fibonacci_aggregator.rb

require 'test/unit'
extend Test::Unit::Assertions

class FibonacciAggregator
 def self.sum_of_even_numbers(limit: 4_000_000)
    sum, lower, higher = 0, 1, 2

    while lower <= limit
      sum += lower if lower % 2 == 0
      lower, higher = higher, lower + higher
    end

    sum
  end
end

#tests
assert_equal(FibonacciAggregator.sum_of_even_numbers(limit: 5), 2)
assert_equal(FibonacciAggregator.sum_of_even_numbers(limit: 10), 2 + 8)
assert_equal(FibonacciAggregator.sum_of_even_numbers(limit: 100), 2 + 8 + 34)