edh.rb

#!/bin/ruby

##
# Take a credit card number and determine the type of card
#
# @param [String] card the card number
# @return [String] the card type.
#
def card_type(card)
  if card =~ /^3[47]\d+$/ && card.length == 15
    'AMEX'
  elsif card =~ /^6011\d+$/ && card.length == 16
    'Discover'
  elsif card =~ /^5[1-5]\d+$/ && card.length == 16
    'MasterCard'
  elsif card =~ /^4\d+$/ && [13, 16].include?(card.length)
    'VISA'
  else
    'Unknown'
  end
end

##
# Verify if a credit card number matches the Luhn algorithm.
#
# @param [String] card the card number
# @return [Boolean] true if the card verifies.
#
def luhn_verify?(card)
  total = 0

  card.reverse.each_char.with_index do |num,idx|
    total += if idx % 2 == 0
               num.to_i
             else
               n = num.to_i * 2
               n >= 10 ? n % 10 + 1 : n
             end
  end

  total % 10 == 0
end

#
# There isn't much information about input methods/validation so lets just have it loaded.
#
if __FILE__ == $0

  DATA.each_line do |card|
    card.gsub!(/\s/, '')
    puts "#{card_type card}: #{card} (#{luhn_verify?(card) ? 'valid' : 'invalid'})"
  end

end

__END__
4111111111111111
4111111111111
4012888888881881
378282246310005
6011111111111117
5105105105105100
5105 1051 0510 5106
9111111111111111