func perfect_shuffle(deck) {
     deck/2 -> zip.flat
}

[8, 24, 52, 100, 1020, 1024, 10000].each { |size|
    var deck = @(1..size)
    var shuffled = deck

    var n = (1..Inf -> lazy.first {
        (shuffled = perfect_shuffle(shuffled)) == deck
    })

    printf("%5d cards: %4d\n", size, n)
}