Ruby implementation of Array#flatten

flatten.rb

class Array
  
  ## Returns a new flattened array
  def custom_flatten
    new_array = []

    # Recursively iterate over sub-arrays and other
    # elements and concat them to the new array
    self.each do |obj|
      if obj.is_a?(Array)
        new_array.concat(obj.custom_flatten)
      else
        new_array.push(obj)
      end
    end

    return new_array
  end

  
  ## Mutator method that flattens and updates
  ## an existing array
  def custom_flatten!
    self.replace(custom_flatten)
  end
  
end

flatten_test.rb

#!/usr/bin/env ruby

require 'test/unit'
require './flatten'


class TestFlatten < Test::Unit::TestCase
  def test_if_the_flatten_methods_exist
    array_methods = Array.new.methods

    assert array_methods.member?(:custom_flatten)
    assert array_methods.member?(:custom_flatten!)
  end

  def test_that_it_flattens_arrays_correctly
    test_cases = [
      [],
      [1],
      [nil],
      [1,[]],
      [1,2,3],
      [1,2,[3,4]],
      [[1,2,[3]],4],
      [1,2,[3,[4,5],6]],
      [1,2,3,[4,[5,[]],[6]]],
      [1,[2,[3,[4,[5,[6,[7]]]]]]]
    ]

    test_cases.each do |arr|
      assert_equal(arr.flatten, arr.custom_flatten)
    end
  end

  def test_that_custom_flatten_does_not_change_the_original_array
    array = [1, 2, 3, [4, 5]]
    array.custom_flatten

    assert_equal(array, [1, 2, 3, [4, 5]])
  end

  def test_that_custom_flatten_bang_updates_the_original_array
    array = [1, 2, 3, [4, 5]]
    array.custom_flatten!

    assert_equal(array, [1, 2, 3, 4, 5])
  end
end