How to reverse strings that contain surrogate pairs in Dart?

Question

I was playing with algorithms using Dart and as I actually followed TDD, I realized that my code has some limitations.

I was trying to reverse strings as part of an interview problem, but I couldn't get the surrogate pairs correctly reversed.

const simple = 'abc';
const emoji = '';
const surrogate = '‍♂️‍';

String rev(String s) {
    return String.fromCharCodes(s.runes.toList().reversed);
}

void main() {
    print(simple);
    print(rev(simple));
    print(emoji);
    print(rev(emoji));
    print(surrogate);
    print(rev(surrogate));
}

The output:

abc
cba


‍♂️‍
‍️♂‍

You can see that the simple emojis are correctly reversed as I'm using the runes instead of just simply executing s.split('').toList().reversed.join(''); but the surrogate pairs are reversed incorrectly.

How can I reverse a string that might contain surrogate pairs using the Dart programming language?

Answer 1

When reversing strings, you must operate on graphemes, not characters nor code units. Use grapheme_splitter.

Answer 2

Dart 2.7 introduced a new package that supports grapheme cluster-aware operations. The package is called characters. characters is a package for characters represented as Unicode extended grapheme clusters.

Dart’s standard String class uses the UTF-16 encoding. This is a common choice in programming languages, especially those that offer support for running both natively on devices, and on the web.

UTF-16 strings usually work well, and the encoding is transparent to the developer. However, when manipulating strings, and especially when manipulating strings entered by users, you may experience a difference between what the user perceives as a character, and what is encoded as a code unit in UTF-16.

Source: "Announcing Dart 2.7: A safer, more expressive Dart" by Michael Thomsen, section "Safe substring handling"

The package will also help to reverse your strings with emojis the way a native programmer would expect.

Using simple Strings, you find issues:

String hi = 'Hi ';
print('String.length: ${hi.length}');
// Prints 7; would expect 4

With characters

String hi = 'Hi ';
print(hi.characters.length);
// Prints 4
print(hi.characters.last);
// Prints 

It's worth taking a look at the source code of the characters package, it's far from simple but looks easier to digest and better documented than grapheme_splitter. The characters package is also maintained by the Dart team.

Answer 3

Create an extension on String named reversed

extension on String {
  /// Reverse the string

  String get reversed =>
      GraphemeSplitter().splitGraphemes(this).toList().reversed.join();
}

In order to add GraphemeSplitter class install grapheme_splitter package :

dart pub add grapheme_splitter

Example Program:

import "package:grapheme_splitter/grapheme_splitter.dart";
import "dart:io";

void main(final List<String> $) async {
  test();
}

void test() async {
  final Writer writer = Writer();

  const simple = 'abc';

  const emoji = '';

  const surrogate = '‍♂️‍';

  const hell = "Z͑ͫ̓ͪ̂ͫ̽͏̴̙̤̞͉͚̯̞̠͍A̴̵̜̰͔ͫ͗͢L̠ͨͧͩ͘G̴̻͈͍͔̹̑͗̎̅͛́Ǫ̵̹̻̝̳͂̌̌͘";

  const nightmare = "️‍";

  await writer.print(simple);
  await writer.print(emoji);
  await writer.print(surrogate);
  await writer.print(hell);
  await writer.print(nightmare);

  await writer.print(simple.reversed);
  await writer.print(emoji.reversed);
  await writer.print(surrogate.reversed);
  await writer.print(hell.reversed);
  await writer.print(nightmare.reversed);
}

class Writer {
  final String filePath;
  final File file;

  Writer({this.filePath = "./data.dat"})
      : file = File(filePath)
          ..writeAsString(
              ""); // If File exits lets truncate it

  print(final Object data) async {
    await file.writeAsString("${data.toString()}\n",
        mode: FileMode.append); // Appends to the above file
  }
}

extension on String {
  /// Reverse the string

  String get reversed =>
      GraphemeSplitter().splitGraphemes(this).toList().reversed.join();
}

Output in data.dat file

abc

‍♂️‍
Z͑ͫ̓ͪ̂ͫ̽͏̴̙̤̞͉͚̯̞̠͍A̴̵̜̰͔ͫ͗͢L̠ͨͧͩ͘G̴̻͈͍͔̹̑͗̎̅͛́Ǫ̵̹̻̝̳͂̌̌͘
️‍
cba

‍‍♂️
Ǫ̵̹̻̝̳͂̌̌͘G̴̻͈͍͔̹̑͗̎̅͛́L̠ͨͧͩ͘A̴̵̜̰͔ͫ͗͢Z͑ͫ̓ͪ̂ͫ̽͏̴̙̤̞͉͚̯̞̠͍
️‍

The output is displayed in a file instead of terminal is because most of the terminal will not render these characters properly.