Recursively flatten an array

flattenable_array.rb

class FlattenableArray
  def initialize(array)
    @array = array
  end

  def flatten
    flat = []

    @array.each do |element|
      if element.is_a? Array
        flat.concat as_flattenable(element).flatten
      else
        flat << element
      end
    end

    flat
  end

  private

  def as_flattenable(element)
    self.class.new element
  end
end

describe FlattenableArray do

  it 'raises an exception when a nil array is given' do
    expect{FlattenableArray.new(nil)}.to raise_error 'You need to provide an array'
  end

  it 'returns an empty array when an empty array was given' do
    subject = FlattenableArray.new([])
    expect(subject.flatten).to eq([])
  end

  it 'returns the array if it has only one element' do
    subject = FlattenableArray.new([1])
    expect(subject.flatten).to eq([1])
  end

  it 'returns the array if it is already flat' do
    subject = FlattenableArray.new([1, 2, 5,12])
    expect(subject.flatten).to eq([1, 2, 5, 12])
  end

  it 'returns the first element if it is an array' do
    subject = FlattenableArray.new([[1]])
    expect(subject.flatten).to eq([1])
  end

  it 'returns the flattened array if two items are arrays with one element each' do
    subject = FlattenableArray.new([[1], [2]])
    expect(subject.flatten).to eq([1, 2])
  end

  it 'returns the flattened array if two items are arrays with more than one element each' do
    subject = FlattenableArray.new([[1, 3, 5, 20], [2]])
    expect(subject.flatten).to eq([1, 3, 5, 20, 2])
  end

  it 'returns the flattened array if an array is 3 levels deep' do
    subject = FlattenableArray.new([[1, 3, [45, 89], 20], [2]])
    expect(subject.flatten).to eq([1, 3, 45, 89, 20, 2])
  end

  it 'returns the flattened array if an array is 5 levels deep and has empty arrays within' do
    subject = FlattenableArray.new([[1, 2, [3, 4], 5], [6], [7, [8, 9, [10, [11, 12, 13, []]]]]])
    expect(subject.flatten).to eq([1,2,3,4,5,6,7,8,9,10,11,12,13])
  end

  it 'handles nil elements within the given array' do
    subject = FlattenableArray.new([[1, 3, 5, 20, nil], [2, nil], nil])
    expect(subject.flatten).to eq([1, 3, 5, 20, nil, 2, nil, nil])
  end

  it 'flattens the given example' do
    subject = FlattenableArray.new( [[1,2,[3]],4])
    expect(subject.flatten).to eq([1,2,3,4])
  end
end