import 'dart:collection';

/// Sliding-window packets-per-second counter.
///
/// Measures wall-clock arrival rate, which intentionally differs from the
/// timestamp_us field in the packet (server time). Network jitter makes the
/// two diverge.
class PpsCounter {
  PpsCounter({this.window = const Duration(seconds: 1)});

  final Duration window;
  final Queue<DateTime> _arrivals = Queue();

  void recordArrival([DateTime? now]) {
    _arrivals.add(now ?? DateTime.now());
    _evictOld(now);
  }

  /// Current PPS measurement.
  double current([DateTime? now]) {
    _evictOld(now);
    final secs = window.inMilliseconds / 1000.0;
    return _arrivals.length / secs;
  }

  void reset() => _arrivals.clear();

  void _evictOld(DateTime? now) {
    final cutoff = (now ?? DateTime.now()).subtract(window);
    while (_arrivals.isNotEmpty && _arrivals.first.isBefore(cutoff)) {
      _arrivals.removeFirst();
    }
  }
}