My own implementation of Array#flatten

flatten.rb

module Flatten
  def flatten(array)
    result = []

    array.each do |element|
      if element.is_a? Array
        flatten(element).each { |inner_element| result.push(inner_element) }
      else
        result.push(element)
      end
    end

    result
  end
end

flatten_test.rb

require 'minitest/autorun'
require_relative 'flatten'

class TestFlatten < Minitest::Test
  include Flatten

  def test_with_already_flat_array
    array = [1, 2, 3, 4, 5]

    assert_equal array, flatten(array)
  end

  def test_with_nested_array
    array = [[1, 2, [3]], 4]

    assert_equal [1, 2, 3, 4], flatten(array)
  end

  def test_random_array_has_expected_length
    length = rand(16)

    array = random_nested_array(length)

    flattend = flatten(array)

    message = "#{array} was flattened into #{flattend} which is not exactly #{length} elements long!"

    assert_equal length, flattend.length, message
  end

  def test_random_array_is_flat
    array = random_nested_array(rand(16))

    flattend = flatten(array)

    is_flat = flattend.none? { |element| element.is_a? Array }

    assert is_flat, "#{array} was flattened into #{flattend} which is not flat!"
  end

  def random_nested_array(length)
    result = []

    current = result
    (1..length).each do |i|
      case rand(4)
      when 1
        temp = [i]
        current.push temp
        current = temp
      when 2
        current = result
        current.push i
      when 3
        current.push [i]
      else
        current.push i
      end
    end

    result
  end
end