BadNote/lib/services/undo_manager.dart

import '../models/ink_stroke.dart';

/// Manages undo/redo state for ink strokes.
///
/// Each action records a stroke that was added or removed.
/// [undo] returns the inverse of the last action (remove if add, add if remove).
/// [redo] re-applies the undone action.
class UndoManager {
  final List<_UndoAction> _undoStack = [];
  final List<_UndoAction> _redoStack = [];
  final List<InkStroke> _strokes = [];

  /// The current list of strokes (read-only view).
  List<InkStroke> get currentStrokes => List.unmodifiable(_strokes);

  bool get canUndo => _undoStack.isNotEmpty;
  bool get canRedo => _redoStack.isNotEmpty;

  /// Records that a new stroke was added to the canvas.
  void addStroke(InkStroke stroke) {
    _strokes.add(stroke);
    _undoStack.add(_UndoAction(type: _ActionType.add, stroke: stroke));
    _redoStack.clear();
  }

  /// Records that a stroke was removed from the canvas.
  /// Also handles partial-eraser replacements: removes [stroke] and adds
  /// [replacements] (which may be empty if fully erased, or 1-2 sub-strokes).
  void removeStroke(
    InkStroke stroke, {
    List<InkStroke> replacements = const [],
  }) {
    _strokes.removeWhere((s) => s.id == stroke.id);
    _strokes.addAll(replacements);
    _undoStack.add(
      _UndoAction(
        type: _ActionType.remove,
        stroke: stroke,
        replacements: replacements,
      ),
    );
    _redoStack.clear();
  }

  /// Undoes the last action. Returns the stroke that was affected and needs
  /// to be reversed on the canvas, or `null` if nothing to undo.
  ///
  /// For add actions: the stroke should be removed from the canvas.
  /// For remove actions: the stroke (and its replacements) should be restored.
  InkStroke? undo() {
    if (_undoStack.isEmpty) return null;

    final action = _undoStack.removeLast();
    _redoStack.add(action);

    switch (action.type) {
      case _ActionType.add:
        _strokes.removeWhere((s) => s.id == action.stroke.id);
        return action.stroke;
      case _ActionType.remove:
        // Remove the replacements that were added during the original remove
        for (final r in action.replacements) {
          _strokes.removeWhere((s) => s.id == r.id);
        }
        _strokes.add(action.stroke);
        return action.stroke;
    }
  }

  /// Redoes the last undone action. Returns the stroke that was affected, or
  /// `null` if nothing to redo.
  InkStroke? redo() {
    if (_redoStack.isEmpty) return null;

    final action = _redoStack.removeLast();
    _undoStack.add(action);

    switch (action.type) {
      case _ActionType.add:
        _strokes.add(action.stroke);
        return action.stroke;
      case _ActionType.remove:
        _strokes.removeWhere((s) => s.id == action.stroke.id);
        _strokes.addAll(action.replacements);
        return action.stroke;
    }
  }
}

enum _ActionType { add, remove }

class _UndoAction {
  final _ActionType type;
  final InkStroke stroke;
  final List<InkStroke> replacements;

  _UndoAction({
    required this.type,
    required this.stroke,
    this.replacements = const [],
  });
}