feat: integrate test file generation

day_generator.dart

@@ -38,6 +38,12 @@ void main(List<String?> args) async {
     File('solutions/$dayFileName').writeAsString(dayTemplate(dayNumber)),
   );
 
+  // Create test file
+  final testFileName = 'day${dayNumber}_test.dart';
+  unawaited(
+    File('test/$testFileName').writeAsString(_testTemplate(dayNumber)),
+  );
+
   final exportFile = File('solutions/index.dart');
   final exports = exportFile.readAsLinesSync();
   final content = "export 'day$dayNumber.dart';\n";
@@ -105,3 +111,69 @@ class Day$dayNumber extends GenericDay {
 
 ''';
 }
+
+String _testTemplate(String day) {
+  return '''
+import 'package:test/test.dart';
+
+import '../solutions/day$day.dart';
+
+// *******************************************************************
+// Fill out the variables below according to the puzzle description!
+// The test code should usually not need to be changed.
+// *******************************************************************
+
+/// Paste in the small example that is given for the FIRST PART of the puzzle.
+/// It will be evaluated again the `_exampleSolutionPart1` below.
+const _exampleInput1 = \'''
+\''';
+
+/// Paste in the small example that is given for the SECOND PART of the puzzle.
+/// It will be evaluated against the `_exampleSolutionPart2` below.
+///
+/// In case the second part uses the same example, uncomment below line instead:
+// const _exampleInput2 = _exampleInput;
+const _exampleInput2 = \'''
+\''';
+
+/// The solution for the FIRST PART's example, which is given by the puzzle.
+const _exampleSolutionPart1 = 0;
+
+/// The solution for the SECOND PART's example, which is given by the puzzle.
+const _exampleSolutionPart2 = 0;
+
+/// The actual solution for the FIRST PART of the puzzle, based on your input.
+/// This can only be filled out after you have solved the puzzle and is used
+/// for regression testing when refactoring.
+const _puzzleSolutionPart1 = 0;
+
+/// The actual solution for the SECOND PART of the puzzle, based on your input.
+/// This can only be filled out after you have solved the puzzle and is used
+/// for regression testing when refactoring.
+const _puzzleSolutionPart2 = 0;
+
+void main() {
+  group(
+    'Day $day - Example Input',
+    () {
+      test('Part 1', () {
+        final day = Day$day()..inputForTesting = _exampleInput1;
+        expect(day.solvePart1(), _exampleSolutionPart1);
+      });
+      test('Part 2', () {
+        final day = Day$day()..inputForTesting = _exampleInput2;
+        expect(day.solvePart2(), _exampleSolutionPart2);
+      });
+    },
+  );
+  group(
+    'Day $day - Puzzle Input',
+    () {
+      final day = Day$day();
+      test('Part 1', () => expect(day.solvePart1(), _puzzleSolutionPart1));
+      test('Part 2', () => expect(day.solvePart2(), _puzzleSolutionPart2));
+    },
+  );
+}
+''';
+}

utils/generic_day.dart

@@ -1,3 +1,4 @@
+import 'package:meta/meta.dart';
 import 'package:timing/timing.dart';
 
 import 'input_util.dart';
@@ -10,7 +11,14 @@ typedef SolutionWithDuration = (int, Duration);
 abstract class GenericDay {
   GenericDay(this.day) : input = InputUtil(day);
   final int day;
-  final InputUtil input;
+  InputUtil input;
+
+  /// This setter must only be used to mutate the input of an existing day
+  /// implementation for testing purposes.
+  @visibleForTesting
+  // ignore: avoid_setters_without_getters
+  set inputForTesting(String example) =>
+      input = InputUtil.fromMultiLineString(example);
 
   dynamic parseInput();
   int solvePart1();