Custom Ruby refinement to demonstrate flattening of an Array with using Array#flatten

FLATTEN-REFINEMENT.md

FlattenRefinement

A Ruby refinement to demonstrate flattening of a nested array without using Array#flatten. Flattening logic is identical to my original implementation done as a Ruby module.

Exposes Array#custom_flatten when used as a refinement; one advantage to using a refinement is not having to check for non-array argument types. I'd be less inclined to use refinements in a production app given they haven't gained a lot of traction in the Ruby community, but I believe they're good for experimentation.

Usage

using FlattenRefinement

[1, [2, [3]]].custom_flatten
=> [1, 2, 3]

Tests

Code was developed using Ruby 2.3.1. To run tests, run bundle install and bundle exec ruby flatten_refinement_test.rb

flatten_refinement.rb

module FlattenRefinement
  refine Array do
    # Returns new one-dimensional array from nested array, recursively
    #
    # @example
    #   using FlattenRefinement
    #
    #   [1, [2, [3]]].custom_flatten
    #   => [1, 2, 3]
    #
    # @return [Array]
    #
    def custom_flatten
      recursively_flatten(self)
    end

    private

    # @private handles array flattening recursively while detecting cycles
    #
    def recursively_flatten(array, visited = {})
      array.each_with_object([]) do |element, new_array|
        case element
        when Array
          object_id = element.object_id
          raise ArgumentError, "Cannot flatten array with cycles" if visited[object_id]
          visited[object_id] = true

          new_array.concat recursively_flatten(element, visited)
        else
          new_array << element
        end
      end
    end
  end
end

flatten_refinement_test.rb

require 'minitest/autorun'
require 'minitest/pride'
require_relative './flatten_refinement'

class TestFlattenRefinement < Minitest::Test
  using FlattenRefinement

  def test_flattens_nested_array_to_one_dimension
    assert_equal [1, 2, 3, 4, 5, 1, 2, 3, 4, 5], [[1,2,[3]], [4, 5], [], [1, [2, [3, 4, [5]]]]].custom_flatten
  end

  def test_empty
    assert_equal [], [].custom_flatten
  end

  def test_already_flat
    assert_equal [3, 3, 3], [3, 3, 3].custom_flatten
  end

  def test_does_not_modify_array
    array = [1, [2]]
    array.custom_flatten

    assert_equal [1, [2]], array
  end

  def test_raises_error_array_with_cycles
    array = [1]
    array << array

    assert_raises(ArgumentError) { array.custom_flatten }

    array_1 = [1]
    array_2 = [2]
    array_1 << array_2
    array_2 << array_1

    assert_raises(ArgumentError) { array.custom_flatten }
  end
end

Gemfile

source "https://rubygems.org"

gem "minitest"

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    minitest (5.8.4)

PLATFORMS
  ruby

DEPENDENCIES
  minitest

BUNDLED WITH
   1.12.0