#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.
describe "problem two" do
  def fib
    Enumerator.new do |yielder|
      x, y = 1, 2
      loop do
        yielder.yield x
        tmp = x
        x = y
        y = tmp + y
      end
    end
  end

  def sum_of_first(n)
    fib.take(n).inject(0) do |memo, x|
      memo + x
    end
  end

  it "computes the sum" do
    result = sum_of_first(10)
    expect(result).to eql([1, 2, 3, 5, 8, 13, 21, 34, 55, 89].inject(0) {|memo, x| memo + x })
  end

  it "accumulates" do
    items = fib.take_while { |n| n < 4_000_000 }.find_all(&:even?)
    result = items.inject(0) { |memo, x| memo + x }
    expect(result).to eql(4613732)
  end

  it "can accumulates manually" do
    total = 0
    enumerator = fib
    current = enumerator.next
    loop do
      break if current >= 4_000_000

      total += current if current.even?
      current = enumerator.next
    end
    expect(total).to eql(4613732)
  end
end