```Here is my solution to Quiz 122:

<code>
class CardChecker
def initialize(card_num)
@card_num = card_num
@issuer = case
when visa? then 'VISA'
when mastercard? then 'MasterCard'
when amex? then 'AMEX'
when discover? then 'Discover'
else 'UNKNOWN'
end
@valid = valid?
end
def visa?
(@card_num.size == 13 || @card_num.size == 16) && @card_num
=~ /^4/
end
def mastercard?
@card_num.size == 16 && @card_num =~ /^5[1-5]/
end
def amex?
@card_num.size == 15 && @card_num =~ /^3[47]/
end
def discover?
@card_num.size == 16 && @card_num =~ /^6011/
end
def valid?
digits = @card_num.reverse.split('')
sum = 0
digits.each_with_index do |e, i|
d = e.to_i
if i & 1 == 0
sum += d
else
q, r = (d + d).divmod(10)
sum += q + r
end
end
sum % 10 == 0
end
def to_s
@issuer + (@valid ? " " : " IN") + "VALID"
end
end

if \$0 == __FILE__
puts CardChecker.new(ARGV.join)
end
</code>

This was a very easy (I like easy) but fun quiz. I believe this is
the first time I actually completed a quiz solution, including
testing, in less than an hour.

Regards, Morton

```