Improve outlines containing non-sequential level increments

@@ -6,6 +6,7 @@
 - Depreciate Font.stringSize
 - Implement fallback font
 - Implement Emoji support
+- Improve outlines containing non-sequential level increments [Roel Spilker]
 
 ## 3.6.6
 

@@ -623,24 +623,28 @@ class Outline extends Anchor {
       color: color,
       style: style,
       page: context.pageNumber,
-    );
+    )..effectiveLevel = level;
 
-    var parent = context.document.outline;
-    var l = level;
+    final root = context.document.outline;
 
-    while (l > 0) {
-      if (parent.effectiveLevel == l) {
-        break;
+    // find the most recently added outline
+    var actualLevel= -1;
+    var candidate = root;
+    while (candidate.outlines.isNotEmpty) {
+      candidate = candidate.outlines.last;
+      actualLevel++;
     }
 
-      if (parent.outlines.isEmpty) {
-        parent.effectiveLevel = level;
+    // find the latest added outline with a level lower than ours
+    while (candidate != root) {
+      final candidateLevel = candidate.effectiveLevel ?? actualLevel;
+      if (candidateLevel < level) {
         break;
       }
-      parent = parent.outlines.last;
-      l--;
+      candidate = candidate.parent!;
+      actualLevel--;
     }
 
-    parent.add(_outline!);
+    candidate.add(_outline!);
   }
 }

@@ -83,8 +83,144 @@ void main() {
     );
   });
 
+  group('Levels', () {
+    test('Well-formed', () {
+      final generated = _OutlineBuilder()
+          .add('Part 1', 0)
+          .add('Chapter 1', 1)
+          .add('Paragraph 1.1', 2)
+          .add('Paragraph 1.2', 2)
+          .add('Paragraph 1.3', 2)
+          .add('Chapter 2', 1)
+          .add('Paragraph 2.1', 2)
+          .add('Paragraph 2.2', 2)
+          .render()
+          .join('\n');
+      const expected = '''null
+  Part 1
+    Chapter 1
+      Paragraph 1.1
+      Paragraph 1.2
+      Paragraph 1.3
+    Chapter 2
+      Paragraph 2.1
+      Paragraph 2.2''';
+
+      expect(generated, expected);
+    });
+
+    test('Does not start with level 0', () {
+      final generated = _OutlineBuilder()
+          .add('Part 1', 1)
+          .add('Chapter 1', 2)
+          .add('Chapter 2', 2)
+          .render()
+          .join('\n');
+      const expected = '''null
+  Part 1
+    Chapter 1
+    Chapter 2''';
+
+      expect(generated, expected);
+    });
+
+    test('Contains non-sequential level increment', () {
+      final generated = _OutlineBuilder()
+          .add('Part 1', 0)
+          .add('Chapter 1', 2)
+          .add('Paragraph 1.1', 4)
+          .add('Paragraph 1.2', 4)
+          .add('Paragraph 1.3', 4)
+          .add('Chapter 2', 2)
+          .add('Paragraph 2.1', 4)
+          .add('Paragraph 2.2', 4)
+          .render()
+          .join('\n');
+
+      const expected = '''null
+  Part 1
+    Chapter 1
+      Paragraph 1.1
+      Paragraph 1.2
+      Paragraph 1.3
+    Chapter 2
+      Paragraph 2.1
+      Paragraph 2.2''';
+
+      expect(generated, expected);
+    });
+
+    test('Reverse leveling', () {
+      final generated = _OutlineBuilder()
+          .add('Paragraph 2.2', 2)
+          .add('Paragraph 2.1', 2)
+          .add('Chapter 2', 1)
+          .add('Paragraph 1.3', 2)
+          .add('Paragraph 1.2', 2)
+          .add('Paragraph 1.1', 2)
+          .add('Chapter 1', 1)
+          .add('Part 1', 0)
+          .render()
+          .join('\n');
+
+      const expected = '''null
+  Paragraph 2.2
+  Paragraph 2.1
+  Chapter 2
+    Paragraph 1.3
+    Paragraph 1.2
+    Paragraph 1.1
+  Chapter 1
+  Part 1''';
+
+      expect(generated, expected);
+    });
+  });
+
   tearDownAll(() async {
     final file = File('widgets-outline.pdf');
     await file.writeAsBytes(await pdf.save());
   });
 }
+
+class _OutlineBuilder {
+  final List<String> _titles = <String>[];
+  final List<int> _levels = <int>[];
+
+  _OutlineBuilder add(String text, int level) {
+    _titles.add(text);
+    _levels.add(level);
+    return this;
+  }
+
+  List<String> render() {
+    final pdf = Document();
+    pdf.addPage(
+      MultiPage(
+        build: (Context context) => <Widget>[
+          for (int i = 0; i < _titles.length; i++)
+            Outline(
+              name: 'anchor$i',
+              title: _titles[i],
+              level: _levels[i],
+            )
+        ],
+      ),
+    );
+    return _collectOutlines(pdf.document.catalog.outlines!);
+  }
+}
+
+List<String> _collectOutlines(
+  PdfOutline outline, [
+  List<String>? output,
+  int indent = 0,
+]) {
+  final result = output ?? <String>[];
+  final intentation = List.filled(indent, '  ').join();
+  result.add('$intentation${outline.title}');
+  for (var child in outline.outlines) {
+    _collectOutlines(child, result, indent + 1);
+  }
+  return result;
+}