Add utils diff_delta

app/android/gradle.properties

@@ -1,3 +1,4 @@
 org.gradle.jvmargs=-Xmx1536M
 android.useAndroidX=true
 android.enableJetifier=true
+android.enableR8=true

lib/utils/diff_delta.dart

@@ -0,0 +1,73 @@
+import 'dart:math' as math;
+
+import 'package:quill_delta/quill_delta.dart';
+
+// Diff between two texts - old text and new text
+class Diff {
+  // Start index in old text at which changes begin.
+  final int start;
+
+  /// The deleted text
+  final String deleted;
+
+  // The inserted text
+  final String inserted;
+
+  Diff(this.start, this.deleted, this.inserted);
+
+  @override
+  String toString() {
+    return 'Diff[$start, "$deleted", "$inserted"]';
+  }
+}
+
+/* Get diff operation between old text and new text */
+Diff getDiff(String oldText, String newText, int cursorPosition) {
+  int end = oldText.length;
+  int delta = newText.length - end;
+  for (int limit = math.max(0, cursorPosition - delta);
+      end > limit && oldText[end - 1] == newText[end + delta - 1];
+      end--) {}
+  int start = 0;
+  for (int startLimit = cursorPosition - math.max(0, delta);
+      start < startLimit && oldText[start] == newText[start];
+      start++) {}
+  String deleted = (start >= end) ? '' : oldText.substring(start, end);
+  String inserted = newText.substring(start, end + delta);
+  return Diff(start, deleted, inserted);
+}
+
+int getPositionDelta(Delta user, Delta actual) {
+  DeltaIterator userItr = DeltaIterator(user);
+  DeltaIterator actualItr = DeltaIterator(actual);
+  int diff = 0;
+  while (userItr.hasNext || actualItr.hasNext) {
+    int length = math.min(userItr.peekLength(), actualItr.peekLength());
+    Operation userOperation = userItr.next(length);
+    Operation actualOperation = actualItr.next(length);
+    if (userOperation.length != actualOperation.length) {
+      throw ('userOp ' +
+          userOperation.length.toString() +
+          ' does not match ' +
+          ' actualOp ' +
+          actualOperation.length.toString());
+    }
+    if (userOperation.key == actualOperation.key) {
+      continue;
+    } else if (userOperation.isInsert && actualOperation.isRetain) {
+      diff -= userOperation.length;
+    } else if (userOperation.isDelete && actualOperation.isRetain) {
+      diff += userOperation.length;
+    } else if (userOperation.isRetain && actualOperation.isInsert) {
+      String operationTxt = '';
+      if (actualOperation.data is String) {
+        operationTxt = actualOperation.data as String;
+      }
+      if (operationTxt.startsWith('\n')) {
+        continue;
+      }
+      diff += actualOperation.length;
+    }
+  }
+  return diff;
+}

pubspec.lock

@@ -81,6 +81,20 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.8.0-nullsafety.1"
+  quill_delta:
+    dependency: "direct main"
+    description:
+      name: quill_delta
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.0.0"
+  quiver_hashcode:
+    dependency: transitive
+    description:
+      name: quiver_hashcode
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.0.0"
   sky_engine:
     dependency: transitive
     description: flutter

pubspec.yaml

@@ -12,6 +12,7 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
+  quill_delta: ^2.0.0
 
 dev_dependencies:
   flutter_test: