adcousin

class Luhn
  def self.checksum(number)
    digits = number.to_s.reverse.scan(/\d/).map { |x| x.to_i }
    digits = digits.each_with_index.map { |d, i|
      d *= 2 if i.even?
      d > 9 ? d - 9 : d
    }
    sum = digits.inject(0) { |m, x| m + x }
    mod = 10 - sum % 10
    mod==10 ? 0 : mod

adcousin

class Sudoku
  SIZE = 9
  NUMBERS = (1..9).to_a

  def initialize
    @board = Array.new(SIZE) { Array.new(SIZE, nil) }
  end

  def [](x, y)
    @board[y][x]