flatten_array.rb

class FlattenArray
  attr_reader :result

  def initialize(array)
    @array = array
  end

  def call
    @result = flatten(@array) || []
    @result
  end

  private

  def flatten(element)
    return [element] unless element.is_a?(Array)

    element.map{|e| flatten(e) }.inject(:+)
  end
end

class TestFlattenArray
  def self.run
    # test case 1
    array = []
    flatten_array = FlattenArray.new(array).call
    assertion = flatten_array == []
    puts "Test case 1: #{assertion} should be true"

    # test case 2
    array = [[1,2,[3]],4]
    flatten_array = FlattenArray.new(array).call
    assertion = flatten_array == [1, 2, 3, 4]
    puts "Test case 2: #{assertion} should be true"
  end
end

TestFlattenArray.run