Ruby Array Flattener

flattener.rb

require 'rspec'

# class used to flatten arrays
# Input:
# @param [Array] arr: the array that needs flattening
# Output when flatten method is invoked:
# @param [Array] flattened array
class Flattener
  def initialize(arr)
    raise 'Pass an array' unless arr.kind_of? Array
    @arr = arr
  end

  def flatten
    get_flattened_array @arr
  end

  private

  def get_flattened_array arr
    arr.reduce([]) do |result, item| 
      if item.is_a? Array
        result + get_flattened_array(item)
      else
        result << item
      end
    end
  end
end

flattener_spec.rb

require './spec_helper'
require './flattener'

RSpec.describe Flattener do
  context 'valid input' do
    TEST_CASES = [
      [1, 2, 3],
      [],
      [1, 2, 3, [1, 2, 3]],
      [1, [1], [[1]], [[2], [3]]]
    ]
    
    TEST_CASES.each do |test_case|
      #context needed otherwise it can retain values from other context and we get random failures
      context "#{test_case}" do
        subject { Flattener.new(test_case) }

        it "returns the right result for #{test_case}" do
          expect(subject.flatten).to eq(test_case.flatten)
        end
      end
    end
  end

  context 'invalid input' do
    subject { Flattener.new('something') }

    it 'checks if array was passed' do
      expect {
        subject.flatten
      }.to raise_error('Pass an array')
    end
  end
end

Gemfile

source 'https://rubygems.org'

gem 'rspec', '~> 3.4'

spec_helper.rb

require 'rspec'