Theorem

array_flattener.rb

class ArrayFlattener

    #recursive function
    def flatten_this(arr)
        raise ArgumentError, 'Argument is not an array' unless arr.is_a? Array 
        arr.each_with_object([]) do | element, flat_array |
            flat_array.push *( element.is_a?(Array) ? flatten_this(element) : element )
        end
    end

end

array_flattener_test.rb

require 'minitest/autorun'
require_relative 'array_flattener'

class ArrayFlattenerTest < Minitest::Unit::TestCase

    def setup
        @array_flattener = ArrayFlattener.new
    end

    def test_with_empty_array
        assert_equal [], @array_flattener.flatten_this([[]])     
    end

    def test_with_a_string
        assert_raises ArgumentError do
            @array_flattener.flatten_this("1,2,3,4")
        end
    end

    def test_with_nested_array
        assert_equal [1,2,3,4], @array_flattener.flatten_this([[1,2,[3]],4])
    end

    def test_with_flat_array
        assert_equal [1,2,3,4], @array_flattener.flatten_this([1,2,3,4])
    end

    def test_with_recurring_elements
        assert_equal [1,1,1,1,2,3,4], @array_flattener.flatten_this([[[1],[1]],1,[1],2,3,4])
    end
end