BadNote/test/undo_stack_test.dart

// test/undo_stack_test.dart
//
// Pure unit tests for UndoStack<T>. No widgets, no DB, no Flutter framework.

import 'package:flutter_test/flutter_test.dart';
import 'package:badnote/editor/engine/undo_stack.dart';

void main() {
  group('UndoStack', () {
    // ------------------------------------------------------------------ basics
    test('starts empty with canUndo=false and canRedo=false', () {
      final stack = UndoStack<int>();
      expect(stack.canUndo, isFalse);
      expect(stack.canRedo, isFalse);
    });

    test('undo on empty stack returns null', () {
      final stack = UndoStack<int>();
      expect(stack.undo(42), isNull);
    });

    test('redo on empty stack returns null', () {
      final stack = UndoStack<int>();
      expect(stack.redo(), isNull);
    });

    // -------------------------------------------- record / undo / redo cycle
    test('record A, B, C then undo yields B then A', () {
      final stack = UndoStack<int>();

      // Simulate: current starts at A=1
      // Before mutating to B=2 we record the pre-mutation state A=1
      stack.record(1); // snapshot before →2
      // Before mutating to C=3 we record the pre-mutation state B=2
      stack.record(2); // snapshot before →3
      // Current live state is C=3

      expect(stack.canUndo, isTrue);
      expect(stack.canRedo, isFalse);

      // First undo: push current(3) to redo, pop B(2) from undo
      expect(stack.undo(3), equals(2));
      expect(stack.canUndo, isTrue);
      expect(stack.canRedo, isTrue);

      // Second undo: push current(2) to redo, pop A(1) from undo
      expect(stack.undo(2), equals(1));
      expect(stack.canUndo, isFalse);
      expect(stack.canRedo, isTrue);
    });

    test('redo after undo restores B', () {
      final stack = UndoStack<int>();
      stack.record(1);
      stack.record(2);
      stack.undo(3); // restore →2
      stack.undo(2); // restore →1

      // First redo: pop 2 from redo, push back to undo
      expect(stack.redo(), equals(2));
      expect(stack.canRedo, isTrue);

      // Second redo: pop 3 from redo
      expect(stack.redo(), equals(3));
      expect(stack.canRedo, isFalse);
    });

    // ------------------------------------------ record after undo clears redo
    test('record after undo clears the redo stack', () {
      final stack = UndoStack<int>();
      stack.record(1); // before →2
      stack.record(2); // before →3
      stack.undo(3);   // back to 2, redo has [3]

      expect(stack.canRedo, isTrue);

      // Now record a NEW snapshot (branching from current=2)
      stack.record(2); // before →99
      // Redo stack must be cleared
      expect(stack.canRedo, isFalse);
      expect(stack.canUndo, isTrue);
    });

    // ------------------------------------------------ capacity / cap behavior
    test('cap drops oldest snapshots beyond capacity', () {
      const cap = 3;
      final stack = UndoStack<int>(cap: cap);

      // Record 4 snapshots — only the last 3 should survive
      stack.record(10);
      stack.record(20);
      stack.record(30);
      stack.record(40); // oldest (10) dropped

      // Undo 3 times from current=50
      expect(stack.undo(50), equals(40)); // most recent
      expect(stack.undo(40), equals(30));
      expect(stack.undo(30), equals(20));
      // Stack is now empty — 10 was dropped
      expect(stack.canUndo, isFalse);
    });

    // --------------------------------------------------- clear
    test('clear empties both stacks', () {
      final stack = UndoStack<int>();
      stack.record(1);
      stack.record(2);
      stack.undo(3);

      stack.clear();

      expect(stack.canUndo, isFalse);
      expect(stack.canRedo, isFalse);
    });

    // ---------------------------------------- works with List<int> snapshots
    test('works with List<int> snapshots (immutable caller discipline)', () {
      final stack = UndoStack<List<int>>();

      final a = List<int>.unmodifiable([1, 2]);
      final b = List<int>.unmodifiable([1, 2, 3]);

      stack.record(a); // before mutation to b
      // live state is now b

      final restored = stack.undo(b);
      expect(restored, equals(a));
      expect(stack.canUndo, isFalse);
      expect(stack.canRedo, isTrue);
    });

    // ------------------------------------------ redo pushes back to undo
    test('redo snapshot is re-undoable (undo stack grows back)', () {
      final stack = UndoStack<int>();
      // undo=[1], redo=[]
      stack.record(1); // before →2
      // undo=[], redo=[2]
      stack.undo(2);   // live=1
      // undo=[2], redo=[]
      stack.redo();    // live=2
      // canUndo must be true because redo() pushed 2 back onto undo
      expect(stack.canUndo, isTrue);
      expect(stack.canRedo, isFalse);
      // Undoing from a new live state 3 pops 2 off undo
      expect(stack.undo(3), equals(2));
    });
  });
}