fib.rb

def time_taken
  start_time = Time.now
  yield
  Time.now - start_time
end

def fib(n)
  if n < 2
    n
  else
   fib(n - 1) + fib(n - 2)
  end
end

#puts time_taken { puts fib(1000) } # too slow

@cache = []
def fast_fib(n)
  if n < 2
    n
  elsif @cache[n]
    @cache[n]
  else
    @cache[n] = fast_fib(n-1) + fast_fib(n-2)
  end
end

puts time_taken { puts fast_fib(1000) }

class LazyFib
  include Enumerable
  
  def initialize
    @lazy_fib = Fiber.new do
      x,y = 0,1
      loop do
        Fiber.yield y
        x,y = y,x+y
      end
    end
  end
  
  def each
    loop { yield @lazy_fib.resume }
  end
end

puts time_taken {  puts LazyFib.new.take(1000).last }