Luhn test of credit card numbers

func luhn (n) {
    static a = {|j| (2*j // 10) + (2*j % 10) }.map(^10)

    var checksum = n.digits.map_kv {|i,j|
        i.is_odd ? a[j] : j
    }.sum

    checksum % 10 == 0
}

for n in [49927398716, 49927398717, 1234567812345678, 1234567812345670] {
    say [n, luhn(n)]
}

Output:

[49927398716, true]
[49927398717, false]
[1234567812345678, false]
[1234567812345670, true]