Recursive Fibonacci with Ruby Hash

fib_hash.rb

fib = Hash.new do |hash, key|
  if key < 2
    hash[key] = key
  else
    hash[key] = hash[key-2] + hash[key-1]
  end
end
#=> {}

fib[3]
#=> 2
fib
#=> {1=>1, 0=>0, 2=>1, 3=>2}