pub fn solve(puzzle: String) -> u64 {
    let mut iters: Vec<_> = puzzle
        .lines()
        .map(|line| line.split_whitespace())
        .collect();

    std::iter::from_fn(move || {
        let column: Option<Vec<&str>> = iters
            .iter_mut()
            .map(|it| it.next())
            .collect::<Option<Vec<_>>>();

        let column = column?;

        Some((
            column[..column.len() - 1].iter().map(|v| v.parse::<u64>().unwrap()).collect::<Vec<_>>(),
            *column.last().unwrap(),
        ))
    }).map(|(vals, op)| {
        match op {
           "+" => vals.iter().sum::<u64>(),
           "*" => vals.iter().product::<u64>(),
           _ => unreachable!(),
       }
    }).sum::<u64>()
}