Using icu from Dart/Flutter

nfkc.dart

import 'dart:ffi';
import 'dart:io';

import 'package:ffi/ffi.dart';
import 'package:unorm_dart/unorm_dart.dart' as unorm_dart; // fallback

String _getLibicuModuleName() {
  if (Platform.isAndroid) {
    return 'libicu.so';
  } else if (Platform.isWindows) {
    return 'icuuc.dll';
  }
  throw Exception('Unsupported platform');
}

final _libicu = DynamicLibrary.open(_getLibicuModuleName());
final _unorm2_getNFKCInstance = _libicu
    .lookupFunction<Pointer Function(Pointer<Uint32>), Pointer Function(Pointer<Uint32>)>('unorm2_getNFKCInstance');

final _unorm2_normalize = _libicu.lookupFunction<
    Int32 Function(Pointer, Pointer<Utf16>, Int32, Pointer<Utf16>, Int32, Pointer<Uint32>),
    int Function(Pointer, Pointer<Utf16>, int, Pointer<Utf16>, int, Pointer<Uint32>)>('unorm2_normalize');

Pointer _getNFKCInstance() {
  return using((arena) {
    final err = arena.allocate<Uint32>(4);
    err.value = 0;
    final unorm = _unorm2_getNFKCInstance(err);
    if (err.value > 0 || unorm.address == 0) {
      throw Exception('unorm2_getNFKCInstance failed: ${err.value}');
    }
    return unorm;
  });
}

bool? _haveIcu;
Pointer _unorm = Pointer.fromAddress(0);

String nfkc(String str) {
  return using((arena) {
    final err = arena.allocate<Uint32>(4);
    try {
      if (_haveIcu == null) {
        _unorm = _getNFKCInstance();
        _haveIcu = true;
      }
    } catch (e) {
      _haveIcu = false;
    }

    if (_haveIcu == false) {
      return unorm_dart.nfkc(str);
    }

    final src = str.toNativeUtf16(allocator: arena);

    err.value = 0;
    final destLength = _unorm2_normalize(_unorm, src, -1, Pointer.fromAddress(0), 0, err);

    // https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utypes_8h_source.html#l00717
    const U_BUFFER_OVERFLOW_ERROR = 15;
    if (err.value > 0 && err.value != U_BUFFER_OVERFLOW_ERROR) {
      throw Exception('unorm2_normalize failed: ${err.value}');
    }
    final dest = arena.allocate<Utf16>((destLength + 1) * 2);
    err.value = 0;
    _unorm2_normalize(_unorm, src, -1, dest, destLength + 1, err);
    if (err.value > 0 && err.value != U_BUFFER_OVERFLOW_ERROR) {
      throw Exception('unorm2_normalize failed: ${err.value}');
    }

    return dest.toDartString();
  });
}