Refactor Auto bullet list in Editor

The earlier code base was a bit fragile, I prefer having this
unit-tested as the logic is a bit tricky, and I still don't support all
the use-cases.

lib/editors/heuristics.dart

@@ -0,0 +1,40 @@
+class EditorHeuristicResult {
+  String text;
+  int cursorPos;
+
+  EditorHeuristicResult(this.text, this.cursorPos);
+}
+
+EditorHeuristicResult autoAddBulletList(
+    String oldText, String curText, int cursorPos) {
+  // We only want to do this on inserts
+  if (curText.length <= oldText.length) {
+    return null;
+  }
+
+  // Only when getting a new line
+  if (curText[cursorPos - 1] != '\n') {
+    return null;
+  }
+
+  // Only add the bullets at the end of the document
+  // FIXME: We should do this anywhere
+  if (curText.length != cursorPos) {
+    return null;
+  }
+
+  var prevLineStart = curText.lastIndexOf('\n', cursorPos - 2);
+  prevLineStart = prevLineStart == -1 ? 0 : prevLineStart + 1;
+  var prevLine = curText.substring(prevLineStart, cursorPos - 1);
+
+  var pattern = RegExp(r'^(\s*)([*\-])');
+  var match = pattern.firstMatch(prevLine);
+  if (match == null) {
+    return null;
+  }
+
+  var indent = match.group(1) ?? "";
+  var text = curText.substring(0, cursorPos) + indent + match.group(2) + ' ';
+
+  return EditorHeuristicResult(text, text.length);
+}

lib/editors/markdown_editor.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 
 import 'package:gitjournal/core/note.dart';
 import 'package:gitjournal/editors/common.dart';
+import 'package:gitjournal/editors/heuristics.dart';
 import 'package:gitjournal/editors/note_title_editor.dart';
 import 'package:gitjournal/settings.dart';
 import 'package:gitjournal/widgets/editor_scroll_view.dart';
@@ -54,7 +55,7 @@ class MarkdownEditorState extends State<MarkdownEditor> implements EditorState {
   TextEditingController _textController = TextEditingController();
   TextEditingController _titleTextController = TextEditingController();
 
-  int _textLength;
+  String _oldText;
 
   bool editingMode = true;
   bool _noteModified;
@@ -62,7 +63,7 @@ class MarkdownEditorState extends State<MarkdownEditor> implements EditorState {
   MarkdownEditorState(this.note) {
     _textController = TextEditingController(text: note.body);
     _titleTextController = TextEditingController(text: note.title);
-    _textLength = note.body.length;
+    _oldText = note.body;
 
     editingMode = Settings.instance.markdownDefaultView ==
         SettingsMarkdownDefaultView.Edit;
@@ -153,7 +154,7 @@ class MarkdownEditorState extends State<MarkdownEditor> implements EditorState {
 
   void _noteTextChanged() {
     try {
-      _insertExtraCharactersOnEnter();
+      _applyHeuristics();
     } catch (e) {
       print(e);
     }
@@ -167,38 +168,23 @@ class MarkdownEditorState extends State<MarkdownEditor> implements EditorState {
     }
   }
 
-  void _insertExtraCharactersOnEnter() {
-    var text = _textController.text;
-    if (text.length <= _textLength) {
-      _textLength = text.length;
-      return;
-    }
-    _textLength = text.length;
-
+  void _applyHeuristics() {
     var selection = _textController.selection;
     if (selection.baseOffset != selection.extentOffset) {
+      _oldText = _textController.text;
       return;
     }
 
-    var cursorPos = selection.baseOffset;
-    if (cursorPos != _textLength || text[cursorPos - 1] != '\n') {
-      return;
-    }
-    var prevLineStart = text.lastIndexOf('\n', cursorPos - 2);
-    prevLineStart = prevLineStart == -1 ? 0 : prevLineStart + 1;
-    var prevLine = text.substring(prevLineStart, cursorPos - 1);
+    var r =
+        autoAddBulletList(_oldText, _textController.text, selection.baseOffset);
+    _oldText = _textController.text;
 
-    var pattern = RegExp(r'^(\s*)([*\-])');
-    var match = pattern.firstMatch(prevLine);
-    if (match == null) {
+    if (r == null) {
       return;
     }
 
-    var indent = match.group(1) ?? "";
-    _textController.text =
-        text.substring(0, cursorPos) + indent + match.group(2) + ' ';
-    _textLength = _textController.text.length;
-    _textController.selection = TextSelection.collapsed(offset: _textLength);
+    _textController.text = r.text;
+    _textController.selection = TextSelection.collapsed(offset: r.cursorPos);
   }
 
   @override

test/editor_heuristics_test.dart

@@ -0,0 +1,23 @@
+import 'package:test/test.dart';
+import 'package:gitjournal/editors/heuristics.dart';
+
+void main() {
+  group('Editor Heuristic Tests', () {
+    test('Does not modify test on newline', () {
+      var origText = "Hello";
+      var newText = "Hello\n";
+
+      var result = autoAddBulletList(origText, newText, newText.length);
+      expect(result, null);
+    });
+
+    test('Adds a bullet item', () {
+      var origText = "Hello\n* One";
+      var newText = "Hello\n* One\n";
+
+      var result = autoAddBulletList(origText, newText, newText.length);
+      expect(result.text, "Hello\n* One\n* ");
+      expect(result.cursorPos, result.text.length);
+    });
+  });
+}