class CustomArray

  def flatten(arr, full = [])
    arr.each do |i|
      if i.is_a? Array
        flatten(i, full)
      else
        full << i
      end
    end
    full
  end
end


require 'rspec'

RSpec.describe CustomArray do

  before(:each) {
    @arr = CustomArray.new
  }

  context "flatten" do
    it "empty array" do
      flattened = @arr.flatten([])
      expect(flattened).to eq([])
    end

    it "1 level array" do
      flattened = @arr.flatten([1, 2, 3, 4, 5])
      expect(flattened).to eq([1, 2, 3, 4, 5])
    end

    it "2 level nested array" do
      flattened = @arr.flatten([1, 2, [3, 4], 5])
      expect(flattened).to eq([1, 2, 3, 4, 5])
    end

    it "3 level nested array" do
      flattened = @arr.flatten([1, [2, [3, 4], 5]])
      expect(flattened).to eq([1, 2, 3, 4, 5])
    end

  end
end