flattener.rb

# A simple Ruby class providing an alternative implementation
# of the *Array#flatten method*
#
# Author:: Svetoslav Blyahov

class Flattener
  # The initializer method needed for the alternative flatten prcedure
  #
  # @param [Array] arr
  def initialize(arr)
    @arr = arr
  end

  # The alternative to Array#flatten method
  # that's performing flattening to arrays provided to this class' initializer
  #
  # @return [Array]
  def flatten
    return [] if @arr.nil?
    @flattened ||= []
    @arr.map do |e|
      @flattened.push(e) && next if e.nil? || Array(e) != e
      @flattened += self.class.new(e).flatten
    end

    @flattened
  end
end

flattener_spec.rb

require 'rspec'
require_relative 'flattener.rb'

describe Flattener do
  describe '#flatten' do
    subject { Flattener.new(preset).flatten }
    let(:flatted_array) { preset.to_a.flatten }

    context 'when empty array' do
      let(:preset) { [] }

      it { is_expected.to eq flatted_array }
    end

    context 'when `nil` passed' do
      let(:preset) { nil }

      it { is_expected.to eq flatted_array }
    end

    context 'when already a flat array' do
      let(:preset) { [1, 2, 3, 4] }

      it { is_expected.to eq flatted_array }
    end

    context 'when a "flattable" array' do
      context 'and the array is heterogeneous' do
        let(:preset) { [1, 2, 3, [4, 5, [8, 7]]]}

        it { is_expected.to eq flatted_array }
      end

      context 'and the array is homogeneous' do
        let(:preset) { [[1, 2], [3], [4, 5], [8, 7, 9]]}

        it { is_expected.to eq flatted_array }
      end
    end
  end
end