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

FLATTEN.md

Flatten

A Ruby module to demonstrate flattening of a nested array without using Array#flatten:

Usage

Flatten.flatten([1, [2, [3]]])
=> [1, 2, 3]

Tests

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

Going Further

For an alternative usage, I also extracted the logic of this custom flatten implementation as a Ruby refinement.

flatten.rb

module Flatten
  # Returns new one-dimensional array from nested array, recursively
  #
  # @param array [Array] the array to be flattened
  #
  # @example
  #   Flatten.flatten([1, [2, [3]]])
  #   => [1, 2, 3]
  #
  # @return [Array]
  #
  def self.flatten(array)
    Flattener.new(array).flatten
  end

  # @private class to handle custom array flattening logic without using Array#flatten
  #
  class Flattener
    def initialize(array)
      @array = array
    end

    def flatten
      raise ArgumentError, "Argument #{@array.inspect} is not an array" unless @array.kind_of?(Array)

      recursively_flatten(@array)
    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_test.rb

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

class TestFlatten < Minitest::Test
  def test_flattens_nested_array_to_one_dimension
    assert_equal [1, 2, 3, 4, 5, 1, 2, 3, 4, 5], Flatten.flatten([[1,2,[3]], [4, 5], [], [1, [2, [3, 4, [5]]]]])
  end

  def test_empty
    assert_equal [], Flatten.flatten([])
  end

  def test_already_flat
    assert_equal [3, 3, 3], Flatten.flatten([3, 3, 3])
  end

  def test_does_not_modify_array
    array = [1, [2]]
    Flatten.flatten(array)

    assert_equal [1, [2]], array
  end

  def test_raises_error_on_non_arrays
    assert_raises(ArgumentError) { Flatten.flatten(nil) }
    assert_raises(ArgumentError) { Flatten.flatten("string") }
    assert_raises(ArgumentError) { Flatten.flatten(12345) }
    assert_raises(ArgumentError) { Flatten.flatten({}) }
  end

  def test_raises_error_array_with_cycles
    array = [1]
    array << array

    assert_raises(ArgumentError) { Flatten.flatten(array) }

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

    assert_raises(ArgumentError) { Flatten.flatten(array) }
  end
end

Gemfile

source "https://rubygems.org"

gem "minitest"

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    minitest (5.9.0)

PLATFORMS
  ruby

DEPENDENCIES
  minitest

BUNDLED WITH
   1.12.0