cached fibonacci sequence generator in ruby

fib.rb

#! /usr/bin/env ruby

require 'forwardable'
require 'minitest/autorun'

class Fibonacci
  extend Forwardable

  def_delegator :generator, :take

  def initialize
    @cache = [1, 1]
  end

  def [](num)
    take(num + 1) unless @cache[num]
    @cache[num]
  end

  def generator
    Enumerator.new do |yielder|
      current_idx = 0

      loop do
        unless @cache[current_idx]
          @cache << @cache[-1] + @cache[-2]
        end

        yielder << @cache[current_idx]

        current_idx += 1
      end
    end
  end
  private :generator
end

if __FILE__ == $PROGRAM_NAME
  describe 'fibonacci sequence generator' do
    before do
      @fib = Fibonacci.new
    end

    it 'returns first item right' do
      @fib[0].must_equal 1
    end

    it 'returns second item right' do
      @fib[1].must_equal 1
    end

    it 'returns third item right' do
      @fib[2].must_equal 2
    end

    it 'returns fourth item right' do
      @fib[3].must_equal 3
    end

    it 'returns list right' do
      @fib.take(10).must_equal [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
    end
  end
end
# >> Run options: --seed 14468
# >>
# >> # Running tests:
# >>
# >> .....
# >>
# >> Finished tests in 0.000463s, 10804.3399 tests/s, 10804.3399 assertions/s.
# >>
# >> 5 tests, 5 assertions, 0 failures, 0 errors, 0 skips