BadNote/lib/utils/stroke_stabilizer.dart

import '../models/ink_point.dart';

/// Stabilization level for hand-drawn strokes.
enum StabilizationLevel { none, light, medium, heavy }

/// Smooths pen input using an Exponential Moving Average (EMA) filter.
///
/// - **none**: no smoothing (passthrough)
/// - **light**: alpha = 0.6 (subtle smoothing)
/// - **medium**: alpha = 0.4 (moderate smoothing)
/// - **heavy**: alpha = 0.25 (strong smoothing, removes most tremor)
///
/// The formula applied to each coordinate independently:
///   x_smoothed = alpha * x_raw + (1 - alpha) * x_prev
class StrokeStabilizer {
  final StabilizationLevel level;

  double? _prevX;
  double? _prevY;

  StrokeStabilizer({this.level = StabilizationLevel.none});

  double get _alpha {
    switch (level) {
      case StabilizationLevel.none:
        return 1.0;
      case StabilizationLevel.light:
        return 0.6;
      case StabilizationLevel.medium:
        return 0.4;
      case StabilizationLevel.heavy:
        return 0.25;
    }
  }

  /// Filters a raw point through the EMA, returning a smoothed point.
  /// Returns the raw point unchanged if level is [StabilizationLevel.none].
  InkPoint filter(InkPoint rawPoint) {
    if (level == StabilizationLevel.none) return rawPoint;

    final alpha = _alpha;

    if (_prevX == null || _prevY == null) {
      _prevX = rawPoint.x;
      _prevY = rawPoint.y;
      return rawPoint;
    }

    final smoothedX = alpha * rawPoint.x + (1 - alpha) * _prevX!;
    final smoothedY = alpha * rawPoint.y + (1 - alpha) * _prevY!;

    _prevX = smoothedX;
    _prevY = smoothedY;

    return InkPoint(
      x: smoothedX,
      y: smoothedY,
      pressure: rawPoint.pressure,
      tilt: rawPoint.tilt,
      timestamp: rawPoint.timestamp,
      pointerDeviceKind: rawPoint.pointerDeviceKind,
    );
  }

  /// Resets the filter state. Call when starting a new stroke.
  void reset() {
    _prevX = null;
    _prevY = null;
  }
}