class Luhn

# File lib/luhn.rb, line 3
def self.checksum(number)
  products = luhn_doubled(number)
  sum = products.inject(0) { |t,p| t + sum_of(p) }
  checksum = 10 - (sum % 10)
  checksum == 10 ? 0 : checksum
end

# File lib/luhn.rb, line 10
def self.luhn_doubled(number)
  numbers = split_digits(number).reverse
  numbers.map.with_index do |n,i|
    i.even? ? n*2 : n*1
  end.reverse
end

# File lib/luhn.rb, line 26
def self.split_digits(number)
  number.to_s.split(//).map(&:to_i)
end

# File lib/luhn.rb, line 17
def self.sum_of(number)
  split_digits(number).inject(:+)
end

# File lib/luhn.rb, line 21
def self.valid?(number)
  numbers = split_digits(number)
  numbers.pop == checksum(numbers.join)
end