Map<String, String> reSwu = {
  'symbol': r'[\\U00040001-\\U0004FFFF]',
  'coord': r'[\\U0001D80C-\\U0001DFFF]{2}',
};

String swu2fsw(String swuText) {
  if (swuText.isEmpty) {
    return '';
  }

  // Initial replacements
  String fsw = swuText
      .replaceAll("𝠀", "A")
      .replaceAll("𝠁", "B")
      .replaceAll("𝠂", "L")
      .replaceAll("𝠃", "M")
      .replaceAll("𝠄", "R");

  // SWU symbols to FSW keys
  RegExp symbolRegex = RegExp(reSwu['symbol']!, unicode: true);
  Iterable<RegExpMatch> symbols = symbolRegex.allMatches(fsw);
  if (symbols.isNotEmpty) {
    for (RegExpMatch sym in symbols) {
      fsw = fsw.replaceAll(sym.group(0)!, swu2key(sym.group(0)!));
    }
  }

  // SWU coordinates to FSW coordinates
  RegExp coordRegex = RegExp(reSwu['coord']!, unicode: true);
  Iterable<Match> coords = coordRegex.allMatches(fsw);
  if (coords.isNotEmpty) {
    for (Match coord in coords) {
      fsw = fsw.replaceAll(
          coord.group(0)!, swu2coord(coord.group(0)!).toString());
    }
  }

  return fsw;
}

String swu2key(String swuSym) {
  int symcode = swuSym.runes.first - 0x40001;
  int base = symcode ~/ 96;
  int fill = (symcode - (base * 96)) ~/ 16;
  int rotation = symcode - (base * 96) - (fill * 16);
  return 'S${(base + 0x100).toRadixString(16)}${fill.toRadixString(16)}${rotation.toRadixString(16)}';
}

int swu2num(String swuNum) {
  return swuNum.runes.first - 0x1D80C + 250;
}

List<int> swu2coord(String swuCoord) {
  return [swu2num(swuCoord[0]), swu2num(swuCoord[1])];
}

void main() {
  String swu = '𝠃𝤟𝤩񋛩𝣵𝤐񀀒𝤇𝣤񋚥𝤐𝤆񀀚𝣮𝣭';
  String fsw = swu2fsw(swu);
  print(fsw);
}