flatten.rb

# business logic file
module DataStructure
  class Array
    def initialize(array)
      @array = array
    end

    def flatten
      _flatten @array
    end

    def flatten!
      @array = _flatten @array
    end

    def to_s
      @array.to_s
    end

    def inspect
      @array.inspect
    end

    private

    def _flatten(array, flatten_array = [])
      array.each do |element|
        if element.kind_of?(::Array)
          _flatten element, flatten_array
        else
          flatten_array.push element
        end
      end

      flatten_array
    end
  end
end

# test file
require 'test/unit'

module DataStructure
  class ArrayTest < Test::Unit::TestCase
    def test_should_flat_the_input_array
      data_provider.each do |test_data|
        assert_equal (DataStructure::Array.new(test_data[:input])).flatten, test_data[:output]
      end
    end

    def data_provider
      [
        {
          input: [],
          output: []
        },
        {
          input: [1,2,3],
          output: [1,2,3]
        },
        {
          input: [1,[2,3]],
          output: [1,2,3]
        },
        {
          input: [[1,2,[3]],4],
          output: [1,2,3,4]
        },
        {
          input: [[1,2,[3,[4,5,6]]],7],
          output: [1,2,3,4,5,6,7]
        }
      ]
    end
  end
end