Luhn algorithm (valid CC check)

Luhn algorithm

# Luhn algorithm to determine if a CC is valid

def valid(cc)
  return false unless cc.respond_to?(:to_i) && cc.to_i != 0
  doubled = []
  cc.reverse.each_char.each_with_index do |c,i|
    v = (i % 2 == 0) ? c.to_i : c.to_i * 2
    doubled << v
  end
  summed = doubled.map do |d|
    d.to_s.each_char.inject(0) { |sum, n| sum + n.to_i }
  end
  total = summed.inject(0) { |sum, n| sum + n }
  total % 10
end