swu2key function
Implementation
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)}';
}