Flatten implementation in ruby without using it own method.

flattener.rb

require 'rspec'

class Flattener
  def initialize(array)
    raise 'An Array must be passed' unless array.kind_of? Array
    @array = array
  end

  def flatten
    do_flatten(@array)
  end

  private

  def do_flatten(current_array)
    acc = []
    current_array.each do |element|
      if element.kind_of? Array
        acc += do_flatten(element)
      else
        acc << element
      end
    end
    acc
  end
end

RSpec.describe Flattener do
  it 'flattens [1, 3, 3]' do
    expect(Flattener.new([1, 3, 4]).flatten).to eq [1, 3, 4]
  end

  it 'flattens [1, [2], 3]' do
    expect(Flattener.new([1, [2], 3]).flatten).to eq [1, 2, 3]
  end

  it 'flattens [1, 2, [ 3, 4, [5]]], 7]' do
    numbers = [1, 2, [ 3, 4, [5]], 7]
    expect(Flattener.new(numbers).flatten).to eq [1, 2, 3, 4, 5, 7]
  end

  it 'checks if array was passed' do
    expect {
      Flattener.new(double)
    }.to raise_error('An Array must be passed')
  end
end