fibo = ->(n) { Enumerator.new { |y, a=[0,1]| loop { y << a[-1]; a << a[-2..-1].reduce(:+) } }.take n } 
fibo[10]

def fibonacci
  # Enumerator.new { |y, a=1, b=1| loop { y << a; a, b = b, a + b } }
  Enumerator.new { |y, a=[0,1]| loop { y << a[-1]; a << a[-2..-1].reduce(:+) } }
end

fibonacci.lazy.select(&:even?).first 10

# using Hash
fib = Hash.new {|h, k| h[k] = k < 3 ? 1 : h[k-1] + h[k-2]}

# https://blog.arkency.com/2014/07/hidden-features-of-ruby-you-may-dont-know-about/
fibbonacci = Hash.new do |accumulator, index|
  accumulator[index] = fibbonacci[index - 2] + fibbonacci[index - 1]
end.update(0 => 0, 1 => 1)