Luhn's algorithm (calculates the last digit of a credit card number)

luhn.rb

def luhn_calc(num)
    sum = 0
    num.to_s.gsub(/\s/, "").each_char.to_a.reverse.each_with_index do |d,i|
        multiplier = 2 - (i&1)
        sum += (multiplier * d.to_i).divmod(10).reduce(:+)
    end
    sum * 9 % 10
end