http://xkcd.com/710/

collatz.rb

#!/usr/bin/ruby

def collatz(n)
  raise "#{n} does not compute" if n < 1
  return n if n == 1
  if n.odd?
    n = 3 * n + 1
  elsif n.even?
    n = n/2
  end
  return n
end

x = ARGV[0].to_i
c = []

begin
  x = collatz(x)
  c << x
end until x == 1

puts c.join(","), "Steps = #{c.size}"