leetcode/source/integer_to_roman/mod.rs

pub fn int_to_roman(number: i32) -> String {
  let mut result = String::new();

  for (index, digit) in number.to_string().chars().rev().enumerate() {
    if digit == '0' {
      continue;
    }

    let digit = digit.to_digit(10).unwrap() as usize;
    let magnitude = 10_usize.pow(index as u32);

    let (unit_1, unit_5, unit_10) = match magnitude {
      1 => ("I", "V", "X"),
      10 => ("X", "L", "C"),
      100 => ("C", "D", "M"),
      1000 => ("M", "", ""),
      _ => unreachable!(),
    };

    result += &match digit {
      1..=3 => unit_1.repeat(digit),
      4 => format!("{unit_5}{unit_1}"),
      5 => unit_5.to_string(),
      6..=8 => format!("{}{unit_5}", unit_1.repeat(digit - 5)),
      9 => format!("{unit_10}{unit_1}"),
      _ => continue,
    }
  }

  result.chars().rev().collect()
}