toy implementation of damm algorithm

damm.rb

# https://en.wikipedia.org/wiki/Damm_algorithm

module Damm
  extend self

  OP_TABLE = [
    [0, 3, 1, 7, 5, 9, 8, 6, 4, 2],
    [7, 0, 9, 2, 1, 5, 4, 8, 6, 3],
    [4, 2, 0, 6, 8, 7, 1, 3, 5, 9],
    [1, 7, 5, 0, 9, 8, 3, 4, 2, 6],
    [6, 1, 2, 3, 0, 4, 5, 9, 7, 8],
    [3, 6, 7, 4, 2, 0, 9, 5, 8, 1],
    [5, 8, 6, 9, 7, 2, 0, 1, 3, 4],
    [8, 9, 4, 5, 3, 6, 2, 0, 1, 7],
    [9, 4, 3, 8, 6, 1, 7, 2, 0, 5],
    [2, 5, 8, 1, 4, 3, 6, 7, 9, 0],
  ]

  def check_digit(num)
    num.to_s.chars.map(&:to_i).inject(0) {|c,i|
      OP_TABLE[c][i]
    }
  end

  def verify(num)
    check_digit(num) == 0
  end

  def encode(num)
    num * 10 + check_digit(num)
  end

  def decode(num)
    if verify(num)
      num / 10
    end
  end

end

p Damm.check_digit(572)
p Damm.verify(5724)
p Damm.encode(572)
p(1000.times.to_a.all? {
  n = rand(100000000)
  Damm.decode(Damm.encode(n)) == n
})