exercism 0.1.9

My exercism
Documentation 
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/// Check a Luhn checksum.
pub fn is_valid(code: &str) -> bool {
    code.chars()
        .rev()
        .filter(|c| !c.is_whitespace())
        .try_fold((0, 0), |(sum, count), val| {
            val.to_digit(10)
                .map(|num| if count % 2 == 1 { num * 2 } else { num })
                .map(|num| if num > 9 { num - 9 } else { num })
                .map(|num| (num + sum, count + 1))
        }).map_or(false, |(sum, count)| sum % 10 == 0 && count > 1)
}