1#Each new term in the Fibonacci sequence is generated by adding the previous two terms. 
 2#By starting with 1 and 2, the first 10 terms will be:
 3
 4#1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
 5
 6#By considering the terms in the Fibonacci sequence whose values do not exceed 
 7#four million, find the sum of the even-valued terms.
 8describe "problem two" do
 9  def fib
10    Enumerator.new do |yielder|
11      x, y = 1, 2
12      loop do
13        yielder.yield x
14        tmp = x
15        x = y
16        y = tmp + y
17      end
18    end
19  end
20
21  def sum_of_first(n)
22    fib.take(n).inject(0) do |memo, x|
23      memo + x
24    end
25  end
26
27  it "computes the sum" do
28    result = sum_of_first(10)
29    expect(result).to eql([1, 2, 3, 5, 8, 13, 21, 34, 55, 89].inject(0) {|memo, x| memo + x })
30  end
31
32  it "accumulates" do
33    items = fib.take_while { |n| n < 4_000_000 }.find_all(&:even?)
34    result = items.inject(0) { |memo, x| memo + x }
35    expect(result).to eql(4613732)
36  end
37
38  it "can accumulates manually" do
39    total = 0
40    enumerator = fib
41    current = enumerator.next
42    loop do
43      break if current >= 4_000_000
44
45      total += current if current.even?
46      current = enumerator.next
47    end
48    expect(total).to eql(4613732)
49  end
50end