rustで3桁区切りなど、文字列に区切り文字を入れる

add_separator.rs

#![allow(dead_code)]

fn add_separator(txt: &str, n: usize, separator: &str) -> String {
    txt.chars()
        .rev()
        .collect::<Vec<char>>()
        .chunks(n)
        .map(|cs| cs.iter().collect::<String>())
        .collect::<Vec<String>>()
        .join(separator)
        .chars()
        .rev()
        .collect::<String>()
}

#[cfg(test)]
mod tests {
    use super::add_separator;
    #[test]
    fn add_separator_test() {
        assert_eq!(add_separator("1234567890", 3, ","), "1,234,567,890");
        assert_eq!(
            add_separator("10110111101111011110110110101100", 8, " "),
            "10110111 10111101 11101101 10101100"
        )
    }
}