Even Fibonacci Numbers

# "Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
# 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
# By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms."

# Instructions: 
# Construct a solution using Ruby that could be incorporated into a larger project. Solutions will be evaluated for readability, testability, and extensibility.

# Calculate sum of even-valued terms in fibonacci
class Fibonacci
  attr_accessor :limit

  def initialize(limit = 4_000_000)
    @limit = limit
  end

  def even_sum
    @a = 1
    @b = 2

    sum = 0

    loop do
      sum += @b if @b.even?
      advance
      
      break if @b > limit
    end

    sum
  end

  private
  
    def advance
      @a, @b = @b, @a + @b
    end
end

puts Fibonacci.new.even_sum