flatten.rb

require 'minitest/autorun'

module Flatten
  def flatten_arr arr
    return error_msg unless arr.is_a? Array

    new_arr = []
    deep_flat(arr, new_arr)
  end

  def deep_flat arr, new_arr
    arr.map do |item|
      if item.is_a? Array
        deep_flat(item, new_arr)
      else
        new_arr << item
      end
    end
    new_arr
  end

  def error_msg
    return 'Array is required'
  end
end

class FlattenTest < Minitest::Test
  include Flatten

  def test_flatten_arr_with_no_array
    assert_equal 'Array is required', flatten_arr(1)
  end

  def test_flatten_arr_with_array_like_item
    arr = [1,2,[3],4]
    assert_equal [1,2,3,4], flatten_arr(arr)
  end

  def test_flatten_arr_with_simple_array
    arr = (1..4).to_a
    assert_equal [1,2,3,4], flatten_arr(arr)
  end

  def test_flatten_arr_with_nested_array
    arr = [[1,2,[3],4]]
    flat_arr = flatten_arr arr
    assert_equal [1, 2, 3, 4], flat_arr
  end

  def test_flatten_arr_with_more_nested_array
    arr = [[1,2,[[3]],4]]
    flat_arr = flatten_arr arr
    assert_equal [1, 2, 3, 4], flat_arr
  end
end