final minutes = offset.abs().inMinutes % 60;
      expect(
        iso.substring(iso.length - 2),
        minutes.toString().padLeft(2, '0'),
        reason: 'the minutes component must come from the real offset',
      );
    });

    test('a UTC input is converted, not relabelled', () {
      // Passing an already-UTC DateTime must not stamp it with the local
      // offset while leaving the UTC wall clock in place — that would recreate
      // the original bug in reverse.
      final instant = DateTime.utc(2026, 8, 5, 7, 19, 28);

      final parsed = DateTime.parse(Formatters.isoWithOffset(instant));

      expect(parsed.toUtc(), instant);
    });
  });
}