Ruby manual flatten method

aux.rb

# frozen_string_literal: true

##
# This class is an auxiliary class with helpful methods useful for common and necessary operations.
require 'rspec/autorun'
class Aux
  # Helper method to flatten deeply nested arrays
  # Input can either be a deeply nested array, or even an element.
  # @return [Array] of elements
  def self.flatten(input, result = [])
    if input.is_a? Array
      input.each do |element|
        Aux.flatten(element, result)
      end
    else
      result << input
    end
    result
  end
end

# In order to use rspec, you must install the gem rspec first, run: gem install rspec
describe Aux, '.flatten' do
  it 'should return a flat array' do
    unflattened_array = [[1, 2, [3]], 4]
    flattened_array = Aux.flatten(unflattened_array)
    expect(flattened_array.length).to equal(4)
  end
  it "should return an array of length 1 if it's a single element" do
    unflattened_array = 3
    flattened_array = Aux.flatten(unflattened_array)
    expect(flattened_array.length).to equal(1)
    expect(flattened_array).to be_instance_of(Array)
  end
end