My solution to Roman Numeral Kata code challenge

roman_numeral_kata.dart

void main(List<String> arguments) {
  print("TOTAL ==== ${convert("MCMXLIV")}"); //Expect 1944
}

final romanRepresentation = {
  'I': 1,
  'V': 5,
  'X': 10,
  'L': 50,
  'C': 100,
  'D': 500,
  'M': 1000,
};

int convert(String figure) {
  figure = figure.toUpperCase();

  final retainedList = <int>[];

  final figureList = figure.split("");

  for (int i = 0; i < figureList.length; i++) {
    String? nextRoman = _hasNoNext(i, figureList) ? null : figureList[i + 1];
    String currentRoman = figureList[i];

    if (nextRoman == null) {
      retainedList.add(romanRepresentation[currentRoman]!);
      break;
    }

    if (isLesser(currentRoman, nextRoman)) {
      final retainValue =
          romanRepresentation[nextRoman]! - romanRepresentation[currentRoman]!;
      retainedList.add(retainValue);

      /// Skip to next
      i = i + 1;
    } else {
      retainedList.add(romanRepresentation[currentRoman]!);
    }
  }

  int total = retainedList.fold<int>(
      0, (previousValue, element) => previousValue + element);

  return total;
}

bool _hasNoNext(int i, List<String> figureList) => i + 1 == figureList.length;

bool isLesser(String current, String target) {
  return romanRepresentation[current]! < romanRepresentation[target]!;
}