1#A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
 2
 3#Find the largest palindrome made from the product of two 3-digit numbers.
 4describe 'problem four' do
 5  class Palindrome
 6    def largest(digits:1)
 7      min = ([1]  + ([0] * (digits - 1))).flatten.join('').to_i
 8      max = ([9] * digits).flatten.join('').to_i
 9      puts [min, max].inspect
10      results = []
11
12      max.downto(min) do |m|
13        max.downto(min) do |n|
14          result = m * n
15          results.push(result) if palindrome?(result)
16        end
17      end
18      results.max
19    end
20
21    private
22
23    def palindrome?(number)
24      number.to_s == number.to_s.reverse
25    end
26  end
27
28  subject { Palindrome.new }
29
30  it 'can find the largest palindrome that is the product of two one digit numbers' do
31    expect(subject.largest(digits: 1)).to eql(9)
32  end
33
34  it 'can find the largest palindrome that is the product of two digits' do
35    expect(subject.largest(digits: 2)).to eql(9009)
36  end
37
38  it 'can find the largest palindrome that is the product of two three digit numbers' do
39    expect(subject.largest(digits: 3)).to eql(906609)
40  end
41end