flatten an array of arbitrarily nested arrays of integers into a flat array of integers

array_extensions.rb

require 'minitest/autorun'

class CustomFlattenTest < MiniTest::Test
  def test_custom_flatten
    assert_equal [1,2,3,4], [[1,2,[3]],4].custom_flatten
  end
end

module ArrayExtensions
  def custom_flatten
    inject(Array.new) do |flat_array, element|
      flat_array + Array(element.is_a?(Array) ? element.custom_flatten : element)
    end
  end
end

Array.send :include, ArrayExtensions