1require "spec_helper"
 2
 3describe "problem six" do
 4  #Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.
 5
 6  class NaturalNumbers
 7    include Enumerable
 8    attr_reader :limit
 9
10    def initialize(limit)
11      @limit = limit
12    end
13
14    def each
15      0.upto(limit) do |n|
16        yield n
17      end
18    end
19
20    def sum_of_squares
21      sum { |n| n*n }
22    end
23
24    def square_of_the_sum
25      result = sum
26      result * result
27    end
28
29    def difference
30      square_of_the_sum - sum_of_squares
31    end
32
33    private
34
35    def sum
36      inject(0) do |memo, n|
37        memo += block_given? ? yield(n) : n
38      end
39    end
40  end
41
42  it "returns the sum of first 10 natural numbers" do
43    subject = NaturalNumbers.new(10)
44    expect(subject.sum_of_squares()).to eql(385)
45  end
46
47  it "returns the square of the sum of the first 10 natural numbers" do
48    subject = NaturalNumbers.new(10)
49    expect(subject.square_of_the_sum()).to eql(3025)
50  end
51
52  it "returns the different between the sum of the squares and the square of the sum" do
53    subject = NaturalNumbers.new(10)
54    expect(subject.difference).to eql(2640)
55  end
56
57  it "solves the problem" do
58    subject = NaturalNumbers.new(100)
59    expect(subject.difference).to eql(25164150)
60  end
61end