chain.rb

#!/usr/bin/env ruby

# Solve a Joel problem

def chain k
  s, c, l = k, 0, [] # start, carry, digit list
  begin
    l.unshift k
    d = 2 * k + c
    c, k = d / 10, d % 10
  end until k == s and c == 0
  l
end

(1..9).each {|k| puts chain(k).join}