pub fn to_pascal_case(s: &str) -> String {
    // we get the first item later so this is to make it impossible for that
    // to error
    if s.is_empty() {
        return String::new();
    }

    let mut result = String::new();
    let mut prev_was_underscore = true; // set to true by default so the first character is capitalized
    if s.chars().next().unwrap().is_numeric() {
        result.push('_');
    }
    for c in s.chars() {
        if c == '_' {
            prev_was_underscore = true;
        } else if prev_was_underscore {
            result.push(c.to_ascii_uppercase());
            prev_was_underscore = false;
        } else {
            result.push(c);
        }
    }
    result
}

pub fn to_snake_case(s: &str) -> String {
    let mut result = String::new();
    let mut prev_was_uppercase = true;
    for c in s.chars() {
        if c.is_ascii_uppercase() {
            if !prev_was_uppercase {
                result.push('_');
            }
            result.push(c.to_ascii_lowercase());
            prev_was_uppercase = true;
        } else {
            result.push(c);
            prev_was_uppercase = false;
        }
    }
    result
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_snake_case() {
        assert_eq!(to_snake_case("HelloWorld"), "hello_world");
        assert_eq!(to_snake_case("helloWorld"), "hello_world");
        assert_eq!(to_snake_case("hello_world"), "hello_world");
    }
}