Chore: improve getDiff perfomance for the editor

CHANGELOG.md

@@ -10,6 +10,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Improve `getDiff` perfomance for the editor [#2517](https://github.com/singerdmx/flutter-quill/pull/2517).
+
 ## [11.2.0] - 2025-03-26
 
 ### Added 

lib/src/delta/delta_diff.dart

@@ -1,7 +1,6 @@
 import 'dart:math' as math;
-import 'dart:ui' show TextDirection;
 
-import 'package:flutter/foundation.dart' show immutable;
+import 'package:flutter/material.dart';
 
 import '../../quill_delta.dart';
 import '../document/attribute.dart';
@@ -16,6 +15,33 @@ class Diff {
     required this.inserted,
   });
 
+  const Diff.insert({
+    required this.start,
+    required this.inserted,
+  }) : deleted = '';
+
+  const Diff.noDiff({
+    this.start = 0,
+  })  : deleted = '',
+        inserted = '';
+
+  const Diff.delete({
+    required this.start,
+    required this.deleted,
+  }) : inserted = '';
+
+  /// Checks if the diff is just a delete
+  bool get isDelete => inserted.isEmpty && deleted.isNotEmpty;
+
+  /// Checks if the diff is just replace
+  bool get isReplace => inserted.isNotEmpty && deleted.isNotEmpty;
+
+  /// Checks if the diff is just an isnertion
+  bool get isInsert => inserted.isNotEmpty && deleted.isEmpty;
+
+  /// Checks if the diff has no changes
+  bool get hasNoDiff => inserted.isEmpty && deleted.isEmpty;
+
   // Start index in old text at which changes begin.
   final int start;
 
@@ -31,30 +57,222 @@ class Diff {
   }
 }
 
-/* Get diff operation between old text and new text */
-Diff getDiff(String oldText, String newText, int cursorPosition) {
-  var end = oldText.length;
-  final delta = newText.length - end;
-  for (final limit = math.max(0, cursorPosition - delta);
-      end > limit && oldText[end - 1] == newText[end + delta - 1];
-      end--) {}
-  var start = 0;
-  //TODO: we need to improve this part because this loop has a lot of unsafe index operations
-  for (final startLimit = cursorPosition - math.max(0, delta);
-      start < startLimit &&
-          (start > oldText.length - 1 ? '' : oldText[start]) ==
-              (start > newText.length - 1 ? '' : newText[start]);
-      start++) {}
-  final deleted = (start >= end) ? '' : oldText.substring(start, end);
-  // we need to make the check if the start is major than the end because if we directly get the
-  // new inserted text without checking first, this will always throw an error since this is an unsafe op
-  final inserted =
-      (start >= end + delta) ? '' : newText.substring(start, end + delta);
-  return Diff(
-    start: start,
-    deleted: deleted,
-    inserted: inserted,
+/// Get text changes between two strings using [oldStr] and [newStr]
+/// using selection as the base with [oldSelection] and [newSelection].
+///
+/// Performance: O([k]) where [k] == change size (not document length)
+Diff getDiff(
+  String oldStr,
+  String newStr,
+  TextSelection oldSelection,
+  TextSelection newSelection,
+) {
+  if (oldStr == newStr) return Diff.noDiff(start: newSelection.start);
+
+  // 1. Calculate affected range based on selections
+  final affectedRange =
+      _getAffectedRange(oldStr, newStr, oldSelection, newSelection);
+  var start = affectedRange.start
+      .clamp(0, math.min(oldStr.length, newStr.length))
+      .toInt();
+  final end = affectedRange.end
+      .clamp(0, math.max(oldStr.length, newStr.length))
+      .toInt();
+
+  // 2. Adjust bounds for length variations
+  final oldLen = oldStr.length;
+  final newLen = newStr.length;
+  final lengthDiff = newLen - oldLen;
+
+  // 3. Forward search from range start
+  var hasForwardChange = false;
+
+  while (start < end && start < oldStr.length && start < newStr.length) {
+    if (oldStr[start] != newStr[start]) {
+      hasForwardChange = true;
+      break;
+    }
+    // Force forward if the change comes only from the cursor position
+    if (start >= oldSelection.baseOffset && start >= newSelection.baseOffset) {
+      break;
+    }
+    start++;
+  }
+
+  // 4. Backward search from range end
+  var oldEnd = math.min(end, oldLen);
+  var newEnd = math.min(end + lengthDiff, newLen);
+
+  var hasBackwardChange = false;
+
+  while (oldEnd > start && newEnd > start) {
+    if (oldStr[oldEnd - 1] != newStr[newEnd - 1]) {
+      hasBackwardChange = true;
+      break;
+    }
+    // Breaks if the cursor still into the same position
+    if (oldEnd - 1 <= oldSelection.baseOffset &&
+        newEnd - 1 <= newSelection.baseOffset) {
+      break;
+    }
+    oldEnd--;
+    newEnd--;
+  }
+
+  // This is a workaround that fixes an issue where, when the cursor
+  // is between two characters, that are the same ("s|s"), when you
+  // press backspace key, instead removes the "s" character before the cursor,
+  // it just moves to left without removing nothing
+  if (!hasForwardChange && !hasBackwardChange) {
+    if (oldStr.length > newStr.length) {
+      return Diff.delete(
+        start: oldSelection.baseOffset < newSelection.baseOffset
+            ? oldSelection.baseOffset
+            : newSelection.baseOffset,
+        deleted: ' ',
+      );
+    }
+  }
+
+  final safeOldEnd = oldEnd.clamp(start, oldStr.length);
+  final safeNewEnd = newEnd.clamp(start, newStr.length);
+
+  // 5. Extract differences
+  final deleted = oldStr.substring(start, safeOldEnd);
+  final inserted = newStr.substring(start, safeNewEnd);
+
+  // 6. Validate consistency
+  if (_isChangeConsistent(
+      deleted, inserted, oldStr, oldSelection, newSelection)) {
+    return _buildDiff(deleted, inserted, start);
+  }
+
+  // Fallback for complex cases
+  return _fallbackDiff(oldStr, newStr, start, end);
+}
+
+TextRange _getAffectedRange(
+  String oldStr,
+  String newStr,
+  TextSelection oldSel,
+  TextSelection newSel,
+) {
+  // Calculate combined selection area
+  final start = math.min(oldSel.start, newSel.start);
+  final end = math.max(oldSel.end, newSel.end);
+
+  // Expand by 20% to capture nearby changes
+  //
+  // We use this to avoid check all the string length
+  // unnecessarily when we can use the selection as a base
+  // to know where, and how was do it the change
+  final expansion = ((end - start) * 0.2).round();
+
+  return TextRange(
+    start: math.max(0, start - expansion),
+    end: math.min(math.max(oldStr.length, newStr.length), end + expansion),
+  );
+}
+
+bool _isChangeConsistent(
+  String deleted,
+  String inserted,
+  String oldText,
+  TextSelection oldSel,
+  TextSelection newSel,
+) {
+  final isForwardDelete = _isForwardDelete(
+    deletedText: deleted,
+    oldText: oldText,
+    oldSelection: oldSel,
+    newSelection: newSel,
   );
+  if (isForwardDelete) {
+    return newSel.start == oldSel.start &&
+        deleted.length == (oldSel.end - oldSel.start);
+  }
+  final isInsert = newSel.start == newSel.end && inserted.isNotEmpty;
+  final isDelete = deleted.isNotEmpty && inserted.isEmpty;
+
+  // Insert validation
+  if (isInsert) {
+    return newSel.start == oldSel.start + inserted.length;
+  }
+
+  // Delete validation
+  if (isDelete) {
+    return oldSel.start - newSel.start == deleted.length;
+  }
+
+  return true;
+}
+
+/// Detect if the deletion was do it to forward
+bool _isForwardDelete({
+  required String deletedText,
+  required String oldText,
+  required TextSelection oldSelection,
+  required TextSelection newSelection,
+}) {
+  // is forward delete if:
+  return
+      // 1. There's deleted text
+      deletedText.isNotEmpty &&
+
+          // 2. The original selection is collaped
+          oldSelection.isCollapsed &&
+
+          // 3. New and original selections has the same offset
+          newSelection.isCollapsed &&
+          newSelection.baseOffset == oldSelection.baseOffset &&
+
+          // 4. The removed character if after the cursor position
+          oldText.startsWith(deletedText, oldSelection.baseOffset);
+}
+
+Diff _fallbackDiff(String oldStr, String newStr, int start, [int? end]) {
+  end ??= math.min(oldStr.length, newStr.length);
+
+  // 1. Find first divergence point
+  while (start < end &&
+      start < oldStr.length &&
+      start < newStr.length &&
+      oldStr[start] == newStr[start]) {
+    start++;
+  }
+
+  // 2. Find last divergence point
+  var oldEnd = oldStr.length;
+  var newEnd = newStr.length;
+
+  while (oldEnd > start &&
+      newEnd > start &&
+      oldStr[oldEnd - 1] == newStr[newEnd - 1]) {
+    oldEnd--;
+    newEnd--;
+  }
+
+  // 3. Extract differences
+  final deleted = oldStr.substring(start, oldEnd);
+  final inserted = newStr.substring(start, newEnd);
+
+  return _buildDiff(deleted, inserted, start);
+}
+
+Diff _buildDiff(String deleted, String inserted, int start) {
+  if (deleted.isEmpty && inserted.isEmpty) return const Diff.noDiff();
+
+  if (deleted.isNotEmpty && inserted.isNotEmpty) {
+    return Diff(
+      inserted: inserted,
+      start: start,
+      deleted: deleted,
+    );
+  } else if (inserted.isNotEmpty) {
+    return Diff.insert(start: start, inserted: inserted);
+  } else {
+    return Diff.delete(start: start, deleted: deleted);
+  }
 }
 
 int getPositionDelta(Delta user, Delta actual) {

lib/src/editor/raw_editor/raw_editor_state_selection_delegate_mixin.dart

@@ -15,18 +15,26 @@ mixin RawEditorStateSelectionDelegateMixin on EditorState
   }
 
   set textEditingValue(TextEditingValue value) {
-    final cursorPosition = value.selection.extentOffset;
     final oldText = widget.controller.document.toPlainText();
     final newText = value.text;
-    final diff = getDiff(oldText, newText, cursorPosition);
-    if (diff.deleted == '' && diff.inserted == '') {
+    final diff = getDiff(
+      oldText,
+      newText,
+      widget.controller.selection,
+      value.selection,
+    );
+    if (diff.hasNoDiff) {
       // Only changing selection range
       widget.controller.updateSelection(value.selection, ChangeSource.local);
       return;
     }
 
     widget.controller.replaceTextWithEmbeds(
-        diff.start, diff.deleted.length, diff.inserted, value.selection);
+      diff.start,
+      diff.deleted.length,
+      diff.inserted,
+      value.selection,
+    );
   }
 
   @override

lib/src/editor/raw_editor/raw_editor_state_text_input_client_mixin.dart

@@ -228,9 +228,13 @@ mixin RawEditorStateTextInputClientMixin on EditorState
     _lastKnownRemoteTextEditingValue = value;
     final oldText = effectiveLastKnownValue.text;
     final text = value.text;
-    final cursorPosition = value.selection.extentOffset;
-    final diff = getDiff(oldText, text, cursorPosition);
-    if (diff.deleted.isEmpty && diff.inserted.isEmpty) {
+    final diff = getDiff(
+      oldText,
+      text,
+      widget.controller.selection,
+      value.selection,
+    );
+    if (diff.hasNoDiff) {
       widget.controller.updateSelection(value.selection, ChangeSource.local);
     } else {
       widget.controller.replaceText(