Even Fibonacci Numbers

fibonacci_sequence.rb

class FibonacciSequence
  DEFAULT_MAX = 4_000_000
  DEFAULT_FILTER_BY = :even?

  def initialize(max: DEFAULT_MAX, filter_by: DEFAULT_FILTER_BY)
    @max = max
    @filter_by = filter_by

    validate_arguments

    @enumerator = build_enumerator
  end

  def sum
    to_a.reduce(&:+)
  end

  def to_a
    @enumerator.to_a
  end

  private

  def validate_arguments
    raise(ArgumentError, "max must be a Numeric.") unless @max.is_a?(Numeric)
    unless @filter_by.is_a?(String) || @filter_by.is_a?(Symbol)
      raise(ArgumentError, "filter_by must be a String or a Symbol.")
    end
  end

  def build_enumerator
    Enumerator.new do |yielder|
      previous = 1
      following = 2
      yielder << previous if previous.send(@filter_by)
      yielder << following if following.send(@filter_by)
      loop do
        previous, following = following, previous + following
        next unless following.send(@filter_by)
        break if following > @max
        yielder << following
      end
    end
  end
end