card.rb

$ cat card.rb
# Set your card number here. 16 digits.
card_no = '1234123412341234'
reverse_card_no = card_no.reverse

index = 1
total = 0
reverse_card_no.each_char do |c|
  multi = (index % 2 == 1) ? 1 : 2

  sub_total = c.to_i * multi
  sub_total -= 9 if sub_total >= 10
  total += sub_total
  index += 1
end

puts "total: #{total}"
if total % 10 == 0
  puts "Valid card no."
else
  puts "Invalid card no."
end