euler_0002.rb

class Fibonacci < Enumerator::Lazy
  def initialize
    @past_numbers = []

    super(1..Float::INFINITY) do |yielder, i|
      if i == 1 || i == 2
        @past_numbers << i
        yielder << i
      else
        fib_value = @past_numbers[0] + @past_numbers[1]

        @past_numbers.shift
        @past_numbers << fib_value
        yielder << fib_value
      end
    end
  end
end

puts Fibonacci.new
  .take_while { |n| n < 4_000_000 }
  .select(&:even?)
  .inject(0) { |a, b| a + b }