#[derive(Debug)] struct User { name:Vec<u8>, age:i64 }
fn parse_age(s:&[u8])->Option<i64> { let n=std::str::from_utf8(s).ok()?.parse().ok()?; if (0..=150).contains(&n) {Some(n)} else {None} }

fn load(text: &[u8]) -> Result<Vec<User>, Vec<u8>> {
    let mut users = Vec::new();
    for (number, line) in text.split(|&b| b == b'\n').enumerate() {
        if !line.is_empty() { users.push(parse_record(line, number + 1)?); }
    }
    Ok(users)
}
fn parse_record(line: &[u8], lineno: usize) -> Result<User, Vec<u8>> {
    let comma = line.iter().position(|&b| b == b',')
        .ok_or_else(|| format!("line {lineno}: expected name,age").into_bytes())?;
    let age = parse_age(&line[comma + 1..])
        .ok_or_else(|| format!("line {lineno}: bad age").into_bytes())?;
    Ok(User { name: line[..comma].trim_ascii().to_vec(), age })
}

fn main() {
    for input in [b"ada,36\ngrace,45".as_slice(),b"ada,36\nbroken",b"ada,invalid"] {
        match load(input) {
            Ok(users) => { assert_eq!((&users[0].name,users[0].age),(&b"ada".to_vec(),36)); println!("loaded {} users",users.len()); }
            Err(error) => { use std::io::Write;let mut out=std::io::stdout().lock();out.write_all(b"error: ").unwrap();out.write_all(&error).unwrap();out.write_all(b"\n").unwrap(); }
        }
    }
}