array.rb

class Array
  # require and extend MiniTest for test_custom_flatten method
  require 'test/unit/assertions'
  extend Test::Unit::Assertions

  # custom_flatten flattens an n-dimensional array and
  # returns a single dimensional array.
  #
  # custom_flatten only works with arrays
  # where each element is either an Integer or a
  # (nested) Array.
  #
  # Examples:
  # [-1,2,[-3],4].custom_flatten        => [-1,2,-3,4]
  # [[[[1,[2],[3]],4]]].custom_flatten  => [1,2,3,4]
  def custom_flatten(array = self, result = [])
    array.each do |el|
      if el.is_a? Array
        # recursively call function if 'el' is a nested Array
        custom_flatten(el, result)
      elsif el.is_a? Integer
        # 'el' is an Integer and is added to the result Array
        result.push(el)
      else
        # raise ArgumentError because 'el' is not of type Array nor Integer
        raise ArgumentError, 'Invalid params'
      end
    end

    result
  end

  # test_custom_flatten tests the custom_flatten function of the Array class
  def self.test_custom_flatten
    # test that return value is an Array
    assert_instance_of(Array, [1,2,3].custom_flatten)

    # test that empty Array returns empty Array
    assert_equal([], [].custom_flatten)

    # test one-dimensional Array
    assert_equal([1,2,3], [1,2,3].custom_flatten)

    # test one-dimensional Array with nested empty Array
    assert_equal([1,2,3], [1,[],2,3].custom_flatten)

    # test two-dimensional Array
    assert_equal([1,2,3,4], [1,2,[3],4].custom_flatten)

    # test two-dimensional Array with negative integers
    assert_equal([-1,2,-3,4], [-1,2,[-3],4].custom_flatten)

    # test n-dimensional Array
    assert_equal([1,2,3,4], [[[[1,[2],[3]],4]]].custom_flatten)

    # test that ArgumentError is raised if Array contains
    # something else than Integers or Arrays
    assert_raise ArgumentError do
      [1,2,3,'abc'].custom_flatten
    end

    assert_raise ArgumentError do
      [1,2,3,4.5].custom_flatten
    end
  end
end