Even Fibonacci Numbers

fibonacci.rb

module Fibonacci

  class << self
    def sum_of_even_upto(max)
      upto(max).inject(0) do |sum, item| 
        item.even? ? sum + item : sum  
      end
    end

    def upto(max)
      generate.take_while { |item| item <= max }
    end

    private

    def generate
      Enumerator.new do |yielder|
        i = j = 1
        loop do
          yielder.yield i
          i, j = j, i + j
        end
      end
    end
  end

end

fibonacci_spec.rb

require "minitest/autorun"
class TestFibonacci < Minitest::Test

  describe '#sum_of_even_upto' do

    describe 'when given positive integer' do
      it 'returns a sum all even fibonacci numbers up to that integer' do
        Fibonacci.sum_of_even_upto(2).must_equal 2
        Fibonacci.sum_of_even_upto(4_000_000).must_equal 4_613_732
      end
    end

    describe 'when given zero' do
      it 'returns zero' do
        Fibonacci.sum_of_even_upto(0).must_equal 0
      end
    end

    describe 'when given negative integer' do
      it 'returns zero' do
        Fibonacci.sum_of_even_upto(-1).must_equal 0
      end
    end

    describe 'when given a string' do
      it 'raises ArgumentError' do
        proc { Fibonacci.sum_of_even_upto('1') }.must_raise ArgumentError
      end
    end

  end

end