pub fn roman_to_int(string: String) -> i32 {
  let mut characters = string.chars().peekable();
  let mut number = 0;

  while let Some(character) = characters.next() {
    let value = match character {
      'I' => 1,
      'V' => 5,
      'X' => 10,
      'L' => 50,
      'C' => 100,
      'D' => 500,
      'M' => 1000,
      _ => panic!("Unknown character: {character}"),
    };

    if let Some(next) = characters.peek() {
      let subtract = match next {
        'V' | 'X' if character == 'I' => true,
        'L' | 'C' if character == 'X' => true,
        'D' | 'M' if character == 'C' => true,
        _ => false,
      };

      if subtract {
        number -= value;
        continue;
      }
    }

    number += value;
  }

  number
}

#[test]
fn test_roman_to_integer() {
  assert_eq!(roman_to_int("III".to_string()), 3);
  assert_eq!(roman_to_int("LVIII".to_string()), 58);
  assert_eq!(roman_to_int("MCMXCIV".to_string()), 1994);
  assert_eq!(roman_to_int("XX".to_string()), 20);
  assert_eq!(roman_to_int("DCXXI".to_string()), 621);
}