flatten-array.rb

# This class provides a collection of class methods to manipulate Arrays
class ArrayUtils
  class << self
    # Recursively flattens its arbitrarily nested array arguement
    # Warning: passing in very big arrays WILL BLOW UP THE STACK!
    def flatten(arr)
      _flatten(arr, [])
    end

    private

    def _flatten(arr, res)
      arr.each do |e|
        if e.is_a?(Array)
          _flatten(e, res)
        else
          res << e
        end
      end

      res
    end
  end
end

require "minitest/autorun"
require "minitest/pride"

class ArrayUtilsTest < Minitest::Test
  def test_flattens_a_simply_nested_array
    res = ArrayUtils.flatten([1, [2], [3, 4]])
    assert_equal [1, 2, 3, 4], res
  end

  def test_flattens_a_deeply_nested_array
    res = ArrayUtils.flatten([1, [2, [3], [[4]]]])
    assert_equal [1, 2, 3, 4], res
  end
end