fibbonacci_sequence.rb

class Seq

  def self.unfold(*initial_state, &producer)
    self.new(*initial_state, &producer)
  end
	
  def take_while(&condition)
    current, state = @producer.call(@initial_state)
    seq = [current]
    while condition.call(current) do
      current, state = @producer.call(state)
      seq << current
    end
    seq
  end

  private
	
  def initialize(*initial_state, &producer)
    @initial_state = initial_state
    @producer = producer
  end
	
end

limit = 10**999
fibonacci = Seq.unfold(0, 1) { |current, upcoming| [current, [upcoming, current + upcoming]] }
term = fibonacci.take_while{ |n| n < limit }.size

puts term