/// Maps real ticker symbols to opaque tokens shown in question text.
class SymbolObfuscator {
  SymbolObfuscator({this.prefix = 'ASSET_'});

  final String prefix;

  /// Stable token for [symbol] among [universe] sorted lexicographically.
  String tokenFor(String symbol, List<String> universe) {
    final List<String> sorted = List<String>.from(universe)..sort();
    final int index = sorted.indexOf(symbol);
    if (index < 0) {
      throw ArgumentError.value(symbol, 'symbol', 'not in universe');
    }
    return '$prefix${_letterForIndex(index)}';
  }

  static String _letterForIndex(int index) {
    if (index < 26) {
      return String.fromCharCode(65 + index);
    }
    return '${_letterForIndex(index ~/ 26 - 1)}${_letterForIndex(index % 26)}';
  }
}